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1

Geological Society of America

Field Guide 6

2005

Basaltic volcanism of the central and western Snake River Plain: A

guide to fi eld relations between Twin Falls and Mountain Home, Idaho

John W. ShervaisDepartment of Geology, Utah State University, Logan, Utah 84322-4505, USA

John D. KauffmanIdaho Geological Survey, University of Idaho, Moscow, Idaho 83844-3014, USA

Virginia S. GillermanIdaho Geological Survey, Boise State University, Boise, Idaho 83725-1535, USA

Kurt L. OthbergIdaho Geological Survey, University of Idaho, Moscow, Idaho 83844-3014, USA

Scott K. VetterDepartment of Geology, Centenary College, Shreveport, Louisiana 71134, USA

V. Ruth HobsonMeghan Zarnetske

Department of Geology, Utah State University, Logan, Utah 84322-4505, USA

Matthew F. CookeScott H. Matthews

Department of Geological Sciences, University of South Carolina, Columbia, South Carolina 29208, USA

Barry B. HananDepartment of Geological Sciences, San Diego State University, San Diego, California 92182-1020, USA

ABSTRACT

Basaltic volcanism in the Snake River Plain of southern Idaho has long been associ-ated with the concept of a mantle plume that was overridden by North America during the Neogene and now resides beneath the Yellowstone plateau. This concept is consistent with the time-transgressive nature of rhyolite volcanism in the plain, but the history of basaltic volcanism is more complex. In the eastern Snake River Plain, basalts erupted after the end of major silicic volcanism. The basalts typically erupt from small shield volcanoes that cover up to 680 km2 and may form elongate fl ows that extend 50–60 km from the central vent. The shields coalesce to form extensive plains of basalt that mantle the entire width of the plain, with the thickest accumulations of basalt forming an axial

Shervais, J.W., Kauffman, J.D., Gillerman, V.S., Othberg, K.L., Vetter, S.K., Hobson, V.R., Zarnetske, M., Cooke, M.F., Matthews, S.H., and Hanan, B.B., 2005, Basaltic volcanism of the central and western Snake River Plain: A guide to fi eld relations between Twin Falls and Mountain Home, Idaho, in Pederson, J., and Dehler, C.M., eds., Interior Western United States: Geological Society of America Field Guide 6, p. xxx–xxx, doi: 10.1130/2005.fl d006(02). For permission to copy, contact [email protected]. © 2005 Geological Society of America

2 J.W. Shervais et al.

fl d006-02 page 2 of 26

INTRODUCTION

The Snake River Plain of southern Idaho is one of the most

distinctive physiographic features of North America (Fig. 1). The

topographic low that defi nes the plain cuts across the structural

grain of both the Idaho batholith and the Basin and Range prov-

ince—even though formation of the plain coincided in time with

Basin and Range extension. Evolution of the eastern Snake River

Plain has been associated with the passage of North America over

a mantle hotspot, forming a time-transgressive volcanic province

that youngs from WSW to ENE (e.g., Morgan, 1972; Suppe et

al., 1975; Armstrong et al., 1975). The onset of hotspot-related

volcanism is marked by a series of overlapping caldera com-

plexes, ignimbrites, and caldera-fi lling rhyolite lavas (Bonnich-

sen, 1982a, 1982b; Bonnichsen and Kauffman, 1987; Pierce and

Morgan, 1992; Christiansen et al., 2002). The early rhyolite com-

plexes were followed by extensive eruptions of plains basalts,

which form a carapace on top of the earlier rhyolites (Malde and

Powers, 1962; Greeley, 1982; Leeman, 1982; Kuntz et al., 1982,

1992; Malde, 1991). The hotspot is currently located under the

Yellowstone Plateau, which also forms the locus of a gigantic

geoid anomaly that underlies much of western North America

(Smith and Braile, 1994; Pierce et al., 2002).

The hotspot or plume model for the Snake River Plain is

supported by studies of tectonic uplift and collapse along the

plume track (Pierce and Morgan, 1992; Anders and Sleep, 1992;

Smith and Braile, 1994; Rodgers et al., 2002), the 1000-km-wide

geoid anomaly centered under Yellowstone (Smith and Braile,

1994), seismic tomography of the underlying mantle (Saltzer

and Humphreys, 1997; Jordan et al., 2004), and helium isotopes

(Craig, 1997). Alternate models have been proposed, however,

such as localized asthenospheric upwelling associated with edge

effects of North American plate motion, and counter fl ow created

by the descending Farallon slab (Humphreys et al., 2000; Chris-

tiansen et al., 2002).

There are two aspects of Snake River Plain volcanism that

are not readily explained by the hotspot model. First, the erup-

tion of basaltic lavas generally postdates passage of the hotspot

in time, and may continue 2–3 m.y. after the onset of hotspot

related volcanism farther to the NE. Second, the hotspot model

does not explain the origin of volcanic rocks that do not lie on

the presumed hotspot track; in particular, basalts of the western

Snake River Plain cannot be directly related to the passage of

North America over the Yellowstone hotspot, although models

that link volcanism in the western Snake River Plain indirectly

to the hotspot have been proposed (Geist and Richards, 1993;

Camp, 1995; Shervais et al., 2002; Camp and Ross, 2004).

Basalts of the Columbia Plateau, thought to represent the “head”

of the Yellowstone plume, also lie well north of the presumed

hotspot track. Several models have been proposed to explain

this anomaly, including defl ection of the plume by the Farallon

plate (Geist and Richards, 1993), compression of the plume head

by North American lithospheric mantle (Camp, 1995; Camp

and Ross, 2004), or location of the plume head below northern

Nevada (Pierce and Morgan, 1992; Pierce et al., 2002).

The central Snake River Plain lies at the junction of the

physiographic western and eastern Snake River Plain and rep-

resents a critical link between the two volcanic provinces. The

high along the length of the plain. In contrast, basaltic volcanism in the western Snake River Plain formed in two episodes: the fi rst (ca. 7–9 Ma) immediately following the eruption of rhyolites lavas now exposed along the margins of the plain, and the second forming in the Pleistocene (≤2 Ma), long after active volcanism ceased in the adjacent eastern Snake River Plain. Pleistocene basalts of the western Snake River Plain are intercalated with, or overlie, lacustrine sediments of Pliocene-Pleistocene Lake Idaho, which fi lled the western Snake River Plain graben after the end of the fi rst episode of basaltic volcanism. The contrast in occurrence and chemistry of basalt in the eastern and western plains suggest the interpretation of volcanism in the Snake River Plain is more nuanced than simple models proposed to date.

Keywords: basaltic volcanism, basalt geochemistry, Snake River Plain, Bonneville

fl ood.Easte

rn SRP

Western SRP

TF

MH

1

23

42º 00’41º 59’

117º 02’ 111º 05’

45º 30’

Figure 1. Physiographic map of the northwestern United States show-ing the Snake River Plain (SRP) and related features. Note the strong topographic expression of the plume track and the absence of Basin and Range extension across the axis of the plain. MH—Mountain Home; TF—Twin Falls.

Basaltic volcanism of the central and western Snake River Plain 3

fl d006-02 page 3 of 26

structural, geophysical, sedimentary, and volcanic features of

these provinces are distinct and require different origins. In this

guidebook, we present a brief synopsis of geologic relations

within and between the central and western Snake River Plain

and then examine the stratigraphy and volcanology of each prov-

ince in the fi eld guide section. A compendium of recent papers on

the Snake River Plain has recently been published by the Idaho

Geological Survey (Bonnichsen et al., 2002), and interested read-

ers are referred to that volume for more detailed information.

THE SNAKE RIVER VOLCANIC PROVINCE: GEOLOGIC SETTING

The central and western Snake River Plain comprise two dis-

tinct provinces with different crustal structure, stratigraphy, and

volcanic history. Both provinces are characterized by crust that

is thicker (40–45 km) than crust in the adjacent Basin and Range

province (≈33 km; Mabey, 1978, 1982; Iyer, 1984; Malde, 1991).

The western Snake River Plain is also characterized by a positive

gravity anomaly that trends parallel to the axis of the plain, and

magnetic anomalies that parallel its southern margin (Mabey,

1982). In contrast, gravity and magnetic anomalies in the eastern

Snake River Plain are subdued and defi ne a NW-trending texture

that is normal to the trend of the eastern plain, but parallels struc-

tural trends in the adjacent Basin and Range province (Mabey,

1982; Malde, 1991). These contrasts refl ect fundamental differ-

ences in the underlying structure and stratigraphy of the two ter-

ranes: the western Snake River Plain is a true graben, whereas the

eastern Snake River Plain is structurally downwarped with little

or no faulting along its margins (McQuarrie and Rodgers, 1998;

Wood and Clemens, 2002; Rodgers et al., 2002).

Central and Eastern Snake River Plain

The ENE-trending central and eastern Snake River Plain

begins as a gentle structural depression on the Owyhee Plateau

that deepens to the NE and merges with the physiographic Snake

River Plain near Twin Falls. This structural depression continues

to the NE until it merges with the Yellowstone Plateau (Rodgers

et al., 2002). The central Snake River Plain is defi ned loosely as

that part of the eastern Snake River Plain trend that lies between

the Owyhee Plateau and the Great Rift, or approximately between

116°W and 114°W along the axis of the plain.

The increase in elevation of the eastern Snake River Plain

from SW to NE is thought to result from thermal buoyancy in

the upper mantle under the hotspot (Dueker and Humphreys,

1990; Pierce and Morgan, 1992; Smith and Braile, 1994). The

progressive decay of this thermal anomaly with time has resulted

in tectonic collapse in the wake of the “deformation parabola”

that emanates from the hotspot (Anders and Sleep, 1992; Pierce

and Morgan, 1992; Smith and Braile, 1994). The elevation dif-

ference between the Owyhee Plateau and the eastern Snake River

Plain probably results from differences in the underlying crust:

the Owyhee Plateau is underlain by the southern extension of the

Idaho batholith, whereas the eastern Snake River Plain transects

older crust of the Basin and Range province (Malde, 1991).

The eastern Snake River Plain is characterized by 1–2 km of

basalt that overlies rhyolite and welded tuff (e.g., Leeman, 1982;

Kuntz et al., 1988, 1992; Greeley, 1982). Scientifi c drill holes at

the Idaho National Laboratory (INL) site show that the basaltic

suite ranges from <100 m to over 1500 m thick, with rhyolite

basement extending to depths in excess of 3000 m (Malde, 1991;

Hughes et al., 1999, 2002). Sedimentary intercalations consist-

ing of fl uvial sands, lacustrine muds, windblown sand, and loess

range from 2 m to ≈25 m thick.

The rhyolite eruptive centers consist of overlapping caldera

complexes and ignimbrites that represent the initial volcanic

activity at any given location along the axis of the Snake River

Plain, and are thought to mark the arrival of the hotspot (e.g.,

Bonnichsen, 1982a; Christiansen, 1982; Draper, 1991; Morgan,

1992). Rhyolite eruptive centers become younger from SW to

NE: the Bruneau-Jarbidge eruptive center (12.5–11.3 Ma), the

Twin Falls eruptive center (10.0–8.6 Ma) the Picabo eruptive

center (10.2 Ma), the Heise eruptive center (6.7–4.3 Ma) and the

Island Park–Yellowstone eruptive center (1.8–0.6 Ma). The oldest

basalts in the central and eastern Snake River Plain are slightly

younger than the eruptive centers they mantle; the youngest

basalts erupted from a series of NW-trending volcanic rift zones

during the Holocene (e.g., the Shoshone lava fl ow, Craters of the

Moon, the Great Rift, Hells Half Acre, Wapi lava fl ow), with fl ows

as young as 2000 yr B.P. reported (Kuntz et al., 1986).

Geophysical studies of the eastern Snake River Plain

have documented differences in both the crustal structure and

lithosphere-asthenosphere boundary relative to both the adjacent

Basin and Range province and the Archean Wyoming craton

(Braile et al., 1982; Iyer, 1984; Dueker and Humphreys, 1990,

1993; Humphreys and Dueker, 1994; Peng and Humphreys,

1998; Saltzer and Humphreys, 1997; Humphreys et al., 2000).

The most signifi cant crustal feature observed in seismic profi les

of the eastern Snake River Plain is a midcrustal “sill” ≈10 km

thick and 90 km wide that underlies the entire width of the east-

ern Snake River Plain, with seismic velocities (≈6.5 km/s) inter-

mediate between the mafi c lower crust and the more felsic inter-

mediate crust (Braile et al., 1982; Peng and Humphreys, 1998).

This mafi c “sill” is inferred to represent basaltic melts that were

intruded into the crust and either fractionated to form the high

temperature rhyolites of the eastern Snake River Plain (McCurry

and Hackett, 1999) or induced partial melting of the crust to form

these rhyolites (Leeman, 1982; Bonnichsen, 1982a, 1982b; Doe

et al., 1982). The crust is also underlain by a low velocity layer

that is thought to be partially molten (Peng and Humphreys,

1998). This low-velocity layer apparently thickens to the NE,

toward Yellowstone, where it dominates the lower crustal section

(Priestley and Orcutt, 1982).

Teleseismic tomography across the eastern Snake River

Plain and adjacent domains show that the subcontinental litho-

sphere directly beneath the plain has been eroded to form a deep

channel parallel to North American plate motion (Dueker and


fl d006-02 page 4 of 26

Humphreys, 1990; Humphreys and Dueker, 1994; Saltzer and

Humphreys, 1997; Humphreys et al., 2000). The channel con-

sists of low velocity, partially molten asthenosphere buttressed by

levees of high-velocity, melt-depleted mantle. The depth of the

lithosphere-asthenosphere boundary beneath the plain is roughly

constant parallel to its axis (Saltzer and Humphreys, 1997). Since

the subcontinental mantle lithosphere is thicker toward the NE,

the sublithospheric channel must be more deeply eroded into the

lithosphere to the NE.

Tomographic images of the Yellowstone plume show that it

dips steeply to the north from Yellowstone and extends to a depth

of 400–600 km (Montelli et al., 2003; Jordan et al., 2004; Dueker

et al., 2004). This is consistent with recent models of deep-seated

mantle plumes, which show that they may tilt signifi cantly from

vertical and follow complex fl ow lines within the mantle (e.g.,

King, 2004; Farnetani and Samuel, 2004).

Western Snake River Plain

In contrast to the eastern Snake River Plain, the western

Snake River Plain is a NW-trending structural graben, ≈70 km

wide and 300 km long, bounded by en echelon normal faults

and fi lled with up to 2–3 km of late Miocene through Pliocene

sediment (Wood and Clemens, 2002; Shervais et al., 2002).

Structural relief, based on deep drill holes that intercept base-

ment, ranges from 2900 m near Mountain Home to over 3500 m

on the southwest side of the valley (Malde, 1991). Sedimentary

deposits in the western Snake River Plain range in age from

Miocene through Quaternary; these deposits are dominantly

lacustrine, with subordinate fl uviatile and phreatomagmatic

deposits (Wood and Clemens, 2002; Godchaux and Bonnichsen,

2002; Shervais et al., 2002). Miocene sediments were deposited

in small, interconnected lakes, precursors to the large Pliocene

“Lake Idaho” (Kimmel, 1982; Smith et al., 1982; Jenks and Bon-

nichsen, 1989; Malde 1991; Godchaux et al., 1992; Jenks et al.,

1993; Wood and Clemens, 2002). The western Snake River Plain

is topographically lower than the eastern Snake River Plain, with

elevations ranging from 670 m to 1100 m (Malde, 1991), but

recent data suggest that an ancestral Snake River system fl owed

southwards during the Miocene, implying higher elevations to

the NW (Smith and Stearley, 1999; Link and Fanning, 1999).

Paleontological evidence similarly suggests southward drainage

in the late Miocene (Repenning et al., 1995).

Volcanic activity in the western Snake River Plain began

with the eruption of the silicic volcanic rocks along the northern

and southern margins of the basin between ca. 11.8 and 9.2 Ma

(Clemens and Wood, 1993; Wood and Clemens, 2002). Major

basaltic activity in the western Snake River Plain occurred in two

time periods: 9–7 Ma, and <2.2 Ma (White et al., 2002; Bon-

nichsen and Godchaux, 2002). The earlier episode is represented

by basalt fl ows and phreatomagmatic vents intercalated with late

Miocene sediments and by the acoustic basement that underlies

much of the western graben (Malde and Powers, 1962, 1972;

Amini et al., 1984; Malde, 1991; Wood and Clemens, 2002;

White et al., 2002; Shervais et al., 2002). The older (7–9 Ma)

lavas and late Miocene to Pliocene sediments comprise the Idaho

Group of Malde and Powers (1962). Young volcanic activity

(<2.2 Ma) in the western Snake River Plain consists of: (1) pla-

teau forming eruptions of tholeiitic basalt that form the volcanic

uplands north and south of the Snake River, (2) tholeiitic shield

volcanoes that sit on top of these uplands, and (3) young shield

and cinder cone vents of alkaline to transitional alkaline basalt

(Shervais et al., 2002; White et al., 2002). The younger lavas

comprise the Snake River Group of Malde and Powers (1962)

and correlate with the more abundant young volcanic rocks of

the eastern Snake River Plain (Leeman, 1982). Volcanic rocks

of similar age and character are also found in the Boise River

drainage 40 km north of Mountain Home (Howard and Shervais,

1973; Howard et al., 1982; Vetter and Shervais, 1992).

The physiographic junction between the western and eastern

portions of the plain is located west of Twin Falls, Idaho (Fig. 1).

Structurally, boundary faults of the western Snake River Plain

graben extend SE into the eastern Snake River Plain plume track

as far as Hagerman, and linears extend as far as Buhl, near Twin

Falls. The axial Bouguer gravity high of the western Snake River

Plain also extends into the plume track, and is defl ected eastward

south of the Mount Bennett Hills (Mabey, 1976, 1982). The

positive magnetic anomaly along the SW margin of the western

Snake River Plain curves to the NE where it intersects the plume

track (Mabey, 1982), possibly outlining the southern margin of

the Bruneau-Jarbidge eruptive center (Bonnichsen, 1982a). The

NW-trending gravity and magnetic anomalies, which character-

ize the eastern Snake River Plain, are absent in this area.

BASALTIC VOLCANISM: FIELD RELATIONS AND GEOCHEMISTRY

Central Snake River Plain

Late Neogene basalts of the central Snake River Plain north

of Twin Falls form large shield volcanoes (Fig. 2) clustered along

the axis of the plain, which overlie rhyolite of the Twin Falls

eruptive center (Bonnichsen and Godchaux, 2002). This area

has been mapped in detail during the past decade by the Idaho

Geological Survey, by graduate students from the University of

South Carolina and Utah State University, and by undergradu-

ates from Centenary College. The results of this work have been

compiled by the Idaho Geological Survey and are being prepared

for publication as the Twin Falls 30′ × 60′ quadrangle geological

map. Readers are referred to the preliminary copy of that map for

the names, and extent of volcanic units in the central Snake River

Plain and their relation to sedimentary units.

All of the hills seen on this part of the plain are constructive

volcanic vents. The shield volcanoes of the oldest vents have sub-

dued topography and radial drainage and a thick cover of loess

with well-developed soils, and they are typically farmed almost

all of the way to the vent summits (e.g., Flat Top Butte, Johnson

Butte). Many of the older vents appear smaller than the largest


fl d006-02 page 5 of 26

young vents because their fl anks have been partially buried by

younger lava fl ows (e.g., Skeleton Butte, Bacon Butte, Lincoln

Butte). The surfaces of fl ows from younger vents are character-

ized by extremely rugged, chaotic topography, with infl ated fl ow

fronts, collapsed fl ow interiors, ridges, and collapse pits (e.g.,

Owinza Butte, Rocky Butte, Notch Butte, Wilson Butte). These

surfaces lack established drainages but thin loess is present in

local depressions. The surfaces of fl ows from vents of intermedi-

ate age have nearly continuous loess mantles of variable thickness

and well-developed soils, but lack well defi ned surface drainage

and are rarely farmed (e.g., Crater Butte, Dietrich Butte). Where

the Bonneville Flood fl owed overland, loess and soil were com-

monly stripped from basalt surfaces yielding surface morphology

that appears younger then it should, which complicates identifi -

cation of basalt units based on surface characteristics.

Shield sizes vary and are diffi cult to constrain with con-

fi dence because the onlapping of younger lava fl ows often

conceals the true size of the older shields. Even so, the largest

are commonly 15–20 km across the main shield edifi ce—not

counting elongate fl ows that may extend 50–60 km from the

vent. Perhaps the most extensive shield in the region is Black

Ridge Crater, located at the eastern edge of the central plain

(113.96°W), which is ≈32 km across and covers ≈680 km2—all

or parts of eight 7.5′ quadrangles.

Shield volcanoes with extremely long fl ows include Black

Ridge Crater, Notch Butte, Wilson Butte, and Rocky Butte. The

Black Ridge Crater vent has a major sinuous lobe that fl owed

over 40 km from the vent, terminating just south of Notch Butte.

Rocky Butte and Wilson Butte both fed extensive fl ow fi elds.

Flows from Notch Butte extend west as far as Hagerman, a dis-

tance of ~42 km.

A few radiometric dates are available for basalts in the cen-

tral Snake River Plain (Armstrong et al., 1975; Tauxe et al., 2004;

Idaho Geological Survey, 2005, unpublished data). These dates,

along with stratigraphic relations, indicate that most of the shield

volcanoes north of the Snake River are Quaternary, whereas

those south of the river are early Quaternary or Tertiary. Older

volcanoes north of the river, such as Lincoln Butte, Johnson

Butte, and Sonnickson Butte, are surrounded and nearly buried

by younger basalt fl ows, and typically have subdued morphology

and radial drainage patterns. Remanent magnetic polarity of most

of the Quaternary basalts north of the river is normal and within

the Bruhnes polarity chron (<0.78 Ma); an exception is Sid Butte,

which has reverse polarity, but because of its geomorphic charac-

teristics it is thought to be early Quaternary and in the later part

of the Matuyama chron. The older basalt units south of the river

have both normal and reverse polarity and represent Tertiary to

early Quaternary polarity events.

Basalts of the central Snake River Plain have MgO similar to

mid-ocean ridge basalt (MORB; 5%–10%) but with higher FeO*

(12%–15%), TiO2 (1.6%–4.3%), P

2O

5 (0.4%–1.2%), and Al

2O

3

(14%–17%). They are also higher in FeO* than similar basalts of

the eastern Snake River Plain (e.g., Idaho National Engineering

Laboratory or INEL site) but less Fe-rich than typical ferrobasalts

of the western Snake River Plain near Mountain Home (Fig. 3).

The wide range in K, P, and K/P ratios at constant MgO implies a

range of parent magmas derived from a similar source by variable

degrees of partial melting. Fe8 values (≈13) imply deep melting

or a source higher in FeO than MORB asthenosphere, while Na8

values (2.4–3.2) imply moderate but variable degrees of partial

melting. Partial melting models based on 18 incompatible trace

elements indicate 5%–10% melting of a spinel lherzolite source

similar in composition to the E-MORB source. Garnet-bearing

sources are ruled out by the slope of the rare earth element pat-

terns, implying pressures less than 20–25 Kb, i.e., within the

sublithospheric channel that has been imaged seismically.

Most of the chemical variation within fl ows from single

vents can be explained by low-pressure fractionation of the

observed phenocrysts (olivine + plagioclase). Line scans of

olivine phenocrysts show no reversals in composition or other

evidence of magma mixing. Line scans of plagioclase pheno-

crysts show minor reversals that indicate fl uctuations in magma

chamber vapor pressures (Cooke, 1999; Matthews, 2000). The

occurrence of cumulate gabbro xenoliths (ol+cpx+plg+oxide) in

the Sid Butte vent is consistent with high-pressure fractionation

at midcrustal levels, within the “basaltic sill” imaged seismically

beneath the eastern plain (Matthews, 2000). The incompat-

ible element ratios K/P and K/Ti decrease with mg# ( = molar

100*Mg/[Mg+Fe]), ruling out extensive assimilation of older,

A

B

Figure 2. Field photographs contrasting the stratigraphy and topogra-phy of the central and western Snake River Plain. (A) Shield volcanoes of the central Snake River Plain north of Twin Falls. (B) Lacustrine sediments of the Glenns Ferry Formation near Mountain Home, over-lain by plateau-forming Pleistocene basalts.


fl d006-02 page 6 of 26

2.02.53.03.54.04.5

121314151617

11 121314151617

89

1011 12

2.5

3.0

3.5

.5

1.0

.5

1.0

1.5

250

300

350

30

40

50

60

5678910

MgO

150200250300350400

ShoshoneMtn Home Tholeiites Mtn Home Alkali

Figure 3. MgO variation diagrams for basalts of the central and western Snake River Plain; data from Cooke (1999), Matthews (2000), and unpublished results (Shervais). The tholeiitic basalts show extensive Fe and Ti enrichment not seen in the alkali series basalts, which are much higher in K. Variations in alumina at low MgO result from plagioclase accumulation or removal.


fl d006-02 page 7 of 26

felsic crust; these trends may be due to assimilation of previously

injected gabbroic dikes at midcrustal depths. We infer that these

basalts represent a mixed asthenospheric-lithospheric source that

formed in response to the Yellowstone melting anomaly; these

melts evolved by a combination of high-pressure and low-pres-

sure crystal fractionation, with possible assimilation of previ-

ously intruded midcrustal ferrogabbros (Shervais et al., 2004).

Western Snake River Plain

The western Snake River Plain graben formed over a short

time period ca. 10–12 Ma, coincident with the eruption of the

extensive rhyolites that now form the fl anks of this structure

(Wood and Clemens, 2002). Late Miocene basalts (9–7 Ma)

underlie the central part of the graben, as shown by geophysical

surveys and deep drilling results (Wood, 1994; Wood and Clem-

ens, 2002; Shervais et al., 2002). Overlying these basalts (and to

some extent interbedded with them) are up to 2–3 km of lacus-

trine and fl uviatile sediments that form the “Idaho Group” of

Malde and Powers (1962): Chalk Hills Formation (oldest), Poi-

son Creek Formation, Glenns Ferry Formation, and the Bruneau

Formation. Phreatomagmatic vents intercalated with sediments

of the Idaho Group are common in the area west of Mountain

Home, between Grand View and Marsing along the Snake River

(Godchaux et al., 1992; Godchaux and Bonnichsen, 2002; Bon-

nichsen and Godchaux, 2002).

Mountain Home occupies an upland plateau above the

Snake River fl oodplain, which is incised into fi ne-grained lacus-

trine sediments deposited by Lake Idaho during the late Mio-

cene–early Pliocene (Fig. 2). Late Pliocene to early Pleistocene

basalts sit conformably on the lacustrine strata, or occur within

the uppermost few hundred feet. Flow of these basalts into the

shallow margin of the lake resulted in deltas of hyaloclastite brec-

cia and pillow lava, which pass upward into subaerial pahoehoe

fl ows. Farther inland, fl ows are separated by deposits of fl uvial

gravel and sand. These plateau-forming basalts are overlain by a

series of younger lavas erupted from 13 central vents that cluster

near the NE margin of the plateau. The shield volcanoes rise

120–210 m above the surrounding plateau; several are capped

by central depressions that probably represent former lava lakes

(pit craters). Pleistocene cinder cones are the youngest volcanic

features, and may cap small shield volcanoes.

Recent tectonic activity is demonstrated by fault scarps with

~2–9 m of throw that crosscut all of the volcanic features. There

are two dominant fault sets: one trends N85W, the other trends

N60W (parallel to the range front faults) and appears to truncate

against the N85W set. All of the faults are vertical or high angle

normal faults that generally dip steeply to the south and are down-

thrown on the south or SW side (a few are downthrown to the N,

forming small grabens with adjacent faults). The young faulting

may be related to Basin and Range extension that has refracted

to more westerly orientation to exploit preexisting fault zones of

the western Snake River Plain. This relationship suggests that

post–Lake Idaho volcanism in the western Snake River Plain

may be related to reactivated partial melting of plume-modifi ed

lithosphere, in response to the onset of Basin and Range exten-

sion (Wood and Clemens, 2002; Shervais et al., 2002).

Basaltic lavas of the western Snake River Plain are generally

distinct from lavas of the eastern Snake River Plain trend, and are

commonly ferrobasalts with up to 17% FeO* (Fig. 3; Shervais et

al., 2002; White et al., 2002). This is clearly seen in the Bruneau-

Jarbidge eruptive center, where basalts of the eastern Snake River

Plain trend are overlain by younger ferrobasalts of Salmon Falls

Butte, which represents the SE extent of western Snake River

Plain magmatism (Vetter and Shervais, 1997). In general, the

oldest (pre–Lake Idaho) basalts are the most Fe-rich; younger

basalts include ca. 2.0–0.7 Ma Fe-Ti basalts (equivalent to the

Boise River Group 1 of Vetter and Shervais, 1992 and group M2

of White et al., 2002) and the <0.7 Ma alkali basalts which are

high in K2O and MgO (equivalent to the Boise River Group 2 of

Vetter and Shervais, 1992 and group M3 of White et al., 2002).

This same sequence of basalts is observed in the Melba area,

100 km west of Mountain Home (White et al., 2002) and along

the Boise River in the Smith Prairie area, 40 km north of Moun-

tain Home (Vetter and Shervais, 1992).

HYDROGEOLOGY OF BASALTIC VOLCANISM IN THE CENTRAL SNAKE RIVER PLAIN

Detailed mapping in the central Snake River Plain has

revealed new details of groundwater recharge and fl ow, and

allows the formulation of new models for the formation of high

conductivity aquifers.

Groundwater fl ow in the Snake River Plain is commonly

thought to be controlled by relatively porous horizons between

lava fl ows (Lindholm and Vaccaro, 1988; Welhan et al., 2002c).

These so-called interfl ow zones are characterized by complex

geometries refl ecting the fractal nature of the pahoehoe infl ation-

ary process, the quasi-stratigraphic layering and interfi ngering of

fl ows within and between lava fi elds (shield volcanoes), and the

development of tension fi ssure networks along the margins of

infl ated fl ow structures (Welhan et al., 2002b). These permeable

zones can be mapped in the subsurface using core and geophysi-

cal logs and their autocorrelation length scales used to construct

stochastic models of subsurface pathways of preferential fl ow

(Welhan et al., 2002a). Direct evidence of the “pipeline” nature

of this fl ow, as refl ected in extremely low dispersivities associated

with mass transport on scales of 25 km, has been documented by

Cecil et al. (2000) using chlorine-36.

The mapping of Cooke (1999) and Matthews (2000) shows

that thick alluvial intervals are most common where adjacent

volcanic edifi ces abut one another and overlap. These overlap-

ping fl ows create moats, which control the location of surface

stream drainages and fi ll with coarse alluvium. Younger lava

fl ows are channeled into these drainages, displacing the streams

and covering the alluvium with relatively thick, semi-permeable

caps, forming elongate aquifers with extremely high conductivi-

ties that follow the preexisting paleo-drainage. Reconstruction of


fl d006-02 page 8 of 26

basaltic volcanism through time using detailed geologic maps

allows us to predict the location of paleo-drainage and elongate

alluvial aquifers. This ability to predict aquifer location will

prove to be a valuable tool as increased demands are placed upon

the groundwater supply by agriculture and population growth.

In addition, our mapping reveals that young basalt vents seldom

exhibit channelized drainage systems that connect with major

through-going streams. Instead, the rugged volcanic topogra-

phy of ridges, fl ow fronts, lava channels, and collapse pits trap

precipitation, which must either evaporate or percolate into the

fractured lavas to recharge the local groundwater. These young

volcanic features constitute “negative basins” of interior drain-

age, despite their topographic emergence. We suggest that these

young basalts represent recharge zones that can be easily mapped

and distinguished from older fl ows that display well-developed

external drainage (Cooke and Shervais, 1999).

On a more regional scale, the eastern and central Snake

River Plain basalts and intercalated units form the Snake River

Plain aquifer, which along with the Snake River system sup-

plies water to southern Idaho and are essential to the agricultural

development of the state. Water recharges from snow melt in the

mountains north of the plain and in local basalt recharge areas.

Groundwater fl ow is from the NE to SW and it discharges as

springs in the Thousand Springs and other reaches of the north

and east wall of the Snake River Canyon between Twin Falls

and King Hill. The spring water is used extensively by the aqua-

culture industry, but quantity and quality of the water has been

declining due to increased groundwater pumping upgradient,

drought, and changes in surface irrigation systems. Spring and

groundwater irrigation is supplemented by surface withdrawals

from the Snake River, but acute water shortages are posing a

political dilemma for Idaho politicians and water experts. Map-

ping of springs in the northwall of the Snake River Canyon by

Covington and Weaver (1989), the Idaho Geological Survey, and

others suggests a geologic control. Covington et al. (1985) (and

others before them) recognized that some springs discharge at

rubble zones at the base of where canyon-fi lling late Quaternary

basalts entered a paleodrainage system. It also appears that the

topography developed on the older, slightly altered (Banbury)

basalts may have infl uenced spring discharge, as many springs

seem to emerge through talus but just above the Quaternary-

Tertiary unconformity. The older units may locally form a more

impermeable horizon and basal aquitard.

FIELD TRIP GUIDE

Day 1 (Half Day) Hagerman Valley

Directions to Stop 1.1

From Hagerman, drive south on Idaho Highway 30 ~7 mi

(11 km) across the bridge over the Snake River to ~1 mi south of

Thousand Springs Resort. Pull off to the right on a small dirt road

across from the entrance gate to a big house on the river. Stop 1

lies ~200 ft (60 m) uphill to the right. Bonneville fl ood effects in

the Hagerman Valley are shown on Figure 4; all Day 1 stops are

shown on Figure 5.

On the way to Stop 1, we will drive by scour features and

deposits created by the Bonneville Flood. The following paragraphs

briefl y describe some of the effects of the fl oodwaters (Fig. 4).

About fi fty years ago, geologists fi rst recognized that giant

gravel bars and stark erosional features common along the Snake

River were caused by cataclysmic lowering of Pleistocene Lake

Bonneville. The most complete descriptions and analyses of the

Bonneville Flood were written by Malde (1968) and O’Connor

(1993). O’Connor (1993, p. 1) writes, “Approximately 14,500

years ago, Pleistocene Lake Bonneville discharged 4750 km3 of

water over the divide between the closed Bonneville basin and

the watershed of the Snake River. The resulting fl ood, released

near Red Rock Pass, Idaho, followed the present courses of

March Creek, the Portneuf River, and the Snake and Columbia

River before reaching the Pacifi c Ocean. For 1100 km between

Red Rock Pass and Lewiston, Idaho, the Bonneville Flood left a

spectacular array of fl ood features.” O’Connor (1993) goes on to

describe the hydrology, hydraulics, and geomorphology of the

fl ood and provides a picture of the history and character of the

fl ood and its many landforms and deposits. A major characteristic

of the fl ood is a stepwise drop in the water surface caused by the

diverse geologic and geomorphic environments along the fl ood

route. A repeated phenomenon is a narrow canyon with con-

stricted but high-discharge fl ow that opens up into a wider valley

with less restricted, lower-discharge fl ow. In many locations, a

constriction back-watered the fl ood causing inundation and over-

land fl ow. East of Twin Falls fl ood water diverted north of the

Snake River canyon scoured a long stretch of basalt surfaces and

rejoined fl ood waters in the canyon for ~17 km forming several

systems of cataracts and plunge pools (see Day 3, Stop 3.6).

The areas of Hagerman and Thousand Springs (Day 1) show

the stepwise nature of the fl ood’s energy and the resulting features.

The Thousand Springs reach was a constriction that overfi lled and

the overland fl ow scoured and plucked basalt at the Sand Springs

Nature Preserve, Box Canyon, and Blind Canyon. South of Thou-

sand Springs a dry valley appears to have been the previous loca-

tion of the confl uence of Salmon Falls Creek and the Snake River.

The fl ood may have scoured a new channel for the Snake River and

subsequently the river abandoned the previous valley. Just down-

stream of Thousand Springs, where the Hagerman valley opens up,

energy dropped and the fl oodwaters deposited a large expansion

bar 5 km long with 4 m boulders at its upstream end. At maximum

discharge, the fl oodwaters would have been ~60 m deep at the

location of the Hagerman Inn. In the bottom of the valley, thick

deposits of Yahoo clay that had buried basalt of Notch Butte were

stripped away and the surface of the basalt was scoured.

Stop 1.1—Banbury Basalt and Bacon Butte Basalt: Basalt Water Interaction [N 42°43.804′, W 114°51.079′]

Mapping and reexamination of the “older” volcanic units

exposed south of Hagerman is one phase of the Idaho Geological


fl d006-02 page 9 of 26

Survey’s STATEMAP project in the Twin Falls 30′ × 60′ quad-

rangle. The area includes Banbury Hot Springs, type locality for

the Tertiary Banbury Basalt, which Malde et al. (1963) mapped

over a wide area of southwestern Idaho. Our work and the excel-

lent mapping of Malde and Powers (1972) show much hetero-

geneity within the “type section.” The lower Banbury Basalt

includes a fi eld of hydrovolcanoes, which appear to be localized

along a NW-trending structural zone that also infl uenced later

graben development and formation of Pliocene Lake Idaho, geo-

thermal activity, and canyon development. The inferred structure

is suspiciously close to a projection (S49E) of the northern mar-

gin of the western Snake River Plain.

Figure 4. Topographic map of Thousand Springs and the southern Hagerman Valley showing erosional and depositional features of the Bonnev-ille Flood. Large blue arrows show inferred fl oodwater fl ow directions. The arrow pointing northwest is placed to show the change from scoured rock to the bouldery expansion bar deposited in the broadening valley.


fl d006-02 page 10 of 26

Stratigraphically overlying Miocene rhyolite, the Miocene-

Pliocene Banbury Basalt consists of a lower sequence of altered

basalt fl ows and vent deposits, a middle layer of basaltic pyro-

clastic deposits overlain by and/or interbedded with sediments,

which thicken to the south, and an upper sequence of basalt fl ows

(Malde and Powers, 1972). The vent facies of the lower Banbury

Basalt includes at least two exposed volcanic centers (Thousand

Hill and Riverside Ferry vents), which are characterized by

tuff breccias with laterally extensive and locally palagonitized

surge and tephra deposits. At Stop 1.2, spectacular 10-m-high

beds of tuff breccia contain blocks larger than 1 m. At Stop 1.1,

distal phreatomagmatic tuffs containing volcanic bombs, stream

pebbles, and siltstone xenoliths are overlain by a 1-m-thick bed

of black spatter. The spatter is overlain by an oxidized tuff with

small glass bombs, and above that are three upward-coarsening

cycles of air-fall tuff. Unconformably overlying the emergent

hydrovolcano is a baked siltstone and the upper Banbury Basalt,

which here consists of notably fresher, plagioclase-phyric, vesic-

ular basalt fl ows. Elsewhere in the map area, the upper Banbury

consists of altered or water-affected basalt (WAB of Godchaux

and Bonnichsen, 2002) with a different magnetic polarity than

the feldspar-rich fl ows. Pending geochronology and chemistry

may improve age constraints and correlations. Even at its type

locality, the Banbury Basalt is lithologically heterogeneous with

lateral facies changes indicating basaltic volcanism within a

lacustrine and fl uvial setting prior to Lake Idaho and probably

over a considerable time span (Gillerman, 2004). The heteroge-

neity, polarity reversals, and spatial distribution of the fl ows also

indicate a variety of sources for the Banbury units. As noted by

others, it is time to revise the nomenclature. The recent Idaho

Geological Survey mapping, which will be displayed on the fi eld

trip, has renamed and more precisely subdivided many units

based on mapping and paleomagnetism, combined with chemis-

try and a very few radiometric age dates.

Stop 1.1A—Exposure of the Lower Banbury Distal Vent Facies Volcaniclastics

The prominent outcrops to the right of the gully are massive

basaltic tuffs in 3–4 upward-coarsening cycles, 2–3 m thick, with

laminated to cross-bedded tuff above and thin (6–8 cm), locally

cross-bedded, fi ne tuff below (Fig. 6A). This sequence overlies

a juvenile conglomerate of basalt, hyaloclastites, and some

rounded stream cobbles, that transitions up to a bleached/oxi-

dized (?) pumice bed (Fig. 6B). Below these beds are vent-like

spatter and pillowy basalt with abundant fresh glass. Although

the base of the sequence here is unexposed, Malde and Powers

(1972) mapped the lower portion of the slope as lower Banbury

sediments and tuff, most likely from better exposures ~1 mi to

the north, near the Sligar’s Thousand Springs Resort. There, a

42º43’ 48” N, 114º 51’ 05” WStop 1.1

Stop 1.2

Stop 1.3

Stop 1.4

42º43’ 35” N, 114º 51’ 21” W

42º43’ 11” N, 114º 51’ 24” W

42º42’ 24” N, 114º 50’ 23” W

Figure 5. Topographic map of the Snake River canyon between Banbury Springs and Sand Springs showing locations for Stop 1 of the fi eld trip, and ero-sional and depositional features of the Bonneville Flood. Relict fl ood features include scoured and plucked basalt and abandoned valley with no established drainage.


fl d006-02 page 11 of 26

section of mixed tuffs, sandstones, and massive basaltic units

are exposed in a gully and in old road cuts. The massive brown

and black mottled basalts are coarse grained, magnetic, and

look almost gabbroic. An exposure of the mottled rock is just

below the Stop 1.1B outcrop. Godchaux and Bonnichsen (2002)

referred to these massive mottled rocks as “spotted” due to black

augite crystals in a lighter brownish groundmass, which might

include glass and fi brous and hydrous minerals such as zeolites,

chlorite, or amphiboles. They interpret the spotted, massive lavas

as water-affected basalt, which formed in deep water (over a

few tens of feet). If so, then the volcaniclastic sediments should

be indicative of such an environment. It may be that volcanic

eruption of fl ows, tectonic activity, and climatic cycles created

a rapidly changing and laterally heterogeneous distribution of

lakes and sedimentary facies during deposition of the “Banbury

Basalt” and related units.

At this stop, the spatter layers dip into the hill (strike N65E,

dip 11°NW to EW and 14°N) and as seen from a distant photo

(Fig. 6C) the entire lower Banbury Basalt section (renamed Tvd

or vent deposits, distal), including the baked siltstone above,

appears to be tilted 10–15°NNW – possibly faulted along a

WNW normal fault. At this location, the upper Banbury lavas are

clearly unconformable on top of the lower Banbury volcaniclas-

tics. The upper Banbury lavas are fl at lying and mostly subaerial,

but the lower part is glassy, popcorn-like, with quasi-pillows and

glass. The fl ows here are distinguished by abundant feldspar phe-

nocrysts and normal magnetic polarity, and have been renamed

the Basalt of Oster Lakes (Tob) for exposures farther east and

several hundred feet topographically lower. Structural relation-

ships, age, and exact correlations of these various exposures are

problematic.

Stop 1.1B—Quaternary Basalt of Bacon Butte: Water Escape Features

The Basalt of Bacon Butte (formerly included in Sand

Springs basalt by Malde and Powers, 1972) fi lls paleo-drainage

here, with evidence for water interaction at base (pillows and hya-

loclastites) and vertical dewatering conduits that penetrate entire

fl ow (Fig. 7). The water-escape structures in this fl ow constitute a

feature of volcanic rocks that have not been described previously.

They consist of subvertical conduits ≈1 m in diameter (0.5–1.5 m

range) and spaced 5–15 m apart, partly fi lled with spicaceous

basaltic rubble. Columnar jointing in the adjacent basalt wraps

from vertical along the base of the fl ow to horizontal adjacent to

the conduits. Vesicles are more common in the wallrock of the

conduits than in parts of the fl ow farther away, and the basalt is

quenched to a glassy or aphanitic texture in the wallrock adjacent

to the conduit (Fig. 7A). In many of the conduits, basalt from the

adjacent wallrock forms a lattice or trellis arrangement within the

conduit that is contiguous with basalt of the wallrock (Fig. 7B).

The basalt in this trelliswork has glassy margins and spikey

projections along its surface. Spicaceous rubble is common in

all conduits, often forming discrete fragments that resemble

popcorn. This spicaceous rubble of “popcorn” basalt does not

represent later fi ll or pieces broken from the wallrock, but rather

blobs of lava that were isolated within the conduit while steam

was actively rising through the fl ow (Fig. 7C).

The upper surface of the lava fl ow contains depressions as

much as 10 m across and 1.5 m deep centered above the water-

escape structures, which appear to represent areas where the

surface lava collapsed into the conduit during water escape. Such

A

B

C

Figure 6. Banbury Basalt in Hagerman Valley. (A) Lower Banbury Basalt, volcaniclastics and phreatomagmatic vent facies (Stop 1.1). (B) Bleached pumice bed in lower Banbury Basalt. (C) Unconformity of upper Banbury Basalt (Basalt of Oster Lakes) over tilted section of lower Banbury phreatomagmatic volcaniclastics with primary and probable structural dip (Stop 1.1). Fence posts for scale.


fl d006-02 page 12 of 26

a feature is well illustrated at another location by the occurrence

of a ropey pahoehoe fl ow surface that is preserved deep inside a

lava fl ow within a steam conduit.

The subaerial lava fl ow with water escape structures is

underlain by pillow lava that fi lls intervening depressions in the

paleosurface. Many of these are mega-pillows as much as 4 m

across, which may contain water-escape structures that continue

up into the subaerial fl ow. In addition, the interface between the

pillow zone and the subaerial fl ow commonly contains small

caves and cavities that represent large steam bubbles (>1 m

across) trapped below the lava fl ow (Fig. 7D).


Continue South 0.3 mi (0.5 km) on Hwy 30 to a prominent

draw on the right with several dirt roads. Turn in to the right, go

through gate and close it, and park near end of drivable jeep trail

to right. Hike northward on cow trail along wire fence ~1/3 mi

(0.5 km) to farthest gully with trees and brush. Hike steeply

uphill 150′–200′ (45–60 m) to large, cliffy exposure of the mega-

breccia at top of ridge on north side of gully.

Stop 1.2—Banbury Basalt: Megabreccias [N 42°43.583′, W 114°51.35′]

As you climb up to the ridgetop, there are poor exposures on

the slope of white breccias and palagonite tuffs, as well as lavas.

The outcrop on the ridge top is an ~10-m-thick “megabreccia”

unit, which can be followed ~600′ to the west, where it termi-

nates, perhaps on the crater rim, or is faulted.

The megabreccia (Tvp) consists of unsorted tuff breccia

with large accidental blocks, many more than 1 m in length

(Fig. 8A). Block compositions include several types of basalt

(lower Banbury Basalt in appearance), tan fi ne-grained lake sedi-

ment, sparse stream pebbles, and some juvenile spattery tephra.

Figure 7. Water-escape structures in basalt of Bacon Butte, Stop 1.1B. (A) Panorama of outcrop showing water-escape conduits, rotated colum-nar jointing, and rubbly fi ll. (B) Close-up of water escape conduit showing trellis structure and spiny rubble with popcorn texture. (C) Popcorn texture basalt in water conduit. (D) Large (1–2 m) scale steam cavities at base of fl ow, just above section of megapillows.


fl d006-02 page 13 of 26

Some clasts show evidence of hot reaction rims on spheroidally

weathered basalt. The matrix is unstratifi ed with ~25% fi ne ash

and many small (<1cm) clasts. The breccia unit appears to grade

upwards into tuff with fewer large clasts. The megabreccia and

associated tuff are interpreted as explosive, subaerial crater-fi ll-

ing deposits of the Thousand Hill vent.

This small drainage basin (~1/2 square mile) is characterized

by outcrops of megabreccia, tuffs, red-orange sediment (which

always overlies the volcaniclastics), and lavas, and by faults. The

area was mapped by Stearns et al. (1938) as “Fault complex of

Hagerman lake beds and interstratifi ed tuff.” Malde and Powers

(1972) mapped it as lower Banbury “vent deposits of basaltic

pyroclastic material,” without showing any internal detail. The

geology is very complex and diverse, and the volcanic and struc-

tural relationships are obscured by a veneer of tan-colored Qua-

ternary Yahoo Clay (Bruneau sediments in earlier work), which

unconformably overlies the Tertiary volcanics.


Return toward vehicles and hike over to small hill nearby,

labeled “Thousand” on topographic map.

Stop 1.3—Banbury Basalt: Tilted Hydromagmatic Vent and Vent Facies Breccias [N 42°43.183′, W 114°51.40′]

The outcrop consists of a 7-m-thick, unsorted and unstrati-

fi ed, matrix-supported volcanic breccia with dull olive-green to

tan color containing accidental blocks of vesicular basalt (as large

as 0.5 m across) and numerous polished stream pebbles. Note the

oblong blocks of undisturbed, fi ne-grained tan water-laid sedi-

ment, which must have been horizontal originally, suggesting that

dip is the result of structural tilting. This is a section of the vent

deposits, dipping 40 degrees north, probably due to faulting.

Hike around to top surface (now dipping) to look at strong

linear features, which are interpreted as elutriation (gas escape)

features.

This outcrop may be the same unit as in Stop 1.2, but on the

opposite side of vent. Note that the Thousand Hill vent and the

Riverside Ferry vent (Stop 1.4) are both along a strong north-

west-trending structural orientation.


Drive south on Hwy 30, ~2 mi (3.2 km), and turn left at

intersection with paved “River Road.” Follow River Road past

houses, ~1 mi (1.6 km) and turn left on paved road near power

line. Drive past sequence of orange-colored palagonitic tuffs on

right roadcut and continue to gravel road at top of hill. Turn right

on gravel road, go ~1/3 mi (0.5 km) and fork left. Drive carefully

northward to edge of vent exposure. Some roads are very sandy

and may need 4WD. There are at least two jeep trails that access

the vent area and overlook the Snake River.

Stop 1.4—Banbury Basalt: Riverside Vent (optional, if time permits) [N 42°42.40′, W 114°50.383′]

This exposure in the north half of the section 29 “island” is a

hydrovolcano bisected by the Snake River and originally named

the Riverside Ferry cone by Stearns et al. (1938). Bonnichsen and

Godchaux (2002) referred to it as the Riverside Ferry cone but

named the phreatomagmatic units “Tuff of Blue Heart Springs”

after a spring located on the north side of the river adjacent to the

volcano. They described the tuff as explosively erupted layers of

cinder to spatter-rich deposits tilted near the vent constructs and

transitioning to subhorizontal, fi ner-grained units more distally.

Chemically they place the basaltic material in the general Snake

River olivine tholeiite compositional range. They also place

some of the Banbury tuff units as intercalating with the Glenns

Ferry Formation sediments, although the mapping of Malde and

Powers (1972) would generally place the Banbury Basalt as

older than the thick pile of lake sediments. Certainly there are

“Banbury-age” lake and fl uvial sediments interstratifi ed with

basaltic units (4–6 Ma) and there are basaltic tuffs and basalt

(3.4–3.8 Ma) interbedded within the Glenns Ferry Formation at

the Hagerman Fossils National Monument (W.K. Hart and M.E.

Brueseke, 1999, personal commun.). Dating the Banbury units is

diffi cult due to the alteration, although a few recent Ar-Ar radio-

metric dates are available, but not defi nitive.

The core of the Riverside Ferry vent is well exposed by

erosion (Fig. 8B). A massive tuff breccia, air-fall tuffs and brec-

cia, cindery beds, surge deposits, and palagonitized tuffs can be

found within a short hike across the 1000 ft of exposed vent.

a b

Figure 8. Vent facies of the Banbury Basalt. (A) Megabreccias that probably represent a vent facies (Stop 1.3). (B) Riverside Ferry vent (Stop 1.4).


fl d006-02 page 14 of 26

On the southeast side of the vent, a basaltic lava fl ow appears

to overlie the vent deposits and may be related. Exposures on

the north side of the river contain abundant red cindery mate-

rial, but these deposits have not been studied in detail. A NW-

trending fault may underlie the course of the Snake River, and

it is probable that there is a signifi cant northwest-trending fault

under the “dry” river course south of the “island.” Evidence for

this structure(s) includes small visible offsets of the older rhyo-

lites and lower Banbury lavas, topographic features emphasized

by the Bonneville Flood (Fig. 4), and alignment of geothermal

wells and hot springs.

Return to Hagerman on Hwy 30.

Day 2 (All Day) Mountain Home


Drive north on I-84 to Mountain Home; take Exit 95 on

Idaho Hwy 20 north toward Camas Prairie and Sun Valley. Con-

tinue N for ~7 mi (11 km) where the highway enters Rattlesnake

Creek canyon; park at the turnout on the right side of highway.

All Day 2 stops are shown on Figure 9A; detailed topography for

stops 2.2 through 2.4 is shown in Figure 9B.

Stop 2.1—Danskin Mountains Rhyolite, Highway 68 North of Mountain Home [N 43°12.052′, W 115°33.2′]

The oldest volcanic rocks exposed in the Mountain Home

area are rhyolite lava fl ows that form the Danskin Mountains

and the Mount Bennett Hills. Clemens and Wood (1993)

mapped the rhyolite here as the Danskin Mountains Rhyo-

lite, and mapped rhyolite which is exposed farther east as the

Mount Bennett Rhyolite. They determined a K-Ar age of 10.0

± 0.3 Ma for sanidine in a vitrophyre from the summit area

on Teapot Dome (Danskin Mountains Rhyolite), and a K-Ar

age of 11.0 ± 0.5 Ma for plagioclase in a rhyolite from near

Mount Bennett. Possible correlative units on the south side of

the western Snake River graben include the Sheep Creek Rhyo-

lite (9.88 ± 0.46 Ma; Hart and Aronson, 1983), the rhyolite of

Tigert Springs, the rhyolite of O X Prong, and the rhyolite of

Rattlesnake Creek (Kauffman and Bonnichsen, 1990; Jenks et

al., 1993). Rhyolite in the eastern Mount Bennett Hills ranges

in age from 9.2 ± 0.13 Ma to 10.1 ± 0.3 Ma (Armstrong et al.,

1980; Honjo et al., 1986, 1992).

The Danskin Mountains Rhyolite is typically a vitrophyre

with abundant phenocrysts of sanidine and quartz set in a red,

gray-brown, or black volcanic glass. Flow banding appears as

laminar variations in the color of the glass, or in its crystallinity.

The fl ow banding is commonly folded ptygmatically, indicat-

ing rheomorphic mobilization of the rhyolite. Flow banding

and axial foliation in the vitrophyre generally trend N50ºW to

N60ºW, and dip 15º–45°NE. There are no indications that these

rhyolites are rheomorphic ignimbrites; they appear to be rhyolite

lavas erupted from fi ssures that were subparallel to the current

range-front faults (Bonnichsen, oral comm., 1996, 1997).


Return to I-84 and proceed west toward Boise; take the next

available exit (exit number 90) then continue across and back on

to I-84 toward Boise (this will slow you down and position you

for the next stop). Pull off to the right immediately after entering

the freeway at the fi rst roadcut encountered.

Stop 2.2—Plagioclase Flotation Cumulates of Lockman Butte, I-84 Roadcut [N 43°10.599′, W 115°45.601′]

This roadcut transects a major fl ow lobe from Lockman

Butte that consists of multiple lava fl ows that are stacked one

on top of (or beneath) another. Many of these fl ow units contain

plagioclase fl otation cumulates that formed within covered lava

channels and aphyric ferrobasalt residues that drained out the

bottom of the cumulates (McGee and Shervais, 1997; Shervais

et al., 2002; Zarnetske and Shervais, 2004). Another interesting

aspect of these fl ows are the gigantic vesicles found in some gas

accumulation horizons; these vesicles are up 2 m long and 0.6 m

high—big enough to lie down in!

Lava fl ows with fl otation cumulates comprise three zones: a

central, plagioclase porphyry with intersertal to intergranular tex-

tures in the groundmass, an upper diktytaxitic zone comprising pla-

gioclase laths with large voids and minor intergranular mafi cs and

glass, and a lower aphyric zone of ferrobasalt, with 16%–17% FeO*

(Fig. 10A). Mass balance calculations show that the diktytaxic

zone contains 30%–50% porosity, represented by the ferrobasalt

base, if the central plagioclase porphyry is assumed to represent

the bulk composition. Detailed outcrop maps show that successive

lava fl ows fl owed beneath previously emplaced fl ows, infl ating and

plastically deforming the aphyric ferrobasalt zone in the overlying

fl ow. Plagioclase fl otation cumulates beneath this ferrobasalt ceil-

ing display a horizontal contact between the diktytaxitic and plagio-

clase porphyry zones; we interpret this horizon to represent the con-

tact between interstitial melt (below) and volcanic gasses (above),

and suggest that interstitial melt was displaced by the rising gasses

(McGee and Shervais, 1997; Zarnetske and Shervais, 2004).

Plagioclase phenocrysts are An65

in both the porphyry

and diktytaxitic zones, suggesting formation of the plagioclase

framework by simultaneous crystallization throughout the fl ow,

followed by sinking of the dense, interstitial ferrobasaltic liquid

to the bottom of the fl ow. This interpretation is supported by

olivine phenocrysts (Fo68

) trapped in the plagioclase framework,

and by partially replaced intergrowths containing Fo80–89

olivine.

The Fo-rich olivine relicts imply a parent magma that was signifi -

cantly more magnesian than observed.

These fl otation cumulates provide an analogue for anor-

thosite formation, both in the lunar magma ocean and in Earth’s

mid-crust. We suggest that planetary anorthosites represent pla-

gioclase saturated melts that crystallize en mass to form buoyant

rafts of plagioclase plus trapped mafi cs; interstitial ferrobasalts

liquids are forced out by rising magmatic gasses, and sink

back into the underlying magma (McGee and Shervais, 1997;

Zarnetske and Shervais, 2004).


fl d006-02 page 15 of 26

Stop 2.1Stop 2.2Stop 2.3

Stop 2.4

Stop 2.5

Stop 2.2

Stop 2.3

Stop 2.4

B

A

Figure 9. (A) Topographic map of the western Snake River Plain around Mountain Home, Idaho, showing Day 2 stops. (B) Detail of Stops 2.2 through 2.4 NW of Mountain Home.


fl d006-02 page 16 of 26


Drive NW on I-84 for ≈14 mi (23 km) to the fi rst exit (Simco

Road, Exit 74); cross the freeway and return to I-84 in the SE

direction. Exit at the fi rst Mountain Home exit (exit number 90)

and take the fi rst right turn encountered on Business I-84 (old

Hwy 30). Proceed along the frontage road parallel to the railroad

tracks for ≈6 mi (≈10 km), then turn left and cross the RR tracks

at a clearly marked crossing. Proceed ≈1.8 mi (2.9 km) along this

wide gravel road to a dirt road turn-off on the left near the crest of

the grade. Turn left and follow this dirt road ≈1.3 mi (2.1 km) to

the rim of Crater Rings. You will go past a stock pond (commonly

dry) and through at least one wire fence; be sure to close all gates

behind you. Note: Crater Rings is rattlesnake heaven, so watch

where you step and put your hands!

Stop 2.3—Crater Rings Pit Craters: Lava Lakes and Spatter Ramparts [N 43°11.715′, W 115°50.866′]

The Crater Rings are a spectacular volcanic feature that con-

sists of two large pit craters at the summit of a broad shield volcano

(Fig. 10B). The western crater is ≈800 m across and 75 m deep;

the eastern crater is ≈900 m across and 105 m deep. The inner

walls of the pit craters consist of welded spatter, agglutinate, and

minor intercalated lava fl ows; no fragmental horizons are exposed

in the crater walls, which argues against phreatomagmatic erup-

tion. The welded spatter and agglutinate are easily identifi ed by

their characteristic textures, oxidized coloration, and hollow ring

when struck with a hammer. The eastern crater is surrounded on

three sides by spatter and agglutinate ramparts.

The Crater Rings represent pit craters that were fi lled episodi-

cally with lava lakes (Shervais et al., 2002). They are equivalent

to similar features in Hawaii, such as Halemaumau pit crater on

the summit of Kilauea volcano and the paired lava lakes of the

1972 Mauna Ulu eruption (Decker, 1987; Tilling et al., 1987). Fire

fountain eruptions in the lava lakes fed spatter to the rims, which

were occasionally mantled by lava fl ows when the lava lakes over-

fl owed their ramparts. The fi nal eruptive phase was confi ned to the

eastern vent, where fi re fountains built ramparts on three sides of

the vent that were not covered by subsequent lava fl ows, although

lava from the eastern vent may have fl owed into the western vent

during lava highstands at this time (Shervais et al., 2002).


Return to the frontage road and turn right; proceed SE along

the frontage road (toward Mountain Home) for ≈5 mi (8 km) and

turn right across the railroad tracks (you will be almost due north

of Union Butte cinder cone). Continue south ~1.1 mi (1.8 km) to

the Union Butte cinder cone.

Stop 2.4—Union Buttes: Holocene Cinder Cones, Late Alkaline Basalts Equivalent to Boise River Group 2 of Vetter and Shervais (1992) [N 43°9.565′, W 115°46.06′]

Union Buttes are the two most prominent volcanic vents west

of Mountain Home. Their stratigraphic position (overlying all older

vents) and relative preservation suggest an age of <500,000 yr.

The basalt of Union Buttes erupted from these two vents but was

confi ned between the large Rattlesnake Springs shield volcano to

the south and the smaller Crater Rings and Lockman Buttes vents

to the west and north. The western Union Butte is larger than the

eastern vent, and fed a small fl ow that fl owed west toward Crater

Rings. Both vents consist of cinder cones built on top of small

shield volcanoes of similar composition basalt.

The basalt of Union Buttes is distinct from almost all other

basalts in the Mountain Home area because it contains large,

clear phenocrysts of olivine but lacks plagioclase phenocrysts.

The basalt of Union Buttes, and the similar basalt of Little Joe

Butte (also known as the basalt of Strike Dam Road), are charac-

terized by high K2O compared to other Mountain Home basalts,

A

B

C

Figure 10. Volcanic features near Mountain Home. (A) Plagioclase fl otation cumulates in covered lava channel, Lockman Butte—note diktytaxitic texture near top of cumulate layer where exsolved gas has forced residual ferrobasaltic liquid from the interstices. (B) The Crater Rings, two large pit craters that fed lava lake eruptions and Pelean-style fi re fountains. (C) Pillow and hyaloclastite delta in basalt of Little Joe Butte (Strike Dam Road), which sits on lacustrine sediments of Glenns Ferry Formation (Lake Idaho); note water escape conduits, steam cavities, and southeast dip of foreset beds


fl d006-02 page 17 of 26

along with lower TiO2 and high-fi eld strength trace element

abundances. In terms of their chemical and age relations, they are

similar to the Boise River Group 2 basalts of Vetter and Shervais

(1992) and to the M3 basalts of White et al. (2002). The transi-

tion from high fi eld strength-rich, alkali-poor tholeiites to potas-

sium-rich, high fi eld strength-poor transitional alkaline basalts

around 600,000–700,000 yr ago is one of the most fundamental

time-dependent transitions observed in the western Snake River

Plain province, with the younger lavas characterized by OIB-like

isotopic compositions. See Vetter and Shervais (1992) and White

et al. (2002) for discussions.


Continue south on gravel and paved roads ~2.6 mi (4.2 km)

to intersection with Idaho Hwy 67. Turn right onto Hwy 67 and

proceed SW toward Mountain Home Air Force Base. Bear right

at the turn-off into the air base (away from the base) and continue

SW on Hwy 67 for 6.5 mi (10.8 km) to a poorly marked intersec-

tion with Strike Dam Road. Turn left onto Strike Dam Road and

proceed south for 6.75 mi (10.9 km) to the rim of the plateau;

park at the top and walk down hill to the next stop.

Stop 2.5—Basalt of Little Joe Butte: Subaerial Basalt on a Pillow Lava + Hyaloclastite Delta on Lake Idaho Sediments [N 42°57.648′, W 115°58.526′]

The basalt of Little Joe Butte is olivine-phyric basalt that

crops out along the western edge of the Mountain Home area

(Cinder Cone Butte, Crater Rings SW quadrangles) and con-

tinues into adjacent quadrangles to the west (Little Joe Butte,

Dorsey Butte) and south (Grand View, C.J. Strike Dam; Jenks et

al., 1993). The source of this fl ow is Little Joe Butte, but it has

also been mapped as the basalt of Strike Dam Road (Jenks et al.,

1993). It consists of at least two major fl ow units, with a mappable

fl ow front preserved in the upper unit, and appears to fl ow south

along a former channel of the Canyon Creek drainage. Collapsed

areas of this fl ow are commonly overlain surfi cially by “intermit-

tent lake deposits,” i.e., dry lake beds (Shervais et al., 2002).

This unit is largely subaerial, but its base is commonly pil-

lowed along its southern margin, demonstrating the fl ow of sub-

aerial lava into standing water of Lake Idaho. The subaqueous

portion of the fl ow consists of pillow lava and hyaloclastite breccia

forming foreset delta sequences with individual “rolled” pillows

interspersed (Fig. 10C). The delta foresets dip to the SE, indicating

the direction the lava fl owed when it entered the lake. The subaque-

ous lava delta is ≈10 m thick in the bluffs overlooking C.J. Strike

reservoir, where it fl owed over fl at-lying lake deposits—indicating

the exact water depth in the lake at the time of eruption.

Like the Bacon Butte Basalt at Stop 1.1B, the subaerial lava

that overlies the pillow delta contains water escape conduits

and giant vesicles (steam bubbles). The conduits and vesicles

can be seen from the road but a close-up examination requires

climbing up the pillow delta, which is inherently unstable—be

careful if you go!

Return to Hagerman

Continue south on Strike Dam Road, across Snake River

at Strike Dam Bridge, to Idaho Hwy 78. Drive E on Hwy 78

through Bruneau to Hammett. As time permits, we will make

stops along this route to view the stratigraphy of the former Lake

Idaho and the effects of the Bonneville fl ood. At Hammett, rejoin

I-84 and return to motel.

Day 3 (Partial Day) Twin Falls2


From Hagerman, drive north on Hwy 30 to Bliss, turn E

(right) onto Hwy 26 to Shoshone. At main intersection in Sho-

shone, turn N (left) on Hwy 75. Drive N ~2.3 mi (3.7 km) and

park on right. Hike E into the Shoshone lava fl ow to the lava chan-

nel. All of the stops for Day 3 are shown on Figure 11. Source

vents for the basalt fl ows are indicated on Figure 12, a hillshade

topographic map of the Twin Falls 30′ × 60′ quadrangle.

Stop 3.1—Lava Channel is Shoshone Lava Flow; a‘a Basalt Fills Channel in Pahoehoe Flow [N 42°58.936′, W 114°18.22′]

The Shoshone basalt fl ow (basalt of Black Butte Crater

on the Idaho Geological Survey Twin Falls 30′ × 60′ geologic

map) erupted from Black Butte Crater located in the Black

Butte Crater quadrangle. The basalt is black, fi ne grained,

vesicular to massive, and aphyric to olivine phyric. Olivine

phenocrysts (Fo58–73

) are visible in hand sample and are up to

2.0 mm in size. Most plagioclase (An31–72

) displays a normal

chemical zoning.

Black Butte Crater is a broad shield volcano that rises

135 m above the surrounding topography. The volcano contains

two large craters that are ~300 m across and 100 m in depth.

This young lava fi eld extends south to the Dietrich Butte quad-

rangle and then west into the Shoshone, Tunupa, and Gooding

quadrangles, and terminates just west of the town of Gooding.

Eruption of lava disrupted the confl uence of the Big and Little

Wood rivers, which was probably near Shoshone, and separated

the two rivers to their present confl uence at Gooding. Radio-

carbon dating of underlying sediment baked by the fl ow (Kuntz

et al., 1986) has yielded a date of 10,130 ± 350 yr B.P. for this

basalt. Aside from a few areas covered by thin loess deposits or

alluvium, most of the basalt is completely exposed, and almost

all of the original fl ow morphology can be seen. The fl ow is

dominated by a series of large lava tube and lava channel sys-

tems that can be seen on aerial photographs and topographic

maps. Most of the Shoshone fl ow consists of smooth pavement

outcrops of pahoehoe, overlain by sporadic a‘a fl ows; a‘a fl ows

also fi ll lava channels and collapsed lava tubes.

We plan to visit a large lava channel in the pahoehoe fl ow

that is now fi lled with a‘a lava. This lava channel has levees that

rise 60–70 ft above the surrounding lava, and the channel is as

much as 70 ft deep (Fig. 13A).


fl d006-02 page 18 of 26

Day One Stops

Stop 3.1Stop 3.2

Stop 3.3

Stop 3.4

Stop 3.5Stop 3.6

Figure 11. Topographic map of central Snake River Plain showing Day 3 stops.

Figure 12. Hillshade of the Twin Falls area showing the names and locations of volcanic source buttes and the topographic expression of different basalt fl ows revealed through vertical exaggera-tion. The solid lines are main highways. The dashed line is the perimeter of the Twin Falls 30′ × 60′ quadrangle.


fl d006-02 page 19 of 26


fl d006-02 page 20 of 26


Return to Shoshone and proceed NE on Hwy 93 toward

Richfi eld. Turn right (S) ≈8.8 mi (14.2 km) from intersection in

Shoshone onto a paved road. Drive south 2.5 mi (4.0 km) to the

second dirt road on the right side of road. Turn right onto this dirt

road and drive to the top of Crater Butte (~0.9 mi/1.5 km). Note:

if we cannot get the vans up Crater Butte, we will drive to the top

of nearby Dietrich Butte instead.

Stop 3.2—Crater Butte [N 42°57.582′, W 114°16.724′]

Crater Butte, located in the northeastern portion of the

Dietrich quadrangle, is a steeply sloping shield volcano that rises

~140 m above the surrounding topography. The most striking

feature of this volcano is the large 80 m deep crater in its center.

This bowling pin-shaped crater is ~1300 m × 1000 m in size

and trends in a NW–SE direction. The inner walls of the crater

show thin layers of spatter interbedded with thicker beds of more

massive basalt (Fig. 13B). The tops of the more massive beds

are highly vesicular which indicates gaseous escape from lava

channels beneath the cooler, more viscous upper crust. When the

volcano was active, the crater probably contained a lava lake that

would periodically spill over the spatter rim to create the inter-

layered spatter and massive fl ows. Most outcrops along the fl anks

of the volcano can be found along terraced ridges concentric to

the main crater. These terraces show the same interlayering of

spatter and massive basalt, indicating that they may mark the

position of older crater rims.

The basalt of Crater Butte is dark gray, vesicular to massive,

and fi ne grained, containing phenocrysts of plagioclase and oliv-

ine. Some of the samples taken from the massive layers display a

slightly diktytaxitic texture but most are intergranular in texture.

Olivine phenocrysts (Fo45–74

) are small (<0.6 mm) and rounded.

Plagioclase phenocrysts (An55–71

) are generally 2.5 mm in size.

Several large plagioclase phenocrysts display an oscillatory zon-

ing, which may be due to pressure fl uctuations during eruption or

to magma mixing. Electron microprobe line scans across plagio-

clase phenocrysts reveal a strong normal chemical zonation.


Return to paved road, turn right (S) and continue 3.3 mi

(5.3 km) to Hwy 24. Turn right (W) onto Hwy 24 and continue

to intersection at south end of Shoshone (≈8.3 mi/13.4 km). Turn

left (S) onto Hwy 93 and drive 2.85 mi (4.6 km) to a gravel road

on the left (east side) of road that leads to Notch Butte.

Stop 3.3—Notch Butte [N 42°53.036′, W 114°24.98′]

Notch Butte is located in the southeastern corner of the

Shoshone quadrangle and rises 110 m above the surrounding

topography. The lava fl ows from this broad shield volcano cover

150 km2, including the southern half of the Shoshone quadrangle,

west into the Tunupa and Gooding SE quadrangles, south into

the Shoshone SW and SE quadrangles, and east into the Dietrich

quadrangle. Several lobes fl owed west to Gooding, continued

south and west around Gooding Butte, and fl owed into the ances-

tral Snake River canyon near Hagerman. The unit is equivalent

to the Wendell Grade basalt of Malde et al. (1963), except for

the canyon fi lling part at Hagerman, which they mapped as Sand

Springs basalt. The unit has been renamed during recent mapping

to follow the convention of naming fl ows after their source vent.

The fl ows display a relatively young volcanic morphology

with many fl ow features such as pressure ridges, collapsed lava

tubes, and fl ow fronts still visible beneath a thin soil and loess

layer. The loess mantle is mostly confi ned to depressions in the

fl ow surfaces. Note the contrast between fl ows from this vent and

those from the older Crater Butte vent and the younger Shoshone

fl ow lavas.

Mapping revealed three to four lobes of varying mineralogy

within the fl ow fi eld. In general, higher lobes closer to the vent

were found to be rich in plagioclase, while those at lower eleva-

tions farther away from the vent contain less plagioclase and are

rich in olivine. The higher lobes are interpreted to be younger,

more fractionated, and therefore more viscous basalt (due to the

plagioclase content), while those found at lower elevations are

older, less fractionated, and less viscous. This difference in vis-

cosity between the older and younger lava accounts for the fact

AA

B

Figure 13. Volcanic features of the central Snake River Plain north of Twin Falls, Idaho. (A) Lava channel in the Sho-shone basalt. (B) Pit crater in Crater Butte.


fl d006-02 page 21 of 26

that the older fl ows spread out over a large area away from the

vent while the younger fl ows did not.

The basalt of Notch Butte is a black, vesicular to massive,

medium- to fi ne-grained, plagioclase- ± olivine-phyric basalt.

Samples taken from some fl ow lobes contain large glomero-

crysts of plagioclase and olivine, which are visible in hand

sample. The basalt is intergranular to intersertal in texture and

contains olivine phenocrysts (Fo35–73

); large glomerocrysts of

plagioclase (An59–71

) and olivine are visible in thin section and

are often up to 5 mm in size. The larger plagioclase phenocrysts

are typically normally zoned.

The view from the top of Notch Butte includes all of the

major volcanoes in this part of the central Snake River plain:

Crater Butte, Dietrich Butte, Owinza Butte, and Black Ridge

Crater to the east; Lincoln Butte to the west; Bacon Butte, and

Flat Top Butte to the south, and Wilson Butte and Rocky Butte to

the southeast. From here, it is easy to contrast the relative ages of

the fl ows, which can be estimated by the amount of cover indi-

cated by farming. We can also see the western fl ow from Notch

Butte, which extends some 40 km to the west where it fl owed

into the ancestral valley of the Snake River. At the time of Notch

Butte eruptions, the ancestral Snake River was already at about

its present position and elevation.


Return to Hwy 93. Turn left (S) onto Hwy 93 and drive south

11.8 mi (19 km) to Hwy 25; turn left (E) onto Hwy 25 and after

≈0.6 mi (1 km) turn left (N) onto paved road; follow for 0.3 mi

(0.5 km), then turn left (N) onto paved/gravel road that goes to

top of Flat Top Butte (~1 mi/1.6 km from highway).

Stop 3.4—Flat Top Butte [N 42°43.722′, W 114°24.75′]

Flat Top Butte is an extremely large (≈450 km2) shield vol-

cano located ~20 km north of Twin Falls in the Falls City 7.5′ topographic map quadrangle. The vent has a large (200 m), shal-

low depression at its summit that represents the former summit

crater; the rim of this crater is now home to a forest of microwave

transceivers. The fl anks of the volcano are covered with a thick

mantle of loess and soil that obscures the underlying basalt. This

butte is farmed almost to its summit, showing that it is one of the

older vents in this part of the plain. Radiometric ages for Flat Top

Butte are in the 0.330–0.395 Ma range (Tauxe et al., 2004; Idaho

Geological Survey, 2005, unpublished data).

The basalt of Flat Top Butte fl owed south and west fi ll-

ing the course of the ancestral Snake River as far as Thousand

Springs, where Malde and Powers (1972) mapped the fl ows as

the Thousand Springs Basalt. Flows from Flat Top Butte defi ned

the present position of the Snake River by forcing the drainage

southward against the regional north slope of the older shield vol-

canoes. Subsequently, the Snake River canyon was cut.

From the summit area of Flat Top Butte we can look east

toward the younger vents Wilson Butte, Rocky Butte, and

Kimama Butte, north toward Bacon Butte and Notch Butte, and

northeast toward Owinza Butte and Black Ridge Crater. Older

buttes to the southeast include Hansen Butte, Skeleton Butte,

Hazelton Butte, Milner Butte, and Burley Butte.

Looking south toward the Snake River canyon and Twin

Falls City, the track of Bonneville Flood overland fl ow was east

to west obliquely across I-84. Along the canyon wall south of

I-84 the overland fl oodwaters rejoined the component follow-

ing the course of the river, forming cataracts in several places.

Although the basalt of Rocky Butte typically has loess fi lling

surface depressions, in the track of the fl oodwaters virtually all

the loess is stripped away and patchy gravel deposits record local

deposition during the fl ood.


Return to Hwy 25; turn left (E) onto Hwy 25 and proceed

4.6 mi (7.4 km) to intersection with paved road. Turn S (right)

and drive 5.4 mi (8.7 km) south—road goes from paved to gravel

after ~2 mi (3.2 km). Continue on dirt road to SE around end of

Wilson Butte fl ow lobe, ≈1 mi (1.6 km). Park anywhere.

Stop 3.5—Wilson Butte Flow Lobe [N 42°37.82′, W 114°21.76′]

Wilson Butte is a large shield volcano (≈10 km across). Its

vent, composed of three small peaks, is rather unimpressive when

compared to the estimated 250 km2 of lava that fl owed down the

slopes of the shield. The fl ows cover the majority of the Shoshone

SE and half of the Star Lake quadrangles. The fl ow continues into

the Shoshone SW and Hunt quadrangles, and it crosses Hwy 25

in the Twin Falls NE quadrangle and continues into the Kimberly

quadrangle, possibly emptying into the Snake River around the

Twin Falls rapids or Devil’s Corral area.

The mode of the basalt is 35%–40% plagioclase, 10%–15%

olivine, 10%–20% pyroxene, 5%–8% oxides, and 15%–20%

glass. Plagioclase shows typical euhedral lath-shaped crystals

with An content ranging from An66

to An36

. Olivine ranges from

Fo71

to Fo33

.

At this stop we will view a major fl ow lobe from Wilson

Butte that stands 10–20 m above the surrounding lava fi elds and

contains a giant complex of lava tubes that fed secondary fl ow

lobes that fl owed primarily west and south. The fl ow lobe is char-

acterized by steep sides and a nearly fl at upper surface that can

be traced for tens of kilometers. This lava tube–channel system

fed fl ows adjacent to the Snake River that have been mapped as

Sand Springs basalt—in addition to fl ows from Rocky Butte (Hill

4526), which may dominate the Sand Springs unit. Wilson Butte

and Rocky Butte have lavas that are nearly identical chemically,

and both appear to have erupted at essentially the same time,

although the Wilson Butte fl ow lobe appears to be younger than

the adjacent fl ows from Rocky Butte. Tauxe et al. (2004) report

radiometric age for “Sand Springs Basalt,” probably from Rocky

Butte, as 0.095 Ma. The lava tube system is exposed in a series of

windows that are open mainly on the eastern margin of the fl ow.

Sitting on the Wilson Butte fl ow lobe here are coarse sedi-

ments deposited by the Bonneville fl ood. These sediments seem


fl d006-02 page 22 of 26

to represent a lag deposit from water fl owing over or around the

end of the lobe. At this elevation (~1173 m) the Bonneville Flood

waters were relatively shallow. The relief of this fl ow lobe was

just suffi cient to divert most of the overland fl oodwater around

the feature, but a few low spots allowed water to spill through and

create small plunge pools and gravel deposits. The fl ood waters

lost energy on the lee side of the fl ow lobe and deposited fi ner

sediment that forms fl at surfaces in broad depressions.


Return to Hwy 25, turn W (left) and return to Hwy 93. Turn

left onto Hwy 93 and drive south 8.6 mi (13.8 km) to Perrine

Bridge in Twin Falls (you will cross I-84). Cross the bridge and

enter turnout into viewpoint-tourist information area immedi-

ately on south side of bridge. Park in visitor center parking lot

next to bridge.

Stop 3.6—Perrine Bridge Overlook [N 42°35.8966′, W 114°27.266′]

The Perrine Bridge viewpoint is one of the classic views

of volcanic stratigraphy in the central Snake River Plain. The

Snake River gorge is over 120 m deep here and contains two golf

courses within the gorge just downstream from the bridge. Evil

Kneivel’s famous attempt to jump the gorge on a rocket-powered

motorcycle took place just upstream from here.

Looking north across the river we can see over 4 m.y. of

volcanic history (Fig. 14). At the base of the section is the Twin

Falls rhyolite (ca. 6.25 Ma; Armstrong et al., 1975) that forms the

basement here, and which underlies Shoshone Falls east of Twin

Falls. Sitting on the rhyolite (and on some patchy exposures of

rhyolite breccia) are sediments that may correlate with the Plio-

cene Glenns Ferry Formation (Covington et al., 1985; Covington

and Weaver, 1989). If this correlation holds, these sediments

would represent lake and fl ood-plain sediments that were depos-

ited in an extension of Lake Idaho. Basalts intercalated with these

sediments just upstream from here have been dated at ca. 4 Ma

by Armstrong et al. (1975).

Sitting on the sediment and rhyolite is a thick section of

basalt erupted from large vents to the south (Hub Butte and

other southern buttes). These fl ows are overlain by fl ows from

Hansen Butte and other volcanoes to the southeast, which pushed

the river farther north and built the north slope of a large shield

complex. Evidence downstream shows subsequent fl ows from

Flat Top Butte burying the preexisting northward slope, presum-

ably fi lling any previous east-west drainage, and reestablishing

the course of the ancestral Snake River at the south edge of the

Flat Top shield. By the time of eruptions from Wilson Butte and

Rocky Butte, the Snake River had incised the older basalts along

the margin of the Flat Top Butte shield. Evidence in the canyon

wall shows local fi lling of the canyon by the younger fl ows.

Subsequent fl ows from Flat Top Butte, Wilson Butte, and Rocky

Butte followed the northern margin of these fl ows. The course

of the Snake River then became established along the contact of

the younger and older fl ows. The basalt of Flat Top Butte was

formerly mapped as Thousand Springs Basalt by Malde and

Powers (1972), while the overlying Sand Springs Basalt (Malde

and Powers, 1972) is now known to represent fl ows from at least

two vents in this area, Wilson Butte and Rocky Butte. However,

valley-fi lling Sand Springs Basalt mapped along Cedar Draw

south of the Snake River (Malde and Powers, 1972) is probably

from Flat Top Butte.

At this vantage point near Perrine Bridge, the Bonneville

Flood at maximum discharge would have been mostly confi ned

to the canyon except that overland fl oodwaters from the north-

east were rejoining the canyon along here and the water depth

exceeded the top of the canyon by ~15 m. Areas of cataracts

and plunge pools on the north side of the canyon, such as the

Blue Lake alcove, show the erosive power of the overland fl ow

(Fig. 15). In the canyon bottom, the golf courses have been built

on fl ood-stripped and molded surfaces of older basalt and rhyo-

lite that have thin deposits of fl ood gravel and sand.

The erosional and depositional features of the Bonneville

Flood are time transgressive, albeit a short time period of only

a few months. Unlike Glacial Lake Missoula that is thought to

have emptied and produced a catastrophic fl ood duration of only

several days, the catastrophic fl oodwater from Pleistocene Lake

Bonneville was more complex (O’Connor, 1993). The early

phases were in low discharge as the lake gently overtopped the

divide at Red Rock Pass and it began to erode. As erosion accel-

Twin Falls Rhyolite

Glenns Ferry Fm.

Hub Butte Basalt (?)

Basalt of Flat Top Butte

Sand Springs Basalt (Rocky or Wilson Buttes)

Figure 14. Volcanic stratigraphy at the Perrine Bridge, Twin Falls, Ida-ho. Dark rock at bottom of canyon is the 6.25 Ma Twin Falls rhyolites. The thickest section of basalt erupted from vents south of the river (Hub Butte, Sonnickson Butte), and is overlain by a thinner section of basalt from north of the current river (mostly Flat Top Butte with basalt of Rocky Butte and/or Wilson Butte) on top. Basalt and rhyolite are separated by thin wedges of clastic sediment in middle of section.


fl d006-02 page 23 of 26

erated, the discharge grew to catastrophic proportions and was

sustained for many weeks. However, as the level of Lake Bonn-

eville dropped, gradually the discharge lowered. Ultimately, an

equilibrium was achieved between the lake and the outlet, and the

lake continued to drain into the Snake River drainage for at least

hundreds of years. Given this backdrop, the overland fl oodwaters

entering the canyon from the northeast represent fl ow during the

greatest discharges, and would have ceased when the canyon

could accommodate the fl ood. Features in the canyon, therefore,

range from high-discharge rock scouring and deposition of giant,

bouldery gravel bars, to lower-discharge thin sand and gravel

deposits in the bottom of the canyon.

Return to I-84. Enter freeway in east-bound direction

(right turn). Return to Salt Lake City (approximate driving

time: 4 hours).

REFERENCES CITED

Amini, M.H., Mehnert, H.H., and Obradovich, J.D., 1984, K-Ar ages of late Cenozoic basalts from the western Snake River Plain, Idaho: Isochron/West, v. 41, p. 7–11.

Anders, M.H., and Sleep, N.H., 1992, Magmatism and extension: the thermal and mechanical effects of the Yellowstone plume: Journal of Geophysical Research, v. 97, p. 15,379–15,393.

Armstrong, R.L., Leeman, W.P., and Malde, H.E., 1975, K-Ar dating, Quater-nary and Neogene volcanic rocks of the Snake River Plain, Idaho: Ameri-can Journal of Science, v. 275, p. 225–251.

Armstrong, R.L., Harakal, J.E., and Neill, W.M., 1980, K-Ar dating of Snake River plain (Idaho) volcanic rocks; new results: Isochron/West, v. 27, p. 5–10.

Bonnichsen, B., 1982a, The Bruneau-Jarbidge eruptive center, southwestern Idaho, in Bonnichsen, B., and Breckenridge, R.M., eds., Cenozoic Geology of Idaho: Idaho Bureau of Mines and Geology Bulletin 26, p. 237–254.

Bonnichsen, B., 1982b, Rhyolite fl ows in the Bruneau-Jarbidge eruptive center, in Bonnichsen, B., and Breckenridge, R.M., eds., Cenozoic Geology of Idaho: Idaho Bureau of Mines and Geology Bulletin 26, p. 283–320.

Bonnichsen, B., and Godchaux, M.M., 2002, Late Miocene, Pliocene, and Pleistocene geology of southwestern Idaho with emphasis on basalts in the Bruneau-Jarbidge, Twin Falls, and western Snake River Plain regions, in Bonnichsen, B., White, C.M., and McCurry, M., eds., Tectonic and Magmatic Evolution of the Snake River Plain Volcanic Province: Idaho Geological Survey Bulletin 30, p. 233–312.

Bonnichsen, B., and Kauffman, D.F., 1987, Physical features of rhyolite lava fl ows in the Snake River plain volcanic province, southwestern Idaho, in Fink, J.H., ed., The Emplacement of Silicic Domes and Lava Flows: Geological Society of America Special Paper 212, p. 119–145.

Bonnichsen, B., White, C.M., and McCurry, M., eds., 2002, Tectonic and Magmatic Evolution of the Snake River Plain Volcanic Province: Idaho Geological Survey Bulletin 30, 482 p.

Braile, L.W., Smith, R.B., Ansorge, J., Baker, M.R., Sparlin, M.A., Prodehl, C., Schilly, M.M., Healy, J.H., Meuller, S., and Olsen, K.H., 1982, The Yel-lowstone–Snake River Plain seismic profi ling experiment: Crustal struc-ture of the eastern Snake River Plain: Journal of Geophysical Research, v. 87, p. 2597–2609.

Figure 15. Topographic map of the area near Perrine Bridge showing features formed by the Bonneville Flood, includ-ing cataracts, plunge pools, scoured canyon walls, and giant gravel bars. Large arrows show inferred fl oodwater fl ow directions.


fl d006-02 page 24 of 26

Camp, V.E., 1995, Mid-Miocene propagation of the Yellowstone mantle plume head beneath the Columbia River Basalt source region: Geology, v. 23, p. 435–438, doi: 10.1130/0091-7613(1995)023<0435:MMPOTY>2.3.CO;2.

Camp, V.E., and Ross, M.E., 2004, Mantle dynamics and genesis of mafi c mag-matism in the intermontane Pacifi c Northwest: Journal of Geophysical Research, v. 109, B08204, doi: 10.1029/2003JB002838.

Cecil, L.D., Welhan, J.A., Green, J.R., Frape, S.K., and Sudicky, E.R., 2000, Use of chlorine-36 to determine regional-scale aquifer dispersivity, east-ern Snake River Plain aquifer, Idaho: Nuclear instruments & methods in physics research, section B, Beam interactions with materials and atoms, v. 172, p. 679–687, doi: 10.1016/S0168-583X(00)00216-0.

Christiansen, R.L., 1982, Late Cenozoic volcanism of the Island Park area, eastern Idaho, in Bonnichsen, B., and Breckenridge, R.M., eds., Ceno-zoic Geology of Idaho: Idaho Bureau of Mines and Geology Bulletin 26, p. 345–368.

Christiansen, R.L., Foulger, G.R., and Evans, J.R., 2002, Upper-mantle origin of the Yellowstone hotspot: Geological Society of America Bulletin, v. 114, no. 10, p. 1245–1256, doi: 10.1130/0016-7606(2002)114<1245:UMOOTY>2.0.CO;2.

Clemens, D.M., and Wood, S.H., 1993, Late Cenozoic volcanic stratigraphy and geochronology of the Mount Bennett Hills, central Snake River plain, Idaho: Isochron/West, v. 60, p. 3–14.

Cooke, M.F., 1999, Geochemistry, Volcanic stratigraphy, and hydrology of Neogene basalts, central Snake River Plain, Idaho [M.S. thesis]: Colum-bia, South Carolina, University of South Carolina, 125 p.

Cooke, M.F., and Shervais, J.W., 1999, Stratigraphic controls of basaltic volca-nism on groundwater recharge and conductivity in the central Snake River Plain, Idaho: Geological Society of America Abstracts with Programs, v. 31, no. 4, p. A8.

Craig, H., 1997, Helium isotope ratios in Yellowstone Park and along the Snake River plain; backtracking the Yellowstone Hotspot: Eos (Transactions American Geophysical Union), v. 78, p. 801.

Covington, H.R., and Weaver, J.N., 1989, Geologic map of the profi le of the northern wall of Snake River canyon: U.S. Geological Survey Map I-1947, A–E, scale 1:24000.

Covington, H.R., Whitehead, R.L., and Weaver, J.N., 1985, Ancestral canyons of the Snake River: Geology and hydrology of canyon-fi ll deposits in the Thousand Springs area, south-central Snake River Plain, Idaho: Boise, Idaho, Geological Society of America, Rocky Mountain Section, April 1985, Guide Book, 30 p.

Decker, R.W., 1987, Dynamics of Hawaiian volcanoes: An overview; in Decker, R.W., Wright, T.L., and Stauffer, P.H., eds., Volcanism in Hawaii: U.S. Geological Survey Professional Paper 1350, v. 2, p. 997–1018.

Doe, B.R., Leeman, W.P., Christiansen, R.L., and Hedge, C.E., 1982, Lead and strontium isotopes and related trace elements as genetic tracers in the upper Cenozoic rhyolite-basalt association of the Yellowstone Plateau volcanic fi eld: Journal of Geophysical Research, v. 87, p. 4785–4806.

Draper, D.S., 1991, Late Cenozoic bimodal magmatism in the northern Basin and Range province of southeastern Oregon: Journal of Volcanology and Geothermal Research, v. 47, p. 299–328, doi: 10.1016/0377-0273(91)90006-L.

Dueker, K., and Humphreys, E., 1990, Upper mantle velocity structure of the Great Basin: Geophysical Research Letters, v. 17, no. 9, p. 1327–1330.

Dueker, K.G., Schutt, D.L., Yuan, H., and Fee, D., 2004, New seismic con-straints for the Yellowstone hotspot: Eos (Transactions American Geo-physical Union), v. 85/47, Fall Meeting Supplement, Abstract 51B-0554.

Farnetani, C.G., and Samuel, H., 2004, Dynamics of thermochemical plumes: Eos (Transactions American Geophysical Union), v. 85/47, Fall Meeting Supplement, Abstract 44B-03.

Geist, D.J., and Richards, M., 1993, Origin of the Columbia River plateau and Snake River Plain: defl ection of the Yellowstone plume: Geology, v. 21, p. 789–792, doi: 10.1130/0091-7613(1993)021<0789:OOTCPA>2.3.CO;2.

Gillerman, V.S., 2004, Diversity in the Banbury Basalt: hydrovolcanoes, sedi-ments and structures of the Banbury and Thousand Springs area, Snake River canyon, Idaho: Geological Society of America Abstracts with Pro-grams, v. 34, no. 6, p. 86.

Godchaux, M.M., and Bonnichsen, B., 2002, syneruptive magma-water and posteruptive lava-water interactions in the western Snake River Plain, Idaho, during the past 12 million years, in Bonnichsen, B., White, C.M., and McCurry, M., eds., Tectonic and Magmatic Evolution of the Snake River Plain Volcanic Province: Idaho Geological Survey Bulletin 30, p. 387–434.

Godchaux, M.M., Bonnichsen, B., and Jenks, M.D., 1992, Types of phre-atomagmatic volcanoes in the western Snake River Plain, Idaho, USA: Journal of Volcanology and Geothermal Research, v. 52, p. 1–25, doi: 10.1016/0377-0273(92)90130-6.

Greeley, R., 1982, The style of basaltic volcanism in the eastern Snake River Plain, Idaho, in Bonnichsen, B., and Breckenridge, R.M., eds., Cenozoic Geology of Idaho: Idaho Bureau of Mines and Geology Bulletin 26, p. 407–422.

Hart, W.K., and Aronson, J.L., 1983, K-Ar ages of rhyolites from the western Snake River Plain area, Oregon, Idaho, and Nevada: Isochron/West, v. 36, p. 17–19.

Honjo, N., McElwee, K.R., Duncan, R.A., and Leeman, W.P., 1986, K-Ar ages of volcanic rocks from the Magic Reservoir eruptive center, Snake River plain, Idaho: Isochron/West, v. 46, p. 15–17.

Honjo, N., Bonnichsen, B., Leeman, W.P., and Stormer, J.C., Jr., 1992, Mineral-ogy and geothermometry of high-temperature rhyolites from the central and western Snake River plain: Bulletin of Volcanology, v. 54, no. 3, p. 220–237.

Howard, K.A., and Shervais, J.W., 1973, Geologic map of Smith Prairie, Elmore County, Idaho: U.S. Geological Survey Map I-818, scale 1:24,000.

Howard, K.A., Shervais, J.W., and McKee, E.H., 1982, Canyon-fi lling lavas and lava dams on the Boise River, Idaho, and their signifi cance for evalu-ating downcutting during the last two million years, in Bonnichsen, B., and Breckenridge, R.M., eds., Cenozoic Geology of Idaho: Idaho Bureau of Mines and Geology Bulletin 26, p. 629–641.

Humphreys, E.D., and Dueker, K.G., 1994, Western U.S. upper mantle struc-ture: Journal of Geophysical Research, B, Solid Earth and Planets, v. 99, no. 5, p. 9615–9634, doi: 10.1029/93JB01724.

Humphreys, E.D., Dueker, K.G., Schutt, D.L., and Smith, R.B., 2000, Beneath Yellowstone; evaluating plume and nonplume models using teleseismic images of the upper mantle: GSA Today, v. 10, no. 12, p. 1–7.

Hughes, S.S., Smith, R.P., Hackett, W.R., and Anderson, S.R., 1999, Mafi c volcanism and environmental geology of the eastern Snake River plain, Idaho, in Hughes, S.S., and Thackray, G.D., eds., Guidebook to the Geol-ogy of Eastern Idaho: Idaho Museum of Natural History, p. 143–168.

Hughes, S.S., McCurry, M., and Geist, D.J., 2002, Geochemical correlations and implications for the magmatic evolution of basalt fl ow groups at the Idaho National Engineering and Environmental Laboratory, in Link, P.K., and Mink, L.L., eds., Geology, hydrogeology, and environmental remediation; Idaho National Engineering and Environmental Laboratory, eastern Snake River plain, Idaho: Geological Society of America Special Paper 353, p. 151–173.

Iyer, H.M., 1984, A review of crust and upper mantle structure studies of the Snake River Plain-Yellowstone volcanic system: a major lithospheric anomaly in the western USA: Tectonophysics, v. 105, p. 291–308, doi: 10.1016/0040-1951(84)90209-9.

Jenks, M.D., and Bonnichsen, B., 1989, Subaqueous basalt eruptions into Pliocene Lake Idaho, Snake River plain, Idaho, in Chamberlin, V.E., Breckinridge, R.M., and Bonnichsen, B., eds., Guidebook of the Geology of Northern and Western Idaho and Surrounding Areas: Idaho Geological Survey Bulletin 28, p. 17–34.

Jenks, M.D., Bonnichsen, B., and Godchaux, M.M., 1993, Geologic maps of the Grand View-Bruneau area, Owyhee County, Idaho: Idaho Geological Survey Technical Report 93-2, 21 p., scale 1:24,000.

Jordan, M., Smith, R.B., and Waite, G.P., 2004, Tomographic Images of the Yellowstone Hotspot Structure: Eos (Transactions American Geophysical Union), v. 85/47, Fall Meeting Supplement, Abstract 51B-0556.

Kauffman, D.F., and Bonnichsen, B., 1990, Geologic map of the Little Jacks Creek, Big Jacks Creek, and Duncan Creek wilderness study areas, Owyhee County, Idaho: U.S. Geological Survey Miscellaneous Field Studies Map MF-2142, scale 1: 50,000.

Kimmel, P.G., 1982, Stratigraphy, age, and tectonic setting of the Miocene-Pliocene lacustrine sediments of the western Snake River plain, Oregon and Idaho, in Bonnichsen, B., and Breckenridge, R.M., eds., Cenozoic Geology of Idaho: Idaho Bureau of Mines and Geology Bulletin 26, p. 559–558.

King, S.D., 2004, Where plumes live: Eos (Transactions, American Geophysi-cal Union), v. 85/47, Fall Meeting Supplement, Abstract 44B-01.

Kuntz, M.A., Champion, D.E., Lefebvre, R.H., and Covington, H.R., 1988, Geo-logic map of the Craters of the Moon, Kings Bowl, and Wapi lava fi elds and the Great Rift volcanic rift zone, south-central Idaho: U.S. Geological Survey Miscellaneous Investigations Series Map I-1632, scale 1:100,000.


fl d006-02 page 25 of 26

Kuntz, M.A., Champion, D.E., Spiker, E.C., Lefebvre, R.H., and McBroome, L.A., 1982, The Great Rift and the evolution of the Craters of the Moon Lava Field, Idaho, in Bonnichsen, B., and Breckenridge, R.M., eds., Cenozoic Geology of Idaho: Idaho Bureau of Mines and Geology Bulletin 26, p. 423–437.

Kuntz, M.A., Spiker, E.C., Rubin, M., Champion, D.E., and Lefebvre, R.H., 1986, Radiocarbon studies of Holocene-latest Pleistocene lava fl ows of the Snake River Plain, Idaho: data, lessons, interpretations: Quaternary Research, v. 25, p. 163–176, doi: 10.1016/0033-5894(86)90054-2.

Kuntz, M.A., Covington, H.R., and Schorr, L.J., 1992, An overview of basaltic volcanism of the eastern Snake River Plain, Idaho, in Link, P.K., Kuntz, M.A., and Platt, L.B., eds., Regional Geology of Eastern Idaho and West-ern Wyoming: Geological Society of America Memoir 179, p. 227–267.

Leeman, W.P., 1982, Development of the Snake River Plain–Yellowstone Pla-teau province, Idaho and Wyoming: An overview and petrologic model, in Bonnichsen, B., and Breckenridge, R.M., eds., Cenozoic geology of Idaho: Idaho Bureau of Mines and Geology Bulletin 26, p. 155–177.

Lindholm, G.F., and Vaccaro, J.J., 1988, Region 2, Columbia Lava Plateau, in Back, W., Rosenshein, J.S., and Seabar, P.R., eds., Hydrogeology, Geology of North America: Geological Society of North America, v. O-2, p. 37–50.

Link, P.K., and Fanning, C.M., 1999, Late Miocene Snake River fl owed south into the Humboldt drainage: detrital zircon evidence: Geological Society of America Abstracts with Programs, v. 31, no. 4, p. A22.

Mabey, D.R., 1976, Interpretation of a gravity profi le across the western Snake River Plain, Idaho: Geology, v. 4, p. 53–55, doi: 10.1130/0091-7613(1976)4<53:IOAGPA>2.0.CO;2.

Mabey, D.R., 1978, Regional gravity and magnetic anomalies in the eastern Snake River Plain, Idaho: U.S: Geological Survey Journal of Research, v. 6, no. 5, p. 553–562.

Mabey, D.R., 1982, Geophysics and tectonics of the Snake River Plain, Idaho, in Bonnichsen, B., and Breckenridge, R.M., eds., Cenozoic Geology of Idaho: Idaho Bureau of Mines and Geology Bulletin 26, p. 139–153.

Malde, H.E., 1968, The catastrophic late Pleistocene Bonneville Flood in the Snake River Plain, Idaho: U.S. Geological Survey Professional Paper 596, 52 p.

Malde, H.E., 1991, Quaternary geologic and structural history of the Snake River Plain, Idaho and Oregon, in Morrison, R.B., ed., Quaternary Non-glacial Geology: Conterminous United States: Boulder, Colorado, Geological Society of America, The Decade of North American Geology, v. K-2, p. 251–281.

Malde, H.E., and Powers, H.A., 1962, Upper Cenozoic stratigraphy of western Snake River Plain, Idaho: Geological Society of America Bulletin, v. 73, p. 1197–1220.

Malde, H.E., and Powers, H.A., 1972, Geologic map of the Glenns Ferry-Hager-man area, west-central Snake River Plain, Idaho: U.S. Geological Survey Miscellaneous Investigations Map I-696, scale 1:48,000, 2 sheets.

Malde, H.E., Powers, H.A., and Marshall, C.H., 1963, Reconnaissance geologic map of west-central Snake River Plain, Idaho: U.S. Geological Survey Miscellaneous Geological Investigations Map I-373, scale 1:125,000.

Matthews, S.M., 2000, Geology of Owinza Butte, Shoshone SE, and Star Lake quadrangles: Snake River Plain, southern Idaho [M.S. thesis]: Columbia, South Carolina, University of South Carolina, 110 p.

McCurry, M., and Hackett, W.R., 1999, Genesis of Quaternary rhyolites in Southeast Idaho; implications for the Yellowstone–Snake River plain hotspot system: Geological Society of America Abstracts with Programs, v. 31, no. 4, p. 24.

McGee, J., and Shervais, J.W., 1997, Flotation cumulate in a Snake River Plain ferrobasalt: Petrologic study of a possible lunar analogue: Geological Society of America Abstracts with Programs, v. 29, no. 6, p. A136.

McQuarrie, N., and Rodgers, D.W., 1998, Subsidence of a volcanic basin by fl exure and lower crustal fl ow; the eastern Snake River plain, Idaho: Tec-tonics, v. 17, no. 2, p. 203–220, doi: 10.1029/97TC03762.

Montelli, R., Nolet, G., Dahlen, F.A., Masters, G., Engdahl, R., and Hung, S.-H., 2003, Finite-frequency tomography reveals a variety of plumes in the mantle: Science, v. 303, p. 338–343, doi: 10.1126/science.1092485.

Morgan, L.A., 1992, Stratigraphic relations and paleomagnetic and geochemi-cal correlations of ignimbrites of the Heise volcanic fi eld, eastern Snake River Plain, eastern Idaho and western Wyoming, in Link, P.K., Kuntz, M.A., and Platt, L.B., eds., Regional Geology of Eastern Idaho and West-ern Wyoming: Geological Society of America Memoir 179, p. 215–226.

Morgan, W.J., 1972, Plate motions and deep mantle convection, in Shagam, R., et al., eds., Studies in earth and space sciences: Geological Society of America Memoir 132, p. 7–22.

O’Connor, J.E., 1993, Hydrology, hydraulics, and geomorphology of the Bonn-eville Flood: Geological Society of America Special Paper 274, 83 p.

Peng, X., and Humphreys, E.D., 1998, Crustal velocity structure across the east-ern Snake River plain and the Yellowstone Swell: Journal of Geophysical Research, B, Solid Earth and Planets, v. 103, no. 4, p. 7171–7186, doi: 10.1029/97JB03615.

Pierce, K.L., and Morgan, L.A., 1992, The track of the Yellowstone hot spot: volcanism, faulting, and uplift, in Link, P.K., Kuntz, M.A., and Platt, L.B., eds., Regional Geology of Eastern Idaho and Western Wyoming: Geologi-cal Society of America Memoir 179, p. 1–53.

Pierce, K.L., Morgan, L.A., and Saltus, R.W., 2002, Yellowstone plume head: postulated tectonic relations to the Vancouver slab, continental boundar-ies, and climate, in Bonnichsen, B., White, C.M., and McCurry, M., eds., Tectonic and Magmatic Evolution of the Snake River Plain Volcanic Province: Idaho Geological Survey Bulletin 30, p. 5–33.

Priestley, K.F., and Orcutt, J., 1982, Extremal travel time inversion of explosion seismology data from the eastern Snake River plain, Idaho, Yellowstone-Snake River plain symposium: Journal of Geophysical Research, v. B, p. 2634–2642.

Repenning, C.A., Weasma, T.R., and Scott, G.R., 1995, The early Pleistocene (latest Blancan-earliest Irvingtonian) Froman Ferry fauna and history of the Glenns Ferry Formation, southwestern Idaho: U.S. Geological Survey Bulletin 2105, 86 p.

Rodgers, D.W., Ore, H.T., Bobo, R.T., McQuarrie, N., and Zentner, N., 2002, Extension and subsidence of the eastern Snake River Plain, Idaho, in Bon-nichsen, B., White, C.M., and McCurry, M., eds., Tectonic and Magmatic Evolution of the Snake River Plain Volcanic Province: Idaho Geological Survey Bulletin 30, p. 121–155.

Saltzer, R.L., and Humphreys, E.D., 1997, Upper mantle P-wave velocity struc-ture of the eastern Snake River Plain and its relationship to geodynamic models of the region: Journal of Geophysical Research, B, Solid Earth and Planets, v. 102, no. 6, p. 11829–11841, doi: 10.1029/97JB00211.

Shervais, J.W., Shroff, G., Vetter, S.K., Matthews, S., Hanan, B.B., and McGee, J.J., 2002, Origin of the western Snake River Plain: Implications from stratigraphy, faulting, and the geochemistry of basalts near Moun-tain Home, Idaho, in Bonnichsen, B., White, C.M., and McCurry, M., eds., Tectonic and Magmatic Evolution of the Snake River Plain Volcanic Province: Idaho Geological Survey Bulletin 30, p. 343–361.

Shervais, J.W., Vetter, S.K., and Hanan, B.B., 2004, Basaltic Volcanism of the Central Snake River Plain, Idaho: Geological Society of America Abstracts with Programs, v. 36, no. 4, p. 98.

Smith, G.R., and Stearley, R.F., 1999, Fish paleoecology and late Cenozoic his-tory of the Snake River Plain: Geological Society of America Abstracts with Programs, v. 31, no. 4, p. A56.

Smith, G.R., Swirydczuk, K., Kimmel, P.G., and Wilkinson, B.H., 1982, Fish biostratigraphy of late Miocene to Pleistocene sediments of the western Snake River Plain, Idaho, in Bonnichsen, B., and Breckenridge, R.M., eds., Cenozoic Geology of Idaho: Idaho Bureau of Mines and Geology Bulletin 26, p. 519–542.

Smith, R.B., and Braile, L.W., 1994, The Yellowstone hotspot: Journal of Volca-nology and Geothermal Research, v. 61, p. 121–187, doi: 10.1016/0377-0273(94)90002-7.

Stearns, H.T., Crandall, L., and Steward, W.G., 1938, Geology and ground-water resources of the Snake River Plain in southeastern Idaho: U.S. Geological Survey Water-Supply Paper 774, 268 p.

Suppe, J., Powell, C., and Berry, R., 1975, Regional topography, seismic-ity, Quaternary volcanism, and the present day tectonics of the western United States: American Journal of Science, v. 275A, p. 397–436.

Tauxe, L., Luskin, C., Selkin, P., Gans, P., and Calvert, A., 2004, Paleomagnetic results from the Snake River Plain: contribution to the time-averaged fi eld global database: Geochemistry Geophysics Geosystems (G3), v. 5, no. 8, QH13.

Tilling, R.I., Wright, T.L., and Millard, H.T., Jr., 1987, Trace element chemistry of Kilauea and Mauna Loa lava in space and time: a reconnaissance, in Decker, R.W., Wright, T.L., and Stauffer, P.H., eds., Volcanism in Hawaii: U.S. Geological Survey Professional Paper 1350, v. 1, p. 641–690.

Vetter, S.K., and Shervais, J.W., 1992, Continental basalts of the Boise River Group near Smith Prairie, Idaho: Journal of Geophysical Research, B, Solid Earth and Planets, v. 97, no. 6, p. 9043–9061.

Vetter, S.K., and Shervais, J.W., 1997, Basaltic volcanism of the Bruneau-Jar-bidge eruptive center, southwest, Idaho: Geological Society of America Abstracts with Programs, v. 29, no. 6, p. A298.


fl d006-02 page 26 of 26

Welhan, J.A., Clemo, T.M., and Gego, E.L., 2002a, Stochastic simulation of aquifer heterogeneity in a layered basalt aquifer system, eastern Snake River Plain, Idaho, in Link, P.K., and Mink, L.L., eds., Geology, Hydroge-ology, and Environmental Remediation: Idaho National Engineering and Environmental Laboratory, Eastern Snake River Plain, Idaho: Geological Society of America Special Paper 353, p. 225–247.

Welhan, J.A., Johannesen, C.M., Davis, L.L., Reeves, K.S., and Glover, J.A., 2002b, Overview and synthesis of lithologic controls on aquifer het-erogeneity in the eastern Snake River Plain, Idaho, in Bonnichsen, B., White, C.M., and McCurry, M., eds., Tectonic and Magmatic Evolution of the Snake River Plain Volcanic Province: Idaho Geological Survey Bul-letin 30, p. 435–460.

Welhan, J.A., Johannesen, C.M., Reeves, K.S., Clemo, T.M., Glover, J.A., and Bosworth, K.W., 2002c, Morphology of infl ated pahoehoe lavas and spa-tial architecture of their porous and permeable zones, eastern Snake River Plain, Idaho, in Link, P.K., and Mink, L.L., eds., Geology, Hydrogeol-ogy, and Environmental Remediation: Idaho National Engineering and Environmental Laboratory, Eastern Snake River Plain, Idaho: Geological Society of America Special Paper 353, p. 135–150.

White, C.M., Hart, W.K., Bonnichsen, B., and Matthews, D., 2002, Geochemi-cal and Sr-isotopic variations in western Snake River Plain basalts, Idaho, in Bonnichsen, B., White, C.M., and McCurry, M., eds., Tectonic and Magmatic Evolution of the Snake River Plain Volcanic Province: Idaho Geological Survey Bulletin 30, p. 329–342.

Wood, S.H., 1994, Seismic expression and geological signifi cance of a lacus-trine delta in Neogene deposits of the western Snake River plain, Idaho: American Association of Petroleum Geologists Bulletin, v. 78, no. 1, p. 102–121.

Wood, S.H., and Clemens, D.M., 2002, Western Snake River Plain—geologic and tectonic history, in Bonnichsen, B., White, C.M., and McCurry, M., eds., Tectonic and Magmatic Evolution of the Snake River Plain Volcanic Province: Idaho Geological Survey Bulletin 30, p. 343–361.

Zarnetske, M.L., and Shervais, J.W., 2004, Plagioclase Flotation Cumulate in Ferrobasalts of The western Snake River Plain: Implications for evolution of planetary magma oceans: Geological Society of America Abstracts with Programs, v. 36, no. 4, p. 95.

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