Igneous Processes I: Igneous Rock

Formation, Compositions, and Textures

Crustal Abundances of Rock Types

Igneous Rocks • Form by the cooling and hardening

(crystallization/glassification) of

magma.

• Most magma crystallizes before it

can reach the surface, producing

bodies called plutons made of

_________ (plutonic) igneous rock.

• Some magma (known as lava)

reaches the surface while still at

least partially molten, producing

volcanic eruptions and _________

(volcanic) igneous rocks.

Classifying Igneous Rocks A magma is a multi-component material with a bulk

composition which almost always changes as it

moves and cools.

• ______________: types and abundances of

different minerals and non-minerals

• ____________: sizes, shapes, and boundary

relationships of the mineral grains and other

components (i.e. flow patterns

• ___________________: Temperature at eruption

and/or rate of cooling in a magma chamber

• ______________________: determines final

product that appears on Earth’s surface

Igneous Composition Various igneous environments will produce

magmas which differ in silica content and the

abundances of metals such as Fe, Mg, Ca, Na,

and K.

• ________: poor in silica (~50%), rich in Fe, Mg,

Ca, poor in Na and K

• ________: rich in silica (~70%), poor in Fe, Mg,

Ca, rich in Na and K

• _____________: between mafic and felsic

• ______________: “beyond mafic,” even more

mafic than mafic

Magma (or lava if erupted to

the surface) is composed of

liquid, solid (mineral crystals)

and gas. Its composition is

largely controlled by its

source.

Obsidian flow, Oregon

Pahoehoe flow, Hawaii

Glassy Scoria

Composition

• Magmas are subdivided largely by ___________________.

As silica content increases, iron (Fe), magnesium (Mg), and

calcium (Ca) content decreases.

• Lighter elements, such as sodium (Na) and potassium (K)

content follow the silica trends. Chemical compositions are

often described in terms of ______________

Recognizing Igneous Composition

• Need to be able to identify the common

minerals in igneous rocks: olivine,

pyroxene, amphibole, micas, feldspars,

and quartz.

• If grains are not apparent, can fall back

on the observation that mafic minerals

tend to be ___________, whereas felsic

minerals tend to be _______________.

• Note that the above point applies to

minerals, not glasses, which can be

strongly colored by submicroscopic

inclusions. Obsidian is ______, but is

usually black in color.

Silicate Behavior

Bowen (1925) recognized that mafic minerals

tend to have __________________ and

less polymerization (chain-forming) between

silicate tetrahedra.

Bowen’s Reaction Series summarizes these

trends, along with the effects of dissolution,

precipitation, and solid-state diffusion in

determining which minerals will be produced

for a magma of a given bulk composition.

As magma cools, minerals form at different _______________. Along the

_______________ series, there are distinct “steps” at which minerals will

begin crystallizing (and perhaps later dissolving). Along the ___________

series, the composition of the plagioclase shifts from Ca-rich to Na-rich.

The steps described by

Bowen’s Reaction Series

may end up interrupted if

temperatures fall too

quickly. Olivine, for

example, may only be

partially dissolved before

the texture and

composition becomes

“frozen” when the

reaction rates are too

slow.

Such features are

themselves useful in

determining the conditions

under which the rock

formed.

The “continuous” replacement of high-temperature Ca-spar by low-

temperature Na-spar often is incomplete, since it relies upon very

slow diffusion of atoms through already-solid crystals. The result is

“zoned” plagioclase feldspar, with Ca-rich centers and Na-rich

rims.

Changes in Bulk Chemistry

• Further complications arise if materials are

removed during solidification.

• Several fractionation processes:

1) ________________________of initial solids

2) __________________as the magma moves

3) ___________________of residual fluid

4) _____________(water, gases) along with

readily-dissolved elements which don’t fit

well in the crystallizing silicate minerals

Differentiation of magma can occur from fractional crystallization involving the

removal of crystals as they accumulate. The solid phase will have a composition

that is relatively more mafic than the remaining melt phase.

Magmatic differentiation of magma by __________________. Note how the

composition of the magma changes as more mineral crystals form. Think of the

yellow atoms forming to Fe-Mg silicate minerals that crystallize first during the

differentiation process. Think of the red atoms comprising the silica-rich melt.

As earlier formed minerals are removed from the magma by fractional crystallization, a

greater proportion of the denser elements (Fe and Mg) are removed leaving a residual

melt that is more enriched in silica and lighter elements. Minerals and rocks that form

later will have a greater proportion of the lighter elements (SiO, Al, Na and K).

Several metals of economic interest, such as gold, silver, and copper, do not

“fit” well in the growing silicate minerals. Instead, they often are carried

away from the magma in aqueous fluids and become deposited in cracks

(veins) as pressures and temperatures decrease towards the surface. Silica

also is carried this way, precipitating as quartz.

Gold ore in a

quartz vein

Igneous Rock Classification

• High silica rocks are light in color (pale grey to pink)

• Low silica rocks are dark (due to more dark

minerals containing ______________)

Low Silica Medium Silica High Silica

Silica Content and Color

Basalt Andesite Rhyolite

Gabbro Diorite

Granite

Extr

usiv

e

Intr

us

ive

Even when molten, the silicate

tetrahedra will polymerize into

chains. These will become

entangled and thereby

____________.

Over the seemingly modest range

of 50% to 70% silica content, this

extent of tangling results in a

change of about _____ orders of

magnitude in viscosity, or a factor

of around ________________.

____________magmas can flow

almost like water. ___________

___________ magmas are far

more sluggish than toothpaste.

Silica Content and Viscosity

Mafic lavas often erupt in a gentle fashion. Their low viscosities

make it less likely that gas pressure will build to the point of

explosiveness.

Due to their low viscosities, basaltic composition magma (lava)

will flow great distances from its vent.

Intermediate and felsic lavas often erupt with great violence in

large part because gases cannot easily escape them. When they

do not explode, they instead ooze slowly and do not travel far.

Rhyolite/dacite flows will retain steep slope fronts because of their high viscosity.

• High silica volcanoes are explosive, due to build-up of pressure within volcano. Viscous lava won’t flow far, so volcanoes are tall and pointy (________________).

• Low silica volcanoes are non-explosive. Lava is runny, so volcanoes are broad and non-pointy (______________)

Silica content and Volcano Type

Summary of Trends with Composition

Mafic (Basalt/Gabbro)

• Density about 3.3 g/cm3

• Crystallization ~1200°C

• Low Silica

• Rock color = dark grey to

black

• Low viscosity

• Typically mild eruptions

• Shield Volcanoes (low,

wide)

Felsic (Rhyolite/Granite)

• Density about 2.7 g/cm3

• Crystallization ~700°C

• High Silica

• Rock color = pale

grey/pink

• High viscosity

• Typically violent eruptions

• Stratovolcanoes (tall,

pointy)

Igneous Textures

• Slow cooling produces large grains, rapid

cooling produces small (or no) grains.

• Terms for Crystal Size:

• ______________: visible to unaided eye, also

called coarse-grained. Usually ____________.

• ____________: crystalline, but not visible, also

called fine-grained. Usually ____________.

• _____________: not crystalline. _____________.

• ____________: coarse grains (phenocrysts)

surrounded by fine grains (groundmass). Began

crystallizing underground, then erupted and finished

solidifying on surface. _____________.

_____________ igneous rocks crystallize slowly (usually underground). Chemical

composition also plays a role in determining the specific rock type.

Gabbro Diorite Granite

Phaneritic grains are distinguishable to the unaided eye. This rock contains

quartz (light gray), plagioclase feldspar (white) and biotite (black) crystals.

A pink granite is dominated by potassium feldspar (pink crystals), quartz (gray

glassy appearance), plagioclase (porcelain white mineral) and biotite (black

sheets).

_________ rocks contain mineral grains which are too small to distinguish

clearly with the unaided eye. Same magnification as the previous image.

Obsidian has a glassy texture. It may contain a few isolated mineral grains

or even an abundance of submicroscopic crystal “seeds” (crystallites), but

it is mostly amorphous, lacking the long-range order of ______________.

Note the characteristic conchoidal fracture diagnostic of obsidian.

__________________is partially coarse and partially fine. The large

phenocrysts formed first, slowly, in the subsurface, whereas the

groundmass crystallized quickly after eruption onto the surface. This is

often referred to a two-stage cooling process.

Other Igneous Textures ______________ “Broken by Fire”:

• Violent volcanic eruptions produce an explosive

spray of lava which hardens (at least partially)

while in flight.

• The resulting fragments may or may not weld to

one another upon landing, but usually retain the

outlines of their initial crusts.

• Individual particles range from dust-sized,

called ____, to building-sized, called ________,

and are typically a mixture of minerals and

glass.

A large pyroclastic eruption of Mount Pinatubo in the Philippines (1992).

The ash and other volcanic derived clasts can become welded together

to form fine-grained _______ or coarse-grained volcanic ____________.

Volcanic ash (tephra) derived from the Mount Mazama

(Crater Lake, Oregon) eruption 6800 years ago.

Welded tuffs in thin section: The triangular

fragments are created when the magma between

gas bubbles is blown apart. The fragments then get

flattened and welded together from the heat and

weight of the flow.

____________________forms from a welded, mixture of large, angular

volcanic clasts within a matrix of fine ash. This photo was taken on

Lipari Island, Italy by Raymond Coveney.

Hand Sample

Volcanic ________:

molten rock

aerodynamically shaped

due lava freezing while in

flight.

___________: As a magma

approaches the surface, it

undergoes decompression and

cooling. This decreases its

ability to hold various gases

(H2O, CO, CO2, etc.) in solution.

These gases will separate as bubbles which will

either escape or remain trapped as the

magma hardens around them. Trapped

bubbles are called vesicles.

Other Igneous Textures

Pumice (shown) or scoria (darker) form when gas bubbles are trapped in

rapidly cooling pyroclastic materials. The rocks are glassy and frothy.

Scoria often forms in basaltic magmas where gases are escaping—

often near the tops of flows. Bubble size can get quite large, since the

lower viscosity lavas allow gases to coalesce into larger bubbles

compared to a felsic lava (which will form pumice).

Scorias can be a deep red when the _______ in the mafic lava is

oxidized by the escaping gases.

Pahoehoe (ropey textured) basalt flows have a _________ viscosity than aa

(blocky textured) flows, which have degassed and __________.

Aa Flow (Think about what

you would say if you had to

walk on this aa flow (ah, ah).

Pahoehoe Flow (Smooth word, smooth flow).

Other Igneous Textures

Other Igneous Textures

________________: when basaltic

lava erupts underwater or flows into

water, it will form into pillow-like

shapes, often with a glassy rind,

since the exterior of the pillow is in

contact with cold water and freezes

rapidly.

Other Igneous Textures

Columnar Jointing: fracture

pattern into the shape of

hexagonal columns that happens

when lava (usually basaltic) cools

and _____________.The

columns will be ______________

to the cooling surfaces, such as

the air and ground.

Columnar Jointing at Devil’s Postpile, near

Mammoth Lakes, CA. The direction of the columns

changes near the front of the flow

Typical Magmatic Sources

• The mantle is __________. Unusually

extensive melting will produce ____________

magmas, but “routine” partial melting

produces _________ magmas.

• Partial melting of subducting oceanic crust

(mafic) and its associated sediments

produces mafic and _____________magmas.

• Interaction with _________________is

required for the production of felsic magmas.

Sources of Magma

• In nearly all of the crust and mantle,

temperatures are too _______ for melting to

occur at the surrounding pressures.

• Magma production occurs when:

– warm rock travels ___________ (decompression

melting), as at divergent zones and hotspots, or

– cold rock is forced _____________ and absorbs

heat from its new surroundings, as at subduction

zones

Mafic magma forms from a partial melt

of the asthenosphere, which occurs at a

depth (100-350 km) where the

geothermal gradient intersects the

melting temperature curve for upper

mantle rock (garnet peridotite).

•Note that the geothermal gradient is

dependent upon ______________while

the melting temperature curve is

dependent upon pressure (depth) and

_____________ of the rock involved.

The curve is for a “dry” melt, with no

water involved.

•Even in the region of melting, only a

small fraction (______) of the rock

actually melts– this is the portion with

the lowest melting point. The product is

a relatively low-density mafic magma

from an ultramafic starting material.

This magma will tend to be displaced

upwards by _______________.

Mafic Magma Formation

Mafic magma forms at four different tectonic settings. Mafic (basaltic) magma

is always derived from a partial melt of the ultramafic asthenosphere.

Felsic (granitic) magma forms from a

partial melt of continental crust, which

contains dissolved water. Dissolved water

content in a magma __________ its

melting temperature with ____________

pressure (water molecules will inhibit the

silicate tetrahedra from forming bonds).

Note that the melting temperature curve

for a wet granitic melt increases with

decreasing pressure (opposite of basaltic

dry melt). Melting occurs at a depth of 35-

45 km within continental crust.

As granitic magma rises it solidifies (point

X) as its melting temperature increases

while the geothermal gradient (actual

temperature) decreases. Granitic

composition magmas rarely reach the

surface as volcanic rhyolite flows because

of the high water content and

corresponding increase in melting

temperature as it rises towards the

surface.

Felsic Magma Formation

Granitic composition magma is

produced at _________________

margins. As the continental crust

thickens it begins to partially melt at

depth. Igneous intrusions (plutons)

form below the mountain belts.

______________ is rare in continental

collision boundaries.

As collisional tectonic mountain

ranges are uplifted the overlying

marine sedimentary and metamorphic

rocks are eroded exposing the

underlying granitic plutons.

The granitic rocks of New Hampshire

and Vermont represent old granitic

plutons that were intruded when the

Appalachian Mountains formed 300

million years ago as North American

continent collided with proto-European

continent.

Felsic Magma Formation

Granitic rock excavated from a quarry in Barre, Vermont formed as plutons

beneath the Appalachian Mountains when North Africa collided with

eastern North America 300 million years ago.

Roof pendant of remnant “country rock” (dark metamorphic rock) lies

above the intruded Sierra Nevada Batholith (light colored granodiorite).

Granitic composition magma reaches to the surface in Yellowstone Park

because the continental crust is being heated closer to the surface by upwelling

magma generated from a hotspot in the asthenosphere.

The Yellowstone Caldera (Wyoming) formed following a very large eruption

~600,000 years ago. The rhyolite flows are very viscous and internal gas

pressures can be very high

Intermediate (andesitic) composition

magma can crystallize below the

surface beneath _________________

and create large coarse-grained

plutonic bodies.

Compositions can range from granite

to diorite.

El Capitan shown on the left is part of

the Sierra Nevada intrusive complex

that formed over 90 million years ago

when a subduction zone existed along

the margin of California.

The plutonic bodies comprising the

Sierra Nevada are similar in origin to

the plutonic bodies forming under the

modern Cascades.

Grano-diorite

rock from the

Sierra Nevada

Intermediate Magma Formation

Andesitic magma is produced from a partial melt of oceanic crust along subduction

zones. Introduction of water forced out of the subducting plate lowers the melting

temperature of the upper mantle, which rises and partially melts the overlying crust. In

an ocean-continental convergent margin it may mix with partially melted continental

crust, increasing the magma’s _____________(becomes more felsic). Mount St. Helens

dacites are more silica rich than Mt. Rainier andesite, likely due to continental source.

Mt. St. Helens is composed of intermediate composition dacitic flows. Dacite is

slightly more felsic (has greater silica content) than andesite, but more mafic

(higher Fe and Mg content) than rhyolite.