III. Igneous RocksMinerals Crystallized from Melts

1. The Rock Cycle

2. Formation of Igneous Rocks

3. Classification of Igneous Rocks

4. Igneous Rocks and Plate Tectonics

The Rock Cycle

Igneous Rock

Solidification

Magma

Partial Melting

Mantle Rock Fig 3.1

Geological Materials

Transformation Processes

See Kehew, Fig. 1-6

Partial Melting and MagmaE.g., partial melting results in Silica enrichment

Partial Melting MagmaEnriched: O, Si, Al, Na, K, Depleted: Ca, Fe, Mg Poor in: O, Si, Al, Na, K, (<50%) Rich in: Ca, Fe, Mg (>50% wt.)

See Kehew, Fig 3-6

Formation of Magma

How are rocks melted? 1. Heating ■ 2. Depressurization 3. Increase water content 4. Increased silica content

Where do rocks melt? Subduction zones (Silicic

and Intermediate) Mantle Plumes (“Hot Spots”) not only at

Divergent

Boundaries

MaficSee Kehew, Fig 3-6

Hot andHigh Pressure

Hot andLow Pressure

See Kehew Fig. 3-8

Solidification of Melts

• Magma, Intruded or Extruded,• Solidifies (crystallizes) to form

• Intrusive or Extrusive • Igneous Rocks

Fig 3.12, 3.13See Kehew, Fig 3-6

Intrusive vs. Extrusive (Silicic Rocks)

Silicic (a.k.a, felsic) Magmas Cool (<700oC) Viscous (sticky, doesn’t flow easily) Gaseous (steam of H2O and C02)

Silicic Rocks Usually intrusive, course-grained, Silicic (Granite) to Intermediate

(Diorite) rock forms plutons If extrusive, fine-grained rocks

formed by explosive volcanoesRhyolite or Andesite Volcanoes

Also injects surrounding rocks with silica laden steam

See Kehew, Fig 3-40

Intrusive Igneous RockE.g., Granite (Silicic, Phaneritic):

Crystallized (Solidified) Silicic Magma Poor in: Fe, Mg, Ca, (<20%) Rich in: Silica (>70%)

Quartz

Na Plagioclase

Biotite

Composite Volcanoes Silicic magma High-viscosity Gaseous Explosive

eruptions Steep sloped,

cone shaped volcanoes

Extrusive vs. Intrusive (Mafic Rocks)

Mafic Magmas Hot (>1000oC) Non-Viscous (runny, flows easily) “Dry” (no H2O or C02)

Mafic Rocks Usually Extrusive, Fine-grained,

Mafic (Basalt) rock forms oceanic crust, Shield Volcanoes and Basalt Floods

If Intrusive, course-grained mafic rocks are formed Gabbro.

If intrusive, Dikes and Sills more common

See Kehew, Fig 3-40

E.g., Mafic Volcanism, Hawaii

Mantle Hot Spot Volcano

See Kehew, 3-8

Hawaii

10.3 12.0

20.6

Midway 27.2 19.9 43.4

42.4

48.1 55.2

56.2 59.6

0-5

Current PlateMotion

HawaiiA long chain of inactive volcanoes Island ages, millions of years

Shield Volcanoes

Mafic Magma Low-viscosity Non-explosive eruptions Gentle slopes Covering large areas

Types of Eruptions (Mafic Volc.)

Types of Eruptions Lava floods Lava fountains Fissure eruptions

Rock Textures (Table 3-1)

Aphanitic Porphyritic Vesicular Glassy (Obsidian)

Surface Textures (Extrusive, Mafic Rocks)

Ahah (Rubbley) Mostly solid when

flowing Pahoehoe (Ropey)

mostly liquid when flowing)

See Kehew, Fig 3-9, 3-10

Volcanic Hazards Relative Hazard (see Kehew, Fig. 3-30, 31)

based on Type of volcano Active Vents Earthquakes Calderas Historical Flows Rift Zones Slopes

Mafic Sill: Intruded between layers Mafic magma is less

viscous and hotter so Does not form plutons

but Cuts along layers (Sills)

or even across layers (Dikes) Also Baked Zones

of adjacent country rock and Chill Zones within the intrusion

Igneous Rock ClassificationIntrusive (Plutonic)

Extrusive (Volcanic)

See Kehew, Fig. 3-5

Min

eral

Per

cen

tag

e

Continental Crust Oceanic Mantle Crust

Igneous Rocks and Silicates

The

Igneous Rock Classification

parallels the

systematic

Silicate Mineral Classification

Frame- Double Single Iso. work Sheet Chain Chain

Fig

. 3-6

Min

eral

Per

cen

tag

e

Igneous Rock Classification Silicic Intermediate Mafic

Granite Diorite Gabbro

Rhyolite Andesite Basalt

(Porphyritic)

Intr

usi

veE

xtru

sive

1200

oC

1000

o

Bowen’s Reaction SeriesTwo series of minerals formed during crystallization of magma

Intrus. Extrus.

Gabbro Basalt

Diorite Andesite

Granite Rhyolite

Low Silica Magma

High Silica Magma

Fra

me-

D

oubl

e

S

ingl

e

Isol

ated

wor

k

S

heet

Cha

in

Cha

in

Temperature ofCrystallization

750

o