Chapter 6 – IGNEOUS ROCKS

How, Why & Where Rocks Melt• Begins as solid• Molecules warm

& begin vibrating = softening

• Molecules may vibrate violently enough to break bonds

How, Why & Where Rocks Melt• Heat & pressure inside

Earth– Magma chamber– Geothermal gradient

• Continental vs. ocean crust• Composition varies melting

point• Pressure increases melting

point• Water decreases melting

point

• Heat and pressure inside Earth– Fractional melt (partial

melting)• Mix of molten & solid rock

– 1 magma body mayproduce several different igneous compositions

How, Why & Where Rocks Melt

How, Why &Where Rocks Melt• Magma (Intrusive)

– Molten rock below Earth’s surface

• Lava (Extrusive)– Molten rock when it

reaches Earth’s surface

• Magma & Lava– Composition

• Silica content varies (~ 45-75%)

• Water vapor & carbon dioxide– Temperature

• Temperature varies (~ 750°C – 1200°C)

– Viscosity (resistance to flow)• Varies in ability to flow• Influenced by silica content &

temperature

How, Why & Where Rocks Melt

• Tectonic setting– Characteristics influenced by

location• Oceanic, divergent margins

– Hot, low viscosity basaltic lava• Subduction (convergent) zones

– Cooler, viscous lavas with more silica• Ocean hot spots

– Hot & basaltic; build giant shield volcanoes

• Continental hot spots– Cooler & granitic; high silica lava

How, Why & Where Rocks Melt

Cooling and Crystallization• Crystallization

– Process where mineral grains form & grow in cooling magma (or lava)

– Classified based on:1. Texture (size of mineral

crystals)– Volcanic (extrusive) =

small grains due to rapid cooling

– Plutonic (intrusive) = large grains due to slow cooling

2. Composition (silica content)

Rate of Cooling• Extrusive Textures

– Glassy• Cools too rapidly

to form crystals

• Example: obsidian

Rate of Cooling• Extrusive Textures

– Aphanitic• Fine grained (small

crystals)• Examples: basalt,

andesite, rhyolite

Rate of Cooling• Extrusive Textures

– Vesicular• Form from trapped

gas bubbles• Examples: pumice, scoria

Rate of Cooling• Extrusive Textures

– Pyroclastic or fragmental• Includes rock fragments • Example: volcanic tuff

Rate of Cooling• Intrusive Texture

– Phaneritic• Course grained (large

crystals); slow cooling inside Earth

• Examples: granite, syenite, diorite, gabbro, peridotite

Chemical composition• Igneous rocks

subdivided into 4 categories based on silica content– Felsic– Intermediate– Mafic– Ultramafic

Igneous Rock ClassificationCommon Igneous Compositions

Composition Type % Silica Other

ElementsMagma

ViscosityTemperature

crystallization begins

Igneous Rocks

Produced

Type of Igneous

Rock

Felsic >65% Al, K, Na High 600-800CGranite Plutonic

Rhyolite Volcanic

Intermediate 55-65%Al, Ca, Na, Fe,

MgMedium 800-1000C

Diorite Plutonic

Andesite Volcanic

Mafic 45-55% Al, Ca, Fe, Mg Low 1000-1200C

Gabbro Plutonic

Basalt Volcanic

Ultramafic <40% Mg, Fe, Al, Ca Very low >1200C

Peridotite Plutonic

Komatiite Volcanic

Igneous Rock Classification

Fractional Crystallization• Crystals separate

from liquids during crystallization– Bowens reaction

series– Predictable

melting & cooling of minerals

Plutons and Plutonism• Plutons

– Any body of intrusive igneous rock, regardless of size or shape

• Massive vs. Tabular

• Concordant vs. Discordant

Plutons & Plutonism• Batholith

– Large, irregular shaped pluton

– Massive & Discordant

• Laccolith– Mushroom-

shaped pluton

– Massive & Concordant

Plutons and Plutonism• Dikes

– Magma squeezes into cross cutting fracture & solidifies

– Tabular & Discordant• Sills

– Magma intrudes between 2 layers; parallel to layers

– Tabular & Concordant

Plutons & Plutonism• Volcanic pipe =

remnant• Volcanic neck =

remnant exposed via erosion

Economics of Igneous Rocks• Uncommon uses:

– Mining– Pumice stone– Lava soap– Fingernail files– Surgical tools