Igneous Rocks: Formation, Composition, and Classification

Igneous rocks

Rocks that form by crystallization and solidification from magma

Two main controls on igneous rocks

Composition and texture

Texture in igneous rocks

Size shape and arrangement of crystals

Main factor controlling texture

Cooling rate of magma

Intrusive igneous rocks

Form below Earth's surface and cool slowly producing coarse crystals

Extrusive igneous rocks

Form at Earth's surface and cool rapidly producing fine crystals

Pyroclasts

Fragments formed during explosive volcanic eruptions

Two main compositional groups of igneous rocks

Felsic and mafic

Felsic igneous rocks

Rich in feldspar and silica

Mafic igneous rocks

Rich in magnesium and iron

Ultramafic igneous rocks

Extremely rich in magnesium and iron and low in silica

Four compositional classes

Felsic intermediate mafic ultramafic

Felsic intrusive rock

Granite

Felsic extrusive rock

Rhyolite

Intermediate intrusive rock

Diorite or granodiorite

Intermediate extrusive rock

Andesite or dacite

Mafic intrusive rock

Gabbro

Mafic extrusive rock

Basalt

Ultramafic rock example

Peridotite

Common felsic minerals

Quartz and feldspar

Common mafic minerals

Olivine pyroxene amphibole

Olivine chemical formula

(Mg Fe)2SiO4

Pyroxene chemical formula

(Mg Fe Ca)SiO3

Basalt composition

Mafic with pyroxene and plagioclase

Granite composition

Felsic with quartz feldspar and mica

Classification of igneous rocks

Based on composition and texture

How igneous rocks form

By crystallization from magma below melting temperatures

What controls melting temperature

Composition temperature pressure and fluids

Decompression melting

Melting caused by a decrease in pressure

Fluid induced melting

Melting caused by addition of water lowering melting temperature

Temperature induced melting

Melting caused by an increase in temperature

Divergent plate boundary melting

Decompression melting of mantle peridotite

Magma at mid ocean ridges

Mafic basaltic magma

Convergent plate boundary melting

Fluid induced melting from water released by subducting slab

Hot spot volcanism

Melting caused by decompression within plates

Example of hot spot

Hawaii

Bowen's reaction series

Order in which minerals crystallize as magma cools

First minerals to crystallize

Olivine and calcium rich plagioclase

Last minerals to crystallize

Quartz and potassium feldspar

Continuous branch of Bowen series

Plagioclase feldspar changes from calcium rich to sodium rich

Discontinuous branch of Bowen series

Olivine pyroxene amphibole biotite

Partial melting

Melting of only some minerals in a rock

Key rule of partial melting

Melts are always more felsic than their parent rock

Fractional crystallization

Process where crystals separate from magma changing magma composition

Result of fractional crystallization

Remaining magma becomes more felsic

Ophiolite suite

Sequence of rocks representing oceanic crust and upper mantle

Components of ophiolite suite

Deep sea sediments pillow basalts sheeted dikes gabbro peridotite

Pillow lava

Bulbous basalt formed by lava erupting underwater

Sheeted dikes

Vertical magma intrusions feeding surface eruptions

Gabbro layer

Coarse grained mafic rock below sheeted dikes

Peridotite layer

Ultramafic mantle rock beneath oceanic crust

Igneous rocks at divergent boundaries

Basalt and gabbro

Igneous rocks at convergent boundaries

Basalt andesite dacite rhyolite

Igneous rocks at hot spots

Mostly mafic basalt

Why melts become felsic

Low temperature minerals melt first

Silica content trend

Felsic rocks have high silica mafic rocks have low silica

Viscosity trend

Felsic magma is more viscous than mafic magma

Density trend

Mafic rocks are denser than felsic rocks

Temperature trend

Mafic magmas crystallize at higher temperatures than felsic magmas