What are Igneous Rocks????

Form when magma cools and minerals crystallize

Composition of magma

• Type of Igneous rock depends on the composition of the magma– Silica is the most abundant compound and has the

greatest effect on magma characteristics• 3 classifications of magma based on the

amount of silica it contains– Basaltic– Andesitic– Rhyolitic

• Chemical composition of lava is a little bit different than the magma it came from– Dissolved gases escape into the atmosphere

Magma Formation

• Two ways of formation– Melting earth’s crust– Melting within the mantle

• Four factors– Temperature

• Increases with depth within earth’s crust• Geothermal gradient

– Pressure• Also increases with depth• Result of the weight of overlying rock• Pressure increases, melting point increases

– Water content• Changes the melting point of the rocks• Water content increases, melting point decreases

– Mineral content• Different minerals have different melting points• Basalt – high melting point• Granite – lower melting point

– Contains more water

• Rocks high in iron and magnesium – high melting point• Rocks high in silicon – low melting point

Partial Melting

• Process whereby some minerals melt at relatively low temperatures while other minerals remain solid– New elements are added during the process• Changing the chemical composition• One way different types of igneous rocks are formed

Bowen’s Reaction Series: minerals form in predicatable patterns

Bowen’s Reaction Series

– Iron- rich minerals (left side)• Go through abrupt changes as magma cools

– Feldspars (right side)• Lighter minerals• Continually change chemical composition as magma

cools

Classification of Igneous Rocks

• Intrusive rocks– Magma cools and

crystallizes below the earth’s crust• Magma injected into

surrounding rock

• Extrusive rocks– Cools and crystallizes on the Earth’s surface– Lava flow of flood basalts– Classified by:• Mineral composition• Physical properties

– Grain size – texture

Classification of Igneous rocks• Based on mineral compositions– Basaltic

• Dark colored• Lower silica content

– Granitic rocks• Light colored• High silica content

– Intermediate• Falls between basaltic and granitic• Feldspar and hornblende

– Ultrabasic• Contain only iron rich minerals • Always dark

Basaltic

• Gabbro

Granitic

• granite

Intermediate

• Feldspar

• Hornblende

Ultra basic

• Pyroxene

Crystal size and cooling rates

• When lava cools on the earth’s surface– Cools quickly– Not enough time for large crystals to form

• Rhyolite – hard to see crystals without magnification

– Sometimes no crystals form• When magma cools beneath earth’s

surface– Cools slowly– Large crystals form

Porphyritic Rocks• Porphyritic texture• Large crystals surrounded by finer crystals of

same or different minerals– Complex cooling• First – slow cooling – big crystals• Then quicker as the magma moves closer to the earth’s

surface (or erupted onto the surface) – small crystals

porphyritic granite

Vesicular rocks

• Vesicular texture– Big holes making the rock look spongy• Dissolved gases escape when pressure lessens• Leaving vesicles behind

Thin sections

• Easier to observe igneous rock composition when viewed in a thin strip under a microscope hornblende

olivine

Igneous rocks in resources

• Veins– Fluid left during magma crystallization contain

high levels of water, silica, and leftover minerals– Fluid fills in the cracks of surrounding rocks– Solidifies into mineral-rich quartz veins

Pegmatites

• Veins of very large-grained minerals• Ores of rare elements form• Can produce beautiful crystals

Kimberlites

• Veins in ultrabasic rock• Diamonds • Most likely form deep in the

crust or mantle– Intrude quickly upward– Forming long, narrow, pipe

like structures

Obsidian

• Normally extrusive• Forms when molten rock

material cools so rapidly that atoms are unable to arrange themselves into a crystalline structure.

• The result is a volcanic glass with a smooth uniform texture that breaks with fracture

Rhyolite

• Light-colored• Fine-grained• Extrusive igneous rock• Typically contains

quartz and feldspar minerals.

Biotite Granite• intrusive, most commonly occurring in

batholiths

Hornblende

• greenish-black to black• extrusive

Basalt

• Basalt is dark-colored and fine-grained

• extrusive

Gabbro

• Gabbro is dark, medium- to coarse-grained • Intrusive igneous rock