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lecture 3

Igneous Rocks

Cooling and solidification of magma.
-ex] granite and basalt


Formed by melting of rocks in the hot deep crust and mantle.
-called lava when at Earth's surface.

Crystallization of Magma

-water changes from liquid to solid at 0%C
-magma changes from liquid to solid over a temp interval of 200-300 C
-as magma cools, different silicate minerals begin to crystallize and grow.

Igneous Rocks


Extrusive or Volcanic Rocks

Formed from lava at the Earth's surface.

Intrusive or Plutonic Rocks

Formed from magma at depth.


Consists of three components:


The liquid portion of magma.

Silicate Minerals

The solid portion of magma.


Gases including water (H2O), carbon dioxide, sulfur dioxide.
-at high pressures at depth in magma=dissolved in the melt.
-at low pressures near surface=form separate gas phase.

Classifying Igneous Rocks

-types of minerals
-chemical composition

Igneous Textures



-Fine-grained rock.
-Crystals too small to identify with naked eye. (<1mm)
-Rapid rate of cooling near surface: volcanic
-May contain vesicles(holes from gas bubbles)


-Coarse-grained rock.
-Large visible crystals. (c. 1-20 mm)
-Slow rate of cooling at depth: plutonic


-Large crystals (phenocrysts) are embedded in a matrix of smaller crystals (groundmass)
-Minerals form at different temperatures.


-Extremely coarse-grained rock.
-Crystal sizes from several cm to several m.
-Form in late stages of crystallization of magma when rich in fluids (H2O)


-Also known as fragmental texture.
-Produced by violent volcanic eruptions.
-Often appear more similar to sedimentary rocks.
-Tuff= ash-sized fragments (<2mm)
-Volcanic breccia=particles larger than ash.


-Very rapid cooling lava resulting rock is called obsidian.
-Used by many ancient cultures to make sharp blades and arrowheads.
-ex] pumice: frothy glass.


-Depressurization forms bubbles in the magma as gases exsolve, rapid cooling freezes the bubbles in glass.
-uses: abrasive & cement additive.

Minerals in Igneous Rocks

Silicate minerals are the most important constituents.
-silicon-oxygen tetrahedron is the fundamental building block.


Dark, mafic
-silicates: olivine, pyroxene, amphibole, and biotite mica.


Light, felsic
-silicates: quartz, muscovite mica, and feldspars.

Granitic Composition

-Mainly light-colored silicates.
-Felsic (FELdspar & SIlica) in composition.
-Major constituent of continental crust.

Basaltic Composition

-Dark silicates and calcium-rich feldspar.
-Mafic (MAgnesium & FErrum, for iron) in composition.
-Major constituent of oceanic crust.


Granite (plutonic)
Rhyolite (volcanic)
-high sticky


Diorite (plutonic)
Andesite (volcanic)


Gabbro (plutonic)
Basalt (volcanic)
-low runny

Earth's mantle


Basaltic Composition

viscosity: low (runny)


viscoscity: high (sticky)

Origin of Magma

-The Earth's crust and mantle are solid rocks.
-Earth's crust does NOT float on Mantle of molten rock.
-Magma only forms in special places where conditions allow pre-existing solid rocks to melt.

Origin of Magma

Three Ways:
1. Increase temperature (thermal)
2. Decrease pressure (decompression)
3. Add water (fluid-induced)
-most magma formed from melting of mantle but some felsic rocks from melting of crust.

Decompression Melting

A decrease in confining pressure causes a decrease in a rock's melting temperature.
-divergent plate margins along mid-ocean ridges.

Fluid-Induced Melting

Adding volatiles (primarily water) causes rocks to melt at lower temperatures.
-convergent plate margins along subduction zones.

Magmatic Differentiation

During crystallization, the composition of the liquid portion of the magma continually changes.
-composition of magma in chamber changes as crystals grow and are then removed by settling.

Magma Compositional Variations

A single volcano may erupt lavas with very different compositions.
-ex] Crater Lake, Oregon

Ore Deposits

Economically-viable sources for many metals.
-ex] chromium, platinum, gold, copper, nickel.

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