Earth Science::Igneous Rocks

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lecture 3

Igneous Rocks

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

Magma

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::volcanic
intrusive::plutonic

Extrusive or Volcanic Rocks

Formed from lava at the Earth's surface.

Intrusive or Plutonic Rocks

Formed from magma at depth.

Magma

Consists of three components:
-liquid
-solid
-gases

Melt

The liquid portion of magma.

Silicate Minerals

The solid portion of magma.

Volatiles

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

-texture
-types of minerals
-chemical composition

Igneous Textures

Aphantic
Phaneritic
Pegmatic
Pyroclastic
Glassy

Aphantic

-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)

Phaneritic

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

Porphyritic

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

Pegmatic

-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)

Pyroclastic

-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.

Glassy

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

Pumice

-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.

Ferromagnesian

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

Nonferromagnesian

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.

Felsic

Granite (plutonic)
Rhyolite (volcanic)
-high sticky

Intermediate

Diorite (plutonic)
Andesite (volcanic)

Mafic

Gabbro (plutonic)
Basalt (volcanic)
-low runny

Earth's mantle

Ultramafic

Basaltic Composition

viscosity: low (runny)

Rhyolitic

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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