Earth Science::Igneous Rocks
|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|
|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:|
|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
|Igneous Textures|| Aphantic|
|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)|
|Intermediate|| Diorite (plutonic)|
|Mafic|| Gabbro (plutonic)|
|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.