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.
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: -liquid -solid -gases
The liquid portion of magma.
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
-texture -types of minerals -chemical composition
Aphantic Phaneritic Pegmatic Pyroclastic Glassy
-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.
-Mainly light-colored silicates. -Felsic (FELdspar & SIlica) in composition. -Major constituent of continental crust.
-Dark silicates and calcium-rich feldspar. -Mafic (MAgnesium & FErrum, for iron) in composition. -Major constituent of oceanic crust.