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.
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.
-Crystals too small to identify with naked eye. (<1mm)
-Rapid rate of cooling near surface: volcanic
-May contain vesicles(holes from gas bubbles)
-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.
-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.
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
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.
A decrease in confining pressure causes a decrease in a rock's melting temperature.
-divergent plate margins along mid-ocean ridges.
Adding volatiles (primarily water) causes rocks to melt at lower temperatures.
-convergent plate margins along subduction zones.
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