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Lab 2: Igneous/Metamorphic Rocks
GEOL- 103
Igneous Rocks
 Form as molten rock cools and solidifies
 General characteristics of magma
 Parent material of igneous rocks
 Forms from partial melting of rocks
 Magma at surface is called lava
 Environments
 Extrusive: rocks formed from lava (volcanic rocks)
 Intrusive: rocks formed from magma (plutonic rocks)
 Can also have both stages
Igneous Textures
 Used to describe overall appearance of rock based on:
 Crystal size
 Arrangement of minerals
 Factors affecting crystal size
 Rate of cooling
 Slow rate: fewer, large crystals
 Fast rate: abundant, small crystals
 Very fast rate: no crystals
 % of silica present
 Dissolved gases
Igneous Rock Textures
 Aphanitic texture
 Fine-grained
 Rapid cooling rate
 Microscopic crystals
 Phaneritic texture
 Coarse-grained
 Slow cooling rate
 Large, visible crystals
Igneous Rock Textures
 Porphyritic texture
 Mineral forms at different
depths and temperatures
 Large crystals (phenocrysts) are
embedded in matrix of smaller
crystals (groundmass)
 Glassy texture
 No crystals
 Very rapid cooling
Igneous Rock Textures
 Pyroclastic texture
 Formed from collisions due to
volcanic eruptions
 Many different sized particles
 Vesicular texture
 Formed during volcanic
eruptions
 Contains holes formed by
captured gases
Mineral Compositions
 Felsic
 Light-colored silicate minerals (quartz, feldspar,
muscovite)
 High amounts of silica; high viscosity; high gas content
 Common in continental crust
 Mafic
 Dark-colored silicate minerals (olivine, pyroxene,
amphibole, biotite, plagioclase)
 More dense than felsic rocks
 Low amounts of silica; low viscosity
 Common in oceanic crust and volcanic islands
Mineral Compositions
 Intermediate
 Contain 25% or more dark silicate minerals
 Associated with explosive volcanic activity
 Mix of mafic and felsic compositions
 Ultramafic
 Rare composition common in mantle
 High in magnesium and iron
 Composed mostly of olivine (green tint)
Bowen’s Reaction Series
Metamorphic Rocks
 Formed through process of metamorphism
 Transition of one rock into another by increasing
temperature and/or pressure conditions
 Produced from:
 Igneous rocks
 Sedimentary rocks
 Other metamorphic rocks
 Progresses incrementally from low-grade to high-grade
 During metamorphism, rock must remain essentially
solid
Metamorphic Settings
 Contact metamorphism
 Body of magma comes into contact with country rock
 Chemical alterations from hot, ion-rich water
 Localized event
 Regional metamorphism
 Occurs during large-scale regional mountain building events
 Produces greatest volume of metamorphic rock
 Rocks usually display zones of contact and/or hydrothermal
metamorphism
Metamorphic Agents
 Heat
 Most important agent
 Recrystallization results in new, stable minerals
 Heat sources: contact metamorphism and/or
geothermal gradient (25°C/km)
 Pressure
 Increases with depth
 Realigns mineral crystals
Metamorphic Agents
 Hydrothermal fluids
 Water containing other volatile compounds
 Enhances migration of ions to affect recrystallization of
existing minerals
 Fluid sources: pore spaces, fractures, and hydrated
minerals
 Parent rocks
 Rock with same overall composition of metamorphic rock
 Mineral make-up of parent rock influences degree of
metamorphism that may occur
Metamorphic Textures
 Refers to size, shape, and arrangement of mineral
grains
 Foliation
 Any planar arrangement of mineral grains or
structural features within a rock
 Formation
 Rotation of platy and/or elongated minerals
 Recrystallization of minerals in direction of preferred
orientation
 Changing shape of equidimensional grains into aligned,
elongated shapes
Metamorphic Textures
 Types of foliation
 Slaty: rocks can be easily split into thin, tabular sheets
 Schistosity: platy minerals visible and exhibit a layered
structure
 Gneissic: distinctive banded appearance
 Nonfoliation
 Develop in low deformation environments
 Composed of minerals with equidimensional crystals
Samples
Igneous Rocks
 Basalt
 Diorite
 Gabbro
 Granite (x2)
 Obsidian
 Peridotite
 Porphyritic Andesite
 Pumice
 Rhyolite
 Scoria
 Volcanic Tuff
Metamorphic Rocks
 Anthracite
 Gneiss
 Marble
 Phyllite
 Quartzite
 Schist (x2)
 Slate