GEOL- 103 Lab 2: Igneous/Metamorphic Rocks

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