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Rocks The Rock Cycle • Magma—molten material that forms inside the Earth – It crystallizes either beneath the surface or as a result of volcanic activity to make • Igneous Rock – It weathers, is transported, and deposited to become • Sediment – It goes through lithification (changes into rock) to become The Rock Cycle • Sedimentary Rock – If it is exposed to great heat and pressure because of mountain building or being intruded with magma, it will change into • Metamorphic Rock – If it is exposed to more heat and/or pressure, it will melt and become • Magma The Rock Cycle • The full cycle does not always take place due to “shortcuts” or interruptions The Rock Cycle Igneous Rock • “Formed by Fire” – Slow cooling results in the formation of large crystals – Rapid cooling results in the formation of very small microscopic crystals Classifying Igneous Rock • Texture—Size and arrangement of the interlocking crystals – Fine-grained—microscopic crystals • Example: rhyolite – Course-grained—able to be seen unaided • Example: granite – Porphyritic—large crystals embedded in a matrix of smaller crystals • Example: andesite porphyry – Glassy—rapid cooling of molten rock ejected into the atmosphere during a volcanic eruption • Examples: obsidian, pumice Classifying Igneous Rock • Mineral Composition – Depends on the chemical composition of the magma from which it crystallizes – Influenced by crystal settling in the magma (density of the components) Naming Igneous Rocks • Basaltic Rocks – Derived from the first minerals to crystallize – Rich in iron and magnesium – Example: basalt • Granitic rocks – From the last minerals to crystallize – Mainly feldspar and quartz – High silica content – Example: granite Naming Igneous Rock • Rocks may have the same compositions, but are named differently because of their different textures. Sedimentary Rocks • Compacted and cemented sediment – Geologists date the Earth based on layers of sediment – Sediment can contain fossils – Economically important • Coal, natural gas, oil • Iron, aluminum, manganese, fertilizer, sand and gravel Classifying Sedimentary Rocks • Detrital – Form from solid particles from weathered rocks – Classified by particle size – Examples: • • • • Shale (most abundant) Sandstone Conglomerate Siltstone Classifying Sedimentary Rocks • Chemical – From material that was once in solution and precipitates to form sediment – Common Rocks: • Limestone (most abundant) • Travertine • Microcrystalline quartz – Chert, flint, jasper, agate • Evaporites – Rock salt, gypsum • Coal – Lignite, bituminous Lithification of Sedimentary Rocks • Compaction – Weight of overlying material compresses deeper sediments • Cementation – Materials such as calcite, silica, and iron oxide are carried in solution by water moving through the pore spaces between particles Features of Sedimentary Rocks • Strata or beds—layers of accumulated sediment • Bedding planes—flat surfaces along which rocks tend to separate or break • Fossils Metamorphic Rocks • Changed in Form – From heat, pressure, and chemically active fluids – Takes place when rock is subjected to conditions unlike those in which it originally formed – Regional metamorphism (mountain building) and contact metamorphism (molten material bake surrounding rock) Three Metamorphic Agents • Heat – Most important agent of metamorphism – Provides energy to drive chemical reactions that rearrange crystal structures – Increases with depth Three Metamorphic Agents • Pressure – Increases with depth – Stress during mountain building – Rock at great depth is warm and becomes plastic-like during deformation Three Metamorphic Agents • Chemically active fluids – Water is contained in pore spaces of virtually every rock – It acts as a catalyst to help ions migrate to form crystalline shapes Metamorphism Changes Texture • Foliated – Some minerals under pressure will recrystallize in an alignment that looks layered • Examples: – Shale becomes slate – Shists—metamorphic rocks that are platy – Gneiss—metamorphic rocks that are elongated and granular Metamorphism Changes Texture • Nonfoliated – Some metamorphic rocks are composed of only one mineral • Examples: – Limestone becomes marble – Quartz sandstone can become quartzite
