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Rocks Section 1 The Rock Cycle Rocks A rock is a coherent aggregate of minerals - a physical mixture. Three categories of rock: Igneous Formed from cooling and crystallization of magma or lava Sedimentary Formed from preexisting rocks subjected to weathering and erosion Metamorphic Formed from preexisting rock transformed by heat, pressure, or chemical fluids Rock—a mixture of minerals, volcanic glass, organic matter, or other material The Rock Cycle—model showing processes that create and change rock – Sedimentary rock can be changed by heat and pressure into metamorphic rock – Metamorphic rock can melt and cool to form igneous rock – Igneous rock can be broken into fragments that may later form sedimentary rock Rock Cycle Conservation of matter—rock cycle never destroys elements of rocks but merely redistributes them James Hutton recognized the rock cycle in 1788 by observing Siccar Point, Scotland Rocks Section 2 Igneous Rocks Igneous rocks form from magma found deep under Earth’s surface. – Magma reaching the surface flows from a volcano as lava – Magma trapped below the surface forms large-grained intrusive igneous rock when it cools. – Magma cooling at or near Earth’s surface forms small-grained extrusive igneous rock Magma to Igneous Rock The mineral makeup of igneous rock is dependent on the chemical composition of the magma from which it crystallizes. Three types of magma: – Basaltic – Andesitic – Granitic Basaltic igneous rocks are darkcolored and dense. – Contain iron and magnesium but very little silica – Basaltic lava flows freely from a volcano Granitic igneous rocks are lower density and lighter color. – Contain more silica and less iron and magnesium – Granitic magma is thick and stiff Andesitic rocks have a more balanced composition of minerals and density than basaltic or granitic rocks. Crystal size, large or small, can help identify an igneous rock as intrusive or extrusive Volcanic glass rocks cool so quickly that few crystals form Some rocks have holes formed around once-trapped air and other gases Igneous Rocks Are Classified by Their Texture Rate of cooling determines crystal size: Slow rate promotes the growth of fewer but larger crystals (coarse-grained). Fast rate forms many small crystals (fine-grained). Very fast rate forms glass (i.e., no crystals). Rocks Section 3 Metamorphic Rocks Metamorphic Rocks produced from: Igneous rocks Sedimentary rocks Other metamorphic rocks Metamorphism is “changed rock”; the transition of one rock into another by temperatures or pressures different from those in which it formed. Metamorphic rocks—changed by temperature, pressure, and hot fluids –Heat and pressure result from one layer of rock on top of another layer. Sometimes temperature and pressure are great enough to melt rock, forming magma Sometimes pressure flattens mineral grains in rocks without melting them Agents of Metamorphism Heat is the most important agent. — Recrystallization results in new, stable minerals Two sources of heat: — Heat from magma (contact metamorphism) — An increase in temperature with depth due to the geothermal gradient (burial metamorphism) Agents of Metamorphism Pressure (stress): —Increases with depth —Confining pressure applies forces equally in all directions —Rocks may also be subjected to differential stress—unequal stress in different directions Leads to foliation As pressure and temperature continue to increase over time, one type of rock can change into several different metamorphic rocks. – Hot, water-rich fluids can move through rock, chemically changing it. Classification of metaphoric rocks—by composition and texture – Foliated texture – minerals grains flatten and line up in parallel layers or bands. – Non foliated texture – mineral grains grow and rearrange but do not form layers. Rocks Section 4 Sedimentary Rocks Sedimentary rocks—mostly found on the exposed surface of the Earth – Rock fragments, mineral grains, and bits of plants and animal remains moved by wind, water, ice or gravity are called sediments – Sedimentary rocks form in layers Sedimentary rocks—classified by what they were made of and how they were formed Detrital sedimentary rocks—made from broken fragments of other rocks – When layers of small sediments stick together because of pressure, compaction occurs – When water and other minerals move through open spaces between larger sediments, gluing them together, cementation occurs. – Detrital rocks often have a granular texture – Rocks are named according to size and shape of sediments Sediment size can be large like gravel or small like clay Sediments can be well-rounded or have sharp edges Chemical sedimentary rocks—nonclastic rocks formed when dissolved minerals came out of the solution – Limestone forms from calcite, which was calcium carbonate in solution – Rock salt forms from halite, which was salt in solution Organic sedimentary rocks—made from remains of once-living plants or animals – Chalk—made of microscopic calcite-shell remains of animals – Coal—made of plant remains, chemically changed by microorganisms and compacted over millions of years Rock cycle—a continuous and dynamic process
