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Rocks There are three main groups of rocks 1.) Igneous 2.) Sedimentary 3.) Metamorphic Igneous Rocks • Igneous rocks are classified by their • Origin • Texture • Mineral composition Origin • Igneous rock may form on or beneath Earth’s surface. • Extrusive rock is igneous rock formed from lava that erupted onto Earth’s surface and cooled. • Intrusive rock is igneous rock that formed when magma hardened beneath the surface of the Earth. Texture • The texture of an igneous rock depends on the size and shape of its mineral crystals. • Rapidly cooling lava forms finegrained igneous rock with small crystals or no crystals at all. • Slowly cooling magma forms coarsegrained rocks with large crystals. Granite Rhyolite Mineral composition • Lava (on Earth’s surface) that is low in the mineral silica usually forms dark-colored rocks such as basalt. • Magma (beneath Earth’s surface) that is high in silica usually forms light-colored rocks such as granite. Sedimentary rocks • Sedimentary rocks are made up of sediment, which is small, solid pieces of material that come from rocks or living things. • Most sedimentary rocks are formed through a sequence of processes: • Weathering: the force by which rocks on Earth’s surface are constantly broken up by forces such as freezing and thawing, plant roots, and acid. • Erosion: the process by which running water, wind, or ice carry away bits of broken-up rock Most sedimentary rocks are formed through a sequence of processes: • Deposition: the process by which sediment settles out of the water or wind carrying it. • Compaction: the process that presses sediments together • Cementation: the process in which dissolved minerals crystallize and glue particles of sediment together What are the three major types of sedimentary rock? 1.) Clastic rocks 2.) Organic rocks 3.) Chemical rocks Clastic rocks • A clastic rock is a sedimentary rock formed when rock fragments are squeezed together. • Clastic rocks are grouped by the size of the rock fragments, or particles, of which they are made. Clastic rocks • Shale forms from tiny particles of clay. • Sandstone forms from the sand on beaches, the ocean floor, riverbeds, and sand dunes. • Conglomerate has rock fragments with rounded edges. • Breccia has rock fragments with sharp edges. Organic rocks • Organic rock forms from the remains of material that was once living (plants and animals), which is deposited in layers. • Coal forms from the remains of swamp plants buried in water. • Limestone forms in the ocean, where many living things, such as coral, clams, and oysters, have hard shells or skeletons that are deposited on the ocean floor. Chemical rocks • Chemical rocks form when minerals dissolved in a water solution crystallize, or from mineral deposits that are left when seas or lakes evaporate. • Example: rock salt is made of the mineral halite, which forms by evaporation. Metamorphic rocks • Any rock that forms from another rock as a result of changes in heat and/or pressure is a metamorphic rock. How metamorphic rocks form • Metamorphic rock can form out of igneous, sedimentary, or other metamorphic rock. • The heat that can change a rock into metamorphic rock can come from pockets of magma rising through the crust, or collisions between Earth’s plates pushing rock down towards the heat of the mantle. How metamorphic rocks form • Very high pressure can also change rock into metamorphic rock, such as from plate collisions during mountain formation, or the great pressure deep under Earth’s surface. How metamorphic rocks are classified • Metamorphic rocks are classified according to the arrangement of the grains making up the rocks. • Foliated rocks have their grains arranged in either parallel layers or bands. shale Slate schist How metamorphic rocks are classified • Nonfoliated rocks have their grains arranged randomly. limestone marble The Rock Cycle • Not only can rock be changed into metamorphic rock, such as from heat and pressure, but any rock can be changed into any of the other three rock types! All of this occurs during the rock cycle. • The rock cycle is a series of processes that occur on Earth’s surface and in the crust and mantle that slowly change rocks from one kind to another. • The changes of the rock cycle are closely related to the movement of Earth’s plates.