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Sedimentary Rock. Photo: Liz Nealon


The material that makes up the outer part, or crust, of the earth. Most kinds of rock are hard but some, such as talc, clay, and volcanic ash, are soft. Rocks are usually mixtures of minerals: For example, granite is composed of the minerals quartz, feldspar, mica, and hornblende. But some rocks are mainly made of single materials: Marble consists of the mineral calcite. Rocks are usually classified into three groups according to the way they were formed. Igneous rock is formed when molten lava or magma from beneath the earth’s surface cools and solidifies. Granite, rhyolite, and obsidian are examples of igneous rocks. Sedimentary rock is formed from the breaking down and settling of other rocks and of inorganic remains, such as shells, of organisms. The sediments are deposited in layers. Over time they are pressed together and harden into sedimentary rocks such as sandstone, shale, and breccia. Metamorphic rocks are produced by heat and pressure affecting other rocks. Slate, for example, is a metamorphic rock that was once shale. Marble, gneiss, schist, and quartzite are all examples of metamorphic rocks.