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Rocks

Rocks make up the earth's crust and are the most fundamental unit in geology. Each rock tells a story of how and where it was formed. They provide clues to the history of our planet; they record changes in the environment that occurred millions of years ago; and rocks containing fossils supply evidence of life forms over the last 3.5 billion years. As well, their economic importance is obvious: mining rocks gives us raw materials, such as iron, petroleum, coal, and gravel that are essential in our everyday lives.

Geologists define a rock as a natural aggregation of one or more minerals. Some rocks do not have minerals but are made up of glasses (obsidian) or organic material (coal). There are three groups of rocks that are classified according to how they were formed: igneous, metamorphic, and sedimentary. Each type can be altered from one kind into another - this is part of the "rock cycle."

Metamorphic rocks form when preexisting rocks, either igneous, sedimentary, or metamorphic, are altered by high temperature, high pressure, or both. Sedimentary rocks form when metamorphic, sedimentary, or igneous rocks are uplifted to the earth's surface where erosion then breaks them down into fragments, called sediments. When these sediments are transported, deposited and buried, they are compacted and cemented together to form sedimentary rocks. When sedimentary, metamorphic, or igneous rocks are buried deep beneath the earth's surface, they melt when the temperature gets hot enough. Igneous rocks are formed when molten rock, called magma, cools and hardens.

Rocks

Rocks