Rock Classification

Geology is the study of Earth’s structure and rocks. Rock is the solid, non-living material that the Earth is formed out of and comes in all shapes, textures and colours. The grains that make up a rock are called minerals made from elements and compounds.

The texture of a rock may be described as crystalline or fragmented. Granite is a crystalline rock with mineral grains that interlock. Sandstone is a fragmented rock with random shapes of mineral grains. The grains are often cemented together by other minerals.

Igneous rocks are formed as red-hot magma cools and solidifies. Slow cooling of magma creates coarse-grained rocks with large crystals such as granite. Faster cooling of lava creates fine-grained rocks with small crystals such as basalt. Rapid cooling of lava forms volcanic glass known as obsidian.

Sedimentary rocks are formed from sediment at the bottom of a body of water. Sediments from weathering and erosion accumulate on the bottom of the ocean, lakes and rivers over millions of years. Different sediments (sand, mud, shells) will produce different types of sedimentary rocks such as sandstone, mudstone or limestone.

Metamorphic rocks are formed when igneous and sedimentary rocks are heated and/or compressed deeps within the crust over millions of years. For example, marble is metamorphosed from limestone, slate from shale.

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Mount Kilimanjaro, Tanzania