Chemistry Revision Explaining the Earth Rocks are made out of a mixture of minerals. The shape of rocks can be changed by weathering and erosion. Weathering can be chemical, physical or biological. The Rock Cycle The Earth is continually changing. Rocks are weathered and eroded and new rocks are being formed. The processes that make rocks, weather them and change them are linked together in the rock cycle. The earth movements that bury rocks as part of the cycle are caused when the plates that form the surface of the earth push into each other. Types of rock Sedimentary Igneous Metamorphic
Sedimentary Rocks Rock fragments, formed as a result of weathering and erosion, are transported by moving water, wind or ices, and the fragments get worn down. Small rock fragments are called grains. When the water, wind or ice slows down, some of the grains are deposited at the bottom of rivers, lakes or seas, as sediment. Layers of sediment collect on the sea bed, and the bottom layers get squashed. The grains of sediment are forced closer together (compacted). Eventually, sedimentary rock is formed. The composition and texture of sedimentary rocks vary and depend on the way the rocks are formed. For example, the sediments that made a rock with large grains must have been moved by a fast-flowing river. If any animals or plants get trapped in the sediment, they may form fossils.
Igneous Rocks Molten rock is called magma. If the molten rock flows out of volcanoes it is called lava. Igneous rocks are formed when magma cools down. Lava cools down quite quickly, and forms igneous rocks with small crystals (like basalt). Magma underground cools down much more slowly and forms rocks with bigger crystals, like granite.
Metamorphic Rocks Sedimentary or igneous rocks can be changed by heat or pressure into new kinds of rock, called metamorphic rocks. Metamorphic rocks have different properties from the sedimentary or igneous rocks they were made from.