Rocks and Soils Rock types • The Earth’s crust has many different types of rocks. • These rocks are usually divided into 3 different types. 1. Igneous Rocks 2. Sedimentary Rocks 3. Metamorphic Rocks Igneous Rocks • These result from volcanic activity. They consists of crystals which formed as the volcanic rock cooled down. • E.g Granite, Basalt. Sedimentary Rocks • These have been layered down in layers. • They usually consist of small particles that have been eroded and transported. • E.g. Sandstone and shale. • They can also be formed from the remains of plants and animals. • E.g. Limestone, Chalk and Coal Metamorphic Rock • These rocks are those that have been altered either by extremes of pressure e.g. Shale is compressed into slate, or by extremes of heat. • E.g. limestone is changed into marble. Uses of rocks Igneous Rock Formation Uses Granite Magma cools within the Earths Crust Buildings, Pottery, sites for reservoirs, some tourism Basalt Sedimentary Metamorphic Fertile soil, tourism, foundation for roads Coal Fossilised remains of trees Fuel for power stations Sandstone Grains compressed together Building material Chalk Remains of shells and skeletons of animals Cement and lime Marble Limestone changed by heat and pressure Monuments Slate Clay changed by pressure Building material for roofs Rock Structure • The structure of a rock can among other things, affect its resistance to erosion and its permeability to water. Resistance – how hard or soft a rock is. Permeability – how much rock allows water to pass through it. Resistance • The harder a rock is, the more resistant it is likely to be to erosion. • Harder rocks thus usually form hills, mountains and steep cliffs. • Soft rocks are more liable to be worn away or broken up. • Valleys are formed in soft rocks. • On coasts, resistant rocks form steep cliffs and stand out as headlands, whereas the softer rocks form bays. Permeability • Rocks can be permeable or impermeable • Permeable rocks allow water to pass through • Impermeable rocks do not allow water to pass through. • Permeable rocks may be porous – meaning it has a lot of small pores. • Or contain areas of weakness, such as bedding planes, along which water flows. Horizontal bedding planes, on the other hand, separate individual layers of rock. Weathering and Mass movement • Weathering is the disintegration (breaking up) and decomposition (decay) of rocks in situ – that is in their place of origin. • Weathering, unlike erosion, does not involve the movement of material. • There are two main types of weathering 1. Physical weathering – rocks are broken down into smaller pieces. There is no change in the chemical composition of the rock. E.g. Freeze-thaw, exfoliation, biological weathering 2. Chemical weathering – rocks are decomposed due to chemical changes in the rock. Mostly found in warm, moist climates. An example is limestone solution.