Rocks

• Natural disasters cannot usually be prevented but if geologists know where and when they may happen, people can be moved out of the danger area
• To study natural disasters, geologists need to study rocks and how they are formed
• Rocks

  • Made of different grains that fit together
  • Grains are made from one or more chemical compounds
• The combination of sizes and shapes of grains is called the texture of the rock
• Geologists classify rocks by the minerals they contain and by looking at the texture
• The properties of a rock depend on its texture and the minerals it contains
• Rocks made of rounded grains


  Can absorb water because it can get into gaps between the grains. These rocks are said to be porous
• Most porous rocks are also permeable, water can run through them
• Limestone


  Neutralises acidic soils
• Oolite is more like sandstone or granite
• The high temperature of this lava in Hawaii has set the house on fire
• Lava


  Molten rock that reaches the Earth's surface
• Magma or lava can contain different combinations of substances. The kind of rock that is formed depends on the substances and on how fast it cools
• When the rock is a liquid, the particles are free to move about

  As the liquid cools down, forces between the particles slowly start to form the particles into a regular grid pattern, which grows and grows them
• If the rock cools slowly


  There is plenty of time for a large grid pattern to form, making large crystals
• If the rock cools down quickly

  There is much less time for the particles to become ordered and so smaller crystals are made
• Metamorphic rocks usually have different properties to the rocks from which they were made
• Schist often looks shiny because flakes of a mineral called mica in the rock are lined up
• Gneiss usually has bands of different coloured minerals
• When rain falls onto rocks it can react with the minerals in those rocks. This is called chemical weathering
• The new substances formed may be soluble and wash away or they may be more crumbly than the original rock
• Plant roots can grow into cracks in the rocks. When the roots grow bigger they make the cracks wider and the rock can break apart. This is called biological weathering
• Rocks expand (get bigger) and contract (get smaller) when the temperature changes. If the rock is heated and cooled over and over again, cracks can form. This kind of physical weathering is common in deserts and often causes sheets of rock to peel off
• As the water moves, the rock fragments knock against each other and wear away. This is called abrasion
• The size of sediment particles that can be carried by a river depends on how fast the water is moving. Faster-moving water can carry bigger pieces of rock
• Sediments can also be carried by the wind. The wind can only move very small particles
• Gravity can also cause erosion. Sometimes a lot of rock is moved at once, in a landslide
• Glaciers are rivers of ice. They move very slowly, but they can transport very large pieces of rock. Rocks carried by glaciers also scrape away bits of rock from the land they are moving over. Rocks below the glacier are abraded into very small particles
• You can show what happens when rocks are carried by water by shaking some sugar cubes
• Sediment


  Rock fragments and particles carried and deposited by water, wind or ice
• An 'erratic' is a rock moved by glaciers and left up to hundreds of miles from its original location. The Big Rock in Alberta, Canada has an estimated mass of 16 500 tonnes and is the world's biggest erratic.
• Modelling abrasion


  • Shaking sugar cubes in a jar
• Cementation

  The minerals dissolved in the water in the sediment act as a glue to cement the grains together
• Sedimentary rocks


  • Mudstone
  • Conglomerate
• Fossils


  The remains of dead plants or animals that become covered in a layer of sediment and turn into rock
• Archaeopteryx was one of the first bird-like animals, living around 150 million years ago.
• Metamorphic rocks


  • Sedimentary rocks that are heated or compressed can turn into metamorphic rocks, e.g. mudstone turning into slate, slate turning into schist or gneiss
• Metamorphic rocks


  • Slate
  • Schist
  • Gneiss
• Marble


  A metamorphosed limestone with smaller crystals than the grains in limestone, allowing for very fine details to be carved
• Sedimentary rocks are formed by the deposition and cementation of sediments, metamorphic rocks are formed by the heating and compression of existing rocks, and igneous rocks are formed by the cooling and solidification of molten magma.