 In this video you will learn about the dynamic rock cycle. The earth is constantly changing. Shake some soil in a jar of water. Sand and small stones settle to the bottom, then a layer of clay and floating on the top is organic matter called humus. Plant roots help to create new soil from the solid rock and they get nutrients and support from the soil. This solid rock is often sandstone or granite. They're made up of grains. The granite has interlocking grains like this, but sandstone has rounded grains like this. Weigh each rock and then place it in water. You will see bubbles coming from the sandstone and it gets heavier. No bubbles from the granite and no increase in weight. Why do you think the sandstone made bubbles and gained weight? Pause and think. Well, there are air spaces between the grounded grains so the water pushes the air out, forming bubbles, making it heavier. The granite has no air spaces. These are the two main types of rock. Granite is formed by molten rock solidifying and igneous rock, that in word meaning fire. Think of the word ignite. But sandstone was made by bits of other rock carried by rivers to the sea. The bits get smoothed and rounded like the pebbles on a beach. These grains settle on the seabed forming sedimentary rocks from that incedere to sit. Rocks get broken up by a process called weathering. Pause and think what sort of weather might break rocks. Mechanical weathering. The heat of the sun expands the rock. It contracts at night causing it to crack. If water enters and freezes it will crack more. With water and wind bits of rock knock into each other breaking them. Chemical weathering is when rain which is slightly acidic dissolves some minerals in the rock. Biological weathering happens as lichens and then the roots of plants grow on the rocks forcing their grains apart. Weathered rock forms the soil but it is also eroded that means transported either by wind or more usually by water to the sea. Notice the dark and light coloured grains in granite. These are minerals the pure substances that make up rocks. The dark grains are feldspar and the light grains are quartz. After weathering the grains are carried to the sea where the harder quartz settles as in a jar of soil forming sand. The feldspar settles more slowly forming rich beds of clay containing most of the trace metals plants need to grow. During the movement of the great tectonic plates, see our related video, the rocks get folded and squashed often ending up high in the air as new mountains form. If rocks are heated and compressed enough they change their form and are called metamorphic rocks from the Greek meta change and morph form. From sandstone we get quartzite and from clay we get first shale then slate. Sedimentary and metamorphic rocks get weathered and eroded too. Tectonics carry the rocks down into the earth's hot mantle where they melt and emerge once again as igneous rocks completing the rock cycle. We must also mention chalk and limestone made of calcium carbonate which are sedimentary rocks formed in the sea from the shells of dead sea creatures. Under heat and pressure they form the metamorphic rock marble. Another rock formed by living things is coal and the related fossil fuels. Pete is formed as vegetation in bog land is unable to rot under the heat and pressure that Pete metamorphises into coal. So to summarize molten rock solidifies forming igneous rock like granite. They get weathered that's broken apart then eroded, transported to form sedimentary rocks like sand and clay. These can get heated and compressed to form metamorphic rocks like shale and slate. The movement of the earth's tectonic plates carries rocks into the mantle where they melt to start the cycle all over again.