Rocks originate from three major sources and are thus classified as coming from one of the three rock types; Igneous rocks Sedimentary rocks Metamorphic rocks Igneous Rock Igneous rock is formed when the Earth's magma cools and the minerals that make it up crystallize. The cooling can be quick if the magma has been extruded onto the earth's surface and this gives BASALT which contains very small crystals because of the speed of cooling which in many cases can be seconds if on the ocean floor. If cooling takes place low down in the earths crust then the crystals have much longer to form and grow so this rock has large crystals. Cooling times can be thousands and millions of years. GRANITE is formed this way. Sedimentary Rock Sedimentary rock is formed when minerals and particles in water drop out and settle on the bottom. Over the course of time layers of sediment are built up and by the pressure of the layers above and the water rock is formed. The sediment can originate form three sources; Weathering of rocks.  Weathering or erosion of prexisting rocks on the earth's surface causes particles to be removed from the rocks surface. These particles are then removed by rain, rivers or wind and eventually end up in the sea where they sediment out forming the layers. The eroded particles can be of any size. Very small particles form clay and mud, slightly larger particles form silt and slightly larger still form sand. Particles larger than sand are called gravel. These particles are transported by rivers and the flow rate of the river will determine where and what is deposited. Fast flowing rivers can carry much larger particles further than slower flowing rivers. Flow rates therefore end up sorting particles out. The smallest particles will settle out of the water when the flow rates fall off to nearly zero which is often at the mouth of the river. This is where sand, silt and mud often accumulate blocking up the channels. If enough sediment is formed and the downward pressure is sufficient then the particles become cemented together to form a rock. Mud gives mudstone, sand gives sandstone, clay gives shale, and gravel gives conglomerate. Organic sedimentation.  Formed from the deposition of animal and plant material. Animal skeletons depositing on the seabed build up to form LIMESTONE. Plant remains compacting together form COAL. Chemical sedimentation. Seawater is saturated with calcium carbonate which is constantly sedimentating out of the sea and settling to the bottom as conditions change. This builds up pure limestone deposits. Metamorphic Rock Metamorphic rocks are formed when any rock type is subjected to intense temperatures and pressures. This heat and pressure induces a change in the rock so it is transformed from one rock type to another. For example sandstone metamorphosis into quartzite and limestone becomes marble. One of the possible products of shale after metamorphosis is SLATE.