 Sedimentary rocks form from layers of mud, sand, gravel and other material that is deposited at the bottom of lakes, rivers, and oceans. Over millions of years these layers are compressed, and through movement of the Earth's crust they may end up exposed to the air, or buried deep underground. Eventually they become a solid mass of rock. You can model a sedimentary rock by combining layers of plasticene with small differently coloured bits embedded in it.   If sedimentary rock layers are shifted deep into the earth's crust, they will be subjected to very high temperature, which makes them soft. They will also experience very high pressure. You can model this by rolling your plasticene model of a sedimentary rock with a can of pop. Press very hard to force the 'rock' flat. 
 Now slice open your flattened model with a sharp knife. Examine the interior. This is a model of a metamorphic rock ... a sedimentary rock that has been heated and put under intense pressure. 
 Igneous rocks, which are created from molten material deep within the earth, can also be squeezed like this to form metamorphic rocks. Metamorphic rocks can be created from either sedimentary or igneous rocks. |