Types of Rocks

Igneous

Igneous rocks are formed by the cooling of molten lava.  If the rocks cool below the surface, they are called intrusive.  If they cool above the surface, they are extrusive.  

Sedimentary

Sedimentary rocks are formed by layers of earth deposited over time.   The deeper it gets, the more pressure forces the layers to become hardened.  

Metamorphic

Heat and pressure can change igneous and sedimentary rocks into something else.   The heat is caused by magma rising through the crust.  Pressure is from the weight of rock above.