Rocks and their formation.

== Igneous rocks== Igneous Rocks:
 * are formed when magma or lava solidifies
 * are made of interlocking crystals, which makes them hard, and resistant to erosion.
 * have small crystals if the liquid rock has cooled quickly (e.g. basalt)
 * have large crystals if the liquid rock cooled slowly (e.g. granite)

Sedimentary rocks
Sedimentary Rocks: ​
 * are formed when layers of sediment are compacted together, over a very long time by being buried under more layers
 * erode aways easily, compared to metamorphic and igneous rock
 * are made up of rounded grains and may contain fossils (the remains of living organisms
 * Include chalk and limestone, whichare natural forms of calcium carbonate

Metamorphic rocks
Metamorphic Rocks:
 * Metamorphic rocks are formed from existing rocks by the action of heat (from being buried or from nearby magma) and/or pressure (from being buried), which causes new interlocking crystals to grow. Marble is a metamorphic rock which is formed from chalk or limestone.
 * Marble is also a natural form of calcium carbonate