Igneous
Igneous rocks are formed by volcanoes. When the lava cools, it forms rock. A common igneous rock is obsidian. Obsidian is formed from a lava called rhyolite lava. Rhyolite lava is a thick form of lava that has lots of silica inside of it, which causes it to be less heavy than other lava. Obsidian is formed when rhyolite lava cools down very quickly.
Metamorphic
When rock gets really hot, or is under a lot of pressure, it causes it to change, creating metamorphic rock. Metamorphic is the Greek word for “change of shape.” Metamorphic rocks can take thousands of years to form. The metamorphic rock slate is sometimes made into roofs. Many mountains are made up of metamorphic rock.
Sedimentary
Sedimentary rocks are pieces of debris that have been pressed together to form one rock. These rocks usually look like a bunch of rock slabs that have been stacked on top of each other. The debris that is pressed together is called sediment. Sedimentary rock that is buried deep inside of the earth, it is melted into magma, which then cools into igneous rock.
Tyler's Geology 