Types of Rocks
By Jack Hess
Igneous rock forms when lava cools and hardens. Igneous rock includes pegmatite, granite, porphyry, obsidian, and pumice. The quicker it cools, the more rough it is. The slower it cools, the smoother it is. You might know that igneous rock forms around volcanoes, but I bet you didn't know that it can also form inside of the earth.
Sedimentary Rock is rock that forms when sediments layer up. Pressure compresses the sediments into one big layer of rock. Over millions of years, the layers build up into many layers. If an animal or plant is in the sediments, it will create a fossil.
Metamorphic rock is rock that changes with excessive heat and pressure. One example of metamorphic rock is shale. It turns to slate, then to schist. The word metamorphic is a Greek word that means to change form.