Kinds of Rock
Toothpaste is made up of a sedimentary rock called limestone. When you clean your teeth, a powdered form of limestone helps to remove bits of food. So brush on! And don't worry if you swallow a little - it's harmless.
Scientists place rocks in three classes, according to the way they are formed: igneous, sedimentary, and metamorphic.
1. Igneous rock - When the magma from a volcano cools, it forms igneous rock. Granite is an igneous rock.
2. Sedimentary rock - When layers of minerals or decayed plants are squeezed together by water pressure, sedimentary rock forms. Sandstone is a sedimentary rock.
3. Metamorphic rock - Forces within the earth, such as pressure or temperature, change igneous or sedimentary rock into another form. A rock that has been changed is called a metamorphic rock. Marble is a metamorphic rock.