The basic ingredients of the earth are known as elements. There are 90 known natural elements that exist. These elements combine with one another in a number of natural ways to create minerals. There are around 3,800 known minerals. Minerals combine to form rocks. Rocks are grouped by how they were formed. There are three groups of rocks: igneous, sedimentary, and metamorphic.
Igneous rock
All rocks on Earth were igneous at first. Igneous rocks form as liquid magma cools. Scientists divide igneous rocks into two different categories. Rocks that formed below the surface of the earth are intrusive rocks. It takes much longer for these rocks to cool. Those that formed on the surface are called extrusive rocks and they cool quicker. The rate of cooling causes the rock to look different.
Sedimentary rock
Over the course of millions of years, igneous rocks are weathered down by forces of wind and water. Fine particles of dirt begin to cover the landscape. Often, these small rock particles find their way to the bottom of lakes, streams, and the ocean.
Slowly, the layer of sediment on the bottom of these bodies of water grows deeper and deeper, reaching depths of thousands of feet. The weight of all the sediment becomes very heavy. It pushes down on lower layers of sediment with tremendous force. In addition, a number of minerals, which act like cement, bond the sediment together this causes the formation of sedimentary rock.
Metamorphic rock
Metamorphic rocks form deep within the Earth, when heat and pressure are applied to either igneous rocks, or sedimentary rocks. This heat and pressure cooks the rocks and changes their structure. The rocks are partially melted, and the chemicals within them are rearranged. This causes the final rock to be very different from how it started. The last stage of forming a metamorphic rock depends on the amount of heat and pressure the rock was under and how much time the rock was under these conditions.
