SC.4.E.6.1 Three Types of Rock

SC.4.E.6.1: Identify the three categories of rocks: igneous, (formed from molten rock); sedimentary (pieces of other rocks and fossilized organisms); and metamorphic (formed from heat and pressure).

How Are Rocks Made?

Scientists classify rocks into three categories. Those categories are based on how the rocks were formed. The three categories of rock are:

·      igneous: formed from molten rock

·      metamorphic: formed from heat and pressure

·      sedimentary: formed from pieces of other rocks and fossilized organisms

At one time in Earth’s history, billions of years ago, all minerals on Earth were dissolved in magma and lava. Magma is melted rock found underground. Lava is melted rock on the Earth’s surface. At that time, there were no solid rocks on Earth. As lava on the surface began to slowly cool, the minerals in the lava began to crystalize. Minerals are crystalline solids formed in the Earth. As minerals cooled and crystalized, the first rocks were formed. Because these rocks were formed from molten, or melted rocks, these first rocks were all igneous rock.

Over time, all rocks are broken apart by wind, water, ice, temperature change and plants. this process of weathering breaks rocks apart into smaller and smaller pieces. Gravity, wind, water, and ice move broken rock from one place to another. The process of erosion (the movement of rock) deposits broken rock. Sedimentary rock is formed from the deposits of pieces of other rocks and fossilized organisms. As these deposits build up, the particles of rock that have been deposited at the bottom of an ocean for example become cemented together by minerals dissolved in the water. While rock particles are being deposition on the bottom of an ocean, the remains of plants, animals, and other organisms are being deposited as well. The fossilized remains of organisms also become part of the sedimentary rock that is formed.

The crust, or surface, of the Earth is the solid rock on which we stand. Far below ground rock becomes molten (melted). The center of the Earth produces a large amount of heat which cause rocks to melt deep below ground. when melted rock is underground, it is called magma. Magma will sometimes break through the surface of the Earth. When melted rock breaks through to the surface of the Earth, it is called lava. Magma and lava can both cool to form rock. When rocks are formed from melted rock (magma or lava), the are called igneous rocks. Igneous rock is formed from melted rock.

Metamorphic rocks are formed under heat and pressure under the Earth’s surface. Some rocks on the surface of the Earth can be pushed deep enough underground to be partially melted. When these rocks cool again, they are like the original rock from which they were made. They have been changed under heat and pressure. These new rocks are called metamorphic rock.