A rock is a solid made up of a bunch of different minerals. Rocks are generally not uniform or made up of exact structures that can be described by scientific formulas. Scientists generally classify rocks by how they were made or formed. There are three major types of rocks; Metamorphic, Igneous, and Sedimentary. Metamorphic Rocks Metamorphic rocks were formed by great heat and pressure. They are generally found inside the earth's crust where there is, or was, more heat and pressure to form them. Metamorphic rocks are often formed from other types of rock. For example, shale, a sedimentary rock, can be changed or metamorphosed, into a metamorphic rock such as slate or gneiss. Igneous Rocks Igneous rocks are formed by volcanoes. When a volcano erupts, it spews out hot molten rock called magma or lava. Eventually the magma will cool down and harden, either when it reaches the Earth's surface or somewhere within the crust. This hardened magma or lava rock is igneous rock. Examples of igneous rocks are basalt and granite. Sedimentary Rocks Sedimentary rocks are formed by years and years of sediment compacting together and becoming hard. Generally, something like a stream or river will carry lots of small pieces of rocks and minerals to a larger body of water. These pieces will settle at the bottom and over a really long time (perhaps millions of years), they will form into solid rock. Some examples of sedimentary rocks are shale and sandstone. The Rock Cycle Rocks are constantly changing in what is called the rock cycle. It takes millions of years for rocks to change. To describe the rock cycle we'll start with igneous rocks. Melted rock or magma is sent to the earth's surface by a volcano. It cools and forms an igneous rock. Next the weather, or a river, and other events will slowly break up this rock into small pieces of sediment. As sediment builds up and hardens over years, a sedimentary rock is formed. Slowly this sediment rock will get covered with other rocks and end up deep in the Earth's crust. When the pressure and heat get high enough, the sedimentary rock with metamorphose into a metamorphic rock and the cycle will start over again. Space Rocks There are actually some rocks that come from space called meteorites. They may have different elements or mineral make up than a typical earth rock. Some are made up mostly of iron. More Earth science subjects: Atmosphere Composition of the Earth Rocks Volcanoes Earthquakes The Water Cycle Climate Weather Dangerous Weather Seasons Phases of the Moon Back to Kids Science Page Back to Kids Study Page Back to Ducksters Kids Home Page |
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