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 are formed by great heat and pressure. They are generally found inside the Earth's crust where there is enough heat and pressure to form the rocks. Metamorphic rocks are often made 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. Other examples of metamorphic rocks include marble, anthracite, soapstone, and schist.
- 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 is called igneous rock. Examples of igneous rocks include 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, limestone, and sandstone.
Rocks are constantly changing in what is called the rock cycle. It takes millions of years for rocks to change.
Here is an example of the rock cycle describing how a rock can change from igneous to sedimentary to metamorphic over time.
1. Melted rock or magma is sent to the earth's surface by a volcano. It cools and forms an igneous rock.
2. Next the weather, or a river, and other events will slowly break up this rock into small pieces of sediment.
3. As sediment builds up and hardens over years, a sedimentary rock is formed.
4. Slowly this sediment rock will get covered with other rocks and end up deep in the Earth's crust.
5. When the pressure and heat get high enough, the sedimentary rock will metamorphose into a metamorphic rock and the cycle will start over again.
One thing to note is that rocks don't need to follow this specific cycle. They may change from one type to another and back again in practically any order.
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. Typically they are made up mostly of iron.
Interesting Facts about Rocks
- The word "igneous" comes from the Latin word "ignis" which means "of fire."
- Ores are rocks that include minerals that have important elements such as metals like gold and silver.
- Sedimentary rocks form layers at the bottoms of oceans and lakes.
- Marble is a metamorphic rock formed when limestone is exposed to high heat and pressure within the Earth.
- Layers of sedimentary rocks are called strata.
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