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Three Types of Rock
All rocks can be grouped into one of three categories. The categories are based on how they were formed.
Igneous rock is formed when magma (hot liquid rock beneath Earth’s surface) cools and becomes solid. Some igneous rock cools and becomes solid beneath Earth’s surface. Other igneous rock cools and becomes solid on Earth’s surface. Granite and obsidian are igneous rocks.
Sedimentary rock is formed from sediment, which is tiny pieces of other rocks. The sediment settles in layers at the bottom of a lake or ocean. As more and more sediment builds up, the weight of the sediment at the top pushes down on the sediment at the bottom. The pressure from all this weight squeezes together the sediment and turns it in to sedimentary rock. Limestone and sandstone are sedimentary rocks.
Metamorphic rock is made from sedimentary or igneous rock that has been changed by pressure, or heat, or both. Pressure might come from the shifting of Earth’s crust. The heat from magma might “bake” nearby rocks, changing them into metamorphic rocks. Most metamorphic rocks are formed deep within Earth. Marble and slate are metamorphic rocks.
The Rock Cycle
Did you know that Earth constantly recycles rock? Here is how this can happen:
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