A rock is a natural solid made of one or more minerals. Rocks form the Earth's crust and are grouped into three main types based on how they form. The study of rocks, called petrology, is a common physical geography exam topic.
Igneous Rocks
- Formed when magma or lava cools and hardens.
- Also called primary rocks because they form first.
- Examples are granite and basalt.
Sedimentary Rocks
- Formed when sediments settle in layers and harden over time.
- They often contain fossils.
- Examples are sandstone, limestone and shale.
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Metamorphic Rocks
- Formed when existing rocks change due to heat and pressure.
- The process is called metamorphism.
- Examples: limestone becomes marble and coal becomes diamond.
The Rock Cycle
- Rocks can change from one type to another over a long time.
- This continuous process is called the rock cycle.
- Igneous rocks can become sedimentary, then metamorphic, and so on.
Quick Revision Points
- There are three types: igneous, sedimentary and metamorphic.
- Igneous rocks form from cooling magma or lava.
- Sedimentary rocks often contain fossils.
- Metamorphic rocks form by heat and pressure.
- Limestone changes into marble.
- Granite and basalt are igneous rocks.
- The continuous change is the rock cycle.