Post by Na9er on Oct 8, 2012 12:05:56 GMT -5
Rocks are classified (organised) into three main groups: igneous rocks, sedimentary rocks and metamorphic rocks. This classification is based on how they were formed and their characteristics.
Igneous rocks
Igneous rocks are formed by magma from the molten interior of the Earth. When magma erupts it cools to form volcanic landforms. When it cools inside the Earth it forms intrusive rock, which may later be exposed by erosion and weathering. Intrusive rock will have large crystals as it has cooled slowly. Magma that has cooled on the surface is known as extrusive rock. This will have small crystals as it has cooled quickly.
Examples
Sedimentary rocks
A river carries, or transports, pieces of broken rock as it flows along. When the river reaches a lake or the sea, its load of transported rocks settles to the bottom. We say that the rocks are deposited. The deposited rocks build up in layers, called sediments. This process is called sedimentation.
The weight of the sediments on top squashes the sediments at the bottom. This is called compaction. The water is squeezed out from between the pieces of rock and crystals of different salts form.
The crystals form a sort of glue that sticks or cements the pieces of rock together. This process is called cementation.
These processes eventually make a type of rock called sedimentary rock. It may take millions of years for sedimentary rocks to form.
Examples
Metamorphic rocks
Metamorphic rock has been subjected to tremendous heat and / or pressure, which caused it to change into another type of rock. It is usually resistant to weathering and erosion and is therefore very hard wearing.
Examples
Igneous rocks
Igneous rocks are formed by magma from the molten interior of the Earth. When magma erupts it cools to form volcanic landforms. When it cools inside the Earth it forms intrusive rock, which may later be exposed by erosion and weathering. Intrusive rock will have large crystals as it has cooled slowly. Magma that has cooled on the surface is known as extrusive rock. This will have small crystals as it has cooled quickly.
Examples
- Granite
- Basalt
Sedimentary rocks
A river carries, or transports, pieces of broken rock as it flows along. When the river reaches a lake or the sea, its load of transported rocks settles to the bottom. We say that the rocks are deposited. The deposited rocks build up in layers, called sediments. This process is called sedimentation.
The weight of the sediments on top squashes the sediments at the bottom. This is called compaction. The water is squeezed out from between the pieces of rock and crystals of different salts form.
The crystals form a sort of glue that sticks or cements the pieces of rock together. This process is called cementation.
These processes eventually make a type of rock called sedimentary rock. It may take millions of years for sedimentary rocks to form.
Examples
- Limestone
- Clay
- Chalk
- Sandstone
Metamorphic rocks
Metamorphic rock has been subjected to tremendous heat and / or pressure, which caused it to change into another type of rock. It is usually resistant to weathering and erosion and is therefore very hard wearing.
Examples
- Marble
- Schist
- Slate
