Friday, January 20, 2012

GEOLOGICAL TIME: Classification of rocks


Rocks found on the Earth's surface actually come from inside the Earth - so they tell us a lot about the Earth's interior. They are classified (organised) into three main groups: igneous rocks, sedimentary rocks and metamorphic rocks.

Rock types

 

Type of rockExample
Igneous rock
Lichen on granite
Granite
Sedimentary rock
Navajo Sandstone formations in Utah
Sandstone
Metamorphic rock
Slate in the Lake District
Slate




Igneous rock
Igneous rocks are formed by magma from the molten interior of the Earth. When magma erupts it cools to form volcanic landforms. If magma cools inside the Earth it forms intrusive rock, which may later be exposed by erosion and weathering. Examples of igneous rocks include basalt and granite.

Example of igneous rocks
Type of rockExample
Basalt
Basalt colums in the Giant's Causeway
Basalt
Granite
A granite outcrop on Bodmin Moor
Granite



Metamorphic rock
Metamorphic rocks have been subjected to tremendous heat and/or pressure, causing them to change into another type of rock. They are usually resistant to weathering and erosion and are therefore very hard-wearing. Examples of metamorphic rocks include marble, which originates from limestone,slate, which originates from clay, and schists formed from sandstone or shale (sedimentary rocks).

Slate Quarry, Cullipool


Sedimentary rock
Sedimentary rocks are formed from sediments that have settled at the bottom of a lake, sea or ocean, and have been compressed over millions of years. The sediment comes from eroded rocks carried there by rivers or ice, and from the skeletons of sea creatures. Examples of sedimentary rocks include sandstone, limestone, chalk and clay.

Limestone pavement on top of Malham Cove





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