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Igneous Intrusions

Igneous intrusions form when magma cools and solidifies before it reaches the surface.

Three common types of intrusion are sills, dykes, and batholiths (see image below).
Igneous intrusions image  


form when magma intrudes between the rock layers, forming a horizontal or gently-dipping sheet of igneous rock. The Whin Sill (top left image) in N. England provided a defensive cliff-line on which the Romans built Hadrian’s Wall.


form as magma pushes up towards the surface through cracks in the rock. Dykes are vertical or steeply-dipping sheets of igneous rock. This example, (bottom left image) in the Channel Islands, shows several criss-crossing dykes of different ages.


are large, deep-seated intrusions (sometimes called Plutons) that form as thick, viscous magma slowly makes its way toward the surface, but seldom gets there! Dartmoor (bottom right) forms part of a large batholith that extends under Cornwall and beyond.
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