Igneous intrusion refers to a body of igneous rock that has made its way into pre-existing rock (known as country rock). Igneous intrusions are emplaced as magma, which is less dense than solid rock and therefore tends to move upwards. It can then force its way through cracks in the rocks and can wedge them apart or, if it is hot enough, it can melt and replace them. Igneous intrusions can be of a variety of shapes and sizes, ranging from huge batholiths to bodies only one or two meters across. The general term ‘ pluton’ can be applied to any of these. Intrusions may cut across the bedding of the country rock. They are then termed ‘discordant’ or ‘ transgressive’; dykes and laccoliths are examples. Intrusions that follow the bedding of the country rock, such as sills, are termed ‘concordant’.
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