A volcanic intrusion occurs when molten material intrudes between or through beds of rock in the Earth’s crust.
Volcanic activity occurs along the lines between tectonic plates. Tectonic plates are separate areas of the Earth’s crustal rocks which move over time. There are places where plates meet, whether they are pushing against each other, moving away from each other or sliding alongside each other. In all these cases the situation allows molten material to rise from deep below the crust towards the Earth’s surface.
This molten material will be magma, a mixture of molten rock and very hot gases. If the magma reaches the surface, the gases will be given off and the remaining lava will flow down the volcano’s side, often in association with ash and other materials. These external flows are called pyroclastic flows and are dealt with in other articles.
If, however, the molten material does not reach the surface it will eventually cool and solidify into a solid rock, normally, but not always, as granite. Features caused by magma’s intrusion through crustal rocks vary, both in their formation and effect.
Sometimes the contact by a volcanic intrusion with surrounding rocks can melt those rocks, or displace them, to form enormous underground baths of magma, which will eventually cool to form the igneous crystalline rock, granite. This scale of volcanic intrusion is called a batholith.
In some cases, the rocks above a batholith will erode away over time, due to weathering or ice and water action, or a combination of the three. The result will be an extensive exposed area of rock, normally forming a highland, as granite is resistant to weathering processes and most often stands above the surrounding, softer rocks. Examples of highland areas formed in this way are the granite moorlands of Devon and Cornwall in southwest England.
There are times when horizontal volcanic intrusions have deformed surface rock strata, forming large masses of magma below the folded strata - again, eventually cooling into granite. A volcanic intrusion of this type is called a laccolith. Sometimes laccoliths occur in clusters, and one such is thought to have formed the Henry Mountains in South Utah where five laccoliths have been produced between the local Carboniferous and Cretaceous rocks.
Most often, intrusions are much smaller and restricted to the spaces between rock strata. For example, if magma intrudes through the bedding planes between strata, it can form a sheet of igneous rock called a sill. As with other intrusions, the resultant rock will be hard and resistant, and when it becomes exposed it will form an extensive ridge. The classic British example of a sill is the Great Whin Sill in northern England which extends over 1,500 square miles, 3,900 square kilometres. It is made of dolomite, formed from the metamorphosis of magnesium limestone in the pre-volcanic rock sequence of northeast England, and ranges in thickness from 1 metre to 75 metres.
Contrasting with a sill, a dyke is formed when magma intrudes across strata of rock rather than between them, breaking through and separating the previous surface rock series. Dykes also form ridges, but ridges which are narrower than those of sills, hence the name, relating to their similarity to man-made dykes built to hold back water. Good examples of intrusive dykes can be found on the islands of Mull and Arran off the coast of western Scotland.
As their name implies, volcanic intrusions are formed of molten material which has intruded through the previous surface rocks of the Earth. They are made of igneous or metamorphic rocks, always hard and invariably resistant, causing the formation of some spectacular scenery.