HKDSE Geography/M1/Intrusive Vulcanicity

Intrusive vulcanicity occurs when the magma enters and solidifies in lines of weakness or chambers in the earth's crust.

Process

1. The magma is under great pressure.
2. Plate movements and tectonic processes produce lines of weakness in the crust.
3. The lines of weakness extend downwards and to the magma chamber below.
4. The magma pressure is released.
5. Magma and gases in the mantle rise along the cracks.
6. They intrude into and solidify in chambers the earth's crust (intrusive vulcanicity).

Associated Landforms

Refer to the diagram and table below. The features on the left part of the diagram are landforms formed by intrusive vulcanicity.

Name	Description	Formation
Batholith	It is a large, irregularly-shaped reservoir of magma.	Magma intrudes into a large chamber beneath the surface of the earth. It cools and solidifies at a very slow rate there.
Sill	It is a horizontal sheet-like feature between bedding planes.	Magma intrudes between bedding planes. It cools and solidifies at a very slow rate there.
Laccolith	It is a dome-like feature between bedding planes.	Magma intrudes between bedding planes, but its progression is blocked by a structure. The magma pushes up the rock layer above it. It cools and solidifies at a very slow rate there.
Dyke	It is a vertical or diagonal wall-like feature cutting across bedding planes.	Magma intrudes into a line of weakness cutting across bedding planes vertically or at an angle. It cools and solidifies at a very slow rate there.

Note: The precise definition of bedding planes will be discussed in the Dynamic Earth elective. For now, bedding planes are rock layers.

Intrusive rocks

Intrusive rocks are formed by intrusive vulcanicity. When magma cools and solidifies in the earth's crust, rocks with large crystals are formed. If it was formed very deep in the earth's crust, it is a plutonic rock. They can be exposed onto the earth's surface after denudation. Intrusive rocks are a subset of igneous rocks, which include rocks formed from intrusive and extrusive vulcanicity. These will be covered in great detail in the Dynamic Earth elective.