Intrusive Igneous activity and related features.

Key Questions:

i)                   What is intrusive volcanic activity?

ii)                 What are the main types of igneous intrusions?

Igneous intrusions: these are rocks that form when magma moves into the rocks of the Earth’s crust but does not reach the surface.

Types of Igneous intrusions

i)                   Batholith; it is the largest type of igneous intrusion. It forms very deep in the Earth’s crust and the magma cools slowly to form rocks such as granite.

ii)                 Laccolith; a mushroom-shaped structure which forms when magma forces the overlying strata upwards.

iii)               Lopolith; is similar to a laccolith, but the weight of the overlying strata causes the lopolith to sink into a saucer-shaped structure. One of the largest lopoliths in the world is the Bushveld Complex in South Africa.

iv)               Sill; forms where magma intrudes horizontally between layers of rock along a bedding plane.

v)                  Dyke; forms when magma intrudes vertically across layers of rock.

vi)               Volcanic pipe; is a deep, carrot-shaped pipe of solid magma. The igneous rock which forms is often kimberlite, which often contains diamonds.

vii)             Monolith; it is a very large igneous intrusion, usually a batholith. When this intrusion is exposed at the Earth’s surface, it forms a huge area of domed rock. Famous monoliths are the Paarl Rock and the Brandberg in Namibia.

NB: Refer to Platinum Geography, Learners Book Pages 134 and 135.