Landforms resulting from intrusive vulcanicity

Intrusive vulcanicity is the movement of gasses, magma and other materials into the earth’s curst to form interior volcanic landforms. The magma cools and solidifies whilst still within the earth’s crust to form these landforms.

The most common features resulting from intrusive vulcanicity are:

1. Dykes (spelled dikes in the diagram)
2. Sills
3. Batholith
4. Lopoliths
5. Laccoliths

1. Dykes

A dyke. Image Credit Elate Africa.

• When a mass of magma cuts across bedding planes/sediments
• It forms a wall like feature which is called a dyke.
• A dyke is a sheet of magma that solidifies across the bedding plane as shown in the diagram above.
• Dykes can be vertical or inclined.
• Some dykes are easily eroded and form shallow trenches.
• Others resist erosion to form wall like ridges.
• Dykes can sometimes give rise to waterfalls for example the Howick Falls in South Africa.
• An example of a dyke can be found in the Kaap Valley in South Africa.
• Another example is the Jos Plateau in Nigeria

2. Sills

Sill. Image credit Elate Africa.

• When a sheet of magma lies along a bedding plane
• It forms a structure known as a sill when the magma solidifies.
• A sill is a body of tabular rock that  that solidifies along a bedding plane.
• They are igneous intrusions that are formed when magma( typically diabase) squeezes horizontally between bedding planes of crustal rocks.
• Sometimes due to erosion it can form ridge like escarpments.
• Others remain as caps on the top of hills after erosion.
• They can be dissected by rivers to form isolated hills.
• The Three Sisters in Cape Province South African are buttes which have sills for capes.

3. Batholith

Batholith and Laccolith. Image credit Oregonstate.edu

• A deep-seated large, dome like igneous intrusion usually made up of granite
• Is formed when  a very large mass of magma that accumulates in the earth’s crust.
• The magma cools and solidifies within the earth’s crust.
• Sometimes they form the root/core of mountains.
• They are made up of granite rocks.
• They form various land features like dwalas and low lying hills after they have been exposed by denudation processes.
• They are essentially massive underground hills.
• They are formed when magma cools and solidifies near the earth’s surface.
• An example is Domboshava near Harare

4. Lopolith.

Lapolith. Image credit Elate Africa.

• A saucer(concave) shaped sheet of magma that solidifies in the earth’s crust.
• Lopoliths may form depression to hold water or form lakes.
• An example of a lopolith is the Great Dyke in Zimbabwe which as a matter of geological fact is not a dyke at all but indeed a lopolith.

5 Laccoliths

Laccolith. Image credit Elate Africa.

• It is a convex shaped massive sheet of magma that solidifies withing the earth’s crust and usually near the earth’s surface.
• They are arch-like igneous intrusions formed when magma injected between sediments and cools and solidifies therein.
• They typically have a pipe-like feeder coming from the magma pool very deep down.
• Laccoliths force the sedimentary rock to curve up.
• When they are exposed by erosion and other denudation processes they may form low lying hills.
• An example is Mt Mlanje in Malawi

Bedding planes

• They are layers of sedimentary rock.
• They are formed by layers of sedimentary material or fossil deposits.

To access more notes go to the Geography Notes page.