Vulcanicity

1. Intrusive Volcanicity
2. Extrusive Volcanicity

This is when magma intrudes within the earth's interior. The features that result due to the intrusive volcanic eruption are called intrusive features. The intrusive volcanic features are the ones found within the earth's interior.

Dyke

1. This is a wall of rock, which cuts across the bedding planes.
2. It is formed when magma cools and solidifies vertically across bedding planes.

Sill

• This is a rock sheet formed when the magma solidifies horizontally along the bedding plane.
• It is concordant with the rock strata.
• Example: Kinkon Falls found in the Fouta Djallon ranges in Guinea.

Laccolith

1. This is an intrusive feature, which looks like a dome.
2. It is formed when the magma cools and solidifies in an anticline bedding plane.
3. It looks like a mushroom.
4. Example: Morafanobe in Madagascar.

Lopolith

1. This is a saucer-shaped mass of rock formed in the geosyncline.
2. The saucer-like shape may be due to the increased weight of the deposits.
3. Example: The Bushveld Basin in the Transvaal, South Africa.

Phacolith

1. It is a lens-shaped strip of igneous rock formed when the magma solidifies along the anticline or syncline.
2. Example: Cordon Wills in the U.K.

Batholith

1. It is a large mass of solidified rock formed when magma cools plutonically at great depths.
2. Example: At the heart of mountain ranges.
3. Example: Chilu Batholith in Gabon.

These are the features formed when the magma cools and solidifies on the earth's surface.

Landforms Due to Extrusive Vulcanicity

Ash and Cinder Cone (Scoria Cone)

1. It is a cone-shaped accumulation of rock fragments around the vent.
2. The slopes of the cone are always concave due to the spreading tendency of lava at the base of the cone.
3. Example: Busoka and Bitale in South West Uganda.

Composite Cone (Strato Volcano)

1. It is a large cone with alternate layers of pyroclasts (fragments), i.e., ash and cinder on one hand.
2. Example: Mount Kilimanjaro, Meru in Tanzania, and the Virunga ranges in Uganda.

Volcanic Plug (Plug Dome Volcano)

1. It is a rigid cylindrical plug formed when very viscous lava is forced out by very explosive eruptions.
2. The plug is extruded amid clouds of hot blowing ash and cinders.
3. Example: Hoggar Mountains in Algeria.

Acid Lava Cone (Cumulo Dome Volcano)

1. It is a dome-shaped volcano with convex slopes formed when acidic lava solidifies around the vent.
2. Example: Ntumbi Dome located east of Mbeya, Tanzania.

Caldera

1. It is a large crater (large rounded depression) formed when the upper part of the volcano is either bombarded away by violent eruptions or subsides into the crust or in the volcanic cone.
2. Example: Ngorongoro in Tanzania, and Eboga Crater in Cameroon.