(a) List three landforms that are associated with extrusive volcanic action. (b) With the aid of a diagram, describe the characteristics and mode of formati...

(a) Landforms associated with extrusive volcanic action

1. Lava plains or lava plateaux.
2. Shield volcanoes (basic lava cones).
3. Composite cones (strato-volcanoes).

(b) Laccolith

A laccolith is a concordant intrusive igneous body formed when viscous magma is injected between sedimentary rock beds. It has a nearly flat base and a rounded or dome-shaped upper surface, giving it a mushroom-like form.

Diagram of a laccolith

Characteristics

• It is an intrusive igneous landform, formed below the Earth's surface.
• It lies concordantly, that is, generally parallel to the bedding planes of the surrounding sedimentary rocks.
• It has a broad, almost flat floor and a convex, dome-shaped roof.
• It arches and uplifts the overlying sedimentary strata.
• It is commonly fed from below by a narrow pipe-like conduit or dyke.
• It is composed of hard igneous rock and may stand out as a rounded hill or upland after denudation removes the cover rocks.

Mode of formation

1. Viscous magma rises from the interior through a line of weakness or a pipe-like conduit.
2. On reaching a resistant rock layer, the magma cannot easily continue vertically upward.
3. It therefore spreads sideways along a bedding plane between sedimentary strata.
4. Because the magma is thick and does not spread far, it accumulates locally and pushes the overlying beds upward into a dome.
5. The magma cools and solidifies beneath the arched strata to form a laccolith.
6. Subsequent erosion may remove the overlying sedimentary rocks and expose the resistant laccolith as an upland feature.