Geography
WAEC 2021
(a) List three landforms that are associated with extrusive volcanic action.
(b) With the aid of a diagram, describe the characteristics and mode of formation of laccoliths.
Explanation
(a) Landforms associated with extrusive volcanic action:
- Lava plain
- Lava plateau
- Acid lava cone
- Basic lava cone/shield volcano.
- Basaltic plateau
- Parasitic cones
- Lava tongues ( in lava dammed lakes)
- Crater
- Conposite cones ( strata volcanoes)
- Volcanic plug
- Ash and cinder cone
- Pillow lava.
(b) Characteristics and mode of formation of laccoliths:
(i) Characteristics:
- Found in areas of volcanic activity
- Mushroom-shaped
- It extends for several kilometers
- it is often less than l6 km in diameters
- An igneous rock
- Usually has a flat base and a round top
- Fed by pipe-like conduit from below
- Forms base of many great mountains
- Lies concordant or horizontal to the bedding plane
- lt forms a mountain when exposed
- It is resistant to erosion
- Arches the overlying rock stratum
- Examples are El-Koubra Djauf in Algeria, Black hills of south Dakota, Fonjay Massif in Madagascar, and Henry mountains of south Utah in USA
(ii) Mode of Formation:
- Formed in areas of volcanicity
- Formed when pressure on molten magma under the earth is released into the crust
- Thick (viscous) magma is released from the interior of the earth.
- Magma freed through earth's movement
- If line of weakness is obstructed by a thick layer of rock strata, upward movement stops.
- Rising magma may come across a hard layer on its way.
- As it cannot penetrate it, it forces the overlying layer of rock to arch upwards.
- Causing the layer to form a great oval or semi-circular shape.
- Magma then squeezes beneath the pushed-up/semi-circular-shaped rock.
- This cools and solidifies to form a dome-line feature resembling a mushroom.
- Denudation may expose it to form an upland.
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