(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.
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.
