Theory question 307
Earth Movements and Vulcanicity(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 laccolith.
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(a) Volcanic cones (ash, cinder, composite); lava plateau; lava plain; crater; caldera; crater lake; geysers and hot springs; fumaroles.
(b) Diagram: a dome-shaped mass of solidified magma between rock layers, with a pipe feeding it from below and the overlying layers arched upwards.
Characteristics: intrusive feature; dome or mushroom shape with a flat base; lies between bedding planes; arches the overlying strata; may be exposed by erosion as a hill (e.g. Henry Mountains, USA).
Formation: viscous magma rises along a line of weakness; it cannot spread far or break through the overlying hard layers; it accumulates between strata and forces the upper layers to bulge upward; it cools and solidifies there to form a laccolith.
Label pipe, laccolith and arched strata.