Theory question 206
Earth Movements and Vulcanicity(a) Define vulcanicity. (b) Describe the characteristics and mode of formation of any one of the following features: (i) dyke; (ii) composite cone. (c) In what three ways are volcanic rocks important to man?
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(a) Vulcanicity is all the processes by which molten rock (magma), gases and other materials are forced into the earth's crust or onto the surface.
(b) Dyke: a vertical or steeply inclined wall-like sheet of igneous rock cutting across the bedding planes; magma forces its way up a vertical crack or joint and solidifies; often harder than surrounding rock so it stands out as a ridge when exposed; e.g. Cleveland Dyke (England), Great Dyke (Zimbabwe).
Composite cone: tall, symmetrical, steep-sided volcanic mountain made of alternate layers of ash/cinders and lava; built by repeated explosive and quiet eruptions; has a central vent and crater, may have secondary (parasitic) cones; e.g. Mount Cameroon, Kilimanjaro, Fuji, Vesuvius, Etna. Labelled diagram.
(c) Weather into fertile soils for farming; building and road stones (granite, basalt); source of minerals (diamonds, gold, tin, copper); tourist attractions; crater lakes for water; geothermal power; grinding stones.
Describe one feature only, with a diagram.