Theory question 204
Earth Movements and Vulcanicity(a) Explain the term vulcanicity? (b) Describe the characteristics and mode of formation of any one of the following: (i) Batholith; (ii) Dyke; (iii) Composite cone. (c) In what four ways are volcanic rocks important to man?
Show the model answer
(a) Vulcanicity is all the processes by which molten material (magma), gases and solids are forced from inside the earth into the crust or onto the surface.
(b) Batholith: very large dome-shaped mass of intrusive igneous rock (granite), hundreds of km across, deep-seated, often forming the core of fold mountains; formed when a huge body of magma rises into the crust and cools slowly; exposed later by denudation.
Dyke: narrow, vertical or steeply inclined wall of igneous rock cutting across the bedding planes; formed when magma is forced into vertical cracks and cools; may stand out as a ridge or form a trench when eroded; can cause waterfalls (e.g. ring dykes near Jos).
Composite cone: tall, steep-sided conical volcano made of alternate layers of ash and lava with a central crater and side (parasitic) cones; formed by repeated explosive eruptions (Mt Cameroon, Kilimanjaro, Fuji).
(c) Fertile volcanic soils for farming; building and road stone (granite, basalt); minerals (gold, diamond, tin); tourism and hot springs; geothermal power; landforms for defence and scenery.
Choose one feature and give both characteristics and formation.