(a) Explain the term vulcanicity? (b) Describe the characteristics and mode of formation of any one of the following: (i) Batholith; (ii) Dyke; (iii) Compos...

(a) The term vulcanicity

Vulcanicity (or vulcanism) is the process by which solid, liquid or gaseous materials (magma, lava and gases) from the interior of the earth are forced into the earth's crust or poured out onto its surface. It includes both intrusive activity, where magma cools within the crust to form features such as batholiths, dykes and sills, and extrusive activity, where lava reaches the surface to build volcanoes and lava plateaux.

(b) Characteristics and mode of formation (one chosen: the Batholith)

Batholith - characteristics:

• It is a very large intrusive (plutonic) igneous body, usually made of coarse-grained rock such as granite.
• It forms the deep core or root of large mountain ranges and covers a very wide area.
• It becomes exposed at the surface only after long-continued erosion of the overlying rocks, often forming inselbergs and domed uplands.

Mode of formation: A large mass of molten magma rises from deep within the earth and is intruded into the crust, deep below the surface. Because it lies far below ground, it cools and solidifies very slowly, giving large crystals. Over a long time, the overlying rocks are stripped away by weathering and erosion, exposing the massive granite batholith at the surface.

(If the Dyke is chosen: a dyke is a narrow, wall-like sheet of igneous rock that cuts across the bedding of the surrounding rocks; it forms when magma is forced into a near-vertical crack or joint and solidifies. If the Composite cone is chosen: it is a tall, steep-sided volcano built of alternating layers of lava and pyroclastic materials from repeated eruptions of fairly viscous lava.)

(c) Four ways volcanic rocks are important to man

• On weathering they form very fertile soils that are good for agriculture.
• They provide hard, durable building and road-making materials (for example basalt).
• They are associated with valuable minerals and, where volcanic activity persists, with hot springs and geothermal energy.
• Volcanic landscapes and features attract tourists, bringing revenue.