(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?
Explanation
(a) Explain the term vulcanicity; Vulcanicity is the process by which gaseous, liquid and solid substances are forced from interior of the earth into the crust or onto the surface of the earth.
(b) Characteristics of Batholith
(i) Most spectacular of igneous rocks.
(ii) It is a large mass and usually of hundred kilometers in extent.
(iii) It forms longitudinal ridge in the core of fold mountains.
(iv) Batholith may be exposed as a result of denudation.
(v) Examples are Wicklow mountain, Island and Upland Brittany France, along the Rockies in U.S.A. Formation of Batholith It is volcanic in origin and formed at the roof of mountain, massive and resistant upland. Batholith are large mass of magma rising upwards and accumulates in the crust.
Characteristics of Dyke;
(i) They are vertical or inclined across the bedding planes (discordant)
(ii) It has a narrow wall-like feature.
(iii) Dyke is rarely prominent in the landscape.
(iv) A times when exposed forms shallow trenches.
(v) It is a resistant rock.
(vi) It may be circular in shape e.g. Ring Dyke in Jos.
(vii) Dyke is very extensive in length.
(viii) Dyke sometimes gives rise to waterfalls/rapids.
(ix) Dyke often constitutes oil trap or water trap.
(x) Examples are Cleveland dyke of Yorkshire, ring dyke in Jos.
Formation;
(i) Dykes are formed from the intrusion of molten magma into cracks and joints.
(ii) They are formed through cooling and solidification of vertical and narrow walls of igneous rock.
(iii) It has shallow trenches formed when exposed to denudation.
Characteristics of composite cone;
(i) It is the highest form of volcanoes.
(ii) It is the most common type of volcanoes.
(iii) It is conical in shape.
(iv) In structure, it is made of alternative layers of ash and lava.
(v) Composite cone usually build up conelets around the major cone.
(vi) Examples are Etna Sicily, Mountain Fuji; Mountain Chimborazo and Vesuvius.
Formation;
Composite cones are made of alternate layers of ash and lava. The lava pours out forming layers on top of the ash. The lava moves out from the sides of the cone. Conelects are built up in minor eruptions. New eruption forms new layers which grow gradually as height increases. At times the whole top of the volcano blows off or sinks e.g. chimborazo. A large crater called Caldera may finally emerge at the point of volcano.
(c) Importance of volcanic rocks.
(i) It serves as a source of mineral.
(ii) It is a fertile soil for agriculture.
(iii) It serves as hot spring for domestic uses.
(iv) It provides rocks for construction.
(v) It forms basin for underground water.
(vi) It forms basin for crude oil deposits.