GC Leong Test : Volcanism And Earthquakes - 1

• Volcanic activities have a profound influence on the earth's landforms. Solid, liquid or gaseous materials may find their way to the surface from some deep-seated reservoir beneath.

• Molten magma is the mobile rock that forces its way into the planes of the weakness of the crust to escape quietly or explosively to the surface.

• Magma while thrusting its way up to the surface may cool and solidify within the crust as plutonic rocks resulting in intrusive landforms.

• Perhaps the commonest intrusive landforms are sills and dykes. When an intrusion of molten magma is made horizontally along the bedding planes of sedimentary rocks, the resultant intrusion is called a sill.

• Denudation of the overlying sedimentary strata will expose the intrusion which will resemble a lava flow, or form a bold escarpment like the Great Whin Sill of N.E. England.

• Similar intrusions when injected vertically as narrow walls of igneous rocks within the sedimentary layers are termed as dykes.

• A laccolith is a large blister or igneous mound with a dome-shaped upper surface and a level base fed by a pipe-like conduit from below.

A lopolith is another variety of igneous intrusion with a saucer shape. A shallow basin is formed amid the country rocks.

An example of a phacolith is Corndon Hill in Shropshire, England

• As temperature increases with increasing depth below the earth's crust, at an average rate of about 1°F. for every 65 feet of descent, the interior of the earth can be expected to be in a semi-molten state, comprising solid, liquid and gaseous materials, collectively termed magma.

• The gases and vapour increase the mobility and explosiveness of the lavas which are emitted through the orifice or vent of a volcano during a volcanic eruption.

• Basic lavas: These are the hottest lavas, about 1.000°C. (1,830°F.) and are highly fluid. They are dark coloured like basalt, rich in iron and magnesium but poor in silica.

• As they are poured out of the volcano, they flow quietly and are not very explosive. Due to their high fluidity, they flow readily with a speed of 10 to 30 miles per hour.

• They affect extensive areas, spreading out as thin sheets over great distances before they solidify.

• Acid lavas: These lavas are highly viscous with a high melting point. They are light-coloured, of low density, and have a high percentage of silica. They flow slowly and seldom travel far before solidifying. The resultant cone is, therefore, steep-sided. The rapid congealing of lava in the vent obstructs the flow of the out-pouring lava, resulting in loud explosions, throwing out many volcanic bombs or pyroclasts.

• There are three types of volcanoes: active, dormant and extinct. Volcanoes are said to be active when they frequently erupt or at least when they have erupted within recent time.

• Those that have been known to erupt and show signs of the possible eruption in the future are described as dormant.

• Volcanoes that have not erupted at all in historic times but retain the features of volcanoes are termed extinct.

• All volcanoes pass through active, dormant and extinct stages but we can never be thoroughly sure when they are extinct.

• Volcanic cones are most typical of the extrusive features. The highly fluid lavas build up lava domes or shield volcanoes with gently rising slopes and broad, flattened tops. The volcanoes of Hawaii have the best-developed lava domes.

• The less fluid lavas that explode more violently form ash and cinder cones with large central craters and steep slopes. They are typical of small volcanoes, occurring in groups and seldom exceeding 1,000 feet in height, such as Mt. Nuovo, near Naples and Mt. Paricutin in Mexico. The lava flows are so vicious that they solidify after a short distance.

• The highest and most common volcanoes have composite cones. They are often called strato-Volcanoes. The cones are built up by several eruptions of lava, ashes and other volcanic materials from the main conduit which leads down a reservoir of magma.

• Each new eruption adds new layers of ashes or lava to the sides of the volcano, which grows steadily in height. From the main conduit, subsidiary dykes or pipes may reach the surface as feeders to parasitic cones.