THE ORIGIN OF VOLCANOES
The ancient Greeks believed the volcanic eruptions occurred when Vulcan, the God of Fire, stoked his subterranean furnace beneath Vulcano, a small volcanic island off Sicily, from which the present word volcano is derived. Of course, we no longer believe this is true. Geologists and Volcanists have ascertained that volcanic activity is closely connected with crustal disturbances, particularly where there are zones of weakness due to deep faulting or mountain folding. As temperature increases with increasing depth below the earth’s crust, at an average rate of about 1° C for every 20 metres (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 magma is heavily charged with gases such as carbon dioxide, sulphurated hydrogen and small proportions of nitrogen, chlorine and other volatile substances, 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.
There are two main types of lavas.
1) Basic lavas: These are the hottest lavas, about 1000° 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 16-48 km per hour (10-30 miles per hour). They affect extensive areas, spreading out as thin sheets over great distances before they solidify. The resultant volcano is gently sloping with a wide diameter and forms a flattened shield or dome.
2) Acid lavas: These lavas are highly viscous with a high melting point. They are light-coloured, of a 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 outpouring lava, resulting in loud explosions, throwing out many volcanic bombs or pyroclasts.
Sometimes the lavas are so viscous that they form a spine or plug at the crater like that of Mt. Pelee in Martinique.
Some spines are very resistant and while most of the materials of very old volcanoes are removed by erosion the spine may remain e.g. Puy de Dome, France, and Tangale Peak (south of Kaltungo), almost mid-way between Gombe and the Benue river in Nigeria. The Mamelles of Dakar, Senegal, are also well known.
Types of Volcanoes
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. An example is the Cameroon Mountain which erupted in 1909, 1922, 1954 and 1959. There are examples of recent volcanoes in the grasslands of Tsaro in southern Kenya where vegetation is just beginning to establish itself on the solidified erupted material. Those that have been known to erupt and show signs of possible eruption in the future are described as dormant. Like the Jos Plateau of Nigeria, 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. Mt. Vesuvius and Mt. Krakatoa were once thought by people to be extinct and yet both erupted most violently afterwards.
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