Intoduction to Volcano and Classification of volcanoes

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Volcanoes are due to eruption of magma and gases through weaknesses (fissures) in the overlying rocks (Fig. 2.1).  On Earth, there are about 60,000 volcanoes of which at least 50,000 occur as seamounts on the floor of the Pacific Ocean.  There are only about 500 active volcanoes on Earth. Active implies at least one reception has been witnessed and recorded by people. About 62% of the active volcanoes are on the rim of the Pacific Ocean in the circum-pacific “ring of fire” at convergence plate margins, 22% are in Indonesia, 10% are in Atlantic Ocean and remaining are in Mediterranean – Middle East, the African Rift valley, Hawaii and other oceans islands.

There is a close relationship between the distribution of earthquakes and that of active volcanoes.  Both tend to occur along the active plate margins. In general, volcanic activity is likely to take place at known sites, although the extent of its influence is much less predictable.

A)    Classification of volcanoes

Volcanoes are classified according to their activity. Three categories of volcanoes are recognized. These are active, dormant and extinct.

An active volcano is one that has erupted recently or based on volcanologists might erupt soon. Active volcanoes are watched very closely so that people and animals in the area can be moved to a safe place ahead of time. Eldonyo Legai in Arusha is typical example of an active volcano.

A dormant volcano or sleeping volcano is a volcano that has been quiet for a long time, but still has signs that show possibility of eruption in the future. A volcano becomes dormant when the vent is blocked by hardened lava, called a plug, or if the magma seeps back under the earth’s crust. Volcanoes can be dormant for hundreds of years. Eruption might occur suddenly due to pressure build-up under the plug. The plug of a dormant volcano stops the magma from rising. Typical example is Kiejo Volcano in Rungwe District.

An extinct volcano is one that has not erupted for thousands of years. Sometimes it is hard to tell if a volcano is dormant or extinct. They will be listed as extinct until volcanologists are sure there will be no more eruptions. Most scientists believe Kilimanjaro Mountain is an extinct volcano.

                                                                                              

Volcanic products

Products of volcanic eruption may be divided into:

i)              Solid Products (pyroclasts) – Solid fragmental materials torn from the sides of the pipes or feeder system through which lava moved. These fragments range from rocks, volcanic bombs, Lapilli, to ash and dust.

ii)            Liquid Products (Lavas): These are molten liquid silicate masses. The chemical composition of lavas and their content of gaseous compounds largely determine their viscosity, mobility and the nature of volcanic eruptions. For the example, the basic basaltic lavas that reach the surface with the temperature of about 1200^oC are highly mobile and spread over large distances forming flows or sheets. Acidic lavas, on the other hand, are viscous and their mobility is low. Acidic lavas solidify as short flows or tongues on slopes of the volcano.

iii)             Gaseous Products: At all stages, volcanic activity is associated with emanation of steam and various gases. Gases emanated are mostly H₂O, SO₂, CO₂, CO, HCl, HF, NH₄Cl, and NH₃. These gases are emanated at various temperatures. The gaseous emanations at temperature above 180^oC are called fumaroles, at temperature of 180^oC to 100^oC they called sofataras and below the temperature 100^oC are termed mofettes.

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