The majority of central volcanoes are associated with more or less circular vents, clusters of vents, in the earth’s crust, rather than fissures. The vents give rise to central volcanoes, which there are two basic types. The steep sided conical volcano mentioned above is occasionally constructed entirely from rock solid material, or tephra, which ranges in size from ash and cinders to rocks and boulders. The tephra have been ejected explosively in an eruption, or series of eruptions, and have fallen back to the ground in the immediate vicinity of the crater, the external outlet of the vent. A well-known example of such an extraordinary volcano is the Paricutin, in Mexico, which first erupted in a field on February 20th 1943,and within six days had built a cinder cone 150m (492ft) high; by the end of the year the cone was more than 336m(1,100ft) high. The next type is made up of layers of tephra and lava such volcanoes are called composite or stratovolcanoes. An excellent example of this one would be the Kilimanjaro in Tanzania.
Shield volcanoes are very large structures that can be tens of kilometres in diameters, they usually have gentle slopes generally less than 12 degrees, they are usually formed by hundreds of outpourings of fluid, basaltic lava. These volcanoes usually have more than one vent. One can find the best place to find examples of these are along the Hawaiian Islands.
The next type is about surface volcanoes, which are often associated with the destructive boundaries formed by tectonic plates, which are moving together. When two plates converge, the leading edge of one plunge’s beneath that of the other towards the mantle, the semi-molten layer that underlies the lithosphere, the other continental crust. The earth’s subduction zones lie mainly around the edge of the Pacific Ocean, and so do more than ¾ of active, dormant or extinct surface volcanoes. They form a belt known as the “Ring of Fire”, along which earthquakes are also common.
The final type of volcano is the Calderas which is basically the crater through which the volcanic material emerges often remains as a depression even when the volcano is dormant, the result is lava sinks back into the volcanic vent. Sometimes it sinks so far that the top of the volcano collapses into the main vent, forming a larger depression is some 8km in diameter. The best known Crater is probably Crater Lake in Oregon in the U.S.
Tsunami, Japanese word used as the scientific term for a seismic sea wave, a large ocean wave generated by an undersea earthquake. The wave is thought to be triggered when the ocean floor is tilted or offset during the quake. Another possible cause is an undersea landslide or volcanic eruption. Most tsunamis originate along the so-called Ring of Fire, a zone of volcanoes and seismic activity, 32,500 km (24,000 mil) long, that encircles the Pacific Ocean. Since 1819, for example, about 40 tsunamis have struck the Hawaiian Islands.
A tsunami may travel hundreds of kilometres across the deep ocean, reaching speeds of about 725 to 800 km/h (450 to 500 mphs). On entering shallow coastal waters, the wave, which may have been only about half a metre (a foot or two) high out at sea, suddenly begins growing rapidly. By the time it reaches the shore, it may become a towering wall of water 15 m (50 ft) high or more, capable of destroying entire coastal settlements.
Tsunamis have erroneously been called tidal waves, but they have nothing to do with the diurnal pattern of high and low tides. Such waves, however, in combination with meteorological phenomena, can also sometimes be destructive and an example of this happened on a Caribbean island.
Charlestown (St Kitts and Nevis), Nevis Island, St Kitts and Nevis, eastern Caribbean. Charlestown is the main settlement and port on Nevis Island and is situated on a bay on the western coast. The main industry in the area is sugar milling. Charlestown became the chief town on the island after Jamestown (the previous main settlement) was destroyed by a tidal wave in 1680. In the late 18th century Charlestown became a naval base and was almost destroyed by a fire in 1873. Notable buildings include Hamilton House, the birthplace of the 18th-century American statesman Alexander Hamilton, who was involved in drafting the Constitution of the United States, and the ruins of Montpelier Estate where Horatio Nelson was married. Population (1994) 1,411.
By:
Lowell Mason
