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The vast majority of volcanoes lie near the oceans, and, until recently, this aspect of their distribution was rather puzzling. Equally, most volcanoes are closely associated with earthquakes, which are also concentrated in narrow bands. It had long been known that earthquakes and volcanoes were somehow related, but nobody really knew the reason until closer analysis brought some correlations to light. The key points covered in the paper are volcanic chains and earthquakes, a correlation between earthquakes and eruptions.Earthquakes are caused by shift in plates and subduction processes.
Volcanoes do not erupt everywhere. They are restricted to narrow bands in very specific locations. The Pacific Ocean is virtually surrounded by a belt of volcanoes commonly called the “Pacific Ring of Fire” which contains about twothirds of the world’s active volcanoes (Gilluly 26). The “Ring of Fire” is marked by the volcanic chains of Japan, Kamchatka, South Alaska and the Aleutian Islands, the Cascade Range of the United States and Canada, Central America, the Andes, New Zealand, Tonga, Vanuatu, Papua-New Guinea, Indonesia, the Philippines, and finally the Mariana, Izu and Bonin Islands, which complete the circle. In contrast, the Adantic Ocean has few volcanoes around its edges, except those of the Canary Islands, the Cape Verde Islands, and the West Indies (Blaikie 45).
On the other hand, the Atlantic Ocean has many volcanoes along its central ridge, which are largely submerged. Its crest, however, emerges in Jan Mayen and Iceland, and volcanoes have arisen on the ridge flanks in Ascension Island, Tristan da Cunha, and in the Azores. Similarly the mid-ocean ridges of the Pacific and Indian Oceans are also marked by innumerable submerged volcanoes. The Mediterranean Sea, too, has volcanoes in its midst in southern Italy and Greece. Other volcanoes occur in, or alongside, some of the major rift valleys: the East African Rift Valley system and the Rhine Rift Valley. Several major volcanic areas rise in isolated clusters on both land and sea, including the Tibesti Mountains of Saharan Africa, the Hawaiian Islands, and many submerged seamounts (volcanoes not yet built above sea level) in the Pacific Ocean (Blaikie 47).
Earthquakes are weak and usually less than magnitude 4 and are often accompanied by swarms of minor tremors. The severity of earthquakes is measured on the logarithmic Richter scale, based on the strength of shockwaves propagated through the Earth’s crust. Thus, a magnitude 5 earthquake is ten times greater than one of magnitude 4, and so on up to magnitude 10 or more. Such earthquakes are closely tied to volcanoes, both in time and space, because they are generated by the rise of molten material towards the volcano itself. The second correlation was found on the mid-ocean ridges (Davis 49). Earthquakes accompany the quasi-continual volcanic action along the mid-ocean ridges. Many of these earthquakes are weak, but the strongest tend to range from magnitude 5 to magnitude 6 on the Richter scale. They are also generated at a greater depth, commonly some 25km below the crests of the mid-ocean ridges (Gilluly 66).
The correlation between earthquakes and volcanoes occurs around the “Pacific Ring of Fire”, in the Lesser Antilles of the West Indies and also, to a degree, in the Mediterranean Sea. The earthquakes occur closely parallel to the lines of volcanoes. These earthquakes are often vigorous, exceeding magnitude 7 or magnitude 8 on the Richter scale, and are extremely destructive if they happen in built-up areas, as was testified by those at Anchorage, Alaska, in 1964, in the Ionian Islands, Greece, in 1953 and at Messina, Sicily, in 1908 (Gilluly 36).
These are deep-focus earthquakes, generated at greater depths than any others, with their main centres often between 100km and sometimes as much as 500km below the Earth’s surface. They occur, moreover, along sloping planes known as Benioff zones after one of the scientists who discovered them (Davis 49). The volcanoes in the belts parallel to these zones of vigorous earthquakes are also marked by violent eruptions. On any individual volcano, the explosive episodes are often separated by decades or centuries of tranquillity. However, the volcanoes in these zones are so numerous that scarcely a month goes by without one of them being active. But cases are extremely rare of an eruption resulting directly from a deep-focus earthquake. The earthquake that destabilized the north flank of Mount St Helens at the start of the eruption on 18 May 1980 was very shallow and relatively weak at magnitude 5 (Gilluly 67).
The deep-focus earthquakes do not cause eruptions, nor vice versa, but both represent different aspects of the same fundamental relationship. It was increased investigation of the oceans in the 1960s that revealed the most likely connection between earthquakes and volcanoes-and even explained the frequent relationship between volcanoes and the sea (Gilluly 68). The facts known about volcanoes, earthquakes and the ocean floors were then pieced together in another fashion. It was as if the methodological kaleidoscope of the Earth sciences had been given a great jolt. A new pattern emerged which then appeared so obvious in its broad outlines that it now seems amazing that it had never been comprehended before (Degg and Chester 125). The distribution of earthquakes and volcanoes, instead of being the problem, actually provided the solution-and the science of plate tectonics was born. It was discovered that the unstable volcanic and earthquake belts mark the edges of plates that together make up the lithosphere, the Earth’s outermost shell, composed of the crust and the upper rigid part of the mantle. The correlation was that many earthquakes occur beneath, or near volcanoes. These are shallow, mostly less than 5km in depth and happen with increasing frequency before, and sometimes during volcanic eruptions (Hobbs 27).
Sea-floor spreading could only be accommodated on the globe in two ways. Either the Earth must itself be expanding (which is unlikely on all sorts of other grounds), or the ocean floors must somehow be consumed at more or less the same rate as they are being formed along the mid-ocean ridges. This second view was soon verified. The true significance of the Benioff zone of severe deep earthquakes became clear (Degg and Chester 125). The outer, leading edges of the plates converge and one plate descends beneath another as a broad, cold, solid slab into the hot, plastic layer, known as the asthenosphere (“the zone without strength”) situated below the lithosphere. This process is called subduction, and it is the main way in which die global plates are consumed.
The overall speeds of subduction range from 2cm a year to about 10cm a year from plate to plate (White 62). The subduction can sometimes be accomplished by smooth downward sliding, but the slab usually descends intermittendy both in time and space. It is the jerky downward motion, or a sudden temporary rebound, that produces the violent deep-focus earthquakes. Once subduction has been initiated by differences of temperature, pressure or density, it continues as a passive response to the sinking of the dense cold plate into the hotter zone below the lithosphere, where it is eventually assimilated at depths of about 700km (Hobbs 82). Thus, one oceanic plate may be subducted beneath another, or an oceanic plate may be subducted beneath a plate also carrying continental crust. A continent cannot itself be subducted because it is composed of less dense material than the ocean floors and is therefore buoyed up (and hence the continental rocks can be very much older than those of the ocean floors). It is not directly related to volcanicity, plates sometimes also neither converge nor diverge, but slide past each other along major fractures or transform faults (Blaikie 49).
When the magma solidifies as lava, it is added to the plate edge. Further divergence caused by continued movement beneath the plates generates other fractures up which more magma may then rise. Plate divergence, the ascent of magma, and its accretion to the growing edge of the plate are thus going on all the time. The plate is rather like a conveyor belt moving away from the mid-ocean ridge where its diverging edge is constantly being augmented by new magma. This is “sea-floor spreading”.(Blaikie 43). The newest parts of the ocean floors occur on the ridge crests, older bands run parallel to the crest on either side of it, and the oldest parts occur, as might be expected, on the outer edges of the oceans towards which they have spread farthest from the ridges where they originated. The very oldest parts of the present ocean floors are no more than about 180 million years old. Together these relatively youthful ocean floors constitute two-thirds or so of the rock surface of the globe (Blaikie 82).
The zone where subduction begins is marked by long trenches, which are commonly between 5000m and 11000m deep. Such trenches, the deepest part of the oceans, are deeper even than the abyssal plains that form most of the ocean floors. The angle at which the plate is subducted varies according to many factors including the relative densities, temperatures and mineral contents of the slab, compared with those of the materials into which it is sinking (Blaikie 41). The Marianas subduction zone, for example, curves down until it is almost vertical, whereas the subduction beneath the Peruvian Andes takes place at an angle of some 15°. Subduction initiates a series of complex changes which result in partial melting of the asthenosphere and the formation of magma. In subduction zones, the magma rises intermittently to the surface, with sojourns of varying lengths in reservoirs en route, where it undergoes changes that usually cause it to erupt violently above the zone of melting.
The volcanoes often form on the overriding plate about 150-200km from the ocean trench marking the inception of subduction-the distance depending primarily on the angle at which the plate is subducted. They form lines of islands in the oceans or chains on land, which are frequently more than 1000km long and about 50km wide, with volcanoes spaced at intervals ranging from 10km to 100km apart. The magma generated by subduction does not reach the Earth’s surface continuously, nor at the same rate, otherwise the volcanoes in a given subduction zone would all erupt in a prolonged explosive chorus. Two outbursts along the same subduction zone at the same time are, in fact, rare. The eruptions in the West Indies of the Soufriere on St Vincent on 7 May 1902, and of Montagne Pelée in Martinique on the following day, were quite exceptional-at least as far as historic records and absolute dating methods have revealed. Because the magma that rises from subduction zones often remains in reservoirs for a time, eruptions from subduction zone volcanoes are irregular and separated by long dormant periods (Degg and Chester 125).
In spite of technological developments and innovative solutions, researchers do not understand the nature of eruptions and earthquakes. When an oceanic plate is subducted beneath another oceanic plate, the resulting volcanoes rise from the ocean floor to form a gently curving island arc. These may be relatively simple volcanic island series such as the Aleutian arc, or they may have a greater age and complexity derived partly from the addition of continental masses into their make-up. When an oceanic plate is subducted beneath a continent-carrying plate, the magma rises up fractures through the continental crust onto the surface and creates land-based, usually straighter, volcanic chains
Works Cited
Blaikie, P., Cannon, T., Davis, J. At Risk: Natural Hazards, People’s Vulnerability, and Disasters. Routledge, 2003.
Davis, M. Cosmic Dancers on History’s Stage? The Permanent Revolution in the Earth Sciences. New Left Review, a, (1996): 48-56.
Degg, M.R., Chester, D.K. Seismic and Volcanic Hazards in Peru: Changing Attitudes to Disaster Mitigation. The Geographical Journal, 171 (2005): 125.
Gilluly, G., Waters, A.C., Woodford, A.O., Compton, R.R. Principles of Geology. W. H. Fremmil: Sn Francisco, 1958.
Hobbs, W.H. Earth Features and Their Meaning. Macmillan Company, 1991.
White, J.F. Study of the Earth. Prentice-Hall, 1962.
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