Volcanoes

geological formations that appear above channels and cracks in the earth’s crust, through which lava, hot gases, and fragments of rock are expelled on to the surface from deep-lying magmatic sources. Volcanoes are usually individual mountains formed from the products of eruptions.

Volcanoes are distinguished as active, dormant, and extinct. The first category includes volcanoes that are now erupting constantly or periodically, volcanoes for whose eruptions historical data exist, and volcanoes for whose eruptions there is no information but which are discharging hot gases and waters (solfataric stage). Dormant volcanoes are those for whose eruptions there is no information but which have preserved their shape and underneath which local earth-quakes are occurring. Volcanoes that are strongly broken down and eroded and show no manifestations of volcanic activity are called extinct.

Volcanoes are divided into central and fissured types depending on the shape of the volcanic neck.

Deep-lying magmatic sources may be located in the upper mantle at a depth of 50-70 km (Kliuchevskaia Sopka on Kamchatka; Kilauea in the Hawaiian Islands) or in the earth’s crust at a depth of 5-6 km (Mount Vesuvius, Italy) and deeper.

Volcanic phenomena. Volcanic eruptions may be prolonged (lasting several years, decades, or centuries) or short-term (measured in hours). Among the precursors of an eruption are volcanic earthquakes, acoustic phenomena, and changes in magmatic properties and the composition of fumarole gases. An eruption usually begins with an intensification of the discharge of gases, at first mixed with dark, cold fragments of lava and later with red-hot lava. In some cases these discharges are accompanied by lava flows. The height to which the gases, ash-saturated water vapor, and fragments of lava rise varies from 1 to 5 km depending on the force of the explosion. (During the eruption of Bezymiannyi on Kamchatka in 1956 it reached 45 km.) The ejected material is carried for distances ranging from a few to tens of thousands of kilometers. The volume of rock fragments discharged sometimes reaches several cubic kilometers. In some eruptions the concentration of volcanic ash in the atmosphere is so great that darkness like that in a closed room occurs. In 1956 this happened in the settlement of Kliuchi, which is 40 km from Bezymiannyi.

An eruption is an alternation of weak and strong explosions and outpourings of lava. The explosions of greatest force are called the culminating paroxysm; after them the force of the explosions decreases and the eruptions gradually stop. The volume of lava ejected may be as much as dozens of cubic kilometers.

Types of eruptions. Volcanic eruptions are not always the same. Depending on the quantitative ratios of the volcanic products being ejected (gaseous, liquid, and solid) and the viscosity of the lava, four main types of eruptions are distinguished: effusive, mixed, extrusive, and explosive or, as they are more frequently called, Hawaiian, strombolian, dome-type, and Vulcanian. Hawaiian eruptions, which most frequently create shield-shaped volcanoes, are distinguished by a relatively mild outflow of liquid (basaltic) lava, which forms molten lakes and lava flows in the craters. The gases, which are present in small quantities, form fountains that throw out lumps and drops of liquid lava, which are drawn into fine glass threads in flight (Kilauea). In a strombolian eruption (which usually creates stratovolcanoes), in addition to quite abundant outflows of liquid lava of basaltic and andesitebasaltic composition (which usually form very long flows), small explosions predominate. These explosions throw out pieces of slag and various types of twisted and spindle-shaped bombs (Stromboli on the Lipari Islands, Mihara, and certain eruptions of Kliuchevskaia Sopka). Phenomena typical of the dome type include the squeezing and extrusion of viscous lava (andesite, dacite, or rhyolite) from the volcanic neck by the strong pressure of the gases and the formation of certain shapes: domes (Piui-de-Dom and Tsentral’nyi Semiachik on Kamchatka), cryptodomes (Syowa-Sinzan), conical domes (Ivanov), and obelisks (Shiveluch on Kamchatka). In a Vulcanian eruption gaseous substances are important, causing explosions and discharges of enormous black clouds saturated with large quantities of lava fragments. Viscous andesite, dacite, or rhyolite lavas form small flows (Vul’kano, Avachinskaia Sopka, and Karymskaia Sopka on Kamchatka).

Each of the primary types of eruption is subdivided. Particularly outstanding are the Peleean and the Katmaian sub-types, which fall between the dome and Vulcanian types. Characteristics of the Peleean subtype are the formation of domes and directionally concentrated explosions of very hot gas clouds saturated with lava fragments and lumps which explode spontaneously in flight and when rolling down the sides of the volcanoes (Mount Pelee on the island of Martinique). Eruptions of the Katmaian subtype are distinguished by expulsion of a very hot and mobile sand flow (Mount Katmai in Alaska). Dome-forming eruptions are sometimes associated with red-hot or considerably cooled avalanches, as well as with mudflows. The ultra-Vulcanian subtype is marked by very powerful explosions that eject enormous quantities of lava fragments and rock from the neck walls. Eruptions of very deep underwater volcanoes usually go unnoticed because the great water pressure prevents explosive eruptions. In shallow places the eruptions are marked by explosions (expulsions) of enormous quantities of steam and gases filled with small lava fragments. Explosive eruptions continue until the material being erupted forms an island that rises above sea level. After this explosions are replaced by lava flows or alternate with them.

The products of volcanic eruption are gaseous (volcanic gases), liquid (lava), and solid (volcanic rock). Depending on the nature of the eruptions and composition of the magma, structures of various shapes and heights form on the surface. They are the volcanic neck (a tubelike or fissured channel), the mouth (the top of the neck), the thick accumulations of lava and volcanic fragments that surround the neck, and the crater (a cuplike depression on the top of the structure). The most common shapes of the structures are conical, where discharges of fragmentary material predominate; domed, where viscous lava is extruded; and gently sloping shields, where outflows of liquid lava predominate. Eruptions occur not only through the main, top crater but also through subsidiary (parasitic) craters on the slopes and at some distance from them. In the case of one-time eruptions of gases that break a channel through to the surface, craterlike depressions bordered by a circular embankment of clumps of different rocks form frequently; such craters, which are often filled with water, are called maars. Powerful eruptions are sometimes accompanied by destruction of part of the volcanic structure or even of adjacent territory. The depressions formed, which range from a few to 10-30 km in diameter, are called calderas.

Geographical distribution of active volcanoes. Currently active volcanoes are located along the young mountain ranges or along major fractures (grabens) for hundreds and thousands of kilometers in tectonically mobile areas (see Table 1). Almost two-thirds of the world’s volcanoes are concentrated on the islands and shores of the Pacific Ocean—the Pacific Ocean volcanic belt. Among the other regions, the Atlantic Ocean is notable for its number of active volcanoes.

Table 1. Geographic location of active volcanoes (1970 data)
Areas of volcanic activity		Number of volcanoes¹		Total
	surface	underwater	in the solfataric stage
¹ Figures in parentheses indicate volcanoes whose past activity is open to question
Kamchatka ...............	19 (1)	—	8	27 (1)
Kurile Islands ...............	31	1	7	39
Japan ...............	35 (1)	1	14	50 (1)
Izu and Mariana islands ...............	17	7 (1)	2	26 (1)
Taiwan ...............	—	5	—	5
The Philippines ...............	9 (1)	3	10 (4)	22 (5)
South China Sea ...............	—	1 (1)	—	1 (1)
Melanesia ...............	23 (2)	6 (1)	15 (11)	44 (14)
Kermadec Islands, Tonga, and Samoa ...............	8	10	1	19
New Zealand ...............	5 (1)	—	—	5 (1)
Antarctic ...............	8	1	3 (5)	12 (5)
Juan Fernández Islands ...............	1	2	1	4
Galápagos Islands ...............	9	—	2	11
South America ...............	43(1)	—	14 (1)	57 (2)
Central America ...............	25 (19)	—	7 (6)	32 (25)
North America (not including Alaska) ...............	6 (2)	—	2 (1)	8 (3)
Alaska and the Aleutian Islands ...............	36	—	2	38
Hawaiian Islands ...............	4	1	—	5
Islands from Sulawesi to New Guinea ...............	16	2	3 (5)	21 (5)
Java arc (ending with the Banda Sea) ...............	57	3	26 (16)	86 (16)
Indian Ocean (not including the Java arc) ...............	3	(1)	1 (1)	4 (2)
Arabian Peninsula ...............	2 (14)	—	3	5 (14)
Asia Minor and the Caucasus ...............	2	—	6	8
Tung-pei (Manchuria), Tibet, and others ...............	6	—	—	6
Africa ...............	17 (1)	—	21 (4)	38 (5)
Mediterranean Sea ...............	7 (1)	4	3 (1)	14 (2)
Atlantic Ocean ...............	14	6 (4)	2	22 (4)
Iceland and Jan Mayen Island ...............	27	5	6	38
Lesser Antilles ...............	6 (3)	3	5	14 (3)
Total ...............	436 (47)	61 (8)	164 (55)	661 (110)

Causes of volcanic activity. The geographical location of volcanoes indicates a close relationship between belts of volcanic activity and mobile zones of the earth’s crust. The fractures formed in these zones are the channels along which magma moves to the earth’s surface. The movement of magma along fissures and tubelike channels toward the earth’s surface apparently takes place under the influence of tectonic processes. Deep down, when the pressure of the gases dissolved in the magma becomes greater than the over-lying layers, the gases begin to move quickly and draw the magma toward the surface. It is possible that gas pressure is created during crystallization of the magma, when its liquid part is enriched with residual gases and vapor. The magma boils, as it were, and high pressure is created in the chamber as a result of the intense discharge of gaseous substances; this may also be one of the causes of the eruption.

REFERENCES

Rittmann, A. Vulkany i ikh deiatel’nost’. Moscow, 1964. (Translated from German.)
Tazieff, H. Vulkany. Moscow, 1963. (Translated from French.)
Bullard, F. M. Volcanoes: In History, in Theory, in Eruption. [Austin] 1962.
Catalogue of the Active Volcanoes of the World Including Solfatara Fields, parts 1—. Naples, 1951—.

V. I. VLODAVETS

Volcanoes

vol·ca·no

Volcanoes

Volcanoes

Volcanoes

REFERENCES