Volcanoes

(Figure # 1) The crater lake

VOLCANOES (from the Latin word "vulcanus", which means the fire, flame, and originally means the god of fire within the Roman mythology * EN: volcano; DE: Vulkane; FR: volcans; ES: volcanes; RU: вулканы) are the geological formations, which emerge above the channels and cracks within the Earth's crust, through which the lavas, hot gases, and debris of the rock are erupted from the deep magmatic sources onto the Earth's surface.

The volcanoes of the Earth eject onto the surface at least 5-6 cubic kilometres of the volcanic material per year on average, the underwater volcanoes erupt approximately 80% of this material, and the terrestrial volcanoes erupt only 20% of this material. The most intensive ejection of the volcanic material (approximately 4 cubic kilometres per year) proceeds along the rift zones of the ridges at the middle of the oceans. The volcanism is manifested here in the form of the calm outpourings of the lava from the cracks at the depths of 3-4 kilometres, and is practically inaccessible for the immediate observation. The terrestrial volcanoes usually represent by themselves the individual cone-shaped mountains (the volcanic cones) with the central crater, which are formed of the products of the eruptions. The sizes of the volcanoes depend on their hypsometric situation. The maximal relative height (the elevation of the apex of the cone above the foundation) of the active volcanoes reaches 9 kilometres within the oceans, 6 kilometres within the island arcs, and 3 kilometres within the mountainous structures. The average height of the active volcanoes of the Earth is 1.75 kilometres, the volume is 85 cubic kilometres.

The classification. The volcanoes are sub-divided into active, potentially active, conditionally extinct, and extinct. There are categorized as active the volcanoes, which erupted or manifested the solfataric activity (the emission of the hot gases and water) during the last 3500 years of the historical period. The total quantity of these volcanoes is 947 (1980). There are categorized as potentially active the Holocene volcanoes, which erupted 3500-13500 years ago. The total quantity of the known volcanoes amounts to 1343. The volcanoes, which did not manifest the activity during the Holocene, but which have preserved their external shapes (with the age of younger than 100 thousand years), are considered as conditionally extinct. The volcanoes, which have been significantly re-worked with the erosion, semi-destructed, which did not manifest the activity during the last 100 thousand years, are called as extinct.

(Figure # 2) The southern cone within the Tolbachik dale

Depending on the shape of the supplying channels, they divide the volcanoes into central and cracked. According to the depth of the magma chambers, there are distinguished the volcanoes with the feeding of the mantle type (30-70 kilometres and more), of the crust type (5-45 kilometres), and of the mixed type. The products of the eruption of the volcanoes of the first type are represented by the basalts, of the second type, prevalently by the andesites, dacites, and liparites, and of the third type, by all the types of the volcanic rocks. Within the oceans, there are known only the volcanoes with the feeding of the mantle type (the Kilauea within the Hawaiian islands, Mount Teide on the Tenerife island, and others), while within the island arcs and continental cratons, there are known the volcanoes with the feeding of the mantle type, crust type, and mixed type (the Klyuchevskaya Sopka, Shiveluch, and Karymskaya Sopka on the Kamchatka peninsula, Mount Kilimanjaro within Africa, Mount Vesuvius within Italy, and others), and within the mountainous structures, there are known only the volcanoes with the feeding of the crust type (the Mount Elbrus at the Caucasus, Lassen Peak within the North America, and others).

The volcanic phenomena. The eruptions may be prolonged (during several years, decades, and centuries), and brief (which are measured in hours). There belong to the harbingers of the eruption the volcanic earthquakes, the acoustic phenomena, the changes of the magnetic field and of the composition of the fumarole gases, and other phenomena. The eruption of the volcanoes of the central type usually starts with the intensification of the ejections of the gases, initially with the dark cold debris of the lavas, and later with the incandescent debris. These ejections are in certain cases accompanied by the outpourings of the lava. The height of the rise of the gases, and of the vapours of the water, which are saturated with the ash and debris of the lavas, depending on the strength of the explosions, usually ranges from 1 to 5 kilometres (during the eruption of the Bezymianny volcano on the Kamchatka peninsula during the 1956, this height has reached 45 kilometres). The material , which has been ejected, is carried over the distances from several kilometres to tens of thousands of kilometres. The volume of the clastic material, which has been ejected, sometimes reaches several cubic kilometres.

The eruption of the volcanoes of the central type represents by itself the alternation of the weak and strong explosions with the outpourings of the lavas. The explosion of the maximal strength is called the culminating paroxysm. After these explosions, there proceeds the decreasing of the strength of the explosions, and the gradual ceasing of the eruptions. The quantities of the lava, which has been outpoured, are more than ten cubic kilometres. The eruption of the cracked type proceeds differently: there proceeds the calm outpouring of the lavas from the cracks, with the forming of the single or series of the small volcanoes (slag cones) along these cracks; there is characteristical the forming of the lava covers. There are the examples of the cracked eruption the Laki volcano within Iceland (during the 1783), and the Tolbachik volcano on the Kamchatka peninsula (1975-76).

(Figure # 3) The eruption of the volcano on the Kamchatka peninsula

The types of the eruptions. Depending on the quantitative proportion of the volcanic products, which are erupted (gaseous, liquid, and solid), and on the viscosity of the lavas, there have been distinguished four main types of the eruptions: effusive, mixed, extrusive, and explosive, or, as they call them more often, respectively, Hawaiian, Strombolian, Domed, and Vulcanian.

The Hawaiian type of the eruption, which creates most often the shield-shaped volcanoes, is distinguished with the relatively calm outpouring of the liquid (basaltic) lava, which forms within the craters the fiery-liquid lakes and lava flows. The gases, which are contained in the small quantity, form the fountains, which eject the lumps and blobs of the liquid lava, which are stretching during the flight as the thin glass fibers (the Kilauea within the Hawaiian islands).

In case of the Strombolian type of the eruptions, which usually creates the stratovolcanoes, together with the sufficiently abundant outpourings of the liquid basaltic and andesite-basaltic lavas (they sometimes form very long flows), there prevail the small explosions, which eject the pieces of the slag and diverse twisted and spindle-shaped bombs (the Stromboli on the Aeolian islands, Mount Mihara within Japan, and the certain eruptions of the Klyuchevskaya Sopka).

For the Domed type, there is characteristical the squeezing and pushing of the viscous (andesite, dacite, and rhyolite) lava by the strong pressure from the channel of the volcanoes, and the forming of the domes (the Puy-de-Dôme within the Auvergne region, France; the Central Semyachik on the Kamchatka peninsula), of the crypto-domes (the Shōwa-shinzan on the Hokkaido island, Japan), and of the obelisks (the Shiveluch on the Kamchatka peninsula).

In case of the Vulcanian type, there play the great role the gases, which are performing the explosions and the ejections of the huge clouds, which are overfilled with the large quantity of the debris of the rocks, lavas, and ash. The lavas are viscous (andesite, dacite, or rhyolite), they form the small flows (the Vulcano on the Aeolian islands, Avachinskaya Sopka and Karymskaya Sopka on the Kamchatka peninsula).

(Figure # 4) The lightning near to the stream of the ash and gas

Each of the main types of the eruptions is divided into several subtypes. Of these subtypes, there are especially distinguished Peléan and Katmaian, which are intermediate between the Domed and Vulcanian types. The characteristical peculiarity of the first subtype is the forming of the domes, and the directed explosions of the very hot clouds, which are overfilled with the debris and clumps of the lavas, which spontaneously explode during the flight and during the rolling down the slope (the Mount Pelée on the Martinique island). The eruptions of the Katmaian subtype are distinguished with the ejection of the very hot, very movable sandy flow (the Katmai within the Alaska state). The dome-forming eruptions are sometimes accompanied by the incandescent or sufficiently cooled avalanches, and also by the mud flows. The ultra-Vulcanian subtype is expressed as the very strong explosions, with the ejection of the huge quantity of the debris of the lavas and of the rocks from the walls of the channel.

The eruptions of the underwater volcanoes, which are situated at the great depths, are usually invisible, because the great pressure of the water hinders the explosive eruptions. At the lesser depths, the eruptions are manifested as the explosions (ejections) of the huge quantities of the steam and gases, which are overfilled with the pieces of the lava. The explosive eruptions continue till the time, when the material, which has been erupted, forms the island, which rises above the level of the sea. After this event, the explosions alternate with the outpourings of the lava.

The products of the eruptions of the volcanoes may be gaseous (see the "Volcanic gases" article), liquid (see the "Lava (mass)" article), and solid (see the "Volcanic rocks" article). Depending on the character of the eruptions, and on the composition of the magma, there are formed on the surface the structures with different shapes and heights. These structures represent by themselves the volcanic apparatuses, which comprise the tubular or cracked channel, the muzzle (the uppermost part of the channel), the thick accumulations of the lavas and volcanic-clastic products, which surround the channel from the different directions, and the crater (the bowl-shaped depression, which is situated on the apex of the structure).

 (Figure # 5) The lateral eruption of the Klyuchevskaya Sopka volcano

The most widespread shapes of the structures are cone-shaped (in case of the prevalence of the ejections of the clastic material), dome-shaped (in case of the squeezing of the viscous lava), and flat shield-shaped (in case of the prevalence of the outpourings of the liquid lava). The eruptions proceed not only through the apical main crater, but also through the lateral (parasitic) craters, which are situated on the slopes, and at the certain distance from these slopes. In case of the unique eruptions of the gases, which are piercing the channel to the Earth's surface, there are often formed the funnel-shaped depressions, which are fringed with the annular embankment of the clumps of the different rocks; such funnels, which are often filled with the water, are called the maars. The strong eruptions are sometimes accompanied by the collapses of the part of the volcanic structure, and often also of the adjacent terrain; the depressions, which are formed, with the diameter from sever al kilometres to the first tens of kilometres, are called the calderas.

The placement of the active volcanoes on the surface of the Earth. The modern volcanoes are known within all the large geological-structural elements and geographical regions of the Earth (see the Table). However, these volcanoes are distributed extremely unevenly. There is observed the direct dependence between their quantity and the tectonic activity of the region: the greatest quantity of the active volcanoes in case of the calculation per the unit of the area is accounted for the island arcs (the Kamchatka peninsula, Kuril islands, Indonesia, and others), and for the mountainous structures (the South America, and the North America). Here are also concentrated the most active volcanoes of the world, which are characterized by the highest frequency of the eruptions.

The lowest density of the volcanoes is characteristical for the oceans and continental cratons; here the volcanoes are mainly associated with the rift zones, namely, with the narrow and extended regions of the splits and subsidence of the Earth's crust. Such are, for example, the Eastern African rift system (the Mount Nyiragongo, and others), and the Mid-Atlantic ridge with the splits within the axial zone (Iceland).

(Figure # 6) The flow of the lava

The causes of the activity of the volcanoes. The geographic placement of the volcanoes indicates onto the tight association between the belts of the volcanic activity, and the dislocated movable zones of the Earth's crust. The rise of the magma through the asthenosphere and lithosphere onto the Earth's surface proceeds through the cracks and tubular channels, apparently, under the influence of the hydrostatic forces. Upon the reaching of the upper horizons of the Earth's crust and of the Earth's surface by the magmatic melts, the rapid emission of the volcanic gases also becomes the moving force for the process of the eruption.

Because the volcanoes represent the potential danger, there are conducted the systematic observations of their behavior, which permit to predict the next eruptions. The study of the character of the volcanoes includes the registration and analysis of the movements of the Earth's crust, of the earthquakes, of the changes of the composition of the gases, and of the electromagnetic anomalies.

(Table) The placement of the active volcanoes (by the January 1st, 1980)
The volcanic region The underwater volcanoes (*) The terrestrial volcanoes Total
with the identified dates of the eruptions with the approximate dates of the eruptions during the solfataras stage
The Mediterranean sea 9 10 2 6 27
The Caucasus, Turkey, Iran, Syria - 3 3 4 10
The Arabian peninsula, the Red sea 2 7 8 6 23
Africa - 20 3 26 49
The Indian ocean 1 4 - 1 6
The continental Asia (China, Siberia) - 6 1 - 7
The Kamchatka peninsula - 19 4 9 32
The Kuril islands 4 33 2 7 46
The Japanese islands, the Mariana islands, the Taiwan island 33 54 1 16 104
The Philippine islands 7 15 2 14 38
Indonesia 8 80 - 48 136
Melanesia 8 25 - 30 63
Samoa, Tonga, Kermadec, New Zealand 19 16 - - 35
The Antarctic, and the southern part of the Pacific ocean 3 1 - 7 11
The Hawaiian islands 3 4 - - 7
The Aleut islands, the Alaska state 5 45 7 12 69
The North America (Canada, USA) - 6 8 3 17
Mexico, the Revilla Gigedo islands 1 12 - 2 15
The Central America (Guatemala, Salvador, Nicaragua, Costa-Rica) 1 31 2 10 44
Peru, Bolivia, Chile, the San-Felix islands, the Huan-Fernandes islands 2 33 5 21 61
The Southern Shetland islands, the Southern Sandwich islands 1 7 - 6 14
The Atlantic ocean (the central and southern parts) 7 3 - - 10
The Lesser Antillean islands 2 10 3 3 18
The Canary and Azorean islands 10 10 - 2 22
Iceland and the Jan Mayen island 11 26 11 - 48
The Arctic ocean 1 - - - 1
Total: 139 503 65 240 947
(*) With the underwater solfataras.