Lava has been of interest to scientists for a long time. Its composition, temperature, flow rate, shape of hot and cooled surfaces are all subjects for serious research. After all, both erupting and frozen streams are the only sources of information about the state of the bowels of our planet, they constantly remind us of how hot and restless these bowels are. As for the ancient lavas, which have turned into characteristic rocks, the eyes of specialists are focused on them with special interest: perhaps, behind the bizarre relief, the secrets of disasters on a planetary scale are hidden.
What is lava? According to modern concepts, it comes from a chamber of molten material, which is located in the upper part of the mantle (the geosphere surrounding the Earth's core) at a depth of 50-150 km. While the melt is in the depths under high pressure, its composition is homogeneous. Approaching the surface, it begins to "boil", releasing gas bubbles that tend upward and, accordingly, move the substance along the cracks in the earth's crust. Not every melt, or magma, is destined to see the light. The same one that finds an outlet to the surface, pouring out into the most incredible forms, is precisely called lava. Why? Not quite clear. Essentially, magma and lava are the same thing. In the "lava" itself, one can hear both an "avalanche" and a "collapse", which, in general, corresponds to the observed facts: the leading edge of the flowing lava often really resembles a mountain collapse. Only from the volcano roll not cold cobblestones, but hot debris flying off the crust of the lava tongue.During the year, 4 km 3 of lava is poured out of the bowels, which is quite a bit, given the size of our planet. If this number were significantly larger, the processes of global climate change would have begun, which has happened more than once in the past. In recent years, scientists have been actively discussing the following disaster scenario for the end of the Cretaceous period, about 65 million years ago. Then, due to the final disintegration of Gondwana, in some places, the red-hot magma came too close to the surface and broke through in huge masses. Its especially abundant outcrops were on the Indian platform, covered with numerous faults up to 100 kilometers long. Almost a million cubic meters of lava flowed over an area of 1.5 million km 2. In some places, the covers reached a thickness of two kilometers, which is clearly visible from the geological sections of the Deccan plateau. Experts estimate that lava filled the territory for 30,000 years - fast enough for large portions of carbon dioxide and sulfur-containing gases to separate from the cooling melt, reach the stratosphere and cause a decrease in the ozone layer. The ensuing sharp climate change led to the mass extinction of animals at the border of the Mesozoic and Cenozoic eras. More than 45% of the genera of various organisms have disappeared from the Earth.
Not everyone accepts the hypothesis about the influence of the outflow of lava on the climate, but the facts are obvious: the global extinction of the fauna coincides in time with the formation of vast lava fields. So, 250 million years ago, when there was a mass extinction of all living things, the most powerful eruptions took place on the territory of Eastern Siberia. The area of lava sheets was 2.5 million km 2, and their total thickness in the Norilsk region reached three kilometers.
Black blood of the planetThe lavas that caused such large-scale events in the past are represented by the most common type on Earth - basalt. Their name indicates that they subsequently turned into a black and heavy rock - basalt. Basalt lavas are half composed of silicon dioxide (quartz), and half of aluminum oxide, iron, magnesium and other metals. It is the metals that provide the high temperature of the melt - more than 1200 ° C and the mobility - the basalt flow usually flows at a speed of about 2 m / s, which, however, should not be surprising: this is the average speed of a running person. In 1950, during the eruption of the Mauna Loa volcano in Hawaii, the fastest lava flow was measured: its leading edge moved through a rare forest at a speed of 2.8 m / s. When the path is laid, the next streams flow, so to speak, in hot pursuit, much faster. Merging, lava tongues form rivers, in the middle course of which the melt moves at a high speed - 10–18 m / s.
Basaltic lava flows are characterized by small thickness (first meters) and long length (tens of kilometers). The surface of the flowing basalt most often resembles a bundle of ropes stretched along the movement of the lava. It is called the Hawaiian word "pahoehoe", which, according to local geologists, does not mean anything other than a specific type of lava. The more viscous basalt flows form fields of sharp-angled, spike-like lava debris, also referred to as "aa-lavas" in the Hawaiian manner.
Basalt lavas are widespread not only on land, they are even more characteristic of the oceans. The bottom of the oceans are large basalt slabs 5-10 kilometers thick. According to the American geologist Joey Crisp, three quarters of all lavas erupting on Earth in a year are due to underwater eruptions. Basalts constantly flow from the cyclopean size of ridges that cut through the ocean floor and denote the boundaries of the lithospheric plates. No matter how slow the plate movement is, it is accompanied by strong seismic and volcanic activity on the ocean floor. Large masses of melt coming from ocean faults do not allow the plates to thin out, they are constantly increasing.
Submarine basaltic eruptions show us another type of lava surface. As soon as the next portion of lava splashes to the bottom and comes into contact with water, its surface cools down and takes the form of a drop - a "pillow". Hence the name - pillow lava, or pillow lava. Pillow lava is formed whenever a melt enters a cold environment. Often during a subglacial eruption, when the stream rolls into a river or other body of water, lava solidifies in the form of glass, which immediately bursts and crumbles into lamellar fragments.
Vast basalt fields (traps), hundreds of millions of years old, hide even more unusual shapes. Where ancient traps come to the surface, as, for example, in the cliffs of Siberian rivers, you can find rows of vertical 5- and 6-sided prisms. This is a columnar jointing, which is formed when a large mass of a homogeneous melt is slowly cooled. Basalt gradually decreases in volume and cracks along strictly defined planes. If the trap field, on the contrary, is exposed from above, then instead of the pillars, the surfaces are opened, as if paved with giant paving stones, - "giants' bridge". They are found on many lava plateaus, but the most famous are in the UK.
Neither the high temperature nor the hardness of the solidified lava serve as an obstacle to the penetration of life into it. In the early 90s of the last century, scientists found microorganisms that inhabit the basalt lava that poured out on the ocean floor. As soon as the melt cools down a little, microbes "gnaw" the passages in it and arrange colonies. They were discovered by the presence in the basalts of certain isotopes of carbon, nitrogen and phosphorus - typical products excreted by living things.
The more silica in the lava, the more viscous it is. The so-called medium lavas with a silicon dioxide content of 53–62% are no longer flowing as fast and not as hot as basaltic ones. Their temperature ranges from 800-900 ° C, and the flow rate is several meters per day. The increased viscosity of lava, or rather magma, since the melt acquires all the basic properties even at depth, radically changes the behavior of the volcano. It is more difficult to release the gas bubbles accumulated in it from viscous magma. On approaching the surface, the pressure inside the bubbles in the melt exceeds the pressure on them outside, and the gases are released explosively.
Typically, a crust forms at the leading edge of the more viscous lava tongue, which cracks and crumbles. The fragments are immediately crushed by the hot mass pressing behind them, but they do not have time to dissolve in it, but freeze like bricks in concrete, forming a rock of a characteristic structure - lavobreccia. Even after tens of millions of years, lavobreccia retains its structure and indicates that a volcanic eruption once took place in this place.In the center of Oregon, USA, there is the Newberry volcano, which is interesting for its intermediate lavas. It was last activated more than a thousand years ago, and at the final stage of the eruption, before falling asleep, a lava tongue 1,800 meters long and about two meters thick emerged from the volcano, solidified in the form of pure obsidian - black volcanic glass. Such glass is obtained when the melt cools quickly, without having time to crystallize. In addition, obsidian is often found at the periphery of a lava flow, which cools faster. Over time, crystals begin to grow in the glass, and it turns into one of the rocks of an acidic or medium composition. That is why obsidian is found only among relatively young products of eruption; it is no longer in ancient volcanics.
From fucking fingers to fiamme
If the amount of silica occupies more than 63% of the composition, the melt becomes quite viscous and clumsy. Most often, such lava, called acidic, is generally incapable of flowing and solidifies in the supply channel or is squeezed out of the vent in the form of obelisks, "devil's fingers", towers and columns. If acid magma still manages to reach the surface and pour out, its flows move extremely slowly, several centimeters, sometimes meters per hour.
Unusual rocks are associated with acidic melts. For example, ignimbrites. When the acidic melt in the near-surface chamber is saturated with gases, it becomes extremely mobile and is quickly ejected from the vent, and then, together with tuffs and ash, flows back into the cavity formed after the ejection - the caldera. Over time, this mixture hardens and crystallizes, and against the gray background of the rock, large lenses of dark glass are clearly distinguished in the form of irregular patches, sparks or tongues of flame, which is why they are called "fyamme". These are traces of separation of the acidic melt when it was still underground.
Sometimes acidic lava is so strongly saturated with gases that it literally boils and becomes pumice. Pumice is a very light material, with a lower density than water, so it happens that after underwater eruptions, sailors observe whole fields of floating pumice in the ocean.
Many questions related to lavas remain unanswered. For example, why lavas of different composition can flow from the same volcano, as, for example, in Kamchatka. But if in this case there are at least convincing assumptions, then the appearance of carbonate lava remains a complete mystery. It, half composed of sodium and potassium carbonates, is erupted at the present time by the only volcano on Earth - Oldoinyo Lengai in Northern Tanzania. The melt temperature is 510 ° C. It is the coldest and liquid lava in the world, it flows like water on the earth. The color of hot lava is black or dark brown, but after a few hours of exposure to air, the carbonate melt brightens, and after a few months it becomes almost white. Solidified carbonate lavas are soft and brittle, easily dissolve in water, which is probably why geologists do not find traces of similar eruptions in ancient times.Lava plays a key role in one of the most pressing problems of geology - what heats the bowels of the Earth. What causes pockets of molten material in the mantle, which rise upward, melt the earth's crust and generate volcanoes? Lava is only a small part of a powerful planetary process, the springs of which are hidden deep underground.
Ecology
Volcanoes on our planet are geological formations on the earth's crust.
From here magma comes out to the surface of the earth , which forms lava, as well as volcanic gases, rocks and mixtures of gas, volcanic ash and rocks. Such mixtures are called pyroclastic flows.
It is worth noting that the very word "volcano" came to us from Ancient Rome, where the god of fire was called Vulcan.
A lot of interesting things are known about volcanoes, and below you can find a few facts about them.
25. Strongest volcanic eruption (Indonesia)
Of all the documented volcanic eruptions, the largest was recorded at the Tambora stratovolcano on the island of Sumbawa, Indonesia, in 1815.
In terms of volcanic explosiveness, the force of the eruption reached 7 points (out of 8).
This eruption lowered the average temperature on Earth by 2.5 ° C over the next year, which was called the "year without summer."
It should be noted that the volume of emissions into the atmosphere was approximately 150-180 cubic meters. km.
24. Long-term effects of a volcanic eruption
Gas and other particles released into the atmosphere during the eruption of Mount Pinatubo on Luzon Island, Philippines, in 1991, lowered the global temperature by about 0.5 degrees Celsius over the next year.
23. Lots of volcanic ash
During the 1991 eruption of Mount Pinatubo, 5 cubic kilometers of volcanic material was thrown into the air, creating a column of ash 35 km high.
22. Big volcano explosion
The largest explosion of the 20th century occurred in 1912 during the eruption of Novarupta, one of the Alaska volcano chain - part of the Pacific Ring of Fire. The force of the eruption reached 6 points.
21. The prolonged eruption of Kilauea
One of the most active volcanoes on Earth, Hawaiian Kilauea has been erupting continuously since January 1983.
20. Deadly volcanic eruption
The colossal magma chamber that was inside the Taupo volcano continued to fill up for a very long time, and finally the volcano exploded.
After the eruption in April 1815, the force of which reached 7 points, from 150 to 180 cubic meters were thrown into the air. km of volcanic material.
Volcanic ash also flooded the remote islands, resulting in a huge number of deaths. Their number was approximately 71,000. About 12,000 people died directly from the eruption, while the rest died as a result of hunger and disease, which were the result of eruptive fallout.
19. Big mountains
18. Active volcanoes today
The Hawaiian volcano Mauna Loa is the largest active volcano in the world, rising 4,1769 meters above sea level. Its relative height ( from the ocean floor) - 10 168 meters. Its volume is about 75,000 cubic kilometers.
17. The surface of the earth covered with volcanoes
More than 80 percent of the Earth's surface above and below sea level is of volcanic origin.
16. Ash everywhere (St. Helens volcano)
During the eruption of the St Helens stratovolcano in 1980, about 540 million tons of ash covered an area in excess of 57,000 square kilometers. km.
15. Disaster from the volcano - landslides
The eruptions of St. Helens led to the largest landslides on Earth. As a result of this eruption, the height of the volcano decreased by 400 meters.
14. Eruptions of an underwater volcano
The deepest recorded volcanic eruption occurred in 2008 at a depth of 1,200 meters.
The reason was the West Mata volcano, located in the Lau Basin near the Fiji Islands.
13. Lava lakes of the volcano in Antarctica
The southernmost active volcano is Erebus, located in Antarctica. It should be noted that the lava lake of this volcano is the rarest phenomenon on our planet.
Only 3 volcanoes on Earth can boast of "non-healing" lava lakes - Erebus, Kilauea in Hawaii and Nyiragongo in Africa. And yet, the lake of fire in the midst of eternal snow is truly an impressive phenomenon.
12. High temperature (what comes out during a volcanic eruption)
The temperature inside a pyroclastic flow - a mixture of high-temperature volcanic gases, ash and rocks that forms during a volcanic eruption - can exceed 500 degrees Celsius. This is enough to burn and carbonize the wood.
11. The first in history (Nabro volcano)
On June 12, 2011, the active volcano Nabro, located in the southern Red Sea, near the borders of Eritrea and Ethiopia, woke up for the first time. According to NASA, this was his first recorded eruption.
10. Volcanoes of the Earth
There are about 1,500 volcanoes on Earth, not counting the long volcanic belt on the ocean floor.
9. Pele's tears and hair (parts of the volcano)
Kilauea is mythically home to Pele, the Hawaiian goddess of volcanoes.
Pele's Tears
Several lava formations have been named after her, including "Pele's Tears" (small drops of lava cooled in the air) and "Pele's Hair" (a spray of lava cooled by the wind).
Pele Hair
8. Supervolcano
Modern man could not witness the eruption of a supervolcano (8 points), which is capable of changing the climate on Earth.
The last eruption occurred about 74,000 years ago in Indonesia. In total, there are about 20 supervolcanoes on our planet known to scientists. It should be noted that, on average, such a volcano erupts once every 100,000 years.
»Lava movement
The speed of lava movement is different, depending on its density and the slope of the terrain where it makes its way. Relatively small lava flows pouring down steep slopes move forward extremely rapidly; the stream ejected by Vesuvius on August 12, 1805, rushed along the steep slopes of the cone at an amazing speed and in the first four minutes made 5 ½ km, and in 1631 another stream of the same volcano reached the sea within one hour, i.e. passed at this time 8 km. Especially liquid lavas are distinguished by the open basaltic volcanoes of Hawaii; they are so mobile that they form real lavas on the cliffs and can move at the most insignificant slope of the soil, even in G. It was repeatedly observed how these lavas passed 10-20 and even 30 km per hour. But such speed of movement is in any case one of the exceptions; even the lava observed by Scrop in 1822 and which within 15 minutes managed to descend from the edge of the Vesuvius crater to the foot of the cone is far from ordinary. On Etna, the movement of lava is already considered fast if it occurs at a speed of 1 km in 2-3 hours. Usually lava moves even more slowly and in some cases only travels 1 m per hour.
The lava flowing from the volcano in a molten state has a white-glowing sheen and retains it for a long time inside the crater: this can be clearly seen where, thanks to cracks, deep parts of the stream are exposed. Outside the crater, the lava cools rapidly, and the stream quickly becomes covered with a hard crust consisting of a dark slag mass; within a short time, it becomes so strong that a person can calmly walk on it; sometimes, along such a crust, still covering a moving stream, one can climb to the place where the lava flows out. The solid slag crust forms something like a pipe, inside which a liquid mass moves. The front end of the lava flow is also covered with black hard crust; with further movement, the lava presses this crust to the ground and flows further along it, being covered in front with a new slag shell. This phenomenon does not take place only when the lava is moving very quickly; in other cases, by dumping and moving slags, a layer of solidified lava is formed, along which the flow moves. The latter is a rare sight: the front part of it is compared by Pullet Scrop to a huge heap of coals, which, under the influence of some pressure from behind, are piled up on top of each other. Its movement is accompanied by a noise similar to the ringing of pouring metal; this noise occurs due to the friction of individual lava lumps, their fragmentation and contraction.
The hard crust of a lava flow is usually not a flat surface; it is covered with many cracks, through which liquid lava sometimes flows; the blocks formed as a result of the fragmentation of the original cover collide with each other, like ice floes during an ice drift. It is difficult to imagine a wilder and sullen picture than the one presented to us by the outer surface of a blocky lava flow. Even more peculiar is the form of the so-called undulating lava, which is observed less often, but is well known to every visitor to Vesuvius. The road from Rezina to the observatory is laid over a considerable length along such a lava; the latter was thrown out by Vesuvius in 1855. The cover of such streams is not broken into pieces, but is a solid mass, the uneven surface of which in its peculiar appearance resembles intestinal plexus.
Volcanic activity, one of the most formidable natural phenomena, often brings enormous disasters to people and the national economy. Therefore, it must be borne in mind that although not all active volcanoes cause misfortune, nevertheless, each of them can be to one degree or another a source of negative events, volcanic eruptions are of varying strength, however, only those that are accompanied by the death of people are catastrophic. and material values.
General understanding of volcanism
"Volcanism is a phenomenon due to which, during geological history, the outer shells of the Earth - crust, hydrosphere and atmosphere, that is, the habitat of living organisms - the biosphere were formed." This opinion is expressed by the majority of volcanologists, but this is far from the only idea of the development of the geographic envelope. Volcanism covers all phenomena associated with the eruption of magma to the surface. When magma is deep in the earth's crust under high pressure, all of its gas components remain in a dissolved state. As the magma moves to the surface, the pressure decreases, gases begin to evolve, as a result, the magma poured onto the surface is significantly different from the original. To emphasize this difference, the magma that poured out to the surface is called lava. The eruption process is called eruptive activity.
Fig. 1. Mount St. Helens eruption
Volcanic eruptions are not the same, depending on the composition of the eruption products. In some cases, eruptions proceed calmly, gases are released without large explosions, and liquid lava flows freely onto the surface. In other cases, eruptions are very violent, accompanied by powerful gas explosions and squeezing out or outpouring of relatively viscous lava. The eruptions of some volcanoes consist only in grandiose gas explosions, as a result of which colossal clouds of gas and water vapor, saturated with lava, rise to great heights. According to modern concepts, volcanism is an external, so-called effusive form of magmatism - a process associated with the movement of magma from the interior of the Earth to its surface.
At a depth of 50 to 350 km, in the thickness of our planet, foci of molten matter - magma are formed. Along the sites of crushing and faults of the earth's crust, magma rises and pours out to the surface in the form of lava (differs from magma in that it contains almost no volatile components, which, when pressure drops, are separated from magma and go into the atmosphere. , volcanoes-mountains, composed of lavas and their dispersed particles - pyroclasts According to the content of the main component - magma silicon oxide and volcanic rocks formed by them - volcanics are divided into ultrabasic (silicon oxide less than 40%), basic (40-52%), medium ( 52-65%), acidic (65-75%) The most widespread is basic, or basaltic, magma.
Types of volcanoes, lava composition. Classification by the nature of the eruption
The classification of volcanoes is based mainly on the nature of their eruptions and on the structure of volcanic apparatus. And the nature of the eruption, in turn, is determined by the composition of the lava, the degree of its viscosity and mobility, temperature, and the amount of gases contained in it. Three processes are manifested in volcanic eruptions: 1) effusive - the outpouring of lava and its spreading over the earth's surface; 2) explosive (explosive) - explosion and release of a large amount of pyroclastic material (solid eruption products); 3) extrusive - squeezing, or squeezing, magmatic matter onto the surface in a liquid or solid state. In a number of cases, mutual transitions of these processes and their complex combination with each other are observed. As a result, many volcanoes are characterized by a mixed type of eruption - explosive-effusive, extrusive-explosive, and sometimes one type of eruption is replaced by another in time. Depending on the nature of the eruption, the complexity and variety of volcanic edifices and forms of occurrence of volcanic material are noted. Among volcanic eruptions, the following are distinguished: eruptions of the central type, fissure and areal.
Fig. 2. Hawaiian type eruption
1 - Ash plume, 2 - Lava fountain, 3 - Crater, 4 - Lava lake, 5 - Fumaroles, 6 - Lava flow, 7 - Lava and ash layers, 8 - Rock layer, 9 - Sill, 10 - Magma channel, 11 - Magma chamber, 12 - Dyke
Volcanoes are of the central type. They have a shape close to rounded in plan, and are represented by cones, shields, domes. At the top, there is usually a bowl-shaped or funnel-shaped depression called a crater (Greek 'crater' bowl). From the crater into the depths of the earth's crust, there is a magma supply channel, or a volcanic vent, which has a pipe-like shape, along which magma from a deep chamber rises to the surface. Among the volcanoes of the central type, polygenic, formed as a result of repeated eruptions, and monogenic - once manifested their activity, stand out.
Polygenic volcanoes. These include most of the known volcanoes in the world. There is no unified and generally accepted classification of polygenic volcanoes. Various types of eruptions are most often designated by the name of the famous volcanoes in which this or that process manifests itself most characteristically. Effusive, or lava, volcanoes. The predominant process in these volcanoes is effusion, or the outpouring of lava onto the surface and its movement in the form of flows along the slopes of a volcanic mountain. The volcanoes of the Hawaiian Islands, Samoa, Iceland, etc. can be cited as examples of this nature of the eruption.
Fig. 3. Plinian type of eruption
1 - Ash plume, 2 - Magma channel, 3 - Volcanic ash rain, 4 - Lava and ash layers, 5 - Rock layer, 6 - Magma chamber
Hawaiian type. Hawaii is formed by the merged peaks of five volcanoes, of which four were active in historical time (Fig. 2). The activity of two volcanoes is especially well studied: Mauna Loa, which rises almost 4200 meters above the Pacific Ocean level, and Kilauea, which is more than 1200 meters high. The lava in these volcanoes is basic, basaltic, easily mobile, high-temperature (about 12,000). In the crater lake, lava boils all the time, its level goes down and up. During eruptions, lava rises, its mobility increases, it floods the entire crater, forming a huge boiling lake. Gases are released relatively calmly, forming splashes above the crater, lava fountains, rising in height from several to hundreds of meters (rarely). The gas-foamed lava is sprayed and solidified in the form of thin glass filaments ‘Pele's hair’. Then the crater lake overflows and lava begins to overflow over its edges and flow down the slopes of the volcano in the form of large flows.
Effusive underwater. Eruptions are the most numerous and least studied. They are also confined to rift structures and are distinguished by the dominance of basaltic lavas. At the bottom of the ocean at a depth of 2 km or more, the water pressure is so great that no explosions occur, which means that no pyroclasts appear. Under water pressure, even liquid basaltic lava does not spread far, it forms short domed bodies or narrow and long streams covered from the surface with a glassy crust. A distinctive feature of underwater volcanoes located at great depths is the abundant release of fluids containing high amounts of copper, lead, zinc and other non-ferrous metals.
Mixed explosive-effusive (gas-explosive-lava) volcanoes. Examples of such volcanoes are the volcanoes of Italy: Etna - the highest volcano in Europe (over 3263 m), located on the island of Sicily; Vesuvius (about 1200 m high), located near Naples; Stromboli and Vulcano from the Aeolian Islands group in the Strait of Messina. Many volcanoes of Kamchatka, the Kuril and Japanese islands and the western part of the Cordillera mobile belt belong to the same category. The lavas of these volcanoes are different - from basic (basaltic), andesite-basaltic, andesite to acidic (liparite). Several types are conventionally distinguished among them.
Fig. 4. Ice type eruptions
1 - Water vapor cloud, 2 - Lake, 3 - Ice, 4 - Lava and ash layers, 5 - Rock layer, 6 - Ball lava, 7 - Magma channel, 8 - Magma chamber, 9 - Dyke
Strombolian type. It is characteristic of the Stromboli volcano, which rises in the Mediterranean Sea to an altitude of 900 m. The lava of this volcano is mainly of basalt composition, but it has a lower temperature (1000-1100) than the lava of the volcanoes of the Hawaiian islands, therefore it is less mobile and saturated with gases. Eruptions occur rhythmically at regular intervals, ranging from a few minutes to an hour. Gas explosions throw hot lava to a relatively small height, which then falls on the slopes of the volcano in the form of spirally wound bombs and slag (porous, bubbly lava lumps). It is characteristic that very little ash is thrown out. A cone-shaped volcanic apparatus consists of layers of slag and solidified lava. Such a well-known volcano as Isalco belongs to the same type.
Volcanoes are explosive (gas-explosive) and extrusive-explosive. This category includes many volcanoes, in which large gas explosions are predominant, with the release of a large amount of solid eruption products, almost without the outpouring of lava (or in limited sizes). This nature of the eruption is associated with the composition of the lavas, their viscosity, relatively low mobility and high gas saturation. In a number of volcanoes, gas explosive and extrusive processes are simultaneously observed, which are expressed in the squeezing of viscous lava and the formation of domes and obelisks towering over the crater.
Peleus type. It was especially clearly manifested in the Mont Pele volcano on the island. Martinique, part of the Lesser Antilles group. The lava of this volcano is predominantly medium, andesitic, highly viscous and gas-rich. When solidified, it forms a solid plug in the volcano's vent, which prevents the free escape of gas, which, accumulating under it, creates very high pressures. Lava is squeezed out in the form of obelisks, domes. Eruptions occur as violent explosions. Huge clouds of gases are formed, oversaturated with lava. These incandescent (with temperatures over 700-800) gas-ash avalanches do not rise high, but slide down the slopes of the volcano at high speed and destroy all living things on their way.
Fig. 5. Volcanic activity in Anak Krakatau, 2008
Krakatoa type. Dedicated by the name of the Krakatau volcano, located in the Sunda Strait between Java and Sumatra. This island consisted of three intergrown volcanic cones. The oldest of them, Rakata, is composed of basalts, and the other two, younger ones, are composed of andesites. These three merged volcanoes are located in an ancient vast underwater caldera formed in prehistoric times. Until 1883, Krakatoa was not active for 20 years. In 1883, one of the largest catastrophic eruptions occurred. It began with explosions of moderate intensity in May, after some breaks, it resumed again in June, July, August with a gradual increase in intensity. On August 26, there were two big explosions. On the morning of August 27, a giant explosion occurred, which was heard in Australia and on the islands in the western part of the Indian Ocean at a distance of 4000-5000 km. An incandescent gas and ash cloud rose to a height of about 80 km. Huge waves up to 30 m high, arising from the explosion and shaking of the Earth, called tsunamis, caused great destruction on the adjacent islands of Indonesia, they washed away about 36 thousand people from the shores of Java and Sumatra. In places of destruction and human casualties were associated with a blast wave of enormous force.
Katmay type. It is distinguished by the name of one of the large volcanoes in Alaska, near the base of which in 1912 there was a large gas-explosive eruption and a directed release of avalanches, or streams, of a hot gas-pyroclastic mixture. The pyroclastic material was acidic, rhyolitic, or andesite-rhyolite. This incandescent ash-gas mixture filled a 23 km deep valley located northwest of the foot of Mount Katmai. In place of the former valley, a flat plain about 4 km wide was formed. For many years, from the stream that filled it, massive emissions of high-temperature fumaroles were observed, which served as the basis for calling it "The Valley of Ten Thousand Smokes".
Ice type of eruptions(Fig. 4) is possible when the volcano is under ice or an entire glacier. Such eruptions are dangerous because they provoke the most powerful floods, as well as their ball lava. Until now, only five such eruptions are known, that is, they are a very rare phenomenon.
Monogenic volcanoes
Maar type. This type unites only once erupted volcanoes, now extinct explosive volcanoes. In relief, they are represented by flat saucer-shaped hollows framed by low ramparts. The shafts contain both volcanic slags and fragments of non-volcanic rocks that make up this territory. In vertical section, the crater has the form of a funnel, which in the lower part is connected to a pipe-like vent, or explosion tube. These include volcanoes of the central type, formed during a single eruption. These are gas-explosive eruptions, sometimes accompanied by effusive or extrusive processes. As a result, small slag or slag-lava cones (from tens to a few hundred meters in height) with a saucer-shaped or bowl-shaped crater depression are formed on the surface.
Such numerous monogenic volcanoes are observed in large numbers on the slopes or at the foot of large polygenic volcanoes. Monogenic forms also include gas explosive funnels with a supply pipe-like channel (throat). They are formed by one gas explosion of great strength. Diamond pipes belong to a special category. Explosion tubes in South Africa are widely known as diatremes (Greek "dia" - through, "tram" - hole, hole). Their diameter ranges from 25 to 800 meters, they are filled with a kind of brecciated volcanic rock called kimberlite (after the city of Kimberley in South Africa). This rock contains ultrabasic rocks - garnet-bearing peridotites (pyrope is a diamond satellite), which are characteristic of the Earth's upper mantle. This indicates the undercurrent formation of magma and its rapid rise to the surface, accompanied by gas explosions.
Fissure eruptions
They are confined to large faults and cracks in the earth's crust, which play the role of magma conduits. The eruption, especially in the early phases, can occur along the entire mother-in-law or in certain areas of her sections. Subsequently, along the fault line or cracks, groups of close volcanic centers arise. The poured out main lava, after solidification, forms basalt covers of various sizes with an almost horizontal surface. In historical time, similar powerful fissure outpourings of basaltic lava were observed in Iceland. Fissure eruptions are widespread on the slopes of large volcanoes. They are apparently widespread within the faults of the East Pacific Rise and in other mobile zones of the World Ocean. Especially significant fissure eruptions were in the past geological periods, when thick lava sheets were formed.
Areal type of eruption. This type includes massive eruptions from numerous closely spaced volcanoes of the central type. They are often confined to small cracks, or the nodes of their intersection. In the process of eruption, some centers die off, while others arise. The areal type of eruption sometimes covers vast areas, where the products of the eruption merge, forming continuous covers.
During the eruption of volcanoes, a hot melt of rocks - magma - is poured out. In air, the pressure drops sharply, and the magma boils - gases leave it.
The melt starts to cool. In fact, it is only these two properties - temperature and “carbonation” - that lava differs from magma. For a year, 4 km³ of lava is poured across our planet, mainly at the bottom of the oceans. Not so many, on land there were regions filled with a lava layer 2 km thick.
The initial temperature of the lava is 700–1200 ° C and higher. Dozens of minerals and rocks are melted in it. They include almost all known chemical elements, but most of all silicon, oxygen, magnesium, iron, aluminum.
Depending on the temperature and composition, lava comes in different colors, viscosity and fluidity. Hot, she is shiny bright yellow and orange; cooling down, it becomes red, and then black. It happens that blue lights of burning sulfur are running over the lava flow. And one of the volcanoes of Tanzania spews out black lava, which solidifies, becomes like chalk - whitish, soft and brittle.
The flow of viscous lava is clumsy, it flows barely (a few centimeters or meters per hour). Along the way, solidifying blocks are formed in it. They slow down the movement even more. Such lava freezes in mounds. But the absence of silicon dioxide (quartz) in lava makes it very liquid. It quickly covers vast fields, forms lava lakes, rivers with a flat surface, and even "lava falls" on cliffs. There are few pores in such lava, since gas bubbles easily leave it.
What happens when lava cools?
As the lava cools, the molten minerals begin to form crystals. The result is a mass of compressed grains of quartz, mica and others. They can be large (granite) or small (basalt). If the cooling went very quickly, a homogeneous mass is obtained, similar to black or dark greenish glass (obsidian). 
Gas bubbles often leave many small cavities in the viscous lava; this is how pumice is formed. Different layers of cooling lava flow down the slopes at different speeds. Therefore, long wide voids are formed inside the stream. The length of such tunnels sometimes reaches 15 km.
Slowly cooling lava forms a hard crust on the surface. It immediately slows down the cooling of the mass lying below, and the lava continues to move. In general, cooling depends on the massiveness of the lava, initial heating and composition. There are cases when even after a few years (!) The lava still continued to crawl and ignited the branches stuck into it. Iceland's two powerful lava flows remained warm centuries after the eruption.
The lava of underwater volcanoes usually solidifies in the form of massive "pillows". Due to the rapid cooling, a strong crust forms on their surface very quickly, and sometimes gases burst them from the inside. The fragments are scattered at a distance of several meters.
Why is lava dangerous for people?
The main danger of lava is its high temperature. She literally burns living beings and buildings on the way. A living thing perishes, without even coming into contact with it, from the heat with which it radiates. True, high viscosity restrains the flow rate, allowing people to be saved, to preserve values.
But liquid lava ... It moves quickly and can cut off the path to salvation. In 1977, during the night eruption of the Nyirango volcano in Central Africa. The explosion split the crater wall, and lava gushed out in a wide stream. Very fluid, it rushed at a speed of 17 meters per second (!) And destroyed several sleeping villages with hundreds of inhabitants.
The damaging effect of lava is aggravated by the fact that it often carries clouds of poisonous gases released from it, a thick layer of ash and stones. It was such a stream that destroyed the ancient Roman cities of Pompeii and Herculaneum. A catastrophe can turn into a meeting of a hot lava with a reservoir - the instantaneous evaporation of a mass of water causes an explosion. 
Deep cracks and dips form in the streams, so one must walk carefully on cold lava. Especially if it is glassy - sharp edges and debris hurt painfully. The fragments of the cooling underwater "pillows" described above can also injure overly curious divers.
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