Essentially, a volcano is a hole in the earth's crust. When a volcano erupts from the depths of the Earth, hot rocks erupt to the surface through this hole. Volcanoes that are often active are called active. Volcanoes that may become active in the future are called dormant. An extinct volcano is a volcano whose activity has ceased forever.
Where are the volcanoes?
There are approximately 840 active volcanoes in the world. Usually there are only 20-30 eruptions per year. Most volcanoes are located near the edges of the giant plates that together make up the outer layers of the Earth. An earthquake occurs every 30 seconds in the world, and only a few of them pose a real danger.
Structure of the volcano
For those who want to find out what the volcano is made of, we advise you to study the following images in detail and carefully:
What is the largest volcano in the world?
The largest volcano in the world is Mauna Loa in Hawaii in the USA, the dome of which is 120 km long and 50 km wide. Volcano Lo'ihi is an active volcano off the Hawaiian Islands. It goes under water for 900 m and will rise to the surface in the period from 10 thousand to 100 thousand years. You can see this volcano in the photo below:
What are high-speed waves called?
Speed waves are deep seismic waves traveling through the earth at speeds of thousands of km/h. They are much faster than sound.
What is the greatest lava?
In Iceland in 1783 there was a very strong fissure eruption. At the same time, the red-hot one spread over a distance of 65-70 km.When did people walk on the sea?
The Kat Mai volcano in Alaska, USA, erupted so much floating pumice in 1912 that people walked on the sea.
How many active volcanoes are there on earth?
There are currently approximately 1,300 active volcanoes on land. There are also many of them under water, but their number fluctuates, as some cease their activity, while others arise. Every dormant volcano can suddenly explode. Consequently, those volcanoes that have been active at least once over the past 10 thousand years are considered active.
What is a volcanic eruption? Volcanic eruptions are a series of cannon-like explosions. They continue at intervals of hours and minutes, and occur as a result of the accumulation of a large volume of gas under the lava. During such eruptions, parts of the crater fly off, the size of which can reach the size of a bus.
What is a Plinian eruption?
When the hot gas is saturated with gas and fills the volcano, its crater explodes, throwing it out at twice the speed. The eruption is so violent that the magma breaks up into tiny pieces, and within a few hours the ground may be covered in a layer of ash. The eruption in 79 had the same character. At the same time, the Roman writer Pliny could not escape, so this type of eruption is Plinian.What is the Stomboli eruption?
If the magma is liquid enough, a crust can form above the lava lake in the volcano's crater. At the same time, large gas bubbles float out and explode the shell, splashing out volcanic bombs from the floor of molten lava and lava debris. This type of eruption is Strombolian from the Italian volcanic island of Stromboli.What was the most powerful volcanic eruption?
The most powerful volcanic eruption occurred approximately 20 thousand years ago, when the Toba volcano raged on the island of Sumatra in Indonesia. A 100 km crater formed in its center, and the other part of the island was buried under a layer of volcanic rock more than 300 m thick.
Why did Pompeii perish?
Throughout human history, volcanoes have been dangerous for people living near them. In 79 AD, the Roman city of Pompeii was razed to the ground by the erupting volcano Vesuvius. Even today the strongest eruptions cause harm to people.
When did the legend of Atlantis originate?
Around 1645 BC. e. The Greek island of Santorini exploded. As a result, the Minoan civilization was destroyed. This fact served as the beginning of the legend about the missing continent of Atlantis.
Useful information about volcanoes, geysers, photos of volcanoes
The most dangerous and unpredictable objects on the earth's surface are volcanoes- geological formations that arise above cracks in the earth’s crust, through which hot magma erupts into the earth, burning all living things in its path, hot and fragments of rocks.
In this case, volcanoes are divided into active fallen asleep and extinguished. Erupted magma is called lava. At times it slowly pours out of cracks, and at other times the volcano erupts in an explosion of steam, ash, dust and volcanic ash. It is these processes that lead to consequences that do not benefit people. Man today has no means to resist a volcanic eruption other than escape.
What are pyroclastic flows? When a volcano's crater is exposed, it breaks up the rocks and creates enormous amounts of debris, ash and pumice - pyroclastic materials. During eruptions, they are the first to rise up the vent. After the hole expands, magma begins to pour out of it. In this case, the pyroclastic cloud becomes so thick that it cannot mix with the air to rise upward. Because of this, it flows out as hot pyroclastic flows that move at enormous speeds reaching 200 km/h. They can cover the territory with eruption products.
What types of volcanoes are there?
Where tectonic plates move apart, magma flows through the gaps, forming fissure volcanoes. Rapidly solidified thick lava forms mound volcanoes. During powerful volcanic eruptions, a caldera crater occurs. Water often flows into it, and then a lake is formed. The most specific are stratovolcanoes, which are composed alternately of layers of lava and ash.
Lava erupting from focal and fissure volcanoes is usually fluid. As it cools, it creates basaltic rocks such as basalt, gabbro and dolerite. In situ it becomes rocks such as andesite, trachyte and rhyolite.
Formations from volcanic eruptions
Basalt columns. A dense flow of lava, when hardened, can break into hexagonal basalt columns, reminiscent of those at the Great Dyke in Northern Ireland.
Pahoehoe lava. Sometimes rocks on the surface quickly harden, creating a thin crust over the still viscous and hot lava. If the crust is several centimeters thick, then it cools down to such an extent that you can walk on it. However, if the lava continues to flow, the crust begins to wrinkle. The Hawaiians nicknamed this lava “pahoehoe,” which means “wavy.”
Lava aa. If lava rapidly solidifies into a rough mass, it is called "aa". During underwater volcanic eruptions, such as at mid-ocean ridges, the water instantly cools and breaks the lava into small, smooth particles called “pillows.”
Focal volcanoes. Most volcanoes lie along crustal plate boundaries, as they sit above a single accumulation of magma flowing to the surface. Even when the plate moves, such a source continues to remain in place, burning and burning through it at various points, forming a chain of volcanoes.
What kind of lava can volcanoes have?
Volcanoes can erupt lava of two types: aa-lavu And wavy lava.
Aa-lava is thicker and petrifies sharp rocks - volcanic slag.
Wavy lava is lava that is more fluid and rich in gases. When hardened, it creates rocks with a smooth surface, and sometimes flows down to form long stalactites. The clouds of ash emitted are lava powder.
How geysers appear
Hot spots and geysers are formed by boiling magma. When it leaks, rainwater seeps underground and encounters hot magma. Due to the pressure, its temperature will increase, and then the magma will rise again. If, when rising up, hot water mixes with cold water, it flows to the surface in the form of hot water. If it encounters an obstacle on its way, it remains under pressure and then splashes out in a strong stream called a geyser.
Eruption force
volcanoes can explode more powerfully than an atomic bomb. As a rule, this happens if the magma thickens and becomes so viscous that it plugs the volcano's mouth. Inside it, the pressure gradually increases until the magma dislodges such a plug. The strength of the eruptions is measured by the amount of ash that was thrown into the air. As magma flows underground, it takes on a variety of forms thanks to rocks. Typically, flowing magma flows into cracks within rocks, a process called conformable intrusion. In this case, saucer-shaped rocks are formed, such as lopoliths, lens-shaped ones - phacolites, or flat ones - sills. Viscous magma can push rock hard enough to create cracks, a process called unconformity intrusion.
Eruption forecast. How realistic?
It is extremely difficult to predict the time when the volcano will wake up. Hawaii's eruptions are quite calm, frequent and relatively predictable, but most natural ones are difficult to predict. A tiltmeter is used as one of the means to determine an upcoming eruption. It is a device for determining the steepness of the slopes of a volcano. If it increases, the magma located in the center of the volcano swells and an eruption may occur. But it should be remembered that such changes are only shortly before the eruption, making this type of forecasting dangerous.
In Ancient Rome, the name Vulcan was borne by the mighty god, the patron of fire and blacksmithing. We call volcanoes geological formations on the surface of the land or on the ocean floor, through which lava emerges from the deep bowels of the earth to the surface.
Often accompanied by earthquakes and tsunamis, large volcanic eruptions have had a significant impact on human history.
Geographical object. The importance of volcanoes
During a volcanic eruption, magma comes to the surface through cracks in the earth's crust, forming lava, volcanic gases, ash, volcanic rocks and pyroclastic flows. Despite the danger that these powerful natural objects pose to humans, it was thanks to the study of magma, lava and other products of volcanic activity that we were able to gain knowledge about the structure, composition and properties of the lithosphere.
It is believed that thanks to volcanic eruptions, protein forms of life were able to appear on our planet: the eruptions released carbon dioxide and other gases necessary for the formation of the atmosphere. And volcanic ash, settling, became an excellent fertilizer for plants due to the potassium, magnesium and phosphorus it contained.
The role of volcanoes in regulating the climate on Earth is invaluable: during an eruption, our planet “releases steam” and cools, which largely saves us from the consequences of global warming.
Characteristics of volcanoes
Volcanoes differ from other mountains not only in their composition, but also in their strict external outlines. From the craters at the top of the volcanoes, deep narrow ravines formed by flows of water stretch down. There are also entire volcanic mountains formed by several nearby volcanoes and the products of their eruptions.
However, a volcano is not always a mountain breathing fire and heat. Even active volcanoes can appear as straight-line cracks on the surface of the planet. There are especially many such “flat” volcanoes in Iceland (the most famous of them, Eldgja, is 30 km long).
Types of volcanoes
Depending on the degree of volcanic activity there are: current, conditionally active And extinct (“dormant”) volcanoes. The division of volcanoes by activity is very arbitrary. There are cases when volcanoes, considered extinct, began to exhibit seismic activity and even erupt.
Depending on the shape of volcanoes there are:
- Stratovolcanoes- classic “fire mountains” or volcanoes of the central type, cone-shaped with a crater at the top.
- Volcanic fissures or fissures- fractures in the earth's crust through which lava comes to the surface.
- Calderas- depressions, volcanic cauldrons formed as a result of the failure of a volcanic peak.
- Panel- so called because of the high fluidity of the lava, which, flowing for many kilometers in wide streams, forms a kind of shield.
- Lava domes - formed by the accumulation of viscous lava above the vent.
- Cinder or tephra cones- have the shape of a truncated cone, consist of loose materials (ash, volcanic stones, blocks, etc.).
- Complex volcanoes.
In addition to land-based lava volcanoes, there are underwater And mud(they spew out liquid mud, not magma) Underwater volcanoes are more active than land-based ones; 75% of the lava erupted from the bowels of the Earth is released through them.
Types of volcanic eruptions
Depending on the viscosity of lavas, the composition and amount of eruption products, there are 4 main types of volcanic eruptions.
Effusive or Hawaiian type- a relatively calm eruption of lava formed in craters. The gases released during an eruption form lava fountains from drops, threads and lumps of liquid lava.
Extrusion or dome type- is accompanied by the release of gases in large quantities, leading to explosions and the emission of black clouds from ash and lava debris.
Mixed or Strombolian type- abundant lava output, accompanied by small explosions with the release of pieces of slag and volcanic bombs.
Hydroexplosive type- typical for underwater volcanoes in shallow water, accompanied by a large amount of steam released when magma comes into contact with water.
The largest volcanoes in the world
The tallest volcano in the world Ojos del Salado, located on the border of Chile and Argentina. Its height is 6891 m, the volcano is considered extinct. Among the active "fire mountains" the highest is Llullaillaco- volcano of the Chilean-Argentine Andes with a height of 6,723 m.
The largest (among terrestrial) volcano in terms of area occupied is Mauna Loa on the island of Hawaii (height - 4,169 m, volume - 75,000 km 3). Mauna Loa also one of the most powerful and active volcanoes in the world: since its “awakening” in 1843, the volcano has erupted 33 times. The largest volcano on the planet is a huge volcanic massif Tamu(area 260,000 km2), located at the bottom of the Pacific Ocean.
But the most powerful eruption in the entire historical period was produced by the “low” Krakatoa(813 m) in 1883 in the Malay Archipelago in Indonesia. Vesuvius(1281) - one of the most dangerous volcanoes in the world, the only active volcano in continental Europe - located in southern Italy near Naples. Exactly Vesuvius destroyed Pompeii in 79.
In Africa, the highest volcano is Kilimanjaro (5895), and in Russia it is a double-peaked stratovolcano Elbrus(North Caucasus) (5642 m - western peak, 5621 m - eastern).
The ancient Romans, watching black smoke and fire bursting into the sky from the top of the mountain, believed that before them was the entrance to hell or to the domain of Vulcan, the god of blacksmithing and fire. In honor of him, fire-breathing mountains are still called volcanoes.
In this article we will figure out what the structure of the volcano is and look into its crater.
Active and extinct volcanoes
There are many volcanoes on Earth, both dormant and active. The eruption of each of them can last days, months, or even years (for example, the Kilauea volcano, located in the Hawaiian archipelago, awoke back in 1983 and its activity still does not stop). After which the craters of volcanoes are able to freeze for several decades, only to then remind of themselves again with a new eruption.
Although, of course, there are also geological formations whose work was completed in the distant past. Many of them still retain the shape of a cone, but there is no information about exactly how their eruption occurred. Such volcanoes are considered extinct. As an example, Kazbek can be cited, since ancient times covered with shining glaciers. And in Crimea and Transbaikalia there are heavily eroded and destroyed volcanoes that have completely lost their original shape.
What types of volcanoes are there?
Depending on the structure, activity and location, in geomorphology (the so-called science that studies the described geological formations) separate types of volcanoes are distinguished.
In general, they are divided into two main groups: linear and central. Although, of course, this division is very approximate, since most of them are classified as linear tectonic faults in the earth’s crust.
In addition, there are also shield-shaped and dome structures of volcanoes, as well as so-called cinder cones and stratovolcanoes. By activity they are defined as active, dormant or extinct, and by location - as terrestrial, underwater and subglacial.
How do linear volcanoes differ from central ones?
Linear (fissure) volcanoes, as a rule, do not rise high above the surface of the earth - they have the appearance of cracks. The structure of volcanoes of this type includes long supply channels associated with deep splits in the earth's crust, from which liquid magma of basaltic composition flows. It spreads in all directions and, when solidified, forms lava covers that erase forests, fill depressions, and destroy rivers and villages.
In addition, during the explosion of a linear volcano, explosive ditches may appear on the earth's surface, extending several tens of kilometers. In addition, the structure of the volcanoes along the fissures is decorated with gentle shafts, lava fields, spatter and flat wide cones, radically changing the landscape. By the way, the main component of Iceland's relief is lava plateaus, which arose in this way.
If the composition of the magma turns out to be more acidic (increased content of silicon dioxide), then extrusive (i.e. squeezed out) shafts with a loose composition grow around the mouth of the volcano.
The structure of central type volcanoes
A central type volcano is a cone-shaped geological formation, which is crowned on top by a crater - a depression shaped like a funnel or bowl. It, by the way, gradually moves upward as the volcanic structure itself grows, and its size can be completely different and measured in both meters and kilometers.
A vent leads deep into the crater, through which magma rises up into the crater. Magma is a molten fiery mass that has a predominantly silicate composition. It is born in the earth's crust, where its hearth is located, and having risen to the top, it pours out onto the surface of the earth in the form of lava.
An eruption is usually accompanied by the release of small sprays of magma, which form ash and gases, which, interestingly, are 98% water. They are joined by various impurities in the form of flakes of volcanic ash and dust.
What determines the shape of volcanoes
The shape of a volcano largely depends on the composition and viscosity of the magma. Easily mobile basaltic magma forms shield (or shield-like) volcanoes. They tend to be flat in shape and have a large circumference. An example of these types of volcanoes is the geological formation located in the Hawaiian Islands and called Mauna Loa.
Cinder cones are the most common type of volcano. They are formed during the eruption of large fragments of porous slag, which, piling up, build a cone around the crater, and their small parts form sloping slopes. Such a volcano grows higher with each eruption. An example is the Plosky Tolbachik volcano that exploded in December 2012 in Kamchatka.
Structural features of dome and stratovolcanoes
And the famous Etna, Fuji and Vesuvius are examples of stratovolcanoes. They are also called layered, since they are formed by periodically erupting lava (viscous and quickly solidifying) and pyroclastic matter, which is a mixture of hot gas, hot stones and ash.
As a result of such emissions, these types of volcanoes have sharp cones with concave slopes, in which these deposits alternate. And lava flows from them not only through the main crater, but also from cracks, solidifying on the slopes and forming ribbed corridors that serve as support for this geological formation.
Dome volcanoes are formed with the help of viscous granite magma, which does not flow down the slopes, but solidifies at the top, forming a dome, which, like a cork, plugs the vent and is expelled by gases accumulated under it over time. An example of such a phenomenon is the dome that forms over Mount St. Helens in the northwestern United States (it formed in 1980).
What is a caldera
The central volcanoes described above are usually cone-shaped. But sometimes, during an eruption, the walls of such a volcanic structure collapse, and calderas are formed - huge depressions that can reach a depth of thousands of meters and a diameter of up to 16 km.
From what was said earlier, you remember that the structure of volcanoes includes a huge vent through which molten magma rises during an eruption. When all the magma is on top, a huge void appears inside the volcano. It is precisely into this that the top and walls of a volcanic mountain can fall, forming on the earth’s surface vast cauldron-shaped depressions with a relatively flat bottom, bordered by the remains of the crash.
The largest caldera today is the Toba caldera, located in (Indonesia) and completely covered with water. The lake formed in this way has very impressive dimensions: 100/30 km and a depth of 500 m.
What are fumaroles?
Volcanic craters, their slopes, foothills, and the crust of cooled lava flows are often covered with cracks or holes from which hot gases dissolved in the magma escape. They are called fumaroles.
As a rule, thick white steam billows over large holes because magma, as already mentioned, contains a lot of water. But besides this, fumaroles also serve as a source of release of carbon dioxide, all kinds of sulfur oxides, hydrogen sulfide, hydrogen halides and other chemical compounds that can be very dangerous for humans.
By the way, volcanologists believe that the fumaroles included in the structure of the volcano make it safer, since gases find a way out and do not accumulate in the depths of the mountain to form a bubble that will eventually push the lava to the surface.
Such a volcano includes the famous one, which is located near Petropavlovsk-Kamchatsky. The smoke billowing above it can be seen tens of kilometers away in clear weather.
Volcanic bombs are also part of the structure of Earth's volcanoes
If a long-dormant volcano explodes, then during the eruption the so-called volcanoes fly out of its crater. They consist of fused rocks or fragments of lava frozen in the air and can weigh several tons. Their shape depends on the composition of the lava.
For example, if lava is liquid and does not have time to cool sufficiently in the air, a volcanic bomb that falls to the ground turns into a cake. And low-viscosity basaltic lavas rotate in the air, thereby taking on a twisted shape or becoming like a spindle or pear. Viscous - andesitic - pieces of lava after falling become like a bread crust (they are round or multifaceted and covered with a network of cracks).
The diameter of a volcanic bomb can reach seven meters, and these formations are found on the slopes of almost all volcanoes.
Types of volcanic eruptions
As N.V. Koronovsky pointed out in the book “Fundamentals of Geology,” which examines the structure of volcanoes and types of eruptions, all types of volcanic structures are formed as a result of various eruptions. Among them, 6 types stand out in particular.
When did the most famous volcanic eruptions occur?
The years of volcanic eruptions can, perhaps, be considered serious milestones in the history of mankind, because at this time the weather changed, a huge number of people died, and even entire civilizations were erased from the Earth (for example, as a result of the eruption of a giant volcano, the Minoan civilization died in 15 or 16 century BC).
In 79 AD e. Vesuvius erupted near Naples, burying the cities of Pompeii, Herculaneum, Stabia and Oplontium under a seven-meter layer of ash, leading to the death of thousands of inhabitants.
In 1669, several eruptions of Mount Etna, as well as in 1766, of Mayon Volcano (Philippines) led to terrible destruction and the death of many thousands of people under lava flows.
In 1783, the Laki volcano exploded in Iceland, causing a drop in temperature that led to crop failure and famine in Europe in 1784.
And on the island of Sumbawa, who woke up in 1815, the next year left the entire Earth without a summer, lowering the world temperature by 2.5 °C.
In 1991, a volcano in the Philippines also temporarily lowered it with its explosion, albeit by 0.5 °C.
A truly amazing sight - a volcanic eruption. But what is a volcano? How does a volcano erupt? Why do some of them spew out huge streams of lava at different intervals, while others sleep peacefully for centuries?
What is a volcano?
Externally, the volcano resembles a mountain. There is a geological fault inside it. In science, a volcano is a formation of geological rock located on the surface of the earth. Magma, which is very hot, erupts through it. It is magma that subsequently forms volcanic gases and rocks, as well as lava. Most of the volcanoes on earth were formed several centuries ago. Today, new volcanoes rarely appear on the planet. But this happens much less frequently than before.
How are volcanoes formed?
If we briefly explain the essence of the formation of a volcano, it will look like this. Under the earth's crust there is a special layer under strong pressure, consisting of molten rocks, it is called magma. If cracks suddenly begin to appear in the earth’s crust, then hills form on the surface of the earth. Through them, magma comes out under strong pressure. At the surface of the earth, it begins to break down into hot lava, which then solidifies, causing the volcanic mountain to become larger and larger. The emerging volcano becomes such a vulnerable spot on the surface that it spews volcanic gases onto the surface with great frequency.
What is a volcano made of?
In order to understand how magma erupts, you need to know what a volcano is made of. Its main components are: a volcanic chamber, a vent and craters. What is a volcanic source? This is the place where magma is formed. But not everyone knows what a volcano’s crater and crater are? A vent is a special channel that connects the hearth with the surface of the earth. A crater is a small bowl-shaped depression on the surface of a volcano. Its size can reach several kilometers.
What is a volcanic eruption?
Magma is constantly under intense pressure. Therefore, there is a cloud of gases above it at any time. Gradually they push hot magma to the surface of the earth through the crater of the volcano. This is what causes an eruption. However, just a short description of the eruption process is not enough. To see this spectacle, you can use the video, which you need to watch after you have learned what the volcano is made of. In the same way, in the video you can find out which volcanoes do not exist nowadays and what the volcanoes that are active today look like.
Why are volcanoes dangerous?
Active volcanoes pose a danger for a number of reasons. The dormant volcano itself is very dangerous. It can “wake up” at any time and begin to erupt streams of lava, spreading over many kilometers. Therefore, you should not settle near such volcanoes. If an erupting volcano is located on an island, a dangerous phenomenon such as a tsunami may occur.
Despite their danger, volcanoes can serve humanity well.
How are volcanoes useful?
- During the eruption, a large amount of metals appears that can be used in industry.
- The volcano produces the strongest rocks that can be used for construction.
- Pumice, which appears as a result of the eruption, is used for industrial purposes, as well as in the production of stationery erasers and toothpaste.
Having become acquainted with the activities of the most famous volcanoes on Earth, let us now find out the main question that interests us: what is a volcanic eruption?
In ancient times, people imagined volcanoes as burning mountains. In fact, nothing burns in a volcano, since there is nothing there to burn. Plumes of smoke during eruptions represent steam and gases escaping from the volcano, carrying fine dust. And visible fire is a reflection of the molten mass of lava in the clouds of steam above it.
We are accustomed to thinking that the appearance of a volcano is a mountain with a crater at the top. However, this is not always the case. The most important thing in a volcano is not the mountain, which may or may not form above the volcanic outlet, but the outlet itself, or the vent, from where volcanic products emerge from the depths: steam, gases, ash and lava. Gases escaping from the volcano eject loose material that falls around the exit, and lava immediately pours out; It is these loose materials with lava, piling up at the exit, that gradually form a mountain. However, if the lava is very liquid, and the activity of the volcano manifests itself with large explosions, then the ejected material is scattered or easily spreads over long distances and the kind of mountains that we are accustomed to seeing and considering volcanoes are not formed.
Temperature measurements in deep mines and boreholes show that the deeper underground, the warmer it is. Under the earth's crust at very great depths, heat accumulates from certain features of rocks, the so-called radioactivity. The accumulation of this heat in some places reaches such a high temperature that the rocks melt. Such heat is thought to accumulate over time. First, the rocks soften and gases join them from the surrounding, deeper parts. The increase in gases melts the rock masses even more and a center of fiery liquid molten material is obtained. A mass of rock molten by a heater, located somewhere underground at a very great depth, is called magma.
Magma is a Greek word and means dough or mash. This name is suitable for a molten substance that is more or less viscous and thick. Magma, melted to a fiery liquid state, can foam from excess gases and, together with steam, overflows over the edge of the crater. Magma that comes to the surface during an eruption and has already lost a lot of gases is called lava. The high content of gases in magma makes it more liquid and mobile. It not only occupies large areas in the earth's crust, but also spreads along cracks. In them it freezes in the form of veins. If magma enters the upper layers of the earth's crust through cracks, where the pressure of these layers on it is less, gases are released from the magma, expand and make their way to the earth's surface. The lower the pressure in the upper parts of the earth's crust, the easier it is for gases to clear their way upward and finally break out to the surface, sometimes carrying molten material with them. This is the beginning of the eruption.
Lava comes out of a volcano in a molten, liquid state, and when it cools, it hardens like stone. The gases and lava emitted from a volcano are the most important materials during an eruption.
An eruption of lava is reminiscent of the cork being pushed out of a bottle by the gases of sparkling wine, beer or sparkling water, which then pour out. The liquid poured into the bottle is highly saturated with gas. This gas presses against the walls of the bottle and would be released from the liquid if it were not enclosed in a strong, tightly sealed bottle. The liquid in a sealed bottle is completely calm and no different from ordinary water poured into a glass. But as soon as you loosen the cap in the neck of the bottle, the liquid begins to move, releasing gas bubbles abundantly. The gas expands, noisily pushes out the plug and rushes to the exit, carrying with it the liquid, which foams, splashes and escapes through the edges of the neck. The liquid poured into the glass continues to release gas bubbles, which, bursting on the surface, raise splashes of liquid.
The described state of the gas in a bottle of liquid gives an idea of the gases in magma that find their way out through a volcano clogged with hardened lava. Hot gases press on the stone plug of old lava, partially melt it, partially destroy it and explode out of the crater. Coming out forcefully through the resulting hole, they expand it even more, tearing off pieces of old solidified lava from the walls of the exit and the crater. At the same time, the gases also carry out the sprayed lava foam.
It happens that explosions of enormous power completely tear off part of the mountain and completely change the appearance of the volcano, as happened with the Krakatoa and Katmai volcanoes. A similar case occurred with the Baidaisan volcano in Japan. Of course, such explosions that destroy the entire volcanic cone do not happen often, but ordinary explosions also greatly destroy the walls and edges of the crater. Therefore, as soon as the volcano calms down, landslides occur that fill the voids produced by the eruption; This is why the bottom of the crater is always covered with rock fragments.
The debris ejected by a volcano comes in a variety of sizes, from small pieces to huge blocks of several cubic meters weighing tons. Along with this, when a volcano erupts, tiny dust is released, which is called volcanic dust, or ash. It is so small that it can be carried in the air over vast distances. Volcanic ash is a light, fine powder, often grayish in color, which is why they gave it the name ash. It has nothing to do with combustion. These are fragments of old lava crushed into dust and tiny particles of liquid lava thrown out of the volcano by jets of gas.
Eruptions scatter ash, sand and larger debris called lapilli (Italian for pebbles) over an area of hundreds of kilometers. In some places, the ash lies in a thick layer and compacts tightly over time; then it forms layers of more or less hard rock, the so-called volcanic tuff. Just like lava, tuff persists for many millennia after the volcano has long been extinct. It comes in red, black, brown and yellow colors and is used to build houses, such as in Transcaucasia.
The mud flows that occur during some eruptions occur from the simultaneous release of ash and huge clouds of steam from the crater, or from heavy rain falling on masses of volcanic dust. Mud flows often cause disasters during eruptions. They quickly roll down from the top, breaking and flooding everything in their path; this was the case at the foot of Vesuvius, where the city of Pompeii is buried, and at the foot of Mon Pele, where the factory was demolished.
When lava rises from the depths and flows to the surface of the earth, gas is released from it so strongly that the lava foams, as if boiling. If this foam hardens quickly, the remaining gases form voids inside it; This hardened foam of stone is called pumice. The result is a very light porous stone that floats freely on water. And often, by large accumulations of pumice floating on the surface of the sea, sailors learn that somewhere under the water, at the bottom of the sea, a volcanic eruption occurred.
From the descriptions of some volcanic eruptions, we have seen that volcanic eruptions occur in different ways. This depends, firstly, on the force with which gases escape from the magma and, secondly, how liquid or thick the lava is.
If lava is liquid, almost like water, gases escape freely. It boils, seethes and is thrown upward with strong jets of gas, in the form of a fountain, like sparkling water just poured into a glass. This occurs in the lava lake of the Kilauea crater and is observed in few other volcanoes. The fountain's spray of very liquid lava hardens as it flies into droplets and forms rock droplets called "lava tears." During strong explosions, these sprays are pulled out into hair-thin, long glass threads, which the wind carries long distances from the lava fountain.
If the lava is thick, like dough, gases do not escape from it as freely as from liquid lava. They break free with some difficulty and tear the lava into pieces, large and small. Torn pieces of such lava are thrown high into the air by the force of the gas and at this time spin in the form of a top or short spindle. Solidified ejected pieces of lava are called volcanic bombs (Fig. 17).
Rice. 17. Twisted bombs made of doughy lava, thrown out during the eruption.
Finally, lava can be very thick. It cannot even flow, then it sticks out of the volcano in the form of domes, as we saw in the Mont Pele crater. But such thick, hot lava again contains gases, and they can also be released from it. As gases are released, they tear such thick lava into angular pieces. The latter cool from the surface, forming a glassy crust, and the hot inner part of these pieces, continuing to release the remaining gases, bubbles and swells; then the crust cracks and cracks appear, as sometimes happens on the crust of a loaf of bread. These frozen, torn pieces of very viscous lava are also called volcanic bombs (Fig. 18). But, as we see, they have a completely different shape, and from this shape you can tell that the lava was very thick. Bombs of this type were released in large quantities during the eruption of Mont Pele.
Rice. 18. A cracked bomb thrown from the Mont Pele volcano. The bomb's crust resembles the cracked crust on a loaf of bread.
Signs such as the shape of bombs, the type of lava flows, the accumulation of ejections of loose material, and layers of tuff help scientists understand how and in what order eruptions occurred and how this or that extinct volcano was formed.
