The ancient Romans, watching as black smoke and fire erupted from the top of the mountain into the sky, believed that before them was the entrance to hell or into the possession of Vulcan, the god of blacksmithing and fire. In honor of him, fire-breathing mountains are still called volcanoes.
In this article we will understand 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 on the Hawaiian archipelago woke up in 1983 and still does not stop its work). After that, the craters of volcanoes are able to freeze for several decades, and then again remind themselves of a new outburst.
Although, of course, there are also such geological formations, the work of which was completed in the distant past. Many of them still retained the shape of a cone, but there is no information about exactly how their eruption occurred. Such volcanoes are considered extinct. As an example, Mount Elbrus and Kazbek mountains, from ancient times covered with shining glaciers. And in the Crimea and Transbaikalia there are strongly eroded and destroyed volcanoes, which have completely lost their original shape.
What are volcanoes
Depending on the structure, activity and location, in geomorphology (the so-called science that studies the described geological formations), certain types of volcanoes are distinguished.
In general terms, they are divided into two main groups: linear and central. Although, of course, this division is very approximate, since most of them are attributed to linear tectonic faults of the earth's crust.
In addition, they also distinguish between the thyroid and dome structures of volcanoes, as well as the so-called slag cones and stratovolcanoes. By activity, they are defined as active, dormant or extinct, and by location - as terrestrial, underwater and subglacial.
What is the difference between linear volcanoes and central
Linear (fissured) volcanoes, as a rule, do not rise high above the earth's surface - they look like cracks. The structure of volcanoes of this type includes long supply channels connected with deep cleavages of the earth's crust, from which liquid magma having a basaltic composition flows. It spreads in all directions and, solidifying, forms lava covers, erasing forests, filling hollows, destroying rivers and villages.
In addition, during the explosion of a linear volcano, explosive ditches with a length of several tens of kilometers can arise on the earth's surface. In addition, the structure of volcanoes along the cracks is decorated with gentle shafts, lava fields, spatter and flat wide cones, radically changing the landscape. By the way, the main component of the relief of Iceland is the lava plateau that arose in this way.
If the composition of magma turns out to be more acidic (increased content of silicon dioxide), then around the mouth of the volcano extrusive (i.e. squeezed) shafts with a loose composition grow.
The structure of volcanoes of the central type
A volcano of the central type is a cone-shaped geological formation, which is crowned with a crater on top - a depression in the form of a funnel or bowl. Incidentally, it gradually moves upward as the volcanic structure grows, and its size can be completely different and can be measured both in meters and in kilometers.
Volcanic craters form during an eruption and can occur even on the slopes of a volcanic mountain, in which case they are called parasitic or secondary.
Deep into the volcanic mountain leads a vent through which rises upward, into the crater, magma. Magma is a molten mass of fire that has a predominantly silicate composition. She is born in the earth’s crust, where her hearth is located, and rising up, in the form of lava it pours out onto the surface of the earth.
The eruption is usually accompanied by the release of small splashes 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 the volcano is largely dependent on the composition and viscosity of magma. Easy-moving basaltic magma forms shield (or thyroid) volcanoes. They are usually flat in shape and have a large circle. An example representing these types of volcanoes is the geological formation located in the Hawaiian Islands and called Mauna Loa.
Slag 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 with each eruption becomes higher. An example is the Flat Tolbachik volcano that exploded in December 2012 in Kamchatka.
Structural features of domed and stratovolcanoes
And the famous Etna, Fujiyama and Vesuvius are an example of stratovolcanoes. They are also called layered, since they are formed by periodically erupting lava (viscous and rapidly solidifying) and pyroclastic substance, 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 a support for this geological formation.
Dome volcanoes are formed with the help of viscous granite magma, which does not flow down the slopes, but freezes at the top, forming a dome that, like a cork, clogs the vent and is knocked out by the gases accumulated under it over time. An example of such a phenomenon is the dome that forms over the St. Helens volcano in the northwestern United States (it was formed in 1980).
What is a caldera?
The central volcanoes described above are generally conical in shape. But sometimes during the eruption, the walls of such a volcanic structure collapse, and calderas form - huge depressions that can reach a depth of thousands of meters and a diameter of up to 16 km.
From the above, you remember that a huge vent enters the volcanic structure, along which molten magma rises during the eruption. When all the magma is on top, a huge void appears inside the volcano. It is precisely in it that the peak and walls of a volcanic mountain can fall through, forming vast cauldron depressions bordered by the remains of a wreck with a relatively flat bottom.
The largest to date is the Toba caldera, located on the island of Sumatra (Indonesia) and completely covered with water. The lake formed in this way has a very impressive size: 100/30 km and a depth of 500 m.
What are fumaroles
The craters of volcanoes, their slopes, foot, as well as the crust of cooled lava flows are often covered with cracks or holes from which hot gases dissolved in magma burst out. They are called fumaroles.
As a rule, thick white vapor swirls over large openings, because magma, as already mentioned, contains a lot of water. But besides it, fumaroles also serve as a source of emissions for carbon dioxide, all kinds of sulfur oxides, hydrogen sulfide, hydrogen halide and other chemical compounds, which can be very dangerous for humans.
By the way, volcanologists believe that the fumaroles that make up the structure of the volcano make it safer, since gases find a way out and do not accumulate in the bowels of the mountain to form a bubble that eventually pushes the lava to the surface.
The famous Avachinsky hill, which is located near Petropavlovsk-Kamchatsky, can be attributed to such a volcano. The smoke swirling above it is visible in clear weather for tens of kilometers.
Volcanic bombs are also included in the structure of Earth's volcanoes
If a long-sleeping volcano explodes, the so-called volcanic bombs fly out of its mouth during an eruption. They consist of fused rocks or fragments of solidified lava in the air and can weigh several tons. Their shape depends on the composition of the lava.
For example, if the lava is liquid and does not have enough time to cool enough 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 a twisted shape or becoming like a spindle or a pear. Viscous - andesitic - lava pieces after falling fall like bread crust (they are round or multifaceted and are covered with a network of cracks).
The size of the cross section of a volcanic bomb can reach seven meters, and these formations are found on the slopes of almost all volcanoes.
Types of Volcanic Eruption
As pointed out in the book "Fundamentals of Geology", which considers the structure of volcanoes and types of eruptions, Koronovsky N.V., all types of volcanic structures are formed as a result of various eruptions. Among them, 6 types stand out especially.
- The Hawaiian type of eruption is the ejection of very fluid and mobile lava, which forms huge shield volcanoes that have a flat shape.
- The Strambolian type is the emission of a more viscous lava, which is pushed by explosions of different strengths, as a result of which short powerful flows form.
- The Plinian type is characterized by sudden powerful explosions, which are accompanied by the release of a huge amount of tephra (loose material) and the occurrence of its flows.
- The Pelei type of eruption is accompanied by the formation of hot avalanches and scorching clouds, as well as the growth of extrusive domes from viscous lava.
- The gas type is an eruption of only fragments of more ancient rocks, which is associated with gases dissolved in magma, or with overheating of the groundwater included in the structure of the volcano.
- The eruption of heat flux. It is similar to the release of a high-temperature aerosol consisting of pieces of pumice, minerals and fragments of volcanic glass surrounded by a hot shell of gas. Such an eruption was widespread in the distant past, but in modern times it has long been no longer observed by people.
