Mount St Helens is a composite volcano

Volcanism. Origin, manifestations and post-volcanic processes


Formation of a volcano

In the upper mantle, under certain temperature and pressure conditions, a rock melt forms, too magma called. It initially collects in huge magma chambers that push the surrounding rock apart and partially melt it. The magma is under enormous pressure. Cooling down, pressure relief and other processes lead to the release of the gases dissolved in the magma under the high pressure. Such pressure relief occurs mainly in tectonic weak zones. As a result, the gases can penetrate the earth's surface and carry away the magma and other conveyed products.

The volcanic extraction products

The magma source of volcanic extraction products: The volcanic extraction products derive from the magma (Greek = kneadable mass). Magma is only formed in the earth's mantle at certain temperatures and pressures. During the cooling process and the associated solidification, igneous rocks are also formed Igneous rocks called. If igneous masses solidify within the earth's crust, they arise Deep rocks or Plutonite. Depending on the silica content, magma can be basic, intermediate or acidic. One of its special features is that it contains gases in dissolved form. If magma penetrates up to the surface of the earth, it occurs under the name lava out.

The lavas: The term lava is derived from the Italian verb lavare- wash. Lava has a lower proportion of gas than magma, as the gas is released during the eruption. The temperature of a flowing lava can be determined from its color, whereby the following temperature values ​​apply: red glow-540 ° C, dark red glow-650 ° C, light red glow- 870 ° C, yellowish glow-1100 ° C, beginning white glow- 1260 ° C, white glow-1480 ° C. On the basis of these color gradations, the temperatures of the lavas can be measured with a simple device, the pyrometer. Lavas with different SiO2 content have very different properties.

The basaltic lavas: They are dark gray, dark gray to black. The basic lavas have a low content of SiO2. This is below 52%. They have relatively high levels of magnesium, iron and calcium oxides. The basaltic lavas are predominantly thin, give off their gas content easily and can cover large distances as rivers in a short time. It is by far the most widespread type, which is extracted from the depths of the upper mantle in the rift zones of the mid-ocean ridges and the crevices of the deep-sea basins and forms the ocean floors. The acidic lavas: The acidic lavas are highly viscous, flow extremely slowly and give off their gas content with great difficulty. This creates an overpressure. Accordingly, acidic volcanism usually has an explosive character. The silica content in the acidic lavas exceeds 65%. The intermediate lavas: They are of far greater importance than the acidic ones. Their silica content is between 52 and 65%. The viscosity is relatively high and therefore andesitic volcanism is also explosive.

Two types of lava are distinguished according to the nature of the surface. One is that "Pahoehoe type", also called knitting or seillava. It is mainly developed on the surfaces of basic lavas. It arises from the fact that the still flowing lava is covered with a solidification skin, which is pushed together with further movement in such a way that it resembles a carpet of ropes lying next to each other.

The other guy is that Aa lava. It arises from slowly flowing lava flows, the thicker crust of which breaks into sharp-edged blocks. The angular blocks can have sizes in the decimeter range, but they can also reach the size of a house.

Lavas that have solidified to form rock usually have an "intergranular" or "porphyry" structure. The first is typical of solidified basalt lavas, in which interlaced alkaline soda lime feldspars that crystallize out from the melt are embedded in a pile of pyroxene. The porphyry structure, on the other hand, is typical of intermediate and acidic lavas. They show well-developed mineral crystals that lie in a matrix.

The volcanic loose products: In addition to the emission of gas and steam clouds and the outflow of glowing lava flows, this also includes the discharge of loose material in the form of volcanic bombs, lapilli and ash. The name tephra is also used for volcanic extraction products of this type. This involves explosively “cracked” lava, but also fragments of rocks that are torn out of the walls of the conveying channels during volcanic eruptions. The ejected fragments can appear in all sizes, from huge blocks to the finest dust particles. If the ejecta is derived from cracked lava, the term "pyroclastics" is common. Shreds of hot lava that solidify in flight and usually fall as chunks on the slope of the volcano are known as cinders. Volcanic bombs are lava fragments that are thrown high into the air in a glowing state, round off spherically or spindle-shaped during flight and are covered with a solid solidification crust. Pumice stone is also counted among the pyroclastics. It arises from acidic lavas, the gas content of which is very high. The weight of the pumice stone is so light that it can float on the water. So it happens that the pumice stone can be found on almost all coasts. Small lava fragments or foreign rock debris are called "lapilli". Lapilli is lava blown up by the rapid expansion of gases, but also volcanic ash or dust. The volcanic ash is not combustion residues, but natural glass and crystal fragments. A special phenomenon among the volcanic loosening products is "Pele's hair". This is understood to mean fine, wafer-thin glass threads that are blown away by the wind. The name goes back to the Hawaiian goddess Pele, whose home, according to legend, is said to be the Halemaumau crater.

Outline of the volcanoes

Linear volcanoes: In linear volcanoes, the magma uses a deep crevice or crevice zone as a way to ascend the earth's surface. Basic magma emerges from the surface of the earth as thin liquid lava. If the delivery rates are very large, the outflowing lava can flood large areas of land. Everything that gets in the way is destroyed. It arise Lava plains. In doing so, the morphology and the river network of entire landscapes are changed. After the fissure pouring is complete, the fissure is "sealed" by the conveying material that solidifies in it and is usually not active again. However, corresponding effusions can be repeated in the same area by tearing open parallel gaps and stacking on top of each other like a blanket. Fissure effusions are a common phenomenon on the sea floor, especially in the area of ​​the rift zones, and are the most widespread type of volcanism. The continental flood or plateau basalts also belong to this category. If the delivery rate is low, basalt layers of limited size are formed.

Intermediate conveying material combined with mixed explosive - effusive eruptions, forms Back; acidic, viscous melts tend to be explosive. An example of an explosive fissure eruption of intermediate material was the Tarawera eruption in New Zealand. On a ridge made of volcanic rock about 1100m long, an explosion began at one point. More followed in other places, and finally the whole ridge burst. A fissure 14.5 km long tore open and violent explosion activity set in. Explosively disintegrated sour waste material was funded, as well as ash, andesitic slag and lava fragments. The ashes covered an area of ​​200,000 km².

A result of linear breakouts of a special kind are Explosion trenches or. Explosion columns. They arise when larger amounts of gas have freed themselves from the magma in the deeper areas of conveying gaps and a large overpressure has arisen so that the "roof" in the gap is blown away. An example of this type is the Eldgja Fissure in South Iceland. There, a huge explosion tore a narrow trench 30km in length, with about 9km³ of loose material being extracted.

A phenomenon little known in the present is the emergence of gigantic ones volcanic tectonic depression Slump areas, which are bordered by crevices from which powerful clouds of embers poured out with rhyolite material that separated and welded in the form of extensive Ingnimbrit ceilings. As a result of the space deficit created in the subsurface, the clod of the earth's crust, surrounded by the crevices, sank. A well-known example of such volcanic tectonic depressions is Lake Toba.

Central volcanoes: The central volcanoes take the place of the column of the Chimney, a tube that leads from the igneous hearth to the surface of the earth. Your top end is that crater, a chalice-shaped or kettle-shaped extension of the chimney. In the ideal case, the crater is located on the summit of the surrounding deposits of volcanic production products and is relocated increasingly higher with frequent production and the associated enlargement of the volcano. But craters can also assume eccentric positions; several craters can occur in some volcanoes. In the case of very large volcanic structures, such as Mount Etna in Sicily, the slopes can have numerous smaller ones, sometimes hundreds of "Side craters" or "Parasitic craters" be covered, the supply channels of which branch off from the main chimney or directly from the igneous hearth and which sometimes even operate independently of the main crater. Each outbreak creates new parasitic craters while the others become inactive. The craters change their shape almost with every eruption. Explosive eruptions expand them, invading lava fills them again. Activities in the chimney can cause the crater floor to rise and fall between eruptions. Very tough lavas can form damming peaks in craters, which fill it in or even tower over it like a dome and close off the conveying path. Such closures can be the cause of catastrophic explosive outbreaks.

In the case of central volcanoes, too, the shape depends on the flowability and the abundance of gas in the material to be conveyed. Basic, thin and very hot lavas spread very quickly from the central conveying channel. The multiple stacking of such lava flows results in flat, often large areas that are built on top of each other Shield volcanoeswhose flanks have a very slight slope (3 to 6 °). The base diameter is 20 times the height. The craters of the shield volcanoes are mostly extensive, shallow basins with steep walls. Examples of such shield volcanoes can be found in the oceanic area. However, they are visible in the few places where they have outgrown, such as Iceland and Hawaii.

Central volcanoes, which have a mixed effusive-explosive activity, are ideally cone-shaped mountains, which consist of an alternation of effusively conveyed lava flows and explosively conveyed loose substances such as lapilli, ashes, bombs and slag. Because of their layered structure, they are also known as Stratovolcanoes. However, if only loose material is involved in the build-up of the cone mountains, one speaks of embankment cones. With the increase in the silica content of the conveyed products, the explosive discharge of loose substances also increases or - like it does Rittmann expresses the explosive index E. He understands this as the percentage of loose materials in the total material conveyed. In the case of volcanoes, he differentiates lava-rich types (E = 1 ... 33), intermediate or normal types (E = 34 ... 66) and types in which loose masses predominate (E = 67 ... 90 and above). These values ​​can only be viewed as mean values.

The angle of inclination of a stratovolcano depends on the type of material extracted. Fine ashes and slag come to rest at around 30-35 °. Since they can be carried further away from the crater than coarser material, they usually accumulate on the lower slopes of the cone, while coarser material accumulates in the vicinity of the crater at angles of 40 ° and above. Typical cone mountains of this type can be found in almost all volcanic areas on earth that belong to the belts of subduction zones or continental rift volcanism. Caldera: is a crater basin that is usually many times larger than the original eruption crater. Most calderas are formed by the collapse of underground magma chambers after these chambers have been emptied by an eruption. But there are also calderas that were created by a volcanic peak blasted away. A new volcano can later grow back in the caldera.

Compound volcanic structures: There are volcanoes that are more complex than those described above. There are composite volcanoes that have not only emerged in several stages of development, but in which the chemical composition of the extraction products has also changed over time. Vesuvius offers us a good example of a composite volcano. According to Rittmann, four phases of formation can be distinguished at Vesuvius:

1. Ursomma, consisting of trachytic viscous lava flows or dunnage;
2. Old comma, a relatively low-lava stratovolcano made of phonolithic Leucittephrites;
3. Jungoma, a stratovolcano with lava flows, radial ducts
and Mantelsills, whose development was stopped by the eruption in 79.
4. Today's Vesuvius, a fairly lava-rich stratovolcano; Similar developments also apply to many other volcanoes.

Manifestations of volcanism

The New Global Tectonics gives us the opportunity to classify the various manifestations of volcanism in their mechanism. The following groups can be outsourced:

1. Volcanism of the oceanic rift zones
2. Oceanic intraplate volcanism
3. Volcanism of the subduction zones
4. Continental rift volcanism

Volcanism of the oceanic rift zones: Most of the earth's surface, the soils of the oceans, have a different structure than the continents. They consist of basic volcanic rock that rises as magma in the oceanic rift zones that span the earth from the area of ​​the upper mantle. It is attached to both sides of the oceanic lithosphere and pushed off to the side. It then slowly moves away from the rift zones and after an existence of approx. 200 million years in subduction zones in front of continental margins or offshore island arcs it is assimilated again by the earth's mantle. The melts produced in the oceanic rift zones belong to those basalt variants known as tholeiites. They solidify on the sea floor mainly as pillow lavas.

Oceanic intraplate volcanism: Oceanic volcanism is not only restricted to the rift zones, but is also a widespread phenomenon within the oceanic portions of the lithospheric plates. It is noticeable in places where volcanic extraction products have accumulated in such masses that they tower above sea level and form individual islands or entire archipelagos.

The eruption type of the massive Hawaiian shield volcanoes Mauna Loa and Kilauea is considered a prime example of oceanic intraplate volcanism. Outbreaks are usually heralded by a series of earth tremors. The actual eruption consists in the fact that basaltic lava flows out relatively calmly from opening fissures. As with oceanic rift volcanism, the lava is thin and easily releases its gas content.

The eruptions begin with the fact that the rising lava begins to foam up from the opening crevices as a result of sudden pressure relief and the gases contained therein initially expand in such a way that they generate numerous lava fountains that can join together to form real "fire curtains". But then more or less large floods of lava are ejected, which flow in streams and after their solidification add further volume proportions to the mighty lava shields. The eruptions can last a few days or up to a few months. In oceanic intraplate volcanism, the chemical composition of the lava forming the islands changes with increasing distance from the mid-ocean ridges. Studies on islands seem to suggest that there is a cyclical course of education.

Island formation is divided into three phases:

1st phase: In this first phase, huge amounts of weakly alkaline lava are extracted. This first phase of extraction can make up 90% of the entire island volume and is ended by an erosion stage.
2nd phase: Here, strongly alkaline lavas are promoted. Their volume is much smaller, the eruptions take place slowly and at longer intervals. Another stage of erosion follows.
3rd phase: Even smaller amounts of extremely alkaline lavas are pumped. This undersaturation of silica is explained by the shifting of the partial melting into ever deeper areas of the upper mantle of the earth.

Volcanism of the subduction zones: If there were similarities in the forms of volcanic activity described above, the picture changes completely at those parts of the earth where oceanic lithosphere plates slide under continental plate edges or offshore island arches. This applies to the production products as well as the support mechanism and the volcanic structures. The volcanism of the subduction zones is the “ideal” of volcanism. The predominant volcanic structures are cone mountains, the peaks of which are crowned by one or more craters. With this type of volcanism, we distinguish one high explosive type, one mixed explosive-effusive type and one moderate type.

Highly explosive volcanism: This group includes all volcanoes that are characterized by extreme explosiveness. The conveyed products are exclusively loose materials such as ash, lapilli, bombs and slag. One example of this is the Mont Pelee.

Mixed-explosive-effusive volcanism: The volcanoes belonging to this type can be found almost everywhere in the subduction areas of the earth. Explosive ejection of loose material and effusive outflow of lava flows occur side by side or with one another, so that predominantly cone mountains arise, which are built up from an alternation of volcanic loose material and lava flows. The classic volcanoes of this type include Vesuvius and Etna.

Temperate volcanism: Temperate volcanism has little, but in some cases constant, activity. The island volcano Stromboli is seen as a prime example.

Continental rift volcanism: In addition to the volcanism of the subduction zones, which is bound to converging plate boundaries, the continents show another, completely different form of volcanism: that of the continental rift zones. There are zones of crustal expansion, characterized by deep, widely traceable fissure systems that allow igneous melts from the area of ​​the upper mantle to rise. In the landscape they stand out as trench-like depressions of elongated, narrow inguinal clods of the continental crust. The most impressive example is the system of the East African rifts (rift valleys).

Post-volcanic processes

During volcanic eruptions, during rest breaks, during decaying volcanism and a long time after volcanic activity has ceased, volcanic exhalations, geyser and spring activity play an important role. In the stage of the extinction of a volcano or the cooling down of the volcanic center, which under certain circumstances can drag on for millions of years, these phenomena are referred to as post-volcanic or post-volcanic. Volcanic exhalations: In the active volcanoes there is very seldom calm. Water vapor and gases are separated from crevices and channels, some of which escape quietly, but sometimes also hissing violently or whistling loudly from the lava flows or accumulations of loose products. These gas outlets are known as "Fumarole". Water vapor is the essential component of this fumarole.

Geysers and hot springs: If circulating water close to the surface is heated in the "neighborhood" of active volcanoes, secondary volcanic phenomena occur. Such phenomena are geysers and hot springs. A prerequisite for this is a water reservoir close to the surface. The mechanism works in such a way that the water in the reservoir is heated to boiling. In doing so, water vapor collects over the water in the knee-shaped section. Finally, the associated overpressure becomes so great that the column of water above the accumulation of steam is explosively ejected. This leads to a drop in vapor pressure and the process starts all over again. Thus, the periodic repetition of the eruptions of water fountains proves to be a characteristic feature of the geysers.