A volcano is a geological structure, which is formed when magma ( molten rock ) to the surface of a planet (e.g., the ground) rises. All side effects that are associated with the rise and emergence of the red-hot molten rock melt, is called volcanism. In a volcanic eruption not only glutflüssige, but also solid or gaseous substances are released.

In the case of the earth melt at a depth from 100 km, prevail in the temperature 1000-1300 degrees Celsius, rocks to zähplastischem magma that accumulates in large, teardrop-shaped magma foci in 2 to 50 km depth. If the pressure is too great, the magma rises through cracks and crevices of the lithosphere. Magma which reaches in this way to the surface is referred to as lava.

Most volcanoes have approximately the shape of a cone whose slope depends on the viscosity of the lava. The form can also be arched or irregular dome-shaped.

  • 7.1 recording seismic activity, volcanic tremor
  • 7.2 Geodetic Monitoring
  • 7.3 Measurement of gravimetric and magnetometric changes
  • 7.4 Detection of temperature increases
  • 7.5 Analysis of rising gases
  • 7.6 Conclusion


The term " volcano " derives from the Italian island of Vulcano. This is one of the Aeolian Islands in the Tyrrhenian Sea. In Roman mythology, the island was considered the Forge of Vulcan, the Roman god of fire.

Volcano types and designations

Volcanoes can be subdivided according to their external form, the nature of their Magmenzufuhrsystems, the place of occurrence, the nature of their work and by their state.

  • Subdivision by the external form: (Also called stratovolcanoes ) composite volcanoes
  • Shield volcanoes
  • Spatter and cinder cone
  • Lava dome
  • Maar
  • Caldera
  • Panel volcano
  • Ceiling or plateau volcano (see also Trapp and flood basalt)
  • Division according to the type of Magmenzufuhrsystems: Central volcanoes
  • Column volcanoes
  • Subdivision according to the place of their occurrence: subaerial volcanoes ( volcanoes on land or over water)
  • Submarine volcanoes, seamounts, Guyot ( volcanoes in the sea under water)
  • Subglacial volcanoes ( volcanoes under a glacier)
  • Extraterrestrial volcanoes ( volcanoes on other celestial bodies )
  • Subdivision according to the nature of their work: effusive activity ( quiet outflow of lava)
  • Explosive or ejective activity
  • Mixed effusive and explosive activity
  • Volcanoes with particularly violent explosive activity are called super volcanoes
  • Volcanoes can finally classify them by their state or the frequency of their activity in active volcanoes (active volcanism )
  • Inactive or dormant volcanoes ( no active volcanism, conditions for renewed activity, however, are given )
  • Extinct volcanoes (missing magma supply no activity possible)

However, many volcanoes do not follow a "pure " bursting patterns, but show varying behavior either during an eruption or during the millions of years of their activity. An example of this is Mount Etna in Sicily.

A paroxysm (Greek παρα para " beside", οξυς oxys "sharp" ) is a succession of increasing outbreaks of a volcano.

The resulting by the volcanic activity mountain is called, depending on its form volcanic cones or lava, and the opening from which lava rises from the depths, ie volcanic vent. The more or less wide opening at the top of a volcano is the crater. Breaks down a chimney above a shallow magma chamber, and it forms a major slump crater, this is called a caldera.

Magma types

A decisive influence on the formation of a volcano, in addition to the gas and water content, the composition of its magma, especially the content of silicon dioxide (SiO2). The composition of the magma determines the type of volcanic activity. The more SiO2 contains the magma, the more explosive volcanism associated with it. We can distinguish four main types:

  • Felsisches magma contains more than 63 % SiO2
  • Intermediate magma contains 52-63 % SiO2
  • Mafic magma contains 45-52 % SiO2
  • Ultramafic magma containing less than 45% SiO 2

From the four magma types leads to characteristic rocks:

  • Rhyolite
  • Andesite
  • Basalt
  • Pikrit

These four types can be assigned roughly determined geodynamic environments:

  • Melting of continental crust
  • Subduction zones
  • Mid-ocean ridge, hotspot volcanism, rift volcanism
  • No longer occurring


Volcanoes can also be classified otherwise by being described by the color of the emitted lava.

The color of the emerging lava depends on the temperature, but can also be attributed to the chemical composition of rock melt. The resulting shape of the volcano and the eruption behavior are determined by the composition of the molten rock decisive:

  • Red volcanoes are due to the red or orange yellow glowing lava hot so designated, they form shield volcanoes.
  • Grey volcanoes have a comparatively low Lavatemperatur and form composite volcanoes.

Decisive volcanoes triggered events

With the eruption of volcanoes, other processes are triggered by volcanic material mixing with other substances such as water or air, and by the abrupt exit of lava. These include, among others:

  • Lahar (mud and debris flows)
  • Pyroclastic flows ( pyroclastic flow )
  • Base Surge ( low particle density current)
  • Pyroclastic flow
  • Glacier run
  • Tsunami: If large amounts of magma overthrow or even parts of the mountain slopes into the sea in a volcanic eruption, tsunamis can be triggered. Here achieve these often greater heights than the tsunami generated by earthquake.

Earthquakes can occur before or after the eruption of a volcano, as they can influence each other.

Not the volcanism are attributable to other rising materials that are around for the so-called mud volcanoes (better than Schlammdiapire called ) responsible.

Distribution of volcanoes

After volcano type

Today there are about 1500 active world, that is, in the last 10,000 years erupted volcanoes on the Earth's surface, but you do not know yet the number of submarine volcanoes, of which there are probably many cases are more.

Of these, 719 as a stratovolcano, 176 as a shield volcano, 66 as a complex volcano, 86 as Caldera, 147 as individual cinder cones, 27 as column volcano or crater row, 19 as Maar, 137 as a submarine volcano and 100 as volcanic field ( with some several hundred individual volcanoes ) classified.

After Geography

The geographical distribution can be understood with the help of the knowledge of plate tectonics:

  • Volcanoes of the spreading zones are, with few exceptions on the ocean floor where tectonic plates drift apart. The magma is occurring there basaltic and depleted in elements that are difficult to integrate in the crystal lattice (incompatible elements). It concerns mainly include red or volcanoes shield volcanoes.
  • Volcanoes above subduction zones are the most visible volcanoes. They occur at plate collisions involving at least one oceanic lithosphere is involved. Here, the oceanic crust is transported into the mantle into ( subducted ), provided their age-dependent density has reached a sufficiently high value. The subducting oceanic crust is partially melted in depth, since it due to the high water levels in certain minerals to a lowering of the solidus occurs ( onset of melting temperature ). The resulting magma rises because it is less dense than the surrounding rock, and nourishes the volcanism at the surface. The resulting volcanoes are called because of their layered structure as composite volcanoes or stratovolcanoes.
  • Volcanoes on " hotspots" are rare, since there is currently only about 40 clearly identified "hotspots" around the world. A " hotspot" is an over geological time as nearly stationary to Aufschmelzungsbereich sighted in the mantle beneath the lithosphere. The lithospheric plates slide by plate tectonic mechanisms for long periods over a " hotspot" of time. It formed pearl necklace- like one after another new volcanoes, as they would through weld through the crust. The best known example is the Hawaiian islands: the largest island of Hawaii, which is the youngest volcanic island over the " hot spot " is only 400,000 years old, while the oldest of the six volcanic islands Kauai has arisen in the northwest before about 5.1 million years ago. Examples of this rare type of volcanism in Europe can be found in the East and West Eifel ( Eifel ), the Seven Mountains and the Auvergne. There is such a hotspot Even under Iceland.

In the solar system

Volcanism is a normal phenomenon for terrestrial celestial body. On many worlds of the solar system there are traces extinct volcanic activity, such as on the Earth's moon or Mars. Volcanically active world in the solar system is Jupiter 's moon Io. On Saturn 's moon Enceladus, as well as Neptune's moon Triton Kryovulkanismus was observed.

Contrast, can be found on the mass, size and internal structure very Earth-like Venus is little evidence for currently active volcanism and no evidence of plate tectonics.

Prediction of volcanic eruptions

Whether a volcano is definitely expired or perhaps can become active again, especially interested in the people who live in the vicinity of a volcano. In any case, a volcanic eruption has far-reaching consequences because about the personal fate beyond infrastructure and economy of the region are strongly influenced. Therefore, the primary research objective is to predict volcanic eruptions as precisely as possible. Forecast errors alone would cost perspective disastrous (evacuation of thousands of people, decommissioning of economic life, and more. ).

Despite some similarities, no two volcanic eruption in his behavior the other. Thus, observations of periods of rest or seismic activity of a volcano are hardly transferable to another.

In the monitoring of volcanoes in general are five monitoring methods available, which are used depending on the volcano characteristics in different combinations: the recording of seismic activity, the geodetic monitoring of the topography, the measurement of gravimetric and magnetometric changes, the detection of near-surface temperature increases and the chemical analysis ascending volcanic gases.

Recording seismic activity, volcanic tremor

An eruption process is first initiated by the rise of magma. If the magma rises to pre- recorded or new fault lines, columns, or cracks to the surface, leads to characteristic seismic signals by tensions in the country rocks and degassing of magma. Rock breaks here and start cracks to vibrate. The destruction of rock triggers earthquakes with high frequency, the movement of cracks, however, leads to low -frequency earthquakes, the so-called volcanic tremor.

To determine depth and hearth of volcanic tremor, a network set up by highly sensitive seismometers around the volcano in the rule. It is precisely the weak earthquakes that have a thickness of less than 1, are often signs that a volcano is active. For example, about 2,800 minor earthquakes were recorded by the locally installed seismometers as tremor in the affected south-west slope of Mount Etna in the 12 hours before the 1981 eruption. About an automatic transmission system, the data were forwarded directly to the Istituto Internazionale di Vulcanologia in Catania. With the help of modern technology changes the seismic activity are now determined in real time. Structures and processes below the surface can thus be represented and analyzed immediately and accurately.

Geodetic monitoring

Penetrates magma from the depths to the top, so deformations of the earth's surface in the form of Aufbeulungen, dips, slopes, bumps and cracks can occur in certain areas of the volcano. These deformations can usually permanently installed in boreholes of the rock inclinometers ( Klinometern ) and strain gauges ( Extensiometern ) be measured on site. However, these phenomena can already be detected by simple means such as with a tape measure or by spray- lines.

Beginning of August 1982 had geologists discovered many narrow cracks in the soil in the crater floor of Mount St. Helens and marked with colored lines. Two days later the lines were clearly curved, indicating a change in the cracks caused by rising magma. A few days later there was a violent eruption of the volcano. In October 2004, a state bulging of a volcano flank of more than 100 m was observed at Mount St. Helens, which was visible with the naked eye.

A more complex and accurate method for the detection of morphological changes, for example, the measurement of horizontal distances with electronic distance measurement ( EDM). An EDM can send and receive electromagnetic signals. The wave phase shifted in dependence on the distance between the reflecting object and EDM and thus indicates the extent of the resulting shift. EDMs have ranges of up to 50 km and high measurement accuracies of a few millimeters. Surface changes, especially larger areas and remote volcanoes observed with the help of satellite geodetic measuring methods.

As can also alter groundwater and surface water levels relative to each other due to deformations of the site, groundwater monitoring wells are often set up and installed river and lake water levels in waters nearby areas. It now uses a satellite imagery to monitor volcanoes and their deformation or bulging.

Measurement of gravimetric and magnetometric changes

Penetration hot molten rock near the surface layers of soil, so local changes in the gravitational field can be observed. These local variations are caused by differences in density between magma and surrounding rock. Such so-called microgravimetric anomalies can be discovered with the help of highly sensitive gravimeters that come on active volcanoes used.

When magma ascent and local changes in the magnetic field can be registered, which are caused by thermal effects. Already in 1981, two magnetometric stations were taken with automatic remote data transmission in operation on the southern slope of Mount Etna and about 20 km to Mount Etna.

Detection of temperature increases

The rise of approximately 1100 to 1400 ° C hot magma from a magma chamber or directly from the upper mantle is primarily a local increase in temperature of the country rock along. With the help of fixed stations for temperature measurement and infrared images from satellites such thermal heating operations can be found, the aufgedrungener by near-surface melting caused congestion.

Analysis of rising gases

Eruptive gases are the main driving force of volcanic activity. Changes in their quantity, their temperature and their chemical composition are predicting a volcanic eruption of fundamental importance. In general, the variations in the chemical composition of the gases are higher, the hotter the gases and the more lively is the volcanic activity. At high gas emissions can the concentration of certain gases by means of its absorption spectrum in the visible light determined by remote sensing. The geochemical monitoring also extends to the observation of groundwater and of sources. For underground water is often contaminated by volcanic gases leaking from the magma and spread in the soil. Play a special role in helium and radon. Both gases are taken from the Earth's mantle. Rising to a magma chamber, then also increase the levels of these gases. So you have, for example, after the earthquake crisis on the Greek island of Nisyros (1996 ) have begun to monitor the gases and other factors exactly as it was feared that there could be an imminent volcanic eruption. Within the framework of the EU program GEOWARN, European universities have joined together and watch Nisyros, Vesuvius and other potentially dangerous volcanoes in Europe.

As part of the International Decade for " harm reduction in natural disasters 1990-2000 " 15 volcanoes worldwide have been selected as research objects and continuously monitored, including Vesuvius and Etna.


Despite the large number of early warning systems and many new insights in this area a certain unpredictability is never let off completely at volcanic eruptions. Parallel to the prediction of dangerous eruptions protection measures, risk and action plans, education of the population affected and legal regulations for the real thing are necessary. In addition, it might be worthwhile to observe the nature of an endangered region exactly. Often animals are more sensitive and leave a vulnerable zone long before a volcanic eruption.

The largest volcano on Earth

Nature Geoscience published in its September 2013 edition ( Vol 6 No 9 ) published an article in which researchers consider the undersea Tamu Massif east of Japan for the largest volcano on Earth.

Researchers led by William Sager ( University of Houston ) (Texas / USA ) analyzed rock samples of Tamu Massif. These come from a maximum of 175 meters depth; the massif rises several kilometers above the ocean floor. The shield-shaped undersea mountain could have arisen, as leaked huge Lavamengen at one point and during cooling created flat, far-reaching connections. It handle is probably a single volcano 650 kilometers long and 450 kilometers wide.