Magma chamber

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11 - Magma chamber below a volcano
Artist's impression of the hotspot magma chamber of the Yellowstone volcano under the national park . Source: USGS
granite
Former magma chamber: Dendi Caldera, Ethiopia

In the geosciences, areas in the lithosphere that are filled with liquid magma and whose temperature is usually significantly higher than that of their surroundings are referred to as magma chambers (also known as magma hearth ) . These arise when magma formed in deeper layers, which is lighter than the bedrock, rises in the form of magma bubbles or along weak zones and interrupts its ascent there. Overall, however, the knowledge about the magma transport routes is still somewhat rudimentary.

However, since the 1990s, the notion of large cavities filled with semi-liquid rock has no longer prevailed, but rather zoned magma reservoirs . The designations are also not clear in the secondary literature, sometimes a magma reservoir is used to describe an accumulation of magma that lies on the boundary between mantle and crust ( Moho ) and thus deeper than the actual postulated magma chamber. The magmas are said to form disk-shaped accumulations on the roof of the reservoirs under Iceland's central volcanoes, up to 100 km long and about 5–10 km wide.

Characteristics of magma chambers

Magma chambers represent regional anomalies in the structure of the earth's interior , but are also the source of numerous rock formations. They can make themselves felt through tectonic activity and volcanism . If at all, they are primarily detectable using geophysical means , for example using seismological methods - the waves of earthquakes are dampened when they pass through liquid reservoirs - or by gravimetric measurement of gravity anomalies .

The temperature of the magma in these chambers is between 1500 ° C and 900 ° C, sometimes even below.

Fractional crystallization

Certain minerals with very high melting points can precipitate early on in fractional crystallization in molten rock, such as B. Chromite . If these minerals are specifically heavier than the residual melt, they sink to the bottom of the magma chamber, where they can accumulate and thus form deposits , e.g. B. orthomagmatic deposits , one also speaks here of a sediment, an accumulation .

Rise of magmas

Intrusion formation

Of these chambers - the magma can along - which lie between several kilometers and tens kilometers deep fissures and zones of weakness or by so-called transitions up penetrate, and in the slow solidification rock plutonic with different sized crystals form. In connection with this magma solidified in ducts, one speaks of intrusions or - in relation to large accumulations such as solidified magma reservoirs or chambers of plutons .

The actual pattern of such a magma chamber was and is the Skaergaard intrusion . The zoned intrusion is located in East Greenland , which was once on the Iceland - Hotspot was. You can understand the different crystallization phases in three clearly differentiated areas. Since the intrusion has turned a bit, one can nowadays v. a. study well the bottom of the magma chamber with the rock formed there by fractional crystallization .

Volcanic eruption

If the rock melt from the magma chambers penetrates to the surface of the earth because the pressure in the magma chamber is greater than the strength of the rock above, phenomena of volcanism occur , i.e. H. to volcanic eruptions of various shapes.

Crater shapes created by the collapse of a magma chamber near the surface are called caldera .

Section through a central volcano

Zoning of magma reservoirs

Referring to the rheology , d. H. the degree of fluidity of the respective minerals, the magma chambers can be divided into different areas depending on temperature , crystal content and viscosity . Fractional crystallization works here. Initially it was assumed that heavy metals would sink. However, this has been restricted by recent research, so that under certain conditions convection currents in the magma are also assumed. It affects more highly differentiated magmas, in which, especially on the side walls of the reservoir, highly differentiated melt, i.e. H. Melt with a higher crystal content, because of its lower density , rises upwards.

Above all, fallout deposits, which show a clear stratification - often recognizable by the different colors of the rock - also evidence stratification in the magma reservoir. The more highly differentiated products such as rhyolites and phonolites come to lie at the bottom, and the less developed products, such as basalts , for example - one rotation of the arrangement in the reservoir, because the i. d. R. were first expelled. Examples are ignimbrite from Mount Mazama ( Crater Lake ) or from the Laacher See volcano.

Further examples

Mid-ocean ridges

Fissure eruption at Krafla, Iceland, 1984

Thousands of magma reservoirs are suspected on mid-ocean ridges - the finding of gabbro from the depths in particular proves this - but exploring them is difficult. At the end of the 1990s, some details were discovered through research on an ocean ridge off South America.

In places with a high spreading rate (on the Nazca plate 15 cm per year) , one assumes an elongated melting zone along the back, on which a crystal pulp zone rests, above a zone with small magma pockets. The eruptions are initiated by the plate movements and produce Fe-rich, low- viscosity lavas and tephras .

In areas with a medium spreading rate, small, isolated enamel lenses are assumed, for example at the tip of divergent rift zones, where increased differentiation develops.

If the spread rates are low and the magma supply is low, magma reservoirs are unlikely to be created.

Iceland turns out to be a special case here, since the spreading rate is rather low here (approx. 18 mm per year), but on the other hand - presumably because of a hot spot under the island - there is a high magma production rate and eruption rate . Seismic measurements indicate near-surface magma accumulations which are located at a depth of approx. 10–15 km under the Icelandic volcanic zones . Magma reservoirs have been detected under the central volcanoes in even greater proximity to the surface, for example under the Krafla at a depth of approx. 3–7 km. During the Heimaey eruption series in the 1970s, magma movements were detected under the Eldfell volcano at a depth of 15-25 km.

Due to the degree of crystallization of ejected rocks, there are probably really lingering zones for the magmas under the central volcanoes of Iceland. Here, individual tunnels are initially formed, later swarms or sill . Afterwards, these swarms of dykes and intrusions can be expected to densify until a magma chamber emerges at a depth of 3–8 km. Lenticular magma chambers of this kind are presumably present under Krafla, Grímsvötn and Hekla . Their volume should be around 10–100 km 3 . During rift episodes or the penetration of fresh basalt magma from the mantle, the magma from this depth can move very quickly to the surface and erupt.

Hawaii

Kilauea

The Hawaiian volcanoes have been studied comparatively well.

Under Kilauea, for example, one recognizes a columnar magma structure, which is located about 2 to 6 km below the summit area, with an elliptical cross section and an estimated volume of 5–10 km 3 . The feed system is probably made up of many branches and sills, which together ensured the very constant delivery rate of 3 m 3 / s in the years before 2000. The investigation of older magmas that were ejected after longer breaks in the eruption also showed greater differentiation.

In the asthenosphere , the magma rises under Kilauea, presumably in the form of diapirs . The magma from alkali basalt seems to get directly from the mantle to the surface, while that from tholeiite basalt goes through different phases. Initially, shells made of melted mantle material around olivine and pyroxe crystals form at a depth of 60–80 km . After a while, these form small magma pockets, while at the same time the volume increases and the density decreases. These processes push the magma upwards. However, this is temporarily stalled at the boundary between the asthenosphere and the lithosphere, a process known as underplating .

Then the magma, which continues to melt due to the loss of density, rises through cracks and passages, the latter u. a. formed due to the weight of the resting volcanic building. However, it is not a continuous process, but rather it happens in bursts, with used crevices closing again while new ones open, which would explain the continuous earthquake activity under many calderas. During the ascent, the density continues to decrease, and only when this is equal to or higher than that of the surrounding rock can a larger pocket in the form of a magma chamber form. Its roof is about 3 km deep from the summit area of ​​Kilauea, the bottom about 6–8 km deep and 3 km wide. In addition, olivine is believed to be at higher altitudes, which would ensure a quick ascent of the magma. As soon as the magma chamber is filled, which can be seen from the bulge, which is measured with so-called tilt meters, vertical and / or horizontal corridors are formed and often a peak or flank eruption follows, although a large part of the magma solidifies as intrusions.

See also

literature

  • Gerd Simper: Understanding and experiencing volcanism . Feuerland Verlag, Stuttgart 2005, ISBN 3-00-015117-6 .

Web links

Individual evidence

  1. ^ F. Press, R. Siever: General geology. Spektrum Akademischer Verlag, Heidelberg 1995, ISBN 3-86025-390-5 .
  2. ↑ a b c d e f g h i Gerd Simper: Understanding and experiencing volcanism . Feuerland Verlag, Stuttgart 2005, p. 35.
  3. ↑ a b Ari Trausti Guðmundsson: Living Earth. Facets of the geology of Iceland. Mál og Menning, Reykjavík 2007, p. 154.
  4. ↑ a b Kent Brooks: Skaergaard Intrusion. Retrieved September 23, 2012.
  5. ^ HU Schmincke: Vulcanism. 2., revised. u. supplementary edition. Darmstadt 2000, p. 59.
  6. ^ HU Schmincke: Vulcanism. 2., revised. u. supplementary edition. Darmstadt 2000, p. 30.
  7. ^ HU Schmincke: Vulcanism. 2., revised. u. supplementary edition. Darmstadt 2000, p. 29.
  8. ^ G. Fabbro: Beneath the volcano: The magma chamber. Science 2.0, November 5, 2011.
  9. ↑ a b H. U. Schmincke: Vulkanismus. 2., revised. u. supplementary edition. Darmstadt 2000, p. 29ff.
  10. ↑ a b c d H. U. Schmincke: Vulkanismus. 2., revised. u. supplementary edition. Darmstadt 2000, p. 59f.
  11. ^ Þorleifur Einarsson: Geology of Iceland. Rocks and Landscape. Mál og Menning, Reykjavík 1994, p. 119.
  12. ↑ Ari Trausti Guðmundsson: Living Earth. Facets of the geology of Iceland. Mál og Menning, Reykjavík 2007, p. 155.
  13. ^ HU Schmincke: Vulcanism. 2., revised. u. supplementary edition. Darmstadt 2000, p. 72f.
  14. ↑ a b Ken Hon: Ascent of Magma from the Mantle . Evolution of Magma Chambers in Hawaiian Volcanoes. GEOL 205: Lecture Notes, Univ. of Hawaii, Hilo; Retrieved September 23, 2012.