Magma chamber

As magma chamber (also called magma ) areas in the lithosphere are referred to in the geosciences that are filled with liquid magma and its temperature is usually much higher than that of their surroundings. These occur when formed in deeper layers magma, which is lighter than the bedrock which rises in the form of magma bubbles or along zones of weakness and there interrupting his climb. Total knowledge of the magma conveying paths, however, is still somewhat rudimentary.

However, since the 1990s prevails not so much the idea of ​​great filled with semi-liquid rock cavities, but it 's more from zoned magma reservoirs. The names are in the secondary literature also ambiguous, sometimes is referred to as magma reservoir accumulation of magma, the boundary between the mantle and crust ( Moho ) is at the, and thus lower than the actual postulated magma chamber. The magmas idea is the disk-shaped lumps on the roof of the reservoirs of up to 100 km long and 5-10 km wide approximately at Islands Central volcanoes.

• 5.1 mid-ocean ridge
• 5.2 Hawaii

Characteristics of magma chambers

Magma chambers represent regional anomalies in the structure of Earth's interior, but are also the source of many rock formations. You can make itself felt by tectonic activity and volcanism. If anything, they are mainly concerned with means of geophysics detectable, for example, methods of seismology - so the waves of earthquakes during the passage of liquid reservoirs are attenuated - or by the gravimetric measurement of gravity anomalies.

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

Fractional crystallization

Early on, can excrete certain minerals with very high melting points, such as chromite is in the fractional crystallization in red-hot molten rock melts. When these minerals are heavier than the residual melt, they sink to the bottom of the magma chamber, where they are concentrated and so can form deposits, such as ortho magmatic deposits, one also speaks of a sediment, a cumulate.

Ascent of magmas

Intrusion Education

Of these chambers - the lie deep between a few kilometers and dozens of kilometers - can the magma along fractures and weak zones or by so-called transitions up to penetrate and form plutonic rocks with different sizes of crystals in the slow solidification. One speaks in connection with this solidified magma in aisles by intrusions or - with respect to large collections 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 once lay on the Iceland hotspot. One can understand the different stages of crystallisation in three clearly distinct from each other areas. Since the intrusion has turned something that nowadays, you can va well the bottom of the magma chamber with the rock formed there by fractional crystallization study.

Volcanic eruption

Penetrates the rocks melt from the magma chambers up to the surface, because the pressure in the magma chamber is greater than the strength of the overlying rock, it comes to phenomena of volcanism, ie volcanic eruptions various forms.

Due to the collapse of a shallow magma chamber resulting crater caldera forms are called.

Zoning of magma reservoirs

Referring to the rheology, that is, the liquid level of the respective minerals, you can see the magma chambers in different areas divided as a function of temperature, crystal content and viscosity. This affects the fractional crystallization. First, we had assumed a decrease of heavy metals. This however, has been limited by recent research, so that one also assumes convection currents in the magma under certain conditions. It affects more differentiated magmas, in which, because of their lower density rises to the top, especially on the side walls of the reservoir highly differentiated melt, ie melt with higher crystal content.

Especially fallout deposits that a significant stratification - often identifiable by different colors of the rock - show also layers in the magma reservoir situated. The highly differentiated products such as rhyolites and phonolites below, and the less developed, such as basalts come to be at the top - a revolution of the arrangement in the reservoir because the uppermost usually were first launched. Examples include ignimbrites from Mount Mazama ( Crater Lake ) or from the Laacher See volcano.

Other examples

Mid-ocean ridge

Thousands of magma reservoirs are suspected to mid-ocean ridges - especially the discovery of gabbro from the depths proves this - but their research is difficult. In the late 1990s it has been found among other things, research on an ocean backs off South America some details.

Here you go places with high spreading rate (at the Nazcaplatte 15 cm per year) of an elongated melting zone along the back, on the one Kristallbreizone rests above a zone of small magma pockets. The eruptions are initiated by the plate motions and produce Fe - rich, low-viscosity lavas and Tephren.

In areas with medium spreading rate puts you ahead of small isolated melt lenses at about the peak of divergent rift zones, where increasingly developed differentiation.

If the spreading rate is low and the magma supply low, presumably there are no magma reservoirs.

Iceland turns out to be a special case, since the spreading rate is rather low (about 18 mm per year ), on the other hand - presumably because of an existing island hot spots - present a high magma production rate and eruption rate. Seismic measurements refer here to near-surface magma accumulations which are located at about 10-15 km depth under the Icelandic volcanic zones. Magma reservoirs were among the central volcanoes detected in even closer to the surface, such as under the Krafla in about 3-7 km depth. During the Heimaey eruption series in the 1970s about magma movements were detected under the Eldfell Volcano in 15-25 km depth.

Due to the degree of crystallization of ejected rocks there are probably really residence zones for magma under the central volcanoes in Iceland. Here individual gears are first formed, later transition of schools or Sill. After that is to be expected with a compression ratio of this transition swarms and intrusions, until finally a magma chamber is created within 3-8 km depth. Lenticular such magma chambers are present presumably under Krafla, Grímsvötn and Hekla. Their volume is expected to be at 10-100 km3. While Rift episodes or the ingress of fresh basalt magma from the mantle can very quickly move to the surface and erupt the magma from this depth.

Hawaii

The Hawaiian volcanoes have been relatively well studied.

Under the Kilauea about one recognizes a columnar structure Magma, which is about 2-6 km you will find below the summit area, with an elliptical cross-section and an estimated volume of 5-10 km3. The feed system consists probably of many ramifications and Sills together, the total m3/sec for the very constant flow rate of 3. ensured the years before 2000. The investigation of older magmas that were expelled after prolonged eruption breaks, occupied here also greater differentiation.

In the asthenosphere magma rising under the Kilauea probably in the form of diapirs. In this case, the magma of alkali basalt seems to go directly from the mantle to the surface, while that by turns Tholeiitbasalt different phases. First formed in 60-80 km depth shells fused- coat material to olivine and pyroxene crystals around. These form after some time small magma pockets, at the same time, the volume increases and the density decreases. These operations press the magma upward. However, this device temporarily on the border of asthenosphere and lithosphere to a halt, a process which is called underplating.

Subsequently, the density loss due to continued melting magma rises through cracks and transitions upward, with the latter have formed inter alia due to the weight of the volcano aufruhenden building. However, it is not a continuous process, but rather it happens in spurts, with Columns used close again, while opening new, which would explain the continuous earthquake activity among many calderas. As you climb the density continues to decrease, and only if it is equal to or higher than that of the surrounding rock, a larger bag can form in the form of a magma chamber. Their roof is located about 3 kilometers depth from the summit area of ​​Kilauea, the ground about 6-8 km depth with a width of 3 km. In addition, probably located olivine at higher altitudes, which would ensure a rapid ascent of the magma. Once the magma chamber is filled, what is against the bulge, which is measured by the so-called tiltmeters, knows himself, vertical and / or horizontal transitions and often followed by a summit or flank eruption, but solidifies also a large part of the magma as intrusions form.