Iceland plume

The Iceland Plume is an upflow anomalously hot rock in the mantle beneath Iceland whose origin probably lies at the boundary between Earth's core and mantle in about 2880 km depth. He is after the general doctrine, the plume theory of W. Jason Morgan, the cause for the emergence of Iceland and for its volcanic activity that characterizes the island today.

• 5.1 Scientific contributions
• 5.2 Other

Geological development

The plume, which bears the name of Iceland and is now pretty much at the center of the island, is considerably older than Iceland. Volcanic rocks that are associated with him are to be found on both sides of the South Greenlandic coast and date in the case of West Greenland back about 58-64 million years; they fall so that together with the opening of the North Atlantic in the late Paleocene and early Eocene. It is assumed that the volcanism was caused by the fact that hot material of the plume head has flowed into areas under the lithosphere, which was already thinned by previous rifting, where it has produced large amounts of melt. The exact position of the plume during this period is debatable, but was probably under central Greenland; well is not yet clear whether the plume has risen only at that time from the deep mantle or whether he is much older and has also caused the old volcanism in North Greenland, Ellesmere Island and the Arctic Ocean ( Alpha back).

With the progressive opening of the North Atlantic Ocean east of Greenland during the Eocene North America and Eurasia began to drift apart; the Mid-Atlantic Ridge formed as an oceanic spreading center and part of the submarine volcanic system of mid-ocean ridges. In these plate movements, Greenland pushed over the Iceland plume away; various studies locate the plume some 40 million years, or a little east coast of südostgrönländischen ( Scoresby Sund ) and put it in relation to the North Atlantic flood basalt province. In the course of further ocean opening and plate drift to Plume and Mid-Atlantic ridge approached each other, and finally reached a part of the plume head in the area of thin lithosphere at the back, where it now came to increased melting and crust formation; this increased, the more the two snaking each other. The Greenland - Iceland and the Faroe Iceland - back, both areas of greatly thickened oceanic crust, are traces of this stage of convergence prior to the formation of Iceland.

The oldest crust of Iceland itself is about 20 million years old and was an old, now extinct mid-oceanic spreading center in the Westfjords ( Westfjords ) formed. The westward migration of the plates and thus the back above the plume away and the strong thermal anomaly of the plume led to this ancient spreading center died out before 15 million years and further west, in the area of ​​today's peninsulas Skagi and Snæfellsnes, a new formed; in the latter there is the volcano Snæfellsjökul still some residual activity. The spreading center, and thus the main activity, however, moved in front 7-9 million years turn eastward and formed the present-day volcanic zones in the southwest ( VMS; Reykjanes Hofsjökull - Vatnajökull ) and northeast ( NVZ; Vatnajökull Tjörnes ). Currently, a slow decay of activity in the VMS takes place ( Katla Vatnajökull ) develops during the initiated million years ago 3 volcanic zone in the southeast.

Besides the formation of Iceland Plume has also influenced the crusting of the adjacent portions of the Mid-Atlantic Ridge, especially the Reykjanes Ridge southwest of the island. It is observed there a significant thickening of the crust and an anomalous increase of the seabed, which is attributed to a hot, emanating from the plume mantle flow along the thin lithosphere beneath the back; the variations of the crustal thickness, forming a pattern of nested pointing away from Iceland Vs, shows that in this river has not been uniform. One of the different ways to explain this pattern, based on the interaction of the displacement of the spreading center with the Plumekopf.

Geophysical and geochemical observations

Information about the structure of Earth's deep interior can only be obtained indirectly by geophysical and geochemical methods. For the investigation of Iceland plumes as well as other Plumes next to gravimetric and Geoiduntersuchungen especially seismological methods and geochemical analyzes eruptierter lavas have proved successful. Numerical models of the geodynamic processes attempt to integrate these observations into a coherent overall picture.

Seismology

An important method for imaging large-scale structures in the Earth's interior is the seismic tomography, in which the study area " rays" is by earthquake waves are detected by various earthquakes of various possible directions with a network of seismometers. The size of the network is the deciding factor for the size of the area that can be imaged reliably. For the investigation of Iceland plumes is both global tomography, in which the entire mantle is imaged by using data of globally distributed stations with relatively low resolution, as well as regional tomography, where a limited Iceland seismic sensors on the mantle down to 400-450 km depth with higher resolution maps have been used.

Regional studies from the 1990s ( ICEMELT, HOTSPOT ) clearly show that under Iceland to at least 400 km depth roughly cylindrical structure with a radius of 100-150 km exists, in which the velocity of seismic waves by up to 3% ( P waves) or 4 % (S - waves) is reduced compared to the reference model; Most analyzes of these data even suggest an even greater reduction. Converting them using petrophysical models in a temperature anomaly around, it is clear that the mantle there is 150-250 ° C. hotter than normal. Some uncertainty in the models is given by the limited resolution of seismic tomography: a hot, narrow plume is of a less hot, wider difficult to distinguish.

The global tomography confirmed that the upper mantle beneath Iceland a strong anomaly with markedly reduced seismic velocities is present. For the lower mantle (below 660 km depth ), the picture is contradictory. In all studies, the anomaly is there much weaker and has an irregular shape; in some depth ranges it even seems to disappear, this low position, not all studies are equal. In other seismological methods also an abnormally hot area was discovered at the core- mantle boundary beneath Iceland, and also the structure of the seismic discontinuities in 410 and 660 km beneath Iceland indicates elevated temperatures. Therefore, the majority of scientists assume that the weaker signature of the plume in the lower mantle on the one hand, with possible temporal variability of the plume and / or the change in the physical properties of the mantle with depth, on the other hand with the limitations of the method and the available data is explain, but the plume extends in each case through the entire depth interval of the mantle.

Geochemistry

Numerous studies have examined the geochemical signature of the lavas studied that occur in Iceland and the North Atlantic. They produce an extremely complex, sometimes even contradictory picture, but also consistent in a number of important points. So it is not disputed that the source of volcanism in the Earth's mantle is chemically and petrological heterogeneous: in the formation of the melt not only the normal peridotite of the upper mantle is involved, but apparently also eclogite, whose origin is suspected in metamorphisierter, very old oceanic crust that has come in the subduction of an ocean years ago hundreds of millions into the mantle; also can be found eg on the basis of the isotopic ratios of noble gases indications that there is also a contribution of rock from the lower mantle.

The variations in the content of trace elements such as helium, lead, strontium, neodymium show clearly that Iceland is also geochemically an anomaly compared to the rest of the North Atlantic. The ratio of He-3 to He- 4 for example, has a distinctive, well correlated with geophysical anomalies maximum to Iceland, and the decay of these and other geochemical signatures with increasing distance from the plume allows to estimate that the influence of the Plumes approximately 1500 km along the Reykjanes ridge and at least 300 km along the Kolbeinsey - back covers. Depending on which elements you look and how large the area is derived from the sample, one can identify up to six different mantle sources, but never all to meet in one place.

Furthermore, some studies show that the content of water, which is dissolved in the mantle minerals, in the area of ​​Iceland plumes two to six times higher than in mantle under undisturbed parts of the mid-ocean ridge where it is set to about 150 ppm.

Gravity / Geoid

The North Atlantic is characterized by strong, large-scale gravimetric and Geoidanomalien, located in the center of Iceland. The geoid rises there in a roughly circular area of hundreds of kilometers in diameter up to 70 m above the geodetic reference ellipsoid, or up to about 25 m above the hydrostatic reference figure of the earth. This anomalous 25 m, or a part thereof, can be explained by the dynamic effect of the upflowing plume, where the surface which bulges outwardly. The Plume and the thickened crust also cause a positive gravity anomaly of about 60 mgal ( = 0.0006 m / s ² ) ( free air).

Geodynamics

Since the mid- 1990s, several attempts have been made to the observations using numerical geodynamic models of mantle convection to explain. The aim of this modeling was among other things to solve the contradiction that a wide plume at a relatively low temperature anomaly is more compatible with the observed crustal thickness, topography and gravity, while a narrow hot plume to better seismic and geochemical models fits. The latest models suggest that the plume probably 180-200 ° C hotter than the surrounding mantle and its trunk has a radius of about 100 km, ie the seismological models to be confirmed; Crustal thickness, topography and gravity can be explained by such a model, when it is considered that the loss of water dissolved in mantle rock massive change on melting, the flow behavior of the plume, so that the corresponding anomalies are wider and it produces less melt. However, existing models do not or only very simplified account of the petrological heterogeneity.

The mentioned in the section on geological history V- pattern of the crust of the Reykjanes Ridge are of geodynamic models using pulsations of the plume, that is explained by variations in the mass flow through the Plumestamm.

Alternative models

As mentioned at the Plumemodell is the prevailing view for explaining the emergence of Iceland and its volcanoes. In particular, the weak visibility of the plume in tomographic images of the Earth's lower mantle and geochemical evidence of eclogite in the mantle source, however, have cast doubt on the validity of the Plumemodells some scientists like Don L. Anderson. As an alternative mechanisms are proposed, which are limited to processes in the upper mantle.

After one of these models, a large piece of the subducted plate of a former ocean has survived hundreds of millions of years in the uppermost mantle, and what has become eclogite oceanic crust now provides for excessive melt formation and the observed volcanism. This model, however, is not substantiated by dynamic modeling, is not enforced by the data situation and also leaves unanswered questions such as the following the dynamic and chemical stability of such a body over a period of time or after the thermal effect such a massive melt formation.

Another model suggests that the updraft in the area of Iceland is driven by lateral temperature gradient between the shell and the neighboring subozeanischen Greenland craton and is therefore also limited to the upper 200-300 km of the mantle. However, this is Konvektionsmechanismus under the conditions prevailing in the North Atlantic conditions with respect to the spreading rate of the mid-Atlantic ridge probably not big enough and does not offer a simple explanation for the observed eg Geoidanomalie.