Hotspot (geology)

As hotspots [ hɔt.spɔ͡ts ] (English: ' hot spots ') are denoted by Manteldiapiren in connection standing, localized, relatively stationary, particularly hot areas in the asthenosphere, which in the overlying crust by volcanic activity or at least by a increased heat flux density to comment. Hotspot volcanism occurs mostly at a greater distance from plate boundaries. It is also called by intraplate volcanism. The possibility of the existence of hot spots as the cause of intraplate volcanism was first drawn in 1963 by the geologist John Tuzo Wilson considered.

Geodynamics

The mantle in the range of a hotspot is particularly hot since there (possibly from the core - mantle boundary ) rising mantle material from the Earth's deep interior. The about 150 km wide zones of ascent are also called Manteldiapire: referred (English mantle plumes ). The higher temperatures cause an increased melting in the upper mantle. The melts can ascend to the surface due to their low density and cause a mostly basaltic volcanism. The geochemical signature of OIB thus formed (English: ocean Iceland basalt ) often differs significantly from the so-called MORB (English: MidOcean ridge basalt ).

Since the lithospheric plates glide steadily over the largely stationary constant hotspot away, while the hot mantle material through the plate " welded ", is produced by and by several volcanic edifices, each as long as supplied with melt as they are above the mantle anomaly. In this way, volcanic chains form as the Hawaiian Islands. It should be noted that the crustal thickness of the ocean basins is just 6 km on average, while it is mighty among continents in the middle about 30 km. The continental crust is therefore difficult to penetrate, so intense hotspot volcanism occurs mainly in oceanic areas.

Hotspots and plate movements

Hotspot volcanoes on oceanic crust are a source of data for determining "current " (post - Miocene ) or past plate motions. From the apparent migration of hotspots and determining the age of the basalts caused by him volcanic islands, one can reconstruct the direction and speed of movement of the corresponding lithospheric plate. In the case of the Hawaiian archipelago were, inter alia, basalts from Mauna Kea ( Hawaii 's main island, "Big Iceland " ) to 0.20-0.25 million years, from the Haleakala on 0.95-1.0 million years and the of Waianae on Oahu dated to 3.05 to 3.10 million years, which, with the involvement of the youngest member of the chain of islands which still lying below sea level Loihi, an apparent migration of the hotspots of about 10 cm / year (100 km / million years), corresponds to a direction of movement of about 300 ° ( ONO). In comparison with data from other Hotpsots ( the so-called hotspot reference frame ) yields that, depending on the computer model, a " current" mean velocity of the Pacific plate between 8.3 cm / year (83 km / million years) and 10.5 cm / year (105 km / million years).

Taking the basic assumption that hotspots over long geological periods are fixed, can by appropriate investigations even older Vulkanbauten, which go back to the Hawaiian hotspot ( Hawaii -Emperor seamount chain), and movements are reconstructed that are farther in the past. The sharp " kink" in the Hawaiian - Emperor seamount chain then explained by the fact that the sense of movement of the Pacific plate has (about 43 mya ) changed dramatically during the Eocene.

Both laboratory tests and in-depth investigations of the relative movements of hotspot island chains show that hotspots can perform a proper motion of 1-2 mm / year. Just take the example of the " kink " in the Hawaiian - Emperor chain is the subject of a scientific controversy: Such abrupt change in motion of the Pacific plate would have to be accompanied by major tectonic events in the Pacific Ocean about 43 million years ago. Visible signs of this, there is no, so even a relatively large proper motion of the plume must be considered. Paleomagnetic studies support this assumption. Taking into account the rheology of the mantle is much more likely that " Knicks " go back in hotspot island chains to changes sheath currents caused by mantle convection. A comparison paläotektonischer reconstructions confirms this.

Loihi currently is still 900 meters under the sea, but in the opinion of geologists reach a height of 4,000 m above sea level in the next million years. You would then be exactly as large as the Mauna Kea volcano today. This thesis follows the view of Wilson, that the growth of all hotspot islands will always run the same. The different ages of the Hawaiian Islands shows, inter alia, in the various stages of weathering, the youngest island of Hawaii is realized through regular volcanic eruptions still growing and shrink the northwestern neighboring islands due to erosion and subsidence already again.

Known Hotspots

Since the identification of small plumes is very difficult to find the exact number remains unclear. In the technical literature catalogs of observed hotspots worldwide have been published in various numbers. About 50 of them have so far been clearly verified by seismological investigations as mantle plumes or classified as very likely candidates. The best known examples of hotspot volcanism in addition to the Hawaiian Islands and Iceland (where the interaction with the volcanism of mid-ocean ridge ), the Eifel region of Germany, the politically Ecuador belonging Galápagos Islands and Yellowstone National Park in Wyoming.