"Plattenfriedhöfe" dodge gravity
Sunken crustal rocks explain some geoid anomaliesRead out
Old crustal stone sunk into the depths of the plate boundaries not only affects chemistry and currents in the lower mantle, it also creates "dents" in Earth's gravity. This discovery, now published in "Nature Geoscience, " explains for some long time puzzling subsidence in the "geoid, " the gravity mapping of our planet.
In terms of gravity, the earth is more akin to a potato than a sphere: it is dented in some places and bulged in others. These bumps are caused, among other things, by areas of particularly high density in the earth's crust or in the mantle. For example, there are large depressions around the Pacific Ocean, in the west of the Atlantic, under the Ross Sea in the Antarctic, or under the deep-sea trenches of the Indian Ocean. Their cause was still unclear.
"Plattenfriedhof" in the lower coat
Now scientists from the California Institute of Technology (Caltech) in Pasadena, near Sonja Spasojevic, have found, with the help of a new model, that many of these low-gravity sites occur where plate-rock rocks push the earth's crust down into the depths. At the borders of the plates, where two tectonic plates move towards each other, one of them is subducted, because of its high density it sinks down to the lower mantle where it forms a "plate cemetery".
The sinking of the denser crust plates creates turbulence in the mantle, which carries lighter, less dense rocks up from deeper layers. This ascent, possibly associated with chemical reactions between sunken crustal plates and mantle rocks, causes gravity to fall regionally and thus also changes the geoid - the potato gets a dent.
(Nature, 10.05.2010 - NPO) Display