Throughout geologic history, continents have been pulled apart by tectonic forces forming rifts that eventually become new ocean basins.
Sometimes during this process rock layers near the earth's surface are pulled apart and rocks from depths of 35 kilometers or more are exposed at the Earth's surface.
These deep "crustal" rocks are usually metamorphic rocks that have been re-crystallized by heat and pressure. Surface exposure often forms domes called metamorphic core complexes, which are higher than surrounding terrain.
How these formations develop has been a much-debated question in geology. University of Hawai'i researchers Fernando Martinez, Andrew M. Goodliffe, and Brian Taylor have proposed a new explanation for these formations by studying the offshore areas of Papua New Guinea. Their findings were published in the June 21 issue of Nature Journal.
Eastern Papua New Guinea is a present-day example of a continental land-mass in the early stages of rifting and forming a new ocean basin. Here the processes that rift continents and form ocean basins can be studied directly in their active stages.
The heat flowing from the Earth's interior can be used as a measure of the degree of stretching of the upper strong layer of the Earth. Where there has been a great deal of stretching, this upper layer is thin and the hotter deeper layers are closer to the surface of the Earth producing a high flow of heat.
Using sensitive thermal probes, the researchers measured the flow of heat from the Earth's interior in the deep sea sediment.
Thermal measurements were taken between Papua New Guinea and the D'Entrecasteaux Islands because these islands are metamorphic core complexes that are rising at the same time that the floors of the surrounding basins are deepening.
The thermal measurements revealed that the basins do not have elevated heat, but the islands do.
Examining what is known about the geology of the islands and surroundings, the Hawaii researchers formulated their new model for the formation of the islands.
They discovered that the basin and islands are in a region that was once a continuous layer of dense oceanic crust and mantle (the layer of the Earth between the crust and the core), the type of material that is generally present for the formation of the floors of major ocean basins.
This dense oceanic layer was thrust over part of Australia 50 to 60 million years ago. The result was a heavy oceanic layer pressed tightly over a lighter continental layer.
When the current extension began, the upper layer was split and the lower layer was squeezed up and out through the crack like toothpaste, which was the formation of the islands.
The flow and thinning of the lower continental layer caused the upper oceanic layer of the surrounding basins to sink but not stretch. The heat from the thinning lower layer did not have sufficient time to cross over the upper layer, except where the hot lower layer was squeezed out at the islands.
This superposition of a denser layer over a lighter one is called a density inversion, since it goes counter to the general trend observed on Earth of density increasing with depth.
Nevertheless, these density inversions have occurred repeatedly on Earth as a consequence of collisions of different terrains due to the ever-moving tectonic plates.
In other areas, the fact that density decreases with increasing temperature may create such a density inversion in the crust even without collisions and thrusting of heavy layers over lighter ones.
The processes at work in raising the islands of eastern Papua New Guinea may provide a general explanation for the formation of many similar core complexes throughout the world.
University of Hawai'i
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