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Mysterious Origin of Christmas Island Deciphered

German researchers discover hitherto unknown process in which continental material is transported into the upper regions of the Earth’s mantle When it comes to the formation of volcanic islands such as Hawaii, the so-called hotspot theory is usually used as an explanation. However, there are oceanic volcanoes that do not fit into the picture and have so far remained mysterious. These include the extinct underwater volcanoes around Christmas Island in the Indian Ocean. German marine researchers were now able to solve part of the mystery. Like pearls on a string – volcanic islands or underwater volcanoes often lie side by side in the oceans. Such volcanic chains, such as the Hawaii Archipelago, can be explained by the so-called hotspot model. At a fixed location, the hotspot, hot material from the earth’s interior penetrates through the oceanic crust and forms a volcano on it. Since the earth plates move over the hotspot, the volcanic cone also moves away from the hotspot at some point. It no longer receives a supply of lava and goes out, while a new volcano forms directly above the hotspot. In the course of millions of years a whole chain of extinct volcanic cones is formed, which indicates the direction of movement of the respective earth plate. But this simple and conclusive model cannot be applied to all underwater volcanoes.

“The formation of the Christmas Island Seamount province in the eastern Indian Ocean, for example, does not fit in at all,” says Kaj Hoernle of the Leibniz Institute of Marine Sciences (IFM-GEOMAR) in Kiel. The associated volcanic cones extend over an area of 1800 by 600 kilometres in the sea area between Australia and Indonesia. The ocean crust in this region grows in a north-south direction, but the seamounts extend very irregularly in an east-west direction.

So far unknown processes

“We wanted to investigate the origin of these mysterious seamounts in order to find out whether there are processes that we don’t even know yet, says Hoernle. This is why a team of researchers from IFM-GEOMAR under Hoernle’s leadership, the German research vessel SONNE, mapped and sampled the seabed of the sea area comprehensively in 2008. Subsequently, the recovered samples and the survey data were intensively analysed and evaluated at IFM-GEOMAR and the University of Sydney. The results have now been published in the international journal “Nature Geoscience”. It turned out that the investigated seamounts originated 136 to 47 million years ago, whereas the eastern ones tend to be older. In addition, they are only slightly younger than the ocean crust on which they stand. This suggests that they were formed near a mid-ocean ridge where the ocean crust is formed. The biggest surprise for the scientists, however, was the geochemical analysis of the samples. It showed that the source from which the lavas of these volcanoes originate has similarities to continental material. This is very unusual for oceanic volcanoes of this size.

Gondwana breaks apart

“We have linked these results to plate tectonic reconstructions and found that the Christmas Island Seamount province originated exactly at the point where Australia, India and Western Burma separated when the supercontinent of Gondwana broke up about 150 million years ago,” Hoernle explains. A new ocean formed at the site of the rupture and oceanic crust began to form between the fragments. It is possible that continental material has reached the upper mantle under the newly formed ocean floor. “Continental material is easier to melt than normal oceanic mantle. There was a magma surplus, which eventually formed the seamounts – including the continental material we now find in the middle of the ocean,” Hoernle explains. The researchers have thus found a previously largely unknown process that transports continental material into the upper regions of the oceanic mantle. “This finding is another important piece of the puzzle for understanding the processes and material cycles that take place deep inside the Earth. They form the earth on which we live, but unfortunately they are beyond direct observation,” explains the marine geologist.