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February Editorial      

 

Fragment of an ancient super-continent discovered under the Indian Ocean

 

Through Earth's geological history supercontinents have been forming and breaking up. It's a cyclic event. The most recent supercontinent to form has been called Pangaea ('Pan Gaia' means 'all -earth'). It formed about 300 million years ago and 100 million years later it started to break up into the continents we have today.

The super-continent before Pangaea was Rodinia. Plate tectonic reconstruction shows that Rodinia existed between 1.1 and 750 million years ago in what is known as the Neoproterozoic era. It is probable that Rodinia contained most or all of Earth's landmass. Other than that Rodinia was composed from the accretion and re-assembly of fragments from yet another older supercontinent, known as Columbia, relatively little is known about the Rodinia's formation.

However the movement of continents since the breakup of Rodinia some 750 million years ago is relatively well understood. The Neoproterozoic era was a time of extensive volcanic eruptions and lava flows, evidence for which can be found on all continents today. This suggests that about 750 million years ago there was widespread rifting - rifting being linear zones where the Earth’s crust and lithosphere are being pulled apart.

Rodinia is of much interest today because of the major geological and climatic events which followed the supercontinent's breakup. About 700 million years ago the earth cooled so quickly and dramatically that the event known is known today as 'Snowball Earth'. This event was followed by the rapid evolution of primitive life forms. Scientists attribute both the cooling and subsequent evolutionary acceleration to the breakup of Rodinia.

It is thought that one of the first rifts leading to the break-up of Rodinia began as early as 850 to 800 million years ago. On one side of the rift were what are now the continental masses of present-day Australia, eastern Antarctica, India and the Congo and Kalahari cratons. (A craton is a chunk of tectonic plate which has been around since at least the Precambrian era.) The Laurentia, Baltica, Amazonia and the West African and Rio de la Plata cratons were on the other side of the developing split.

Now we jump forward in time to a relatively recent event dated around 170 million years ago when Madagascar split from India. This split was caused by magma plumes under the crust, and these plumes are currently sitting underneath the Indian Ocean Islands of Marion and Reunion. Scientists believe that during the sort of split of land mass which occurred on this occasion, small fragments may be separated from the main craton. These fragments are called microcontinents. The Seychelles are a well-known example of such a continental fragment. But a recent study in Nature Geoscience (see ref 1) provides evidence that another microcontinent was once tucked between India and Madagascar. This microcontinent is now submerged in the Indian Ocean beneath Reunion and Mauritius (two small islands of the Madagascar landmass which are now popular tourist destinations).

The research team led by Professor Trond Torsvik of the University of Oslo, Norway collected sand samples from the beaches of Mauritius and dated them using a uranium-lead dating technique (U-Pb). While much of the beach sand was young lava grains dating back to a volcanic eruption of about nine million years ago, it also contained minerals that were much, much older. These were zircons.

Zircon (ZrSiO4) is an interesting mineral for a number of reasons. For a start it is evenly distributed within the Earth's crust. Another interesting property of Zircon is its durability. Zircons can survive most geologic processes, including erosion, transport, and even high-grade metamorphism, making them invaluable for dating purposes (see ref 2).

Analyses of the zircons extracted from the Mauritius samples showed that they were either Palaeoproterozoic (more than 1,971 million years old) or Neoproterozoic (between 660 and 840 million years old). This suggests that these zircons were brought to the surface by plume-related lava from ancient fragments of the continental lithosphere which now lies somewhere beneath Mauritius island. This date suggests that this fragment may well be a microcontinent old enough to have belonged to the Rodinia supercontinent. The researchers call the microcontinent Mauritia.

The scientists modelled plate tectonics to calculate exactly where and how the Mauritia piece ended up under the Indian Ocean. Prof Torsvik said that he believes pieces of the lost Mauritia microcontinent could be found about 10 km down beneath Mauritius and under a swathe of the Indian Ocean. But to prove that beyond doubt further research is necessary. One way to go about it is to obtain seismic data which can image the structure. Alternatively, one could drill deep, but the latter would be very expensive. Nevertheless such research would bring to light further details of a key period in the Earth's development.

Journal Reference:

  1. Trond H. Torsvik, Hans Amundsen, Ebbe H. Hartz, Fernando Corfu, Nick Kusznir, Carmen Gaina, Pavel V. Doubrovine, Bernhard Steinberger, Lewis D. Ashwal, Bjørn Jamtveit. A Precambrian microcontinent in the Indian Ocean. Nature Geoscience (2013), 6, pp. 223–227.
  2. Wolfgang Siebel, Axel K. Schmitt, Martin Daniík, Fukun Chen, Stefan Meier, Stefan Wei and Sümeyya Erolu. Prolonged mantle residence of zircon xenocrysts from the western Eger rift. Nature Geoscience (2009), 2, pp. 886 - 890.

 

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