A CONTINENT that rose above the Indian Ocean at least three times over 80
million years, only to disappear beneath the waves again each time, has been
discovered on the seabed about 4000 kilometres southwest of Australia. The
events that raised the Earth鈥檚 crust above sea level are so unexpected that
theories of how continents form may have to be revised.
Geologists aboard the drilling ship Joides Resolution, which is working on
the international Ocean Drilling Program, have found that two submerged
features, the Kerguelen Plateau and Broken Ridge, now about 2000 metres below
the surface, were once joined. Together, they formed a continental shelf of 2
million square kilometres.
Only a small part鈥攏o bigger than Britain鈥攁ctually protruded above
the surface. This landmass probably played a role in the migration of animals
after the supercontinent of Gondwana broke up, which started about 130 million
years ago. The new discovery shows that the break-up, which gave rise to
Australia, Antarctica and India, was much more 鈥渕essy鈥� than once thought, says
Mike Coffin of the University of Texas at Austin, one of the leaders of the
drill team.
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Marine geologists have suspected for more than a decade that the Kerguelen
Plateau was part of an ancient continent. Now samples drilled over the past two
months have provided the first direct evidence. At Elan Bank, on the western
side of the plateau near Heard Island, the geologists found garnet-bearing
metamorphic rock, which forms at the high temperatures and pressures found deep
in continental crusts. At other drill sites, layers of sediment were found
containing plant spores, seeds, pollen, wood fragments and charcoal. 鈥淚f there
are continental fragments below the Kerguelen Plateau, then that changes the way
you reconstruct the formation of the continents,鈥� says Philip Symonds of the
Australian Geological Survey Organisation in Canberra.
Based on the plant remains and fossilised marine plankton, the researchers
have identified three periods during which the crust rose above sea level. The
first was about 110 million years ago, when the southern end of the plateau rose
above the surface. Part of the crust broke the surface again about 85 million
years ago and again 35 million years ago, each time farther to the north and
involving a smaller amount of land.
The crust would have been pushed up by intense volcanic activity. As the
magma plume that caused this uplift cooled and contracted, the crust slipped
back below the waves once more. But because this part of the seafloor Antarctic
Plate was moving so slowly over the volcanic 鈥渉ot spot鈥� in the Earth鈥檚 mantle,
this cycle repeated itself. 鈥淭he volcanism was on a scale far greater than any
in recorded history,鈥� says Coffin. 鈥淚t is unknown for an event of this magnitude
to be repeated three times. It means we don鈥檛 quite understand mantle plumes.鈥�
