Plate tectonic movements control Antarctic Circumpolar Current – Dispatch No. 2

Date: January 3, 2013
Place: Drygalski Fjord, South Georgia
Falklands, South Georgia & Antarctica: Dispatch Number Two

JANUARY 3, 2013, 08:39 AM
54° 49.8′ SOUTH AND 35° 59.8′ WEST
A booming voice sounded over the ship’s public address system at 6:30am. Dr. Ian Dalziel, professor of geophysics at the Jackson School of Geosciences (University of Texas at Austin), was on the horn, enticing us to forsake our warm beds for the ship’s bridge. At the very least, Dalziel encouraged us to gaze out of the cabin potholes.
But, it wasn’t charismatic mega fauna – Southern Right Whales, Humpback Whales or Weddell Seals – that catapulted me and my fellow group of explorers to rise so early. For the group of intrepid geologists, it was the opportunity to view something that’s rarely observed in nature: ophiolites or primordial rocks formed at oceanic spreading centres and brought to the Earth’s surface via violent tectonic forces. A world-class outcrop of these ancient sea floor rocks is exposed in the Drygalski Fjord on the southwestern tip of the island of South Georgia.
Suitably inspired, I assembled my camera gear and headed to the Akademik Ioffe’s bridge where I was greeted by a stunning panorama of towering cliffs, glaciers cascading to the ocean and sea birds aloft.
Comprised primarily of the mineral olivine, the olive-green rocks of the Drygalski Fjord Complex represent a classic textbook example of basalts that spewed from the Earth’s mantle during the Cretaceous period, some 150 million years ago. The basalts poured, like molasses, from a fissure in the oceanic crust, creating new sea floor mass in the process.
Towering more than one kilometre above sea level, the exposed ophiolitic sequence is comprised of basalt lava flows that formed in the shapes of pillows – telltale signs that the rocks erupted onto the oceanic floor where the cold waters slowed their viscous flow and created hardened, outer skins characteristic of pillow lavas.
South Georgia is 160 kilometres long and measures between five and 30 kilometres wide. Deeply dissected by glaciers, the island’s central mountain range rises to almost 3,000 metres in elevation. Once connected to the southern part of South America, the micro-continent containing South Georgia is situated some 2,000 kilometres due east of South America’s Cape Horn. Plate tectonic movements, on the order of 6.5 millimetres per year, continue to transport the micro-continent towards Africa.
“This ophiolitic sequence of rocks helps us identify where the South Georgia micro-continent originated in continental South America,” said Professor Dalziel. “At Drygalski Fjord, we see similar rocks of a similar age in a similar tectonic setting, in a place where there is clearly a missing piece of the South American continent.”

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