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The Transantarctic Mountains

The Antarctic has one of the longest mountain chains in the world, the Transantarctic Mountains that extend from the tip of the Antarctic Peninsula to Cape Adare, a distance of 3000 miles (4800 km). In many places the chain is mostly buried, but the exposed peaks often have steep snow free faces.

The mountains rise high above the western shore of McMurdo Sound and are considerably older and of entirely different geologic origin than the Ross Island volcanics. Many interpretations of the origins of Antarctica have resulted from studies in these mountains, particularly where the rocks are best exposed in the Dry Valleys area. The high, glacier-mantled peaks of the Royal Society Range to the south, however, provide a more spectacular backdrop to the McMurdo area, with continually changing patterns of light and shadow through the 24-hour austral day bringing out both the range's gently rounded glacial contours and the abrupt rock faces of the higher peaks. These mountains, and particularly their Dry Valleys, are within easy access by helicopter from McMurdo Station, and scientists have found excellent exposures of various geologic units, which have helped reveal much about the geologic story of the continent.

Did you know? During the Cenozoic Era, which began about 65 million years ago, the rocks forming the present Transantarctic Mountains were uplifted, with the sandstone and dolerite layers being brought to elevations as high as 4000 meters above sea level.

The rocks of the Transantarctic Mountains include mostly a light, buff-colored sandstone-the Beacon Sandstone of Devonian or Silurian to Jurassic age (400 to 200 million years old)--with interlayers of dark-colored dolerite (an igneous rock similar to basalt), which was injected into the sandstone. These injections occurred about 170 million years ago, during the Jurassic Period, when molten rock beneath the Earth's crust forced its way upward into the sandstones.

Where the dolerite split apart and flowed between (parallel to) the sandstone layers, they formed the sills, and where they cut across the sandstone, they formed the more vertically oriented dikes. Locally where the molten rock reached the surface, it flowed out as basaltic lava, or in some cases erupted explosively as ash and pumice to produce widespread volcanic sediments.

Today these rocks are impressively displayed in the steep eastern face of the Royal Society Range and in the peaks of the ice-free "Dry Valleys" area. The ice sheet of the polar plateau was evidently formed during the Miocene Epoch (5-24 million years ago) and probably has been in existence in some form for at least 15 million years.

How were the Dry Valleys formed?

As the Transantarctic Mountains were being uplifted, ice flowing eastward from the margin of the plateau cut deep valleys into the rocks. Thus were formed the valleys which transect the range and provide the pathways for ice flowing from the plateau. Several of these valleys, however, were later uplifted higher than others and ceased to be avenues of glacier flow; these became the "Dry Valleys" of the present day. During the final recession of the ice sheet, its surface dropped to present levels, which allowed a local dome to form on the ice surface west of the Dry Valleys. As a result of this configuration, along with the increased aridity caused by the presence of the ice sheet, very little ice flowed into the valleys. They became dry because ice ablation (down-melting), caused largely by winds, exceeded ice inflow.