Cold fire: in Antarctica's Dry Valleys, the deep chambers and conduits that poured hot lava onto the surface are exposed as nowhere else on Earth

Natural History, July, 2005 by Edmond A. Mathez

The enormous riverlike glaciers, such as the Taylor Glacier, that enter the heads of the major valleys do provide some liquid water. In summer they absorb solar heat, which melts the subsurface ice, leaving, within the glaciers, bowl-like pools called cryoconite holes. Because the summer air temperature is commonly still below freezing, the "holes" remain sealed by ice. Nevertheless, the water collects in small, coalescing passageways and eventually trickles out of each glacier.

Where liquid water does occur, life finds a toehold. The cryoconite holes, encased in ice, host communities of bacteria. The same organisms also occur in the lakes of the Dry Valleys, where they form algal mats. Sometimes pieces of the mats become encased in ice that migrates upward as the top of the ice sublimes. The process takes a long time, and microbial mats as old as 2,800 years have been identified. Besides bacteria, life is limited to a few lonesome lichens, which grow on protected rock surfaces, and to several species of microscopic worms known as nematodes, which live in the soil in the few places that become damp in summer.

Since nearly all of Antarctica is covered by ice, one might reasonably suppose that the Dry Valleys were simply carved by glaciers that overtopped the Transantarctic Mountains. Unlike rivers, which tend to cut valleys that are V-shaped in cross-section, glaciers characteristically carve a U (think of California's Yosemite Valley). The walls and floor of the lower reaches of the Dry Valleys are indeed shaped like a U, but to the practiced eye the U looks too wide and too shallow to have been solely glacial.

The most telling evidence of how the valleys did form occurs at high elevations, where geologists have discovered remnants of flat surfaces interrupted by steep-walled valleys. The latter valleys appear to be relics of river erosion 55 million years ago, when the climate was semiarid and Antarctica had no ice. In those days, in a sense, Antarctica did not exist at all. It was still attached to Australia as part of a larger continent known as Gondwana. There are places on Earth today that may bear a close resemblance to the Antarctic landform back then. One of them is the Colorado Plateau of southern Utah and northern Arizona, where seasonal rivers and streams have carved sinuous valleys with flat floors and steep semicircular walls.

The Antarctic ice cap began to form much later than the valleys did, about 34 million years ago. The fundamental cause was the opening of the Drake Passage, the part of the Earth's great Southern Ocean that separates Antarctica from South America. The newly circumpolar ocean established a circumpolar current that blocks the southerly flow of warm water and thermally isolates Antarctica from the rest of the world. In contrast to the Arctic, which is highly sensitive to global climate change, the Antarctic climate has remained relatively stable for millions of years.

Early on, glaciers did cover much of the Dry Valleys. The glacier traces are hard to miss: rock surfaces have been scoured smooth, and rocky, gravelly debris known as glacial till has been scattered throughout the high valleys. But by about 14 million years ago, the glaciers--except for the small, cold-based glaciers of today--had largely disappeared from the valleys. How do geologists know? On some of the surfaces high above the valley floors, delicate volcanic ash deposits, some as old as 13.6 million years, lie just beneath the desert pavement--the loose rock and gravel too heavy for the wind to have blown away. Those regions of the Dry Valleys have remained hyperdry, hypercold, and glacier-free, at least since the ash fell.