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

Normally one would think nothing of pouring Scotch whiskey over a few chunks of ice. But this ice was more than 18,000 years old! It came from the Taylor Glacier, an enormous river of ice flowing off the polar ice cap into Antarctica's Taylor Valley. The wall of the glacier is marked by a pair of thin, brownish layers of volcanic ash, each containing mineral grains that have been dated by radioisotope decay. The lower layer is 18,000 years old, and since our ice came from below it, we knew the ice must be older than that.

The "we" in this band of Scotch drinkers included me and twenty-four other geologists. At the behest of Bruce Marsh, a geologist at Johns Hopkins University, and with the support of the U.S. National Science Foundation, we had assembled in Wright Valley, one of the McMurdo Dry Valleys of Antarctica--or simply, the Dry Valleys. The region is surely among the most remote, exotic, and starkly beautiful corners of the planet. In spite of the ice and snow that blanket the rest of Antarctica, Wright, Taylor, and other parts of the Dry Valleys are deserts, among the most waterless places on Earth.

For the past decade, Marsh has been studying a labyrinth of basaltic rock spectacularly exposed in the walls of the Dry Valleys. The labyrinth is made up of dikes (sheetlike bodies of rock that cross the surrounding strata) and sills (bodies parallel to the strata). Basalt, a fine-grained black rock, is the most common igneous rock on Earth. It forms when the Earth's upper mantle partially melts, and the resulting magma rises to places where it can solidify rapidly, either within the crust, as dikes and sills, or at the surface, as erupting lava. The dikes and sills of the Dry Valleys are the remnants of a kind of plumbing system through which magma worked its way to the surface in a series of eruptions about 180 million years ago. Volcanic plumbing systems are rarely exposed at the surface. The reason is simply that around active volcanoes, lava covers everything. Even at old, inactive volcanoes that have been deeply dissected by erosion, geologists commonly see only the interior of the volcanic edifice, not the structure of the rocks below.

Exposed to view in various parts of the Dry Valleys, however, is a vertical slice of the dikes and sills immediately beneath the lavas, which cuts across layers of rock two and a half miles thick. Hence along the valley walls, geologists can see much deeper into the volcanic plumbing than they can almost anywhere else. Taken together, the dikes and sills of the Dry Valleys are known as the Ferrar dolerites, after Hartley Ferrar, the geologist who, as a member of Robert F. Scott's 1901-1904 Discovery Expedition, first recognized the valleys' dolerites as such. Dolerite is basaltic magma that solidifies rapidly in sills and dikes near the surface.

The dikes and sills bear on several questions geologists ask. One of them is why volcanoes commonly erupt lavas that vary so widely in composition--a major factor in creating the planet's surface. Marsh had chosen to study the Ferrar dolerites because they are quite variable in composition and also extremely well exposed in the valley walls. Now he had assembled a group of geologists, all experts in the physics and chemistry of magmatic systems, to share insights, help discover the reasons for the compositional diversity, and use the lessons learned in Antarctica to understand complex bodies of rock elsewhere in the world.

Sipping Scotch in our isolated camp, though, we were fascinated not only by the rocks, but also by the enormous scale and alien nature of our surroundings. The Dry Valleys feature rivers that stay bone-dry except for summer trickles that flow inland to frozen lakes many times saltier than seawater; glaciers whose surfaces sublime (turn from solid ice directly into water vapor) but do not melt; microbial communities that live in bubbles of liquid water locked within subliming lake ice for hundreds to thousands of years. The region opens a window into igneous geology, as well as into how the Antarctic ice sheet responded to climate fluctuations in the past, and how it may respond in the future. Finally, the glaciers impinging on the Dry Valleys may act somewhat like icy features discovered on Mars, and the bacteria that occur in the ice and saline lakes may resemble extraterrestrial life, if it exists. So let us take a moment to wander through the hyperdry, hypercold landscape.

Antarctica is a gigantic continent, with an area far larger than that of the United States (5.25 million versus 3.5 million square miles). Obscured by the continent's ice cover, the Transantarctic Mountains divide Antarctica into two parts [see map at left]. The major part of the continent lies in the Eastern Hemisphere, on one side of the mountains, where it is apparently made up of ancient, crystalline bedrock. Most of that rock lies above sea level and is covered by the East Antarctic Ice Sheet, a body of some 6 million cubic miles of ice. On the other side of the mountains is the much smaller (0.8-million-cubic-mile) West Antarctic Ice Sheet, which rests on an ancient bedrock that would be submerged, were it not topped by ice.