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

I don't mean to suggest that nothing has happened "recently." Low parts of the valleys were flooded by the sea: nine-million-year-old sediments, typical of the sediments at the bottoms of fjords, remain in the floor of Wright Valley. Similar deposits, aged between 6 million and 3.4 million years, occur in Taylor Valley. Since then, the region has been uplifted--and so the major valley floors are now mostly a few hundred feet above sea level. There was also a period of glacial advance, about 3 million years ago, when the Taylor and Wright glaciers expanded into the lower parts of their valleys and left deposits of till as their calling cards. At various times, sea ice has also invaded the lower valleys. In brief periods of relative warmth, glacial meltwater accumulated behind this ice to form small, temporary lakes. Their sites are marked today by local beds of sediment in the valley floors. Yet none of these events fundamentally altered the landscape; the vista we gaze on today has remained largely unchanged for at least 14 million years.

In today's valleys, the Ferrar dolerites occupy four massive sills, each between 330 and 1,150 feet thick, interconnected by a series of dikes [see illustration on this page]. The intricacy of the array is accompanied by a wide variation in the composition of the rocks--a variation presumably related to the way the magma solidified. Unlike water, which freezes at a single temperature, magma freezes over a temperature interval. Most basaltic magmas are completely molten at 2,200 degrees F (1,200 degrees C) and do not become completely solid until the temperature falls to about 1,650 degrees F (900 degrees C). As the magma cools between those two temperatures, different minerals crystallize, or freeze out, at different temperatures. These minerals are not the same in composition as the magma itself. Therefore, as crystals form and sink or otherwise separate from the magma, the composition of the residual magma changes.

In a cooling basaltic magma, the first mineral to crystallize is usually olivine, a magnesium silicate ([Mg.sub.2]Si[O.sub.4]) that also includes some dissolved iron. In the case of the Ferrar dolerites, the first to freeze out was orthopyroxene (MgSi[O.sub.3] with about 15 percent dissolved iron). Because orthopyroxene--and, for that matter, olivine--both have a higher proportion of magnesium than magma does, the magnesium content of the residual magma falls as these minerals crystallize. As cooling continues, plagioclase (a mixture of two kinds of aluminum silicates) and the pyroxene mineral augite (a magnesium silicate mixed with some iron) form.

The distribution of orthopyroxene proved to be our most telling clue for understanding the Ferrar dolerites. Looking closely at the rocks, we could see clear differences among the sills. Some are made up of dense, black, homogeneous basalt, with no visible mineral grains. Others have varying abundances of orthopyroxene crystals suspended in otherwise fine-grained rock. In consequence, the rock varies from 3 percent to 12 percent magnesium, depending entirely on the proportion of orthopyroxene.