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

Marsh and his students have meticulously mapped those proportions. The maps show that orthopyroxene crystals are distributed within the sills in great tongues, sometimes extending for tens of miles along the sills. The orthopyroxene-rich tongues are thickest--occupying perhaps half the height of a sill--where the sills appear to have filled from their feeder dikes, and they thin from the filling points toward the more distant parts of the sills.

Given the high melting point of orthopyroxene, its distribution into tongues suggests it froze out of the melt deep in magma chambers, before it reached the sills. Crystals of the mineral were then carried in suspension by the upward-moving magma and forced toward the center of the flowing slurry.

When the flow of magma intruded between layers of rock as a sill, the liquid began to solidify at the sill margins. The "solidification front" then proceeded inward as the basalt continued cooling. Meanwhile, the suspended orthopyroxene crystals, most of which were now near the center of the sill, slowly settled into the lower part of the magma in which they were suspended. When the solidification front moved entirely through the sill, it preserved the tongue-shaped distributions of orthopyroxene crystals we see today in the valley walls.

Elsewhere in the world, geologists have studied vast bodies of well-layered igneous rocks. Some of those rocks hold important deposits of metals such as platinum, for reasons that may be related to the rock-layering process. The layering has thus engaged the attentions of several generations of earth scientists, who continue to wonder not only about ore deposits, but also about the larger lessons the layered rocks may hold. In the Dry Valleys of Antarctica, where the Ferrar dolerites solidified rapidly, geologists have caught the layering process in the act. Thanks to the fortuitous circumstances that keep the landscape free of ice, the Dry Valleys are among the planet's best places to study how various kinds of rocks form, and even how the deep Earth organizes itself.