Dec. 20, 2006
There are a number of theories why meteorites are concentrated on the Antarctic ice sheets by about a factor of 1000 over the intrinsic fall rate [ref 1]. No matter the theory, they all hinge around the fact that the ice sheets are in a balance between growing and shrinking with areas of motion and areas of stagnation. Somehow these processes concentrate the meteorites. The conveyor belt theory relies on the observation that glaciers slowly flow downhill and upon reaching the end of their line, the rock loading, like so many passengers, are dumped in a pile called a moraine. Thousands of years’ accumulation of meteorites are thus heaped among the normal rocks.
The conveyor belt theory uses the basic observations that glaciers have accumulation zones, areas where there is significant snow fall that compacts into ice and adds to the mass of the glacier and ablation zones, places where the glacier shrinks in size usually due to the ice breaking off and falling into the sea. This obviously happens if the glacier terminates at the ocean.
For glaciers that do not reach the ocean, those deep in the Antarctic interior, other ablation processes take place. For ablation zones found in the interior, sublimation is the dominant process. Sublimation is the process of ice evaporating without going through a liquid phase. Ice can turn into gas (water vapor) directly from the solid ice state without first melting into liquid water. We can see the results of sublimation in the freezer section of our refrigerator. We all have a bin of ice there and when this bin has been sitting for a spell, the ice cubes seemingly shrink in size, like a collection of shrunken heads, becoming this distorted figure that is recognizable as once being an ice cube but somehow taking on a diminutive size. This is sublimation at work and is the same process that can cause glaciers, deep in the Antarctic interior to waste away without having temperatures above freezing.
So in the ice balance of an interior glacier, new snow falls and turns into ice in the accumulation zones, thus forcing a slow movement of ice from the accumulation zone to the ablation zone. This slow movement, perhaps a few meters per year, also transports anything else like meteorites that happen to be buried in the ice. If there is an ablation zone where the dominant process is sublimation, then the goodies carried within the ice end up accumulating in a heap on the surface, thus creating a moraine.
The result of this icy conveyor belt concentrates thousands of years of falling meteorites into a rather small jumbled zone of rocks. So the meteorites, along with any other rocks, are found simply laying on the surface of the glacier ice, seemingly waiting to be harvested like some kind of exotic crop.
As with any construct of the human mind, this theory has its limitations and does not explain every instance of where meteorites are found. We will expand upon this in another chronicle.
1. Harvey, R.P., “The Origin and Significance of Antarctic Meteorites”, Chemie der Erde, vol. 63 (2), 93-147 (2003).
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