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Stranded Ice

As a mechanism for meteorite concentration, the lateral movement of glaciers from zones of accumulation to ablation is refereed to as "the conveyor belt model." But this simple concept alone does not explain many occurrences in areas where meteorites are found.

There are areas of glacier ice, blue glacier ice, that seem to be cut off from lateral movement. In these stranded zones, there is no input of new ice to replace that which ablated away. A conveyor belt model does not fit in these circumstances.

Like boiling down tree sap into maple syrup, what appears to be happening in these stranded zones is the concentration of glacier ice goodies simply by the bulk of the ice wasting away. If one looks at simple vertical compaction of snow into ice, the snow with density of about 200 kilograms per cubic meter compacts into glacier ice with density around 850 kilograms per cubic meter. Just from the conversion of snow into ice concentrates things by about a factor of four.

The surface of these blue ice ablation zones have been observed to waste away by sublimation at about 10 centimeters per year [ref 1]. Surprisingly, much of this yearly ablation seems to happen in just the few summertime days when the air temperatures warm up to near the freezing point. If this rate can be extrapolated over long time periods, then each ten years removes one meter of glacier ice, and each thousand years removes 100 meters. So each thousand years of stranded ablation distills down 100 vertical meters of ice and leaves any goodies initially within the glacier spread over the surface. This rock-strewn surface can take on the look of a paved rock garden and is called a lag deposit.

The age of these meteorites is measured in billions of years, near the age of our solar system. However, the time they have spent on Earth is estimated to be in the tens of thousands of years. For a rock that lands on a stranded ice sheet for a few tens of thousands of years, this mechanism could concentrate the meteorites on the surface simply from ablation-driven vertical compaction.The stranded ice and the conveyor belt models are good for a first order explanation of meteorite finds. What is actually taking place is probably more complicated and combines these and phenomena yet to be discovered.

1. Harvey, R.P., “The Origin and Significance of Antarctic Meteorites”, Chemie der Erde, vol. 63 (2), 93-147 (2003).

Aerial view of the Larkman Nanatak area showing the location of camp and the location of the panoramic view point in the next image looking in the direction of camp. The general glacier ice flow is from lower right to upper left. The Larkman Nanatak is obviously sculptured by this ice flow creating a series of stranded ice sheets on the lee-side. The boundaries between the moving and stranded ice sheets are delineated by 100 meter high ridges of ice. A series of moraines are left in the central region of the stranded ice.

Panoramic view from top of Larkman Nanatak looking towards camp. Hundred meter high blue ice ridges (upper right) show the transition between the moving and stagnant ice. The patterned moraines in the central area probably resulted from earlier glacier movement when the ice flowed over the top of the nanatak and has since been concentrated due to ablation.

A blue ice ridge dwarfs a team member and his snowmobile.