Geology: Anchorage's Feet of Clay

The soft, dark grey substance that geologists call "quick clay" is composed primarily of small flaky particles and a great deal of water. It contains very little of the electrolytic salts that tend to bind normal soil particles together.

All of which means that the slippery stuff has another distinctive characteristic: it is thixotropic—a sudden shock can transform it from a solid to a liquid.

Residents of Anchorage, Alaska, saw a dramatic demonstration of that strange phenomenon during the disastrous 1964 earthquake, says Columbia University Geologist Paul Kerr, whose investigation is described in the current issue of Scientific American. While probing...

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