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Mysterious Mima mounds: seismic source?

Mysterious Mima mounds: Seismic source?

A fluke observation involving a doghouse and an eruption of Mount St. Helens may solve a geologic mystery that has puzzled scientists for more than 150 years.

Mima mounds -- rounded piles of soil standing as high as 3 meters--appear clustered in diverse spots around the world and "may have generated a greater variety of hypotheses than any other geologic feature," says Charles G. Higgins of the University of California, Davis. In the past, scientists have attributed these mounds to factors ranging from burrowing gophers to plant roots. But a serendipitous series of events leads geologist Andrew Berg to propose earthquakes as the cause.

While constructing a doghouse in 1980, Berg happened to hammer on a piece of plywood covered by a fine coat of volcanic ash from the Mount St. Helens eruption that spring. Berg, who works for the U.S. Bureau of Mines in Spokane, Wash., noticed that the pounding produced a pattern of bumps in the ash that looked suspiciously like miniature versions of the Mima mounds common near his home. In his off hours, Berg repeated the experiment under more controlled conditions. He observed that the vibrations from several hammer blows sorted the material, causing soft sediments to form mounds separated by coarser-grained material--a feature characteristic of some Mima mounds.

Berg thinks the experimental mounds arise because vibrational waves traveling through the plywood interfere with each other, causing certain locations to vibrate heavily while others remain still. A similar interference pattern of earthquake waves, he reasons, could create Mima mounds in areas where a thin layer of loose soil rests on a flat section of rock or hard soil. Because repeated hammer blows to the plywood did not erase the mounds, Berg believes they are stable once formed and would not fall apart during repeated earthquakes.

The hammer experiment does not prove the earthquake hypothesis, Berg notes in the March GEOLOGY. Nonetheless, he calls the evidence "extremely compelling." His theory would explain why Mima mounds form in many earthquake-prone areas around the world that have markedly different climates.
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Title Annotation:Earth Sciences
Publication:Science News
Date:Apr 14, 1990
Words:346
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