Mysterious Mima mounds: seismic source?Mysterious Mima mounds Mima mounds (also known as Hog-wallows) are mounds of soil some meters tall that occur in various places in the world. Their origin is still unknown. Various theories have been proposed: that they are Indian burial mounds, remnants of glacial activity, interactions of the soil with : Seismic source A seismic source generates controlled seismic energy that is used in both reflection and refraction seismic surveys. A seismic source can be simple, such as dynamite, or it can use more sophisticated technology, such as a specialized air gun. ? A fluke fluke, parasitic flatworm of the trematoda class, related to the tapeworm. Instead of the cilia, external sense organs, and epidermis of the free-living flatworms, adult flukes have sucking disks with which they cling to their hosts and an external cuticle that 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 The University of California, Davis, commonly known as UC Davis, is one of the ten campuses of the University of California, and was established as the University Farm in 1905. . In the past, scientists have attributed these mounds to factors ranging from burrowing gophers to plant roots. But a serendipitous ser·en·dip·i·ty n. pl. ser·en·dip·i·ties 1. The faculty of making fortunate discoveries by accident. 2. The fact or occurrence of such discoveries. 3. An instance of making such a discovery. 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 See under Ashes. See also: 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 interference pattern An overall pattern that results when two or more waves interfere with each other, generally showing regions of constructive and of destructive interference. 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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