Mima Mounds

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Mima Mounds are relatively low, hilly elevations made up of loose, unstratified sediments. The naturally created structures correspond to a very powerful A-horizon . Its origins are still not fully understood, although there are several possible explanations.

etymology

The Mima Mounds or Mima Hills were named after the Mima Prairie in Thurston County in the US state of Washington . They now form the Mima Mounds Natural Area Preserve .

description

Mima Mounds at Olympia , Washington

The individual hills of the Mima Mounds reach a diameter of 3 to 50 meters. They vary in height from 30 to more than 200 centimeters. In their ground plan they are circular to oval, their outline is domed like a dome or also flattened at the top. Mima Mounds occur predominantly together, with their arrangements forming natural patterns. Up to 50 hills can lie on one hectare. Inside, Mima Mounds consist of loose, non-stratified , sandy , sometimes gravel-fractions , to which decomposed, organic material is mixed.

Internal structure

Profiles by Mima Mounds in Washington indicate that the mounds beneath a protective surface layer of prairie grass are mostly a mix of loose sand, fine gravel and decomposed plant matter. Sometimes complex soil profiles are also encountered with an A horizon made of black, sandy loam (the charcoal content of which is due to slash and burn), a B horizon made of gravelly sandy loam and a C horizon made of particularly gravelly sand loam.

The following profile was encountered with the pimple mounds : Here, below the above-average thick A and E horizons made of clay and sand, there is a flat, sometimes clearly sagging B horizon made of clay.

Emergence

So far, four models have been put forward to explain the development process:

Digging by pocket rats

According to this theory, the Mima Mounds were formed by the bioturbation of pocket rats . Investigations in 1940 showed that the hills occur predominantly in areas with poorly draining soils. According to this, generations of pocket rats threw up the mounds to stay dry above the water table.

As a counter-argument it may be put forward that the pocket rats did not build the structures themselves, but only stayed in them. In 1987, metal detectors implanted in a hill field near San Diego were able to prove that pocket rats did cause soil displacement towards the middle of the hill. And in 2013 a study found that the characteristic surface extension of a hill corresponds very well with the individual radius of action of pocket rats.

The results from the implantation study were also entered into a computer model that simulates the burrowing activity of the pocket rats. The simulation showed that it was only after hundreds of years that the hills gradually began to emerge - possibly the reason why no one has yet been able to observe the formation of a mima hill. These very slow growth rates and the emerging spatial distribution patterns agree very well with the actual field observations.

Accumulation as nabkas

Another theory explains the origin of the related pimple mounds and prairie mounds as being of Aeolian origin. As a result, the hills accumulate as nabkas (in English also coppice dunes ) in the slipstream of tufts of vegetation. Based on grain size analyzes and OSL aging of pimple mounds in the central south of the USA, Seifert was able to prove, among other things, that the mounds were made up of wind-blown sediments that had accumulated during the drought at the end of the Holocene . Despite their morphological similarity to the Mima Mounds of the northwestern USA, pimple mounds are therefore likely to originate from a fundamentally different development process.

Seismic ground tremors

According to geologist Andrew Berg, mima mounds and pimple mounds are caused by seismic ground vibrations. He makes use of the analogous phenomenon of vibrating surfaces. For example, jogging circular saw tables spontaneously concentrate sawdust in small piles that are very similar in their arrangement to the Mima Mounds.

Berg also suspects that Mima Mounds can only be found in earthquake zones whose soils are unstable. A major earthquake took place at the type locality around 1000 years ago .

The objection to this theory is that since the formation of the hills countless earthquakes have occurred worldwide, but nowhere did mima mounds arise. Since plowing work in Carrizo Plain , California, has been stopped, Mima Mounds have slowly resurfaced, but without any earthquake impact.

Swelling and shrinking processes of clay-rich layers

The swelling / shrinking model is based on the common explanation of the related Gilgai , which are usually underlain by clay soils . The basis of the Mima Mounds itself consists of swelling clay soils .

Occurrence

Mima Mounds

The Mima Mounds are not limited to the northwestern United States . Similar structures are known in other parts of North America as vernal pools , prairie mounds, or pimple mounds . The much flatter fairy circles of South Africa , the Heuweltjies of the southwestern Cape Province and the Gilgais in general are also likely to be related .

To the landscape of the vernal pools counting hogwallow mounds can be found extending from the south-central part in a belt, Oregon through northern California to the west coast of California and then from San Diego to the Baja California extends. The prairie mounds occur in a north-south belt that runs in the Great Plains from central Wyoming through Colorado to northern New Mexico . The pimple mounds are restricted to east Texas and adjacent Louisiana , southeast Oklahoma, and south Missouri .

Isolated occurrences of Mima Mounds exist in Iowa , eastern North Dakota, and northwest Minnesota .

See also

Individual evidence

  1. ^ AL Washburn: Mima mounds: an evaluation of proposed origins with special reference to the Puget Lowland . In: Report of Investigations . tape 29 . Washington Division of Geology and Earth Resources, Olympia, Washington 1988.
  2. ↑ United States Dept. of Agriculture: Soil Survey of Thurston County, Washington . 1990, p. 283 .
  3. ↑ Walter Wölber Dalquest, Victor B. Scheffer: The origin of Mima mounds of western Washington . In: Journal of Geology . tape 50 , 1942, pp. 68-84 .
  4. ^ GW Cox, DW Allen: Soil translocation by pocket gophers in a Mima moundfield . In: Oecologia . tape 72 , no. 2 , May 1987, ISSN  0029-8549 , pp. 207-210 , doi : 10.1007 / BF00379269 .
  5. ↑ a b c Emmanuel J. Gabet, J. Taylor Perron, Donald L. Johnson: Biotic origin for Mima mounds supported by numerical modeling . In: Geomorphology . tape 206 , February 1, 2014, p. 58-66 , doi : 10.1016 / j.geomorph.2013.09.018 .
  6. ↑ Christopher L. Seifert, Randel Tom Cox, Steven L. Forman, Tom L. Foti, Thad A. Wasklewicz, Andrew T. McColgan: Relict nebkhas (pimple mounds) record prolonged late Holocene drought in the forested region of south-central United States . In: Quaternary Research . tape 71 , no. 3 , May 2009, p. 329-339 , doi : 10.1016 / j.yqres.2009.01.006 .
  7. ^ Berg, AW: Formation of Mima Mounds, A seismic hypothesis . In: Geology . tape 18 , no. 3 , 1990, p. 281-285 , doi : 10.1130 / 0091-7613 (1990) 018 <0281: FOMMAS> 2.3.CO; 2 ( geology.gsapubs.org ).
  8. ↑ Michael D. Cramer, Nichole N. Barger: Are Namibian “Fairy Circles” the Consequence of Self-Organizing Spatial Vegetation Patterning? In: PLOS ONE . tape 8 , no. 8 , August 15, 2013, ISSN  1932-6203 , p. e70876 , doi : 10.1371 / journal.pone.0070876 .
  9. ↑ Jennifer L. Horwath Burnham and Donald Lee Johnson: 0 Mima Mounds: The Case for Polygenesis and Bioturbation . In: Geological Society of America . 2012, ISBN 978-0-8137-2490-4 , pp. 70 f .
  10. ↑ G .W. Cox: The Distribution and Origin of Mima Mound Grasslands in San Diego County, California . In: Ecology . tape 65 , no. 5 , 1984, pp. 1397-1405 .
  11. ^ GW Cox: Mounds of Mystery . In: Natural History . tape 93 , no. 6 , 1984, pp. 36-45 .