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Andrew Alden

Mima Mounds: A Report from AGU

By December 10, 2013

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mima moundsI'm in the middle of a week at the Fall Meeting of the American Geophysical Union, in San Francisco. This is typically a whirl of talks, posters, refreshment breaks (coffee in the morning, beer in the afternoon) and throngs of geoscientists. I typically don't cover the meeting like a journalist does, although I share the press room with them. A lot of what I seek out is background stuff that I hope is ahead of the cutting edge—at least, it's ahead of my cutting edge. A talk I saw yesterday was not like the others.

Manny Gabet, of San Jose State University, has been studying the odd landscape features known as mima ("meema") mounds. They're piles of dirt, a meter or two high, and range in area from dinner-table to living-room size. They occur bunched up together in big groups, and are widespread in the American West. Gabet cited two special facts about them: they're found where the winter rains regularly saturate the ground, and they're occupied by pocket gophers, one on each mound, with the unusual habit of pushing the dirt from their burrows uphill. He made a computer model of a bunch of gophers and set them loose on a flat computer landscape, showing that their behavior, over several centuries, turned the ground into fields of mima mounds.

The advantage of a mound is that it sits above the winter rains, making large areas of marginal land inhabitable for gophers. Gabet told us that by his reckoning, mima mounds are the largest set of animal-made structures on Earth—other than our own, of course. It intrigues me that a single change in behavior brought this about—pushing dirt uphill instead of downhill. That probably arose from a very small genetic change, one that doesn't make a difference east of the Mississippi, where mima mounds aren't found.
Mima mound field from the air — Washington Department of Natural Resources

Comments

December 16, 2013 at 9:14 am
(1) Rick Giardino says:

You need to read the works of John D. Vitek and Don Johnson on mina mounds. They are way ahead of any recent work. Don’s GSA work is one that all need to read.

RG

December 16, 2013 at 9:32 am
(2) Saranne Cessford says:

Another kind of animal-mediated landform occurs in the Western Cape region of South Africa. Known as ‘heuweltjies’ or ‘little hills’ they occur widely and are a result of termite activity. In a good flower year they are dense with daisies.
http://link.springer.com/article/10.1007%2FBF00317612
http://www.researchgate.net/publication/234126462_Distribution_and_formation_of_Mima-like_earth_mounds_in_the_western_Cape_Province_of_South_Africa
and for an image:
http://www.ispot.org.za/node/214075

December 16, 2013 at 9:35 am
(3) John Byrnes says:

Rick, what’s the reference? I have a webpage on curious sizeable mounds in western New South Wales. The last theory on ours was that they were made by members of the extinct megafauna (birds) .

John

December 16, 2013 at 9:36 am
(4) kate-v says:

1. Where did the name ‘Mima’ mound come from? 2. I see pictures of the termite mounds in some parts of Africa and they look huge – some several stories high – these would be larger that a living room sized mima mound and in at least some of the pictures it seems there are several termite mounds within a given area – do these count as made by animals? I have never seen these termite mounds in person, but I have seen areas where the gophers (we called ‘em picket pins) live. Also, (I think) corals are animals and they get fairly large ‘colonies’.

December 16, 2013 at 12:18 pm
(5) Meredith Gibbs says:

Reminds me of the fairy circles in Namibia.

December 16, 2013 at 12:29 pm
(6) Joan Pederson says:

Some of the first mima mounds described in detail occupy the Mima Prairie, for which they may have been named. It’s in Thurston County, not far from Washington’s capital, Olympia.

Citation for Don Johnson’s GSA Special Paper follows below. As many tributes have noted since is death earlier this year, he helped launch many careers, and he readily shared credit where credit was due; the editorship of the paper lists his academic colleague in this work above his own. (His wife, Donna, was another of his key collaborators in this and nearly all of his work.)

Don suggested mima mounds as a research topic to those of us who took his Soil Geomorphology course at U of Illinois at Urbana-Champaign. More broadly, he helped us see in the field how profoundly critters were involved in soil formation, in vertical migration of buried objects (and in the potential geoarchaeological consequences), and in development of surficial features. He profoundly understood the other processes to which mima mounds have been attributed. It’s not very likely that some other explanation will come to supersede his.

GSA Special Papers 490, 2012 Mima Mounds: The Case for Polygenesis and Bioturbation
edited by
Jennifer L. Horwath Burnham, Department of Geography, Augustana College, Rock Island, Illinois 61201-2296, USA
Donald L. Johnson, Department of Geography, University of Illinois, Urbana, Illinois 61801, USA

December 16, 2013 at 12:42 pm
(7) Shane Proctor says:

I’m understanding that the computer model assumes a flat surface. But, I’ve seen these all over <i>relatively</i> steep hillsides in California’s Central and Southern Valleys.

I’m also not sure that winter rains regularly saturate the soil in some of these regions. What are we talking about in terms of “regular”?

Interesting piece to the puzzle, but I’m left with some questions.

December 16, 2013 at 10:13 pm
(8) Geology Guide says:

Shane, Gabet’s talk mentioned the sloping sites. His model produced elongated mounds on sloping surfaces.

The point about winter rains should also include the fact that mima mound sites have a shallow layer of hardpan, such that in the average year the ground fills with water. In mima mound fields in California’s Central Valley, winter rains feed vernal pools in the spaces between mounds.

The question about what’s “regular” is interesting. As far as evolutionary adaptations go, a gopher that lives, say, five years will have an advantage if it can preserve its home even if it floods only once every, say, three years.

December 16, 2013 at 12:49 pm
(9) David Hilpert says:

I’ve lived next to a set of mima mounds in Tenino Wa. most of my life. There are no gophers there although they do have moles. but I have never observed any mole hills on the mounds as the mounds are to rocky and moles prefer softer ground..

December 16, 2013 at 1:50 pm
(10) Joan Pederson says:

The mounds are commonly very old. Gophers need not be present now to have formed them in the past.

Termite mounds are probably the largest of the world’s non-human earthwork constructions, although the extent of the effects of beaver dams can be far more vast.

December 16, 2013 at 2:51 pm
(11) Anthony Tellier says:

“Tenino, WA”
Named for the railroad water tank number: “10-9-0″. Sorta like Novi, Michigan: “No. VI”.

Quack

December 16, 2013 at 5:42 pm
(12) Graham Houghton says:

No. They’re moguls made by skiing gophers.

December 16, 2013 at 6:05 pm
(13) Duane says:

On the face of it, gophers do not seem they could have created the similar feature in the Gulf Coast, especially southwest Louisiana and southeast Texas, we call “pimple mounds”…not because there are no longer gophers. The mound soil is silty, but the soil between the mounds is clay-dominated. Are soils in the Mima mound areas similar?

December 17, 2013 at 2:06 pm
(14) Hamid Sadeghipour says:

Please, take a pan and cook something that has some powder of the bread on it. Some fish, or chicken and leave the oil after cooking. Of course, physicians advise you to not using the used oil again, if left for a long time, it becomes poisonous. some hours later, you try to cook something in the pan again, what happen? A colloidal very small mima mounds appear. It seems to me that magma came up and you know sometimes the petroleum is associated with volcanic activities. The oil appeared on the surface. As the viscosity is high, a larger amount did some colloidal and semispherical domes. As the next time it was heated the soil and oil stabilized. It could be an explanation.

December 17, 2013 at 8:03 pm
(15) Geology Guide says:

Hamid, I can always count on you for a creative idea.

January 16, 2014 at 3:49 pm
(16) Mike Garverich says:

The Gophers credited with building these mounds are likely pocket gophers, not the ground suirrels commonly called gophers or picket pins. They are also different from moles. Pocket gophers spend most of their lives below ground and are rarely observed except when the push dirt from thier burrows. That being said, in Montana’s Big Hole Basin I have observed mima type mounds draped on slopes as well as horizontal surfaces and on upland surfaces as well as flood plains. Upland mounds are on a skelatel soil and floodplain mounds are on sndy or silty soils. Excavated mounds show numerous fossil burrows of various sizes but it seems likely to me that there is some abiogenic process that forms the mounds or at least some of them.

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