MIMA MOUNDS

Mima mounds is a term used for low, flattened, circular to oval, domelike, natural mounds found in the northwestern United States, Idaho, Oregon, and Washington, that are composed of loose, unstratified, often gravelly sediment that is an overthickened soil horizon. These mounds range in diameter from 3 to more than 50 m; in height 30 cm to greater than 2 m; and in density from several to greater than 50 mounds per hectare. The mounds occur throughout the world by the millions and represent a major unexplained phenomenon for scientists.

The origins of the mima mounds has been debated for many years and consists of numerous theories. The reality of the mima mound phenomenon seems to be more polygenetic in origin, meaning there is no single explanation but a multitude of explanations for the origin of the mounds.

Structure:
Excavations made into the Washington mounds show that underneath a blanket of prairie grass lies a mixture of loose sand, fine gravel, and decayed plants. Not all mima mound features have the same structure though. One mound in Washington had a very complex soil profile: A horizon is a black sandy loam (due to charcoal content from aboriginal burning on the prairies), B horizon is a gravelly sandy loam, C horizon is an extremely gravelly sand.Bernard Hallet, a geologist at the University of Washington explains that this is unusual, as most land surfaces have a top layer of organic material, dead plants and animals, no more than half a meter (1.6 ft) thick, while Mima mounds are made of organic-rich soil more than 2 meters thick.

Theories:

Pocket Gophers:

One major theory concerning the origin of Mima mounds argues that they were created by pocket gophers—small, burrowing rodents with fur-lined "pockets," or pouches, in their cheeks. The theory is that gophers tunneling into loose soil run into a gravel layer below. Unable to burrow any farther, the gophers start building upward and outward. The theory's author, George Cox, a retired zoologist at San Diego State University, says that a gopher family often can move up to 5 tons of earth a year.That's one-twentieth of the soil in an average Mima mound.Cox also asserts, most North American mounds are in gopher territory, and many gophers actually live in mounds. However, the only pocket gophers ever spotted on Mima Prairie built their burrows between the mounds, not in them. It is unclear whether gophers created the mounds or migrated to them because the mounds provided handy home sites.

Seismic Activity:

Andrew Berg, a geologist with the U.S. Bureau of Mines in Spokane, proposed that Mima and pimple mounds were the result of very intense ground shaking resulting from major earthquakes.According to Berg, the soil on the Mima Prairie is like volcanic ash, and the layer of rock below that is like a plank of wood. When seismic waves move through the hard ground and bump into faults, or large fractures in the ground, the waves bounce backward. Those ricocheted waves collide with other seismic waves from the quake, and between the collision points, the soil rises and forms mounds. Berg claims that Mima mounds occur only in seismically active areas—areas where the ground is unstable and many earthquakes occur. The area where the Washington Mima mounds are found experienced a major earthquake about 1,000 years ago.

Glaciation:

A 2009 study using lidar data and published by researchers at Washington Department of Natural Resources showed that the retreat of the ice-age glaciers could account for the mima mounds' formation. A feature of this theory is that meltwater from the glaciers carried gravel to sun cups, where it was deposited and formed mounds after glaciers had fully retreated

Vernal Pools:

Vernal Pools are shallow depressions that fill with water during winter rains. Water accumulates below the soil surface and above a layer of hardpan or impermeable substance, creating a seasonally perched water table. Water is prevented from percolating underground, leaving evaporation as the only means of escape.The mima mounds located in the California Floristic Province (west of the Sierra Nevada and Cascade ranges from Southern Oregon to Baja California.Coincidentally, mima mounds are located in Washington and Oregon State, west of the Cascade Range, and likewise throughout California west of the Sierra Nevada Range.

Shrinking and Swelling of Silts:

When clay is exposed to large amounts of water, the water collects between the clay minerals (which are flat planes). Due to the shape of the minerals, the water travels in between the compacted layer, thus "swelling" the clay-bed into mound-like features. Silts are also capable of this type of geomorphologic feature; however, silt is coarser-grained sediment so the minerals do not "hold" water in the same way. Silt is more penetrable than clay is, but since the area around the base of the mima mounds are a loamy soil, the composition allows water to swell and shrink in a similar fashion to clay, but on a smaller scale.