Å®ÉúСÊÓƵ

MIMA mounds – a mysterious type of hillock found in the Americas and
Africa – have long puzzled geologists. Now a researcher in the US believes
he has found an explanation for the mounds’ origins: he suggests they are
formed by earthquake vibrations.

Mima mounds appear to be thousands of years old, but there is no historic
record of one forming. They take their name from a cluster on the Mima Prairie,
south of Olympia, Washington. Measuring up to 3 metres high and between
2.5 and 15 metres in diameter, they are found on flat surfaces of hard material
or packed gravel, spaced at regular intervals. Gophers and other burrowing
animals often make their homes in the mounds.

In the past century, geologists have proposed many explanations for
the mounds’ existence. Some have claimed that they were built by gophers
or that they are the product of selective erosion of the soil. Others believe
they result from glacial deposition or the accumulation of soil around the
roots of plants. Gophers have been the prime suspects, but no consensus
has emerged among geologists. Now Andrew Berg, of the US Bureau of Mines
in Spokane, Washington State, has made a study of the mounds.

Berg stumbled on his idea that the vibrations of earthquakes might cause
Mima mounds 10 years ago, in the aftermath of the eruption of Mount St Helens.
While building a dog kennel, he happened to hammer on a piece of plywood
which was covered with a thin layer of volcanic ash. The vibration caused
the ash to clump together to form miniature mounds.

Berg realised that Mima mounds might form in a similar way. Immediately,
he began a search of the scientific literature to discover whether anyone
else had ever made the suggestion. But he drew a blank. A group of French
geologists had come the closest, when they had suggested that stone polygons
on the island of Spitzbergen might be caused by repeated earthquakes.

Berg suggests that on hard surfaces, seismic waves from earthquakes
which are moving nearly parallel to the ground are reflected. The reflection
may happen wherever there is discontinuity, such as a geological fault.
The reflected waves can ‘interfere’ with each other to produce ‘standing
waves’. In such waves, there are ‘null zones’ – places where the amplitude
of vibration is permanently zero – and places where the amplitude of the
vibration is high.

According to Berg’s theory, particles of material are shaken loose from
high-amplitude areas and pushed towards the null zones. It is here that
they accumulate to form Mima mounds. Unfortunately, Berg does not have a
quantitative model of this phenomenon, and cannot predict how much seismic
energy would be needed.

Some geologists are intrigued by Berg’s theory. Charles Higgins of the
University of California at Davis says: ‘I don’t think (his theory) is right,
but I am impressed that it explains two aspects of mounds that many published
hypotheses don’t’ The vibrations Berg envisages would form regular features,
as researchers have observed in the field. The vibrations would have to
occur on distinct ‘floors’ of gravel or hard material which underlay the
mounds.

Many mounds are in earthquake zones, such as California and the Pacific
Northwest, which would seem to support Berg’s claim. However, Higgins notes
that others are in stable areas, such as eastern Texas.

Other geologists still support the gopher theory. George Cox of San
Diego State University points out that the mounds in North America overlap
the range of the pocket gopher. But Higgins maintains that gophers destroy
mounds not build them.