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Lake Superior's Deep-Water Donut Mystery

sonar image depicting ring-shaped impressions

Mysterious rings (see top of this sonar image) grace the floor of Lake Superior. Scientists at the University of Minnesota and in the United Kingdom are working to discover how they were formed. Image courtesy of Nigel Wattrus, Large Lakes Observatory.

A curious swath of paper covers half of the door inside the office of Minnesota Sea Grant researcher Nigel Wattrus. The computer-generated printout looks lunar, but the image isn't from the moon and the circular shapes aren't craters.

"It's as if someone pressed giant donuts into the soft sediment on the floor of Lake Superior, then took the donuts away," said Wattrus, associate professor of geological sciences at the University of Minnesota's Large Lakes Observatory. "All that remains are huge ring-shaped impressions and unanswered questions."

The image that captured Wattrus' attention and valuable door space was produced from multibeam sonar data collected from one of the deepest parts of Lake Superior. The bluish circles scattered across the beige landscape indicate the location of ring-shaped depressions in the fine-grained clay.

In the image, the rings are smaller than Cheerios™; in reality they are HUGE — 150 to 300 meters (164 to 328 yards) in diameter and up to 5 meters (16 feet) deep. The rings themselves have a width of 20 to 50 meters (22 to 55 yards).

A few things are clear about Lake Superior's mystery rings. These formations are found in the deepest parts of the lake, where the sun doesn't shine and waves can't disturb the soft sediments.

The rings are geologically young; the sediments in which they are found are less than 10,000 years old. The sediment is soft and partly composed of smectite — the slimiest, slipperiest clay around. This type of fine-grained clay acts like a sponge and absorbs water between layers that lie like a deck of cards. This clay is known to exhibit dramatic changes in volume and may be tied to the origins of petroleum and of life itself.

Not as much is known about the processes that formed the rings. To help unravel the mystery, Wattrus is collaborating with Joe Cartwright, a professor at the University of Cardiff in Wales who studies distinctive honeycomb-like Polygonal Fault Systems (PFS) around the world's ocean floors. Using high-resolution seismic data, they are examining the structure of the sediments below Lake Superior's rings, thinking that these rings could be immature PFS.

"The data we are collecting has revealed a complex pattern of faulting in the sediments below the lake floor," said Wattrus. "We think the rings are the surface expression of this developing fault system."

A 3-D view of a Polygonal Fault System (PFS). (polygon-shaped faults)

A 3-D view of a Polygonal Fault System

The data collected by Wattrus and Cartwright in Lake Superior fit many of the criteria for PFS developed by Cartwright and his co-workers at Imperial College, London. Once believed to be insignificant, the PFS found in the North Sea and elsewhere are apparently important to the creation of some oil fields.

Unlike most faults, PFS and Lake Superior's mysterious rings seem to be born of something other than irregularities in the Earth's deep crust or sediments compacting under their own weight.

Wattrus and Cartwright believe that the mechanism responsible for these unique deep-water features involves syneresis. This is the same process that creates curds and whey in cheese production, and the polygonal-shaped cracks in dried mud.

As with cheese proteins, smectite clay molecules prefer to interact with each other, rather than with the water surrounding them. They group together through the process of syneresis, creating a three-dimensional (volumetric) contraction, expelling the water otherwise trapped between the sediment particles. The result is a network of small faults that look like polygons.

Wattrus and his colleagues collected core samples of the sediment in Lake Superior to find out more about the unusual ring features. They discovered that the sediments are over-compacted; the compaction is much higher than might otherwise be expected for a sample that had simply been buried to a similar depth. There is no evidence that significant sediment erosion has occurred or that the sediments have been subjected to external loading (for example by a glacier). Syneresis could create this over-compaction.

Wattrus and his colleagues conclude that the water squeezed out by syneresis through a more compact crust may be the trigger that formed the rings. Geologic conditions associated with retreating glaciers and processes like syneresis caused the lower sediment layer to become mobile and the overlying sediments to collapse.

Wattrus hopes that by studying the immature Lake Superior's deep-water donut rings he and his colleagues will be able to shed light on what triggers volumetric contraction in sediments and more about the processes that shape our planet. But for now, the image on his door remains a testament to unanswered questions and one of Lake Superior's deepest mysteries.

By Sharon Moen
December 2002

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