What does a squashed crater look like?

In the forests north of Lake Huron lies a large basin shaped something like a 60-kilometer-long, 40-kilometer-wide lima bean. Called the Sudbury structure, it contains some of the world's greatest nickel-copper ore deposits, but questions about its origin have long stymied geologists. Now, researchers have uncovered evidence that the oblong basin started off as a circular meteorite crater that was later deformed.

After first discovering the Sudbury structure in the 19th century, geologists theorized that magma rising from Earth's mantle formed the rich ores and unusual igneous rocks in the region. In the 1960s, researchers challenged that

theory after finding evidence linking the Sudbury structure to a meteorite impact. Yet many geologists discounted the impact theory, in part because of the structure's oblate shape.

Recently, a team of geophysicists from the Geological Survey of Canada in Ottawa studied the basin through seismic profiling--a technique that sends vibrations down into the Earth and measures how they reflect off deep geologic features. Information collected by the profiling experiment reveals that the shape of the Sudbury structure has changed much more than scientists previously believed. Faults below the surface are oriented in such a way that their movements have apparently deformed the ring's shape. Although it remains unclear how much the Sudbury structure has changed, it may once have been circular, they contend. This finding opens the possibility that it resulted from a large impact, they say in the September GEOLOGY.

Among the evidence supporting the impact theory, geologists have found grains of shocked minerals in the region. Such grains are often accepted as the hallmark of a meteorite or comet crash. Because the proposed impact occurred 1.85 billion years ago, most of the crater would have eroded away by now. The current basin most likely represents the remnant of an internal ring that formed part of the original crater, says Richard Grieve, one of the coauthors of the paper. He estimates that the original crater measured roughly 200 kilometers in diameter, which would make it the largest impact structure known on Earth.