The larger of the two craters was discovered in 1990 by four scientists from the Geological Survey of Canada, Teske said. While studying magnetic and gravity maps of the area, the scientists discovered the crater more than a mile underneath the bed of Lake Huronjust north of Port Huron. While divers visiting the lake bottom will see no sign of its presence, its 30-mile size and 500-million-year age make it one of Earths largest and oldest impact scars, Teske said.
The second Michigan crater was found in Cass County in 1987 by Randall Milstein of the Michigan Geological Survey. He was examining information from about 100 test wells drilled in the area. This crater lies just south of a small village named Calvin Center, and is buried about 100 to 400 feet beneath the surface. Now covered with farms and forests, it is five miles across and 440 million years old, and was excavated by an object about the size of a football stadium, Teske said.
About 140 meteorite craters are known worldwide, according to Teske. They were blasted out by projectiles the size of a house and larger that arrived here from the asteroid belt between the planets Mars and Jupiter, he said. These objects are almost always stony or rock-like. Usually it takes an expert to distinguish one from ordinary rocks found on Earth.
Once a crater is formed, it gets quickly obliterated by erosion and sedimentation, Teske explained. So scientists only know the most recent history of cratering on our planet, less than a half-billion years into the past.
Finding these buried Michigan craters has helped to teach two exciting lessons. First, recent technical developments of the kind used by the discoverers make it possible to recognize the presence of buried craters. Second, study of the buried craters can let us look further back into the past record of how the Earth has been hit by large pieces of space debris, Teske said.
Only large chunks of rocky material survive the plunge through our atmosphere to reach the ground, according to Teske. Its good news for us that the bigger they are, the fewer there are in space; giant impacts are rare.
Penetrating Earths atmosphere at speeds exceeding seven miles per second, the projectiles are strongly heated by friction with air molecules. Their outer surfaces melt and the resulting white-hot stone droplets are stripped away by the fast wind generated by their high speeds. Originally larger than a mans head, these objects are reduced to fist size by the time they finally reach the ground. Smaller ones never make it at all.
Although one fist-sized object falls to Earth somewhere every two hours or so, chances of getting bopped by anything are very small, Teske said. Only one personMrs. Annie Hodges of Sylacauga, Ala.is on record as being hit by a stone from the sky. Mrs. Hodges was napping on her living room couch in November 1954, when an eight-pound stony meteorite crashed through her roof and then through the bedroom floor beneathfinally bouncing off a radio before badly bruising her arm and leg.
Almost every year close calls are reported as small meteorites drop within a few feet of people, according to Teske.