The Weaubleau Structure and the Rock Balls
The Weaubleau Structure is one of the fifty largest known impact craters on earth and the fourth largest in the United States. The three larger ones in the U.S. either have been glaciated and buried, submerged under water, or deformed by tectonic movement. Therefore, the Weaubleau structure is the largest exposed impact crater in the U.S. that has not been deformed or misshapen.
Surrounding the impact area are spherical rock formations of varying size, called the Missouri rock balls, which probably formed at the time of the impact. Nearly perfectly round, they are also sometimes referred to locally simply as "geodes," "round rocks," or "Weaubleau eggs." The round rocks may have formed when the impact threw shale away from the center of the crater and silica-saturated waters subsequently formed silica around the shale fragments.
The Weaubleau structure is one of a series of known or suspected impact sites along the 38th parallel in Illinois, Missouri, and Kansas. The others include the Hicks Dome in Hardin County, Illinois; the Avon crater in Ste. Genevieve County, Missouri; the Furnace Creek volcanics in Washington County; the Crooked Creek structure in southern Crawford County; the Hazelgreen volcanics near the Laclede-Pulaski county line; the Decatur dome in southern Camden County; and the Rose dome in eastern Kansas. The effects of the impact on the surrounding topography at the Decatur dome can be seen in a road cut that runs along Highway 5 about sixteen miles north of Lebanon.
Because all of these structures are located roughly along the same latitude, one theory holds that all of them were the result of a serial meteorite strike. However, scientists consider such a serial impact a highly unlikely event on earth. The difficulty in dating all of the structures as having originated from the same period also casts doubt on the serial impact theory. In addition, there is evidence that at least some of the structures, such as the Hicks dome, resulted from volcanic eruptions rather than from meteorites. Only the Crooked Creek structure and the Decatur dome have been confirmed as having resulted from impacts.