The area of north-central Iowa around the town of Manson has been known as a region of anomalous geology since the early part of the century, when samples collected during the drilling of a Manson town water well proved to be unlike other rocks in the area. The nature of the anomalous geology remained a mystery until 1953, when a joint Iowa Geological Survey-U.S. Geological Survey investigation led to an initial interpretation as a cryptovolcanic feature (a crater created by a blast of volcanic gas).
The meteorite impact origin for the structure was first proposed by Robert Dietz in 1959 and confirmed in 1966 by Nicholas Short, who published photographs of "parallel deformation features" in quartz grains, including specimens from the Manson Structure. Short concluded that these features constituted incontrovertible evidence of a meteorite impact origin. The so-called "shocked quartz grains" (see photo, below) are produced when a high-energy shock wave generated by an impact passes through a quartz grain, creating thin regularly spaced zones of melting along preferred crystallographic planes. Extraterrestrial impacts are the only known natural force with sufficient energy to create these features.
Shocked Quartz: A microscopic view through a thin-section of rock from the Manson Crater’s Central Peak is shown. Lines in the brownish quartz grains are impact-derived “parallel deformation features.” Such features are found at meteor impact sites around the world and are considered irrefutable evidence of extraterrestrial impacts. Horizontal field-of-view is 3 mm. Photo by Ray Anderson.
It was proposed in the late 1980s that the Manson Impact Structure might be linked to one of the Earth's greatest extinctions of life (including dinosaurs) at the end of the Cretaceous (about 65 million years ago). To test this hypothesis and learn more about the Manson crater, the Iowa Department of Natural Resources, Geological Survey Bureau and U.S. Geological Survey drilled 12 research cores in the structure (totaling over 4,000 feet) in 1991 and 1992. Impact rocks recovered during this drilling have been subjected to numerous investigations to develop a better understanding of the structure. One of the most significant discoveries resulting from this work was a new date for the Manson impact, about 74 million years ago. Study of the cores and other data showed the Manson Impact Structure to be a very well-preserved complex impact structure, with a large central peak, an outer ring of down-dropped strata known as the terrace terrane, and an intermediate crater moat region (see cross-section and overhead views below).
Crater Cross Section
Crater Overhead View
Investigation of the drill cores disclosed the presence of six primary types of impact rocks. Four impact rock units were identified on the Central Peak. These include Proterozoic Basement Blocks (large blocks of granite and gneiss from below the crater floor); Crystalline Clast Breccia with a Sandy Matrix (smaller fragments of granite and gneiss in a matrix of sand-sized rock and mineral grains); Crystalline Clast Breccia with a Melt Matrix (similar to the previous unit, except most of the sandy matrix and many of the larger fragments have been melted); and the Keeweenawan Clast Breccia (broken and partially melted billion-year-old shale). The fifth impact rock type is Ejecta (rocks thrown from the crater during formation), found only in the Terrace Terrane. The sixth impact rock is Phanerozoic Clast Breccia (material originally stripped from the land surface, mixed with ejecta in a ground surge that moved ahead of the growing crater, and then quickly transported back into the crater by returning sea waters). This material was found in all three terranes of the crater.
Rock Layering within the Crater
The effect of such a large impact event is hard to imagine, but it would have had drastic consequences on the plant and animal life at the time. At the end of the Cretaceous Period, this part of Iowa was located in the area of the Western Interior Seaway cutting through the middle of Canada and the United States. Dinosaurs and sea reptiles were the dominant animals. When the meteor struck, it hit with the force of 10 trillion tons of TNT. Everything within 130 miles was immediately incinerated, vegetation up to 300 miles away was knocked flat, and it is thought that most animals within 650 miles were killed.
The shallow seaway eventually retreated from the region of the Manson Structure within a few million years following the impact, exposing the area to erosion. Over the next 70 million years about 1,000 feet of rock layers were removed from the region by erosion, including all impact rocks beyond the crater. However, with its thick cover of Phanerozoic Clast Breccia for protection, only small areas on the Central Peak and Terrace Terrane were eroded. During the last 2.5 million years, continental glaciers covered the Manson area repeatedly. These glaciers further eroded the impact feature before blanketing it with glacial sediments that cover it from view today.
The Manson Impact Structure has a diameter of 37 km (23 miles) making it the largest intact, on-shore meteorite crater in the United States. The structure is present at the bedrock surface, but is buried beneath 20 to 90 m (70 to 300 feet) of glacial till with no apparent surface expression of the structure. The original object was probably a stoney meteorite about 1.5 miles (2 km) in diameter, weighing close to 10 billion tons, and traveling a brisk 45,000 miles per hour. The solar system still contains hundreds of thousands of asteroids similar to the one that struck Iowa that pose a remote, but undoubtedly catastrophic hazard.
Logging requirements
1) Mark your position using your GPS at the center of crater and then visit one of the crater rim points. Determine the distance from the rim to the center of the crater. Send me your answer in an email.
2) Visit the library in Manson (NOTE AVAILABLE HOURS) and view the cores that are on display there. Using information in library send the answers the following questions in an email to me:
a. What are the colors of the granite, gabbro, gneisses, basalts, and sandy matrix in the M-8 core sample collected at 351.5 feet?
b. What color are the feldspar and quartz crystals in the M-10 core from 614.5 feet?
c. Sedimentary rocks are found in two of the cores collected from the M-4 site. What are their names, and what is so unusual about their position? See the hint, if you are having difficulty answering this final question.
Sources:
Iowa Department of Natural Resources (http://www.igsb.uiowa.edu/Browse/manson99/manson.htm)
Special thanks to Ray Anderson at the Iowa Department of Natural Resources and Ann Schlapkohl at the Manson Public Library for help developing this EarthCache.