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Iowa's Manson Impact Structure

Red ball icon Iowa's Manson Impact Structure

by Raymond R. Anderson

 

Seventy-four million years ago, near the end of the Cretaceous Period, central Iowa lay near the shoreline of an inland seaway that separated eastern North America from rapidly rising mountains to the west.  The low-lying Iowa landscape was home to a rich and varied population of plants and animals, including dinosaurs and small mammals.   These organisms lived in a fern-rich, mixed conifer and deciduous forest with a warm, moist climate much like today’s Gulf Coast.  The environment dramatically changed when a stony meteorite, over one mile in diameter, weighing about 10 billion tons and traveling about 45,000 miles per hour, blasted through the atmosphere and crashed to earth.

In the fraction of a second that it took the meteorite to penetrate about one mile into the ground, the shock wave created by the initial contact with the surface reached the back side of the meteorite and its potential energy was transformed to kinetic energy, the equivalent of about 10 trillion tons of TNT.  An electromagnetic pulse moved away from the point of impact at nearly the speed of light, and instantly ignited anything that would burn within approximately 130 miles of the impact (most of Iowa).  The shock wave toppled trees up to 300 miles away (Chicago, Minneapolis, and St. Louis), and probably killed most animals within about 650 miles (Detroit  and Denver).  The blast left a crater over 24 miles in diameter centered in an area of unimaginable death and destruction.

Today there is no land surface expression of the crater that exists 100 to 300 feet below the town of Manson (Calhoun County), which lies near the center of the crater that bears its name.


Location map

Location of Manson Impact Structure and principal features of its crater.

 

The area of the Manson Impact Structure (see map, above) has been known as a geologic anomaly since the early 1900s.  At that time a new water well for the town of Manson encountered an unusual sequence of rocks that yielded the only naturally soft groundwater known in Iowa.  The first investigation of the anomaly, in 1955, consisted of drilling two research cores and studying rock samples collected during water well drilling in the area.  Since meteorite impact craters were almost unknown at that time, the feature was interpreted as a "cryptovolcanic structure," a crater produced by a giant explosion of volcanic gases.  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.

 

Thin section
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.


In 1991 and 1992 the Geological Survey Bureau and U.S. Geological Survey began to investigate the possibility that the Manson impact played a role in the extinction of the dinosaurs and other species at the end of the Cretaceous Period, 65 million years ago.   During the course of this investigation 12 research cores, totaling over 4,000 feet, were obtained from all terranes of the crater.  Study of those cores and other data by scientists throughout the United States and from several other countries produced an understanding of the processes involved in the formation of the Manson Structure.   This investigation identified the Manson Structure as a "complex" impact crater; that is, it includes an outermost "Terrace Terrane" of down-dropped blocks, an inner "Central Peak," and a "Crater Moat" in between (see diagram, below).

 

Crater cross section diagram

Cross-section view of the geologic features of the Manson Impact Structure.



Some of the most important data obtained from the research cores was a more accurate age for the impact of approximately 74 million years.  Also significant was the identification of six types of impact rocks which were emplaced during actual crater formation. 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.

The shallow seaway 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.

Although the Manson Impact Structure is now one of the best preserved and best studied complex impact craters on Earth, many unanswered questions remain about the effects of this impact on life forms.  Additional developments in crater research as well as a detailed model of the crater’s formation can be obtained from the Geological Survey Bureau’s web site.
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Adapted from Iowa Geology 1999, Iowa Department of Natural Resources