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 todays 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
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
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
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.
|A microscopic view through a thin-section of rock from the Manson
Craters 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
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
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 craters formation can be obtained from the Geological Survey
Bureaus web site.
Adapted from Iowa Geology 1999, Iowa Department of Natural Resources