The Manson crater is an

impact crater An impact crater is a circular depression in the surface of a solid astronomical object formed by the hypervelocity impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal collapse, impact crater ...

near the site of

Manson, Iowa Manson is a city in Calhoun County, Iowa, United States. The population was 1,709 at the time of the 2020 census. History Manson was platted in 1872 on the Dubuque and Sioux City Railroad line. Tornado of June 28, 1979 A destructive F4 tornado ...

where an asteroid or comet nucleus struck the Earth during the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

Period, approximately 74 Ma. It was one of the largest known impact events to have happened in North America. Previously it was thought to have led to the extinction of the

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...

s until isotopic ages proved that it was too old.

Description

No surface evidence exists due to comparatively recent coverage by

glacial till image:Geschiebemergel.JPG, Closeup of glacial till. Note that the larger grains (pebbles and gravel) in the till are completely surrounded by the matrix of finer material (silt and sand), and this characteristic, known as ''matrix support'', is d ...

, and the site where the crater lies buried is now a flat landscape. But, hidden about below the surface is a buried structure about in diameter. It lies under the southeast corner of Pocahontas County and extends under portions of three adjoining counties. That an anomalous structure underlaid the area was known from unusual water well drill cuttings in 1912 of deformed rock, "crystalline clast breccia with a melt matrix" as a later report described it. A research investigation was started in 1955, and it was labeled a "cryptovolcanic structure" (a hypothetical

volcanic A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plates a ...

steam explosion). Further investigation was undertaken by Robert S. Dietz who proposed an impact origin in 1959 and by Nicholas Short in 1966 who produced evidence of

shocked quartz Shocked quartz is a form of quartz that has a microscopic structure that is different from normal quartz. Under intense pressure (but limited temperature), the crystalline structure of quartz is deformed along planes inside the crystal. These pl ...

grains which confirmed the impact origin of the structure. In 1991 and 1992 the U.S. Geological Survey along with others including the Iowa Geological Survey conducted detailed research in part to test the possible connection of the Manson Crater with the

Cretaceous–Paleogene extinction event The Cretaceous–Paleogene (K–Pg) extinction event (also known as the Cretaceous–Tertiary extinction) was a sudden mass extinction of three-quarters of the plant and animal species on Earth, approximately 66 million years ago. With the ...

. The / isotope ratio dating of the core from the impact structure gave an age of about 74 Ma, or about 10 Ma older than the K–T boundary. The impactor is considered to have been a stony meteorite about in diameter. The site at the time was the shore of a shallow inland sea, the

Western Interior Seaway The Western Interior Seaway (also called the Cretaceous Seaway, the Niobraran Sea, the North American Inland Sea, and the Western Interior Sea) was a large inland sea that split the continent of North America into two landmasses. The ancient sea ...

. The impact disrupted

granite Granite () is a coarse-grained ( phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies under ...

gneiss Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures a ...

, and shales of the Precambrian basement as well as

sedimentary Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

formations of

Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ' ...

age, Devonian through

Limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

layers that give the rest of Iowa

hard water Hard water is water that has high mineral content (in contrast with "soft water"). Hard water is formed when water percolates through deposits of limestone, chalk or gypsum, which are largely made up of calcium and magnesium carbonates, bicarbo ...

were instantaneously vaporized down to the basement rocks, giving Manson the anomalous soft water that it has today.Bryson 2002:237.

References

Bibliography

* Christian Koeberl and Raymond R. Anderson, eds; 1996, ''The Manson Impact Structure, Iowa: Anatomy of an Impact Crater'', Geological Society of America Special Paper 302,

External links

Aerial Exploration of the Manson Impact Structure
{{Impact cratering on Earth Impact craters of the United States Cretaceous impact craters Cretaceous United States Campanian Stage Geology of Iowa Landforms of Pocahontas County, Iowa Landforms of Iowa

Description

References

Bibliography

Further reading

External links