Manson
Key facts
- Country
- United States
- Diameter
- 35 km
- Age
- 73.8 ± 0.3 million years
- Location
- 42.58° N, 94.55° W
Nothing at all: the crater lies under flat Iowa farmland and the town of Manson, buried beneath tens of metres of glacial clay and gravel.
Manson, in north-west Iowa, is one of the largest impact structures in the United States, a crater about 35 km across hidden completely beneath the corn and soya fields of the Midwest. It formed about 74 million years ago, in the Late Cretaceous, when the area lay near the eastern shore of a shallow inland sea that split North America in two.
What it hit. The impactor struck a layer cake of sedimentary rock, Cretaceous shales and sandstones resting on older limestones and dolomites, all sitting on ancient crystalline basement. The rebound of the crater floor lifted granite and gneiss from about 4 km down to just below today's surface, forming a central peak. Ice sheets of the last ice ages later planed the area flat and blanketed it with glacial debris, which is why nothing shows at the surface.
How it was found. Water wells drilled in the early twentieth century struck unusual rocks, and the area became known as the Manson anomaly. For decades it was thought to be a volcanic or 'cryptovolcanic' structure. In the 1960s shocked quartz, grains crossed by sets of microscopic planes that only a shock wave can make, was identified in drill samples, showing that an impact was responsible.
A suspect cleared. In the 1980s Manson was briefly a prime suspect for the impact that ended the age of the dinosaurs, because early dates seemed to match. In 1991 and 1992 a programme of twelve research boreholes recovered cores through the crater, and careful argon dating of the melt rock gave an age of about 74 million years, some 8 million years too old. The search moved on, and Chicxulub in Mexico was soon identified as the true culprit.
Why it matters. Manson is one of the best-drilled complex craters on Earth, and its cores are a reference for how shocked rock, breccia and melt are arranged inside a buried crater. The impactor is unknown.
Did you know?
Manson was once thought to be the dinosaur-killing crater, until dating showed it was about 8 million years too old.
Granite from about 4 km down was lifted to just below the surface by the rebound of the crater floor.
There is nothing to see: the whole crater is hidden under glacial deposits and farmland.