Des Plaines
Key facts
- Country
- United States
- Diameter
- 8 km
- Age
- less than about 280 million years
- Location
- 42.05° N, 87.87° W
Nothing: the structure lies beneath glacial deposits and the north-western suburbs of Chicago, around the city of Des Plaines, Illinois.
Des Plaines is a buried impact structure about 8 km across beneath the north-western suburbs of Chicago, in Illinois, in the United States. Hundreds of thousands of people live above it without seeing any sign of it.
Discovery. The structure was found from records of water wells. Geologists studying the rocks reached by wells in the area noticed that the normally flat-lying layers were disturbed in a round area, with older rocks raised in the centre. In the 1980s, study of samples from boreholes found shatter cones and quartz grains with shock features, showing that it was an impact structure.
What it hit. The target was a sequence of Palaeozoic sedimentary rocks, mainly limestones, dolomites and shales. The impact disturbed layers to a depth of several hundred metres. Later, ice sheets of the last ice ages eroded the top of the structure and covered it with thick glacial clay and gravel.
Age. The youngest rocks affected are Pennsylvanian, so the impact happened less than about 280 million years ago. Its exact age is unknown.
Why it matters. Des Plaines shows that impact structures can hide beneath even heavily built-up areas, discovered only through careful study of borehole records. The impactor is unknown.
Did you know?
Des Plaines lies beneath the suburbs of Chicago.
It was found from water-well records rather than anything at the surface.
Glacial deposits completely hide the structure.