Chesapeake Bay
A hidden crater that still affects the water people drink.
Key facts
- When
- about 35.5 million years ago
- Where
- Virginia, at the mouth of Chesapeake Bay, United States
- What hit
- an asteroid or comet perhaps 3 to 5 km across
- Era
- Deep time
- Location
- 37.28° N, 76.02° W
Probably a few million megatons of TNT.
Roughly a thousand times all the nuclear weapons in the world.
What happened
In the late Eocene, an impactor struck the shallow sea off what is now the east coast of the United States. It blasted out a crater about 85 km across, which was then buried under hundreds of metres of younger sediment. It was found only in the 1980s and 1990s, from drill cores containing jumbled rock and from seismic surveys that revealed its buried shape.
The crater lies beneath the southern end of Chesapeake Bay, and the bay's shape may partly owe something to the way the crater's fill has settled over time.
Effects
The blast sent a tsunami across the coastal plain and scattered glassy droplets of melted rock, called tektites, across a large region: the North American tektite strewn field, which reaches from Texas to the Caribbean. Today the crater still matters to people living above it. A deep layer of very salty water, trapped inside the broken rock, affects groundwater supplies, and the crater fill settles unevenly, so the land above it is sinking slightly faster than elsewhere.
The evidence
Drill cores of breccia containing shocked quartz, seismic images of the buried crater and its central peak, and tektites of the same age.
Did you know?
The crater was found partly because geologists puzzled over a strange layer of jumbled rock in boreholes across south-eastern Virginia.
Chesapeake Bay is one of the largest impact craters in the United States.
Related pages
How is impact energy measured? See energy explained, or try the impact calculator.