Chesapeake Bay
Key facts
- Country
- United States
- Diameter
- 85 km
- Age
- about 35.5 million years
- Location
- 37.28° N, 76.02° W
Nothing at the surface: the crater is buried beneath hundreds of metres of younger sediment under the southern Chesapeake Bay and Virginia's coastal plain.
Millions of people live and work above the Chesapeake Bay crater without ever seeing it. It lies buried beneath the southern end of the bay and the low coastal plain of Virginia, hidden under hundreds of metres of younger sand and clay. Yet it still shapes the land and the water there today.
The impact. About 35.5 million years ago, in the late Eocene, an asteroid or comet a few kilometres across struck the shallow continental shelf off the east coast of North America. The sea was then much higher, and the coastline lay far inland. The impact blasted a crater about 85 km wide through the sea-floor sediments and into the hard rock below, and sent a huge tsunami racing across the region. Seawater rushed back into the cavity, carrying broken rock and sediment that filled the crater with a thick jumble of debris.
Discovery. Hints of something unusual turned up in the 1980s, when drilling found a puzzling layer of broken rock under the Virginia coastal plain. In the early 1990s the geologist C. Wylie Poag of the United States Geological Survey and colleagues combined borehole data with seismic surveys, which use sound waves to map buried rock layers, and identified the buried crater. Shocked quartz in the debris confirmed an impact. The crater is also linked to the North American tektite strewn field, a scatter of natural glass that fell across a wide area of the south-eastern United States and the Caribbean region.
In 2005 and 2006 a deep scientific borehole was drilled near the centre of the crater, reaching nearly 1.8 km. Its cores revealed the sequence of events after the impact in remarkable detail.
Living on a crater. The crater still matters to people living above it. The jumbled, salty debris inside it affects groundwater, and there is a zone of unusually salty water in the aquifers within the crater's outline, which limits where fresh water can be drawn from wells. The crater may also have influenced the course of rivers and the shape of the bay itself, because the debris-filled crater settled more than the rock around it, creating a low area that rivers followed. Some geologists think this helps explain why the lower Chesapeake Bay lies where it does.
A twin event. Chesapeake Bay formed at almost the same time as the much larger Popigai crater in Siberia. Whether this was coincidence or part of a period of increased impacts is still discussed.
Seeing it today. There is nothing to see on the surface. The crater's story is told through geological maps, drill cores and the curious patterns of groundwater and river courses in south-eastern Virginia.
Did you know?
The crater was unknown until the early 1990s, despite lying beneath a heavily populated region.
Salty groundwater inside the crater affects where drinking water can be drawn.
Its tektites, a kind of natural glass, are found across parts of the south-eastern United States.