There are meteor impact structures all over the world. The impacts may be made by either planetoidal bodies (meteors) or comets. Wikipedia describes impact structures and impact craters as follows:
“An impact structure is a generally circular or
craterlike geologic structure of deformed bedrock or sediment produced by
impact on a planetary surface, whatever the stage of erosion of the structure.
In contrast, an impact crater is the surface expression of an impact structure.
In many cases, on Earth, the impact crater has been destroyed by erosion,
leaving only the deformed rock or sediment of the impact structure behind.
“In an impact structure, the typical visible and
topographic expressions of an impact crater are no longer obvious. Any
meteorite fragments that may once have been present would be long since eroded
away. Possible impact structures may be initially recognized by their anomalous
geological character or geophysical expression. These may still be confirmed as
impact structures by the presence of shocked minerals (particularly shocked
quartz), shatter cones, geochemical evidence of extraterrestrial material or other
methods.”
These structures used to be
called crypto-explosive structures or, in some cases, crypto-volcanic
structures since their origins were often unclear. Most were later confirmed as
meteor impact structures, and the previous terms have fallen out of use. Impact
craters are often eroded, which can further complicate
interpretation.
The Earth Impact Database, begun in 1955, recognizes 190 confirmed impact structures worldwide. The last new one confirmed was in 2019. Other lists contain many more possible and probable impact structure sites. Impact structures have also been mapped in the subsurface.
I know of at least one mapped and interpreted in the Ohio
subsurface by a colleague. I don’t know how big it is deemed to be, but I think
it was discovered through mapping Silurian-aged rocks in the subsurface by
picking formation tops from oil & gas geophysical well logs. In the area of
concern, the wells were closely spaced, which likely improved the mapping. If I
recall correctly, it was in northeastern Ohio.
There are three mapped impact
structures in Kentucky and one in Ohio, not including the one mapped in the
subsurface. Ohio’s Serpent Mound impact structure is well studied. According to
the Earth Impact Database, the Serpent Mound Structure in Ohio and the
Middlesboro Structure in Kentucky are confirmed impact craters. The Jeptha Knob
Structure in Kentucky is a probable impact structure, and the Versailles
Structure in Kentucky is a possible impact structure. The one mapped in the
subsurface would also be a possible impact structure. Below is an old map of faults and structures of Eastern Kentucky with the three impact structures included. The Serpent Mound impact structure is just to the north near the Glenville Front suture zone.
Around the world, some of the
most important meteor impact sites include the impact crater offshore the
Yucatan Peninsula that is considered to be the impact event that destroyed the
dinosaurs and much of Cretaceous-aged life about 66 million years ago. It is
considered one of the most significant mass extinction events. That crater is
very large at 170 km in diameter. The largest one in the U.S. is the 60km
diameter crater in Virginia’s Chesapeake Bay. Canada has two larger than that,
with the largest one in North America at Sudbury, Ontario, at 250km in
diameter.
The largest known meteor
impact that ever hit the Earth is thought to be 520 km (323 miles) in diameter.
This suspected meteor impact is buried in the subsurface of Australia. It was
discovered and analyzed with geophysical methods, including seismic and
aeromagnetic surveys. This impact is buried about 4000 meters below the
surface, and the impact is thought to have occurred between 440 and over 500
million years ago. If confirmed, it would be larger than the previous largest
known one at 300 km in diameter in South Africa. According to a 2023 Forbes
story by geologist David Bressan about the buried crater and other impact
structures:
“About 200 terrestrial impact craters are currently
known. Over half are located in Europe, North America and Australia. The ages
of the great majority of preserved impact structures are less than 200 million
years, and structures smaller than 5 kilometers are greatly underrepresented.
The observed distributions of crater sizes and ages have been biased by
post-impact processes. Erosion tends to quickly destroy or bury craters
(especially the smaller ones) in tectonically active areas, like near fault zones
or on the seafloor. Impact craters are best preserved inside the stable cores
of the continents, like the Canadian-, Fennoscandian- and Australian Cratons.”
Thus, the ones we know are
not the whole story. Many smaller ones have been eroded or deformed so that
they can no longer be observed. Others have likely not been found yet. Many
others are likely buried as well.
Many years ago, I visited the
Serpent Mound region of the impact crater and even collected a few interesting
rocks, which I think are carbonates, either Limestone or Dolomite, likely of
Silurian age. The impressive Native American earthwork is built within the
southern part of the impact crater. The native people likely noticed some kind
of geological anomaly there.
These structures will often
exhibit post-impact rebound. This is perhaps one reason why Jeptha Knob
exhibits the highest elevation in Kentucky’s bluegrass region, rising some 300
feet higher than the surrounding terrain.
Impact Structures of Southern Ohio and Eastern Kentucky
References:
Impact
structure. Wikipedia. Impact
structure - Wikipedia
Earth
Impact Database. Wikipedia. Earth Impact
Database - Wikipedia
The Serpent
Mound Impact Structure. Ohio Department of Natural Resources. The
Serpent Mound Impact Structure | Ohio Department of Natural Resources
Guide
to the Serpent Mound Impact Structure, South-Central Ohio. Keith A. Milam.
Department of Geological
Sciences. Ohio University. Field Trip 1 of the Geological Society of America North-Central
Section 46th Annual Meeting. Dayton, Ohio, April 22–24, 2012. GB22_Milam_2016_EOGS10023.pdf
Did
You Know That Meteorites Have Hit Kentucky? Kentucky Geological Survey. Meteorites Have
Hit Kentucky, Kentucky Geological Survey, University of Kentucky
Geophysical
Evidence Suggests That the World’s Largest Impact Crater Is Buried Deep Beneath
Australia. David Bressan. Forbes. August 11, 2023. Geophysical
Evidence Suggests That The World’s Largest Impact Crater Is Buried Deep Beneath
Australia
List
of impact structures in North America. Wikipedia. List
of impact structures in North America - Wikipedia
Jeptha
Knob. Wikipedia. Jeptha
Knob - Wikipedia
Middlesboro
crater. Wikipedia. Middlesboro
crater - Wikipedia
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