Chesapeake Bay Crater Montgomery College Planetarium at Takoma Park/Silver Spring By Dr. Harold...
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Chesapeake Bay Crater
Montgomery College Planetarium
at Takoma Park/Silver Spring
By Dr. Harold Williams
http://montgomerycollege.edu/Departments/planet/
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Synopsis
• 35.5 ± 0.6 million years ago, when Washington, DC and Richmond VA were on the coast, (sea level was higher then)
• an object hit the earth on the continental shelf in the Atlantic ocean, N 37° 17' W 76° 1',
• causing a crater 85 km in diameter. • After sea level fell, this crater would help
form the Chesapeake Bay. • The impact site is now called the lower
Chesapeake Bay in Virginia.
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Where did this information come from mostly.
http://woodshole.er.usgs.gov/epubs/bolide/index.html
Who did most of the real work and research.
Principal Investigator C. Wylie Poag [email protected]
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Bolide?
• Extraterrestrial body in the 1-10 km size range
• Impacts the earth at velocities of 20-70 km/sec = Mach 75
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Simple Craters
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Earliest identified crater on Earth
• Some Links
• Meteor Crater http://www.meteorcrater.com/
• Barringer Crater http://www.barringercrater.com/
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Complex Craters
Manicouagan, Quebec Canada, 70 km (43.4 miles) across, Formed around 212 million years ago, Lake surrounds the more erosion-resistant melt sheet created by the impact onto metamorphic and igneous rocks on the stable Canadian craton.
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Complex Crater on the far side of the Moon, King Crater
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Shoemaker-Levy 9 striking Jupiter
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Chesapeake Bay Crater
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First Evidence for Impact
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Shocked Quartz intersecting lamellae
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Some Shocked Quartz
• PDF, Planar Deformation Features, Link http://www.unb.ca/fredericton/science/emunit/Trepmann/Shocked%20quartz.html
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S-1 Large shocked quartz grains with planar deformation features (PDFs) identified in the Fraser Park, Sydney Basin Permian-Triassic boundary layer. Inset in upper photo shows close-up of intersecting PDFs. Quartz grains are mounted in oil and photographed in plane polarized light.
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Cores from drilling
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Index microfossils give a date
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Radiometric ages in tektite glass
• This age was confirmed by determining the ratio of two isotopes of argon gas contained in the tektite glass.
• 35.5 ± 0.6 million years ago
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Virginia Core Sites
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Geological Column
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Seismic Profiles
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Ground Instability Due to Faulting
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Summary• During the late Eocene, the formerly
quiescent geological regime of the Virginia Coastal Plain was transformed when a bolide struck in the vicinity of the Delmarva Peninsular
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Produced the following principal consequences:
• The bolide carved a roughly circular crater twice the size of the state of Rhode Island (~6400 km2), and nearly as deep as the Grand Canyon (1.3 km deep).
• The excavation truncated all existing ground water aquifers in the impact area by gouging ~4300 km3 of rock.
• Structural and topographic low formed over the crater. • The impact crater may have predetermined the present-
day location of Chesapeake Bay. • A porous breccia lens, 600-1200 m thick, replaced local
aquifers, resulting in ground water ~1.5 times saltier than normal sea water.
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2004MN4 now 99942 Apophis
• Link http://en.wikipedia.org/wiki/2004_MN4
• Friday, April 13, 2029 near miss of 320 m 4.6×1010 kg asteroid
• Magnitude 3.3 moving at 42 degrees/hour maximum angular speed with a diameter of 2 arc seconds from Europe and Africa
• Change from Aten to Apollo
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