Chesapeake Bay Crater

Montgomery College Planetarium

at Takoma Park/Silver Spring

By Dr. Harold Williams

http://montgomerycollege.edu/Departments/planet/

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.

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 wpoag@usgs.gov

Bolide?

• Extraterrestrial body in the 1-10 km size range

• Impacts the earth at velocities of 20-70 km/sec = Mach 75

Simple Craters

Earliest identified crater on Earth

• Some Links

• Meteor Crater http://www.meteorcrater.com/

• Barringer Crater http://www.barringercrater.com/

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.

Complex Crater on the far side of the Moon, King Crater

Shoemaker-Levy 9 striking Jupiter

Chesapeake Bay Crater

First Evidence for Impact

Shocked Quartz intersecting lamellae

Some Shocked Quartz

• PDF, Planar Deformation Features, Link http://www.unb.ca/fredericton/science/emunit/Trepmann/Shocked%20quartz.html

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.

Cores from drilling

Index microfossils give a date

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

Virginia Core Sites

Geological Column

Seismic Profiles

Ground Instability Due to Faulting

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

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.

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