Aorounga
Impact Crater? Yes
Latitude: N 19° 6'
Longitude: E 19° 15'
Age: 345 million years
Size: 12.6 km in diameter
Location: Sahara Desert of northern Chad, Africa
Kind of Image: Spaceborne Imaging Radar-C/X-Band Synthetic Aperture
Radar (SIR-C/X-SAR)
Description: An object less than 1 kilometer wide made this crater
7-10 miles wide. The original crater was buried by sediments, which
were then partially eroded to reveal the current ring-like appearance.
The dark streaks are deposits of windblown sand that migrate along
valleys cut by thousands of years of wind erosion. The dark band
in the upper right of the image is a portion of a proposed second
crater. Scientists are using radar images to investigate the possibility
that Aorounga is one of a string of impact craters formed by multiple
impacts.
20,000-30,000 years ago, the area that is now the Sahara Desert
was a wetland. Different kinds of forces were shaping this crater
then.
360 million years ago, the Earth was undergoing a period of mass
biological extinction. By way of comparison, the impact that scientists
believed wiped out the dinosaurs 65 million years ago involved an
asteroid or comet 10 times larger than the one that broke up to
form the craters in Chad.
El'gygytgyn
Impact Crater? Yes
Latitude: N 67° 30'
Longitude: E 172° 5'
Age: 3 - 4 million years
Size: 18 km in diameter
Location: Northeastern Siberia
Kind of Image: Landsat 7
Description: This impact crater has been filled in by a lake.
Haughton
Impact Crater? Yes
Latitude: N 75° 22'
Longitude: W 89° 41'
Age: 23 million years
Size: 20 km in diameter
Location: Devon Island, Nunavut, in the Canadian high Arctic
Kind of Image: NASA's Landsat 7
Description: Although Haughton Crater has undergone substantial
erosion, many of its surviving geologic features are exceptionally
well preserved. The good state of preservation is due mostly to
the crater's geographic setting. Erosion in the polar desert of
the high Arctic is particularly sluggish due to the low amount of
liquid water and the presence of continuous permafrost. The absence
of any substantial vegetation cover also limits the weathering of
surface materials, while it optimizes the site's exposure for geologic
surveys from the ground and by remote-sensing.
The impact event punched through the entire stack of Paleozoic sediments
present at the time and excavated material from a depth of over
1.7 km, biting into the gneissic basement.
Manicougan
Impact Crater? Yes
Latitude: N 51° 23'
Longitude: W 68° 42'
Age: 214 million years
Size: 72 km in diameter now; 100 km in diameter when created
Location: northern Quebec, Canada
Kind of Image: NASA's Multi-angle Imaging Spectroradiometer (MISR)
Description: Scientists have proposed that impact of an asteroid
of about 5 km in diameter created this crater, which was originally
about 100 km in diameter. The initial crater rim has disappeared.
A broad central uplift persists. Construction of a hydroelectric
dam downstream from the crater allowed water to fill the surrounding
depression, creating a lake that forms a ring around the central
part.
The Manicougan crater is composed of large pieces of rock embedded
in finer grained material rock ("impact-brecciated" rock).
Geologists have estimated the crater's age by studying the ratios
of various radioactive elements in the rock. The lake is bounded
by erosion-resistant metamorphic and igneous rocks, and shock metamorphic
effects are abundant in the target rocks of the crater floor.
The Manicougan impact event occurred toward the end of the Triassic
period. At that time the Earth experienced a mass extinction event
involving the loss of roughly 60 percent of all species. Some scientists
believe the impact may have been responsible for this mass extinction.
Mount St. Helens
Impact Crater? No. Mount St. Helen's is a volcano.
Latitude: N 46° 16'
Longitude: W 122° 12'
Age: probably less than 1 million years
Size: Several km in diameter
Location: South central part of the State of Washington, northwestern
United States
Kind of Image: NASA's Landsat 7
Richat
Impact Crater? No. Richat is a product of erosion.
Latitude: N 21°04'
Longitude: W 11°22'
Age: Modern age
Size: 38 km in diameter
Location: Central Mauritania, northwest Africa
Kind of Image: NASA photograph taken by astronaut on the Space Shuttle
Description: Richat is a depression or pit about 100 m deep, in
which there is a dome of rock. The rock is actually different kinds
of rock in layers, several hundred million years old. Some kinds
of rock erode faster than others. Made of different materials, the
layers of rock have eroded at different rates, creating a series
of concentric ridges. Fields of sand surround Richat and are encroaching
into the southern part of the structure.
Researchers once thought Richat was an impact crater. But its flat
middle and lack of shock-altered rock indicate otherwise. A volcanic
eruption couldn't have formed it, because there's no dome of igneous
or volcanic rock. Why Richat is nearly circular, nobody knows.
Schooner
Impact Crater? No, Schooner is manmade.
Latitude: N 37deg° 20' 36.1"
Longitude: W 116° 33' 59.9"
Age: 20th century
Size: About 300 m in diameter
Location: Nevada, U.S.
Kind of Image: IKONOS (privately-owned satellite); 1-meter resolution
Description: Nuclear testing resulted in the Schooner crater.
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Thursday, August 14, 2003