Monday, July 28, 2008
This is the Serra da Cangalha Crater in Northern Brazil. It's hidden underneath tropical savanna vegtation, resting on sediments laid down approximately 300 million years ago. Geologists estimate the meteorite struck here about 220 million years ago. The crater's structure shows a series of concentric rings with a diameter of 8 miles and the inner bowl is rimmed by rocks rising about 1,380 feet above the surrounding land.
It took some time to establish Serra da Cangalha as a impact crater. Geologists had to take into account, its circular shape, no volcanic rocks in a drill core and no carbonate or salt layers in nearby sediments which would suggest a salt dome. It also had to have the signs of impact in the form of shatter cones, conical shaped, grooved rocks known only to appear in impact craters.
This image was taken by the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on NASA's Terra satellite on June 23, 2006. It's a simulated-true-colour image with varying shades of green defining the mix of savanna and riparian forest (forests along a river or stream). The occasional patches of purple-grey mean bare ground.
This is another way to image a crater or potential crater. It's a 3.4 mile wide craterlike formation which is buried 4,900 to 5,250 feet below sea level west of Stockton, California. Rocks in the area date to about 37 to 49 million years. This is from a seismic survey data of the Central Valley region which scientists believe was underwater at that time. More important than the circular shape is rock analysis showing shocked quartz that require a high-shock pressure impact to form.
The Victoria Island structure is being added to a database of suspected impacts along with the 0.8 mile Cowell structure to the north. If this can be proven, it'll be the first crater found in California.