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Impact cratering was especially prevalent for the first several hundred million years after the planets formed as the planets swept up left-over material.The last stage of that "sweeping up", called the occurred from about 4.1 to 3.8 billion years ago. Links to external sites will be displayed in another window. Select the photographs to display the original source in another window.See the "Not Round" page from the THEMIS site for what can make an impact crater not round (links will appear in a new window).
Careful studies of how the craters overlap other craters and other features can be used to develop a history or sequence of the bombardment on the moons and planets. Worlds with less volcanism or erosion or tectonic activity in their history will retain more impact craters since the planet formed.Volcano craters are above the surrounding area on mountaintops while the craters from impacts are below the surrounding area with raised rims.The craters on all of the moons except Io, Mercury, and most of the ones on Mars are from impacts.When they get close enough to a planet or moon, they will be pulled in by the large body's gravity and strike the surface at a speed of at least the escape velocity of the planet or moon, i.e., faster than a bullet.
At such speeds, the projecticle explodes on impact and carves out a round bowl-shaped depression on the surface. How can you distinguish an impact crater from a volcanic crater?
Select the image to go to the Selene website for a great flyover of the crater.