We have seen that on the Moon, a complete lack of wind (except the solar wind) and water means that the only processes that will modify impact craters are other impacts and volcanism. On Earth it is an entirely different story. We have an atmosphere, we have gravity six times that of the Moon, and we have water covering 74% of the planet. Add in crustal recycling through plate tectonics and you can appreciate why there are so relatively few craters present on Earth. Craters are not preserved at all well in water, so at the outset we have only just over a quarter of the planet available for cratering. For a dramatic account of the possible consequences of an ocean impact, read the science fiction book Lucifer's Hammer by Larry Niven and Jerry Pournelle.

The first image below is of Meteor crater, Arizona, approximately 1.2 km in diameter. A probable age for the crater is about 50,000 years. As can be seen, it has the bowl shape typical of small craters however, it is squarish in outline rather than being perfectly circular; probably because of pre-existing structural discontinuities in the impactee. The interior of the crater is relatively smooth except for small rivulets running down the walls to a floor which looks like it at times contains water which has left evaporite deposits. Not surprizing when it can be seen that the floor of the crater is well below the surface of the surrounding desert terrain.

The next image is of Lonar crater, about 1.8 km in diameter. Again believed to be about 50,000 years in age. The crater is located in the Deccan lava plateau of India. It's no wonder its impact origin took a while to be established. The crater is fairly obviously filled with water.

The next image is of Clearwater Lakes in Quebec, the left crater being 32 km in diameter, the right 22 km. It is now fairly obvious that we have exceeded the transition from simple bowl-shaped to complex craters; a transition which takes place at about 3 km diameter on Earth. The left crater has a ring approximately 10 km in diameter, while the right crater has a submerged central peak. The impact which produced both craters is dated at approximately 290 m.y.

The next shows Manicouagan crater also in Quebec. It is 70 km in diameter and has been dated at about 210 m.y. The lake was excavated by glaciation into the less resistant impact breccia which surrounds the central impact melt. It is pretty obvious that both it and the previous craters, have been worn down to their "roots". As an aside, the Canadian Geological Survey does much of their research on these craters in the winter when they can take a drilling rig out onto the ice and the flies are less of a nuisance.

The next image is a ground view of our very own Sierra Madera, located just north of Marathon, Texas. It was originally about 13 km in diameter but all that is visible are parts of the central ring structure. It has been dated at 100 m.y. but I think it could be as young as 65 m.y. and possibly another stucture formed at the K/T boundary.

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