|MadSci Network: Earth Sciences|
Dear Bill, The Manicouagan structure is indeed a large impact crater, about 65 km in diameter. It was formed by a meteorite impact about 200 million years ago. The lake filling in the crater is artificial, formed by the Daniel Johnson dam across the Manicouagan river. The crater has a concentric structure picked out by the lake waters, as can be seen in any good aerial photograph. It also has a central peak, which is the high ground in the centre that you have observed. Much work has been done on the structures of impact craters and how these vary with size. Based on observations of hundreds of craters on the Moon, Mars and other bodies in the Solar System, and on laboratory experiments, there is a marked variation of structure with size. Small craters less than 15 km in diameter tend to be deep, bowl shaped depressions. Craters between 15 and 140 km in diameter, into which class the Manicouagan crater falls, are shallower, with flat floors, terraces formed by slumping of material from the crater walls, and a central peak. Above this size craters can have multiple central peaks and numerous concentric rings. The largest crater in the Solar System is the Orientale Basin on the far side of the Moon, which is 900 km in diameter; this crater has numerous large concentric rings, and the centre of the basin has been flooded by lava, probably formed by decompression melting of the lunar mantle when the impact punctured the crust. The central peaks of craters are explained by the effects of the shock waves generated by the impact. In a large impact the central region is at first depressed by the impact and then rebounds. You can try an experiment to see a similar effect by observing water drops dripping into a bowl of water. When the drop hits the surface it generates a compression ring moving out from a central depression. Then the central region rebounds visibly, sometimes ejecting a tongue of liquid. Rock behaves like a fluid (indeed, much is directly melted) under the enormous pressures and temperatures created by an impact of a giant meteorite. All of the effects described here in the formation of a crater will take no more than about 10 seconds or so, even in the formation of the largest craters! I hope this answers your question. Best wishes, David Scarboro
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