Re: Reason for round lake in Quebec

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