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In the northern part of Quebec, among some of the oldest rocks
on the continent, a curious ring of water surrounds a remarkably
circular island. This is the site of Manicouagan crater; with a diameter
of more than 70 km across its prominent rim (the outer shore of
the lake), it is one of the largest and best exposed craters on
the planet. Manicouagan was formed by a meteor’s collision with
Earth more than 200 million years ago. Many of the local rocks were
melted by the heat and pressure of the impact, and their ages were
"reset" to reflect the time of impact. As can be seen in the image,
most of the region is made of Precambrian
rocks (the reds, yellows and oranges), but much of the island is
made up of Triassic rock
(green).
As a result of underground fluid pressure pushing the rock in
the crater upward during a bout of post-impact equilibration, Manicouagan
has a plateau in the center, rather than the characteristic depression
associated with craters. The remaining, ring-shaped depression filled
with water, creating the lakes Manicouagan and Mouchalagane. These
lakes have since been dammed to create the annular Manicouagan Reservoir
around that central plateau, now an island, the Ile René-Levasseur.
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