Crater Formation

This series of cartoons by Denis W. Roy illustrates the sequence of events that formed the Manicouagan crater, Québec, Canada

The approach

The meteorite or comet is approaching the target region, probably at a speed of about 60 kilometres per second. The target consists of a layer of limestone (in brick pattern) overlying metamorphic and igneous rocks of the Canadian Shield.

Explosion of the object.

The object is slowed down by the atmosphere. It cannot get rid of its energy fast enough, hence it explodes before reaching the ground and is completely vaporised. A hemispherical transient crater is formed by the explosion, and a jet of target rock is blasted out sideways.

Formation of the transient crater.

Shatter-cones are formed in the target rocks, all pointing towards the original impact site. The rocks are also shock metamorphosed - forming shock lamellae and isotropic minerals. Part of the target rocks are melted and accumulate in a pool at the bottom of the crater.

Collapse of the walls

The transient crater is unstable - the walls are too steep. Concentric faults (cracks) form and huge blocks rotate towards the center, which is lifted up to form the central peak. An annular lake of lava starts to solidify. The outer ring is formed by the slumping of these blocks. It has a greater diameter than the original transient crater.

214 million years later - The present time.

Erosion has removed about a kilometre of rock from the region. All the limestone that originally covered the crystalline rocks of the shield is now gone. Most of the lava lake formed by the melting of the target rocks is also gone. All that is left is a central highland made of shocked rocks. However, parts of the limestones are now preserved in the annular crater lake.

This is a much larger view of the present situation. the crater still has an overall relief of about 1000 metres. The overall uplift of the crust produced in response to the impact is only about 250 metres.

Author Michael Higgins; diagrams by Denis W. Roy