MIAC

Impact Craters on Earth

Canadian impact structures.

The map shows the locations of meteorite impact structures recognised in Canada. An updated list of Canadian impact sites is included in the annual Observer's Handbook of the Royal Astronomical Society of Canada. (Courtesy Richard Grieve, Geological Survey of Canada)


Manicouagan Crater

The Manicouagan crater lies in northern Quebec, Canada (Lat 51 23N, Long 68 42W). It is one of the largest known, with a diameter of about 100 km. The crater is a multiple-ring structure, but the feature that shows up best in this Landsat satellite photo is the inner ring, which is occupied by a lake (Manicouagan 5 reservoir) with an outer diameter of about 70 km. The impact occurred 214 million years ago. The asteroid probably had a diameter of about 5 km. It may have produced a mass extinction similar to that at the end of the Cretaceous period. (Courtesy NASA)

A series of cartoons show how this crater was formed


Pingualuit Crater (Nouveau / New Quebec, Chubb)

The Pingualuit crater (also known as the Nouveau / New Quebec or Chubb crater) lies near the northern tip of the Ungava peninsula, Quebec, Canada (Lat 61° 17' N, Long 75° 40' W). It is a classic example of a simple relatively small crater. It is about 3.4 km in diameter and almost perfectly circular with a raised rim up to 163 metres above the lake surface in the central flooded depression. The lake is 252 metres deep, the deepest in Quebec. The crater, which occurs in crystalline shield rocks, was first recognised as an impact structure in 1950. The impact that produced this crater occurred about 1.4 million years ago, that is just before the glaciers covered this area  (Courtesy Geological Survey of Canada)


Wolf Creek Crater, Western Australia

The location of this crater is 19°10'S, 127°46'E. The rim diameter is 800 m and the distance from the rim crest to the present crater floor is 60 m. The age approximately 10,000-20,000 years. This is another example of a small simple crater. Meteorite fragments have been recovered from the crater floor, attesting to its origin. (Courtesy Energy Mines and Resources, Canada).


Special features of impacts on rocks

Shatter cones

Shatter-cones are one of the features produced by meteorite impacts most easily observed in the field. They are conical fractures generally produced in fine-grained rocks. Longitudinal striae with horsetail patterns, decorate the fracture surfaces. Complete cones are rare; segments of cones are common and frequently intersect each other. The angle of the apex of the cone is typically about 90 degrees, and they are 1 centimetre to 5 metres high. Occasionally they have a trumpet shape, as on the left above. They can occur singly, or in clusters, and are produced during the impact when a shock-wave passes through the rock. The apex of the cone originally pointed towards the point of impact. However, the huge landslides that follow most impacts, as the sides of the crater slip into the hole, have usually changed their orientation. Shatter-cones are sure evidence of a nearby meteorite impact, as they cannot be produced by other means, except nuclear explosions. The example on the left is from the Ile Rouleau impact crater and that on the right is from the Presque'Ile structure. (Courtesy Denis W. Roy)

Shock-induced lamellae in quartz

The passage of the shock wave through the rock also changes the structure of some of the minerals. This is a photograph of a grain of quartz, 0.03 millimetres thick and 1 millimetre across. The shock wave has produced several different sets of planar features, called shock lamellae. The occurrence of multiple sets of planar lamellae is a diagnostic feature of shock metamorphism. This example is from the Ile Rouleau impact crater. (Courtesy Denis W. Roy)

Diaplectic glass in plagioclase

Another change is possible in the feldspar mineral plagioclase. The shock wave can break down the structure of the mineral, changing parts of it into a diapletic glass (glass formed at high-pressure in the solid-state) which is isotropic, or uniform in all directions. This photograph of a thin slice of plagioclase, 0.03 millimetre thick, is seen here in cross-polarised light, with a 'senstive tint' plate. The original plagioclase is coloured yellow and the shock-changed mineral is purple. This sample is from the Manicouagan impact crater. (Courtesy Denis W. Roy)

Impact breccia

Meteorite impacts also break up large quantities of rock that becomes recemented to form breccia rock. In this sample from Sudbury, about 10 centimetres across, some of the larger blocks in the breccia are themselves made of fragments of rock. (Courtesy Denis W. Roy)


MIAC Slide Group and Michael Higgins