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Above: Daedulus crater on the Moon, photographed by the Apollo 11 crew. Source: NASA.
Asteroids and comets are continually hurtling through our Solar System. Sometimes, one of these Small Solar System Bodies (SSSBs) collides with a larger planetary body, such as a planet or moon. The impact of such a high-velocity object can cause a tsunami, if it lands in water, or massive explosions and dust clouds and even earthquakes if it impacts land. These impact events can result in large depressions on the surface of a planet, moon or other body in the Solar System including other asteroids. These depressions are known as impact craters. Mercury, our Moon, and other planets’ moons (such as Jupiter’s moons Callisto and Ganymede) are good examples of objects with many impact craters.
Impact craters have characteristic features. First, they tend to be roughly circular. This is not because the object that created the crater had a circular shape, but rather because upon impact there is typically a massive explosion which results in material – called ejecta - flying out in all directions. During the explosion, the impacting object itself can be shattered into small pieces or may even melt or vaporize. An elongated (oval) crater can form if an object strikes the surface at a very shallow angle. If an impacting object is large enough, some of the material pushed toward the edges of the crater will slide back down and rock in the centre of the crater will push up, resulting in a central peak (see Figure 1 below).
Impact craters have the following structures (see Figure 1):
- Floor - this is the bottom of a crater. It may be flat or slightly bowl-shaped. The floor is usually below the level of the surrounding ground.
- Walls - these are the interior sides of a crater. They are usually quite steep and may have step-like areas that are created by the collapsing of the walls due to gravity.
- Rim – this is the top edge of the crater. The rim level is usually above the level of the surrounding ground because of material which is pushed up as a result of the impact.
- Central peak – this is a higher area in the center of large craters.
- Ejecta – this is rock material thrown out of the crater. Notice how it can be seen all around the crater. The ejecta is thickest closest to the crater and thinnest further away.
- Rays – these are bright streaks of ejecta that extend away from a crater like rays from the Sun.
Small bowl-shaped, smooth-walled craters are known as Simple craters, while larger craters with features such as central peaks and stepped sides are known as Complex craters (the image above is of a complex crater). Simple and complex craters range in size from a few tens of metres up to about 300 km. Very large impact craters greater than 300 km across are called impact basins. Our Moon has several impact basins which appear as large dark areas. ImpaBackgrounder-Impact Craters.pdfct basins on the Moon were created when huge objects struck the Moon, penetrating the Moon’s crust. This caused molten rock from below the surface to seep out and cover the surface in large flat sheets.
The size and shape of a crater depends on the mass, density and velocity of the impacting object and the geology of the surface (type of rock) on which the object lands. The greater the mass and velocity of the object, the larger the diameter of the crater formed.
On the Moon and other planets, craters look very much as they did when they were formed. On Earth, however, craters tend to get worn away due to weathering and erosion and destroyed through plate tectonics and volcanic activity. Although Earth’s surface processes can eventually erase impact craters, about 170 terrestrial impact craters have been identified. On average, about once every million years our planet experiences from one to three impacts large enough to produce a 20 km diameter crater.
Impact Craters in Canada
Some of the world’s most spectacular impact craters can be found in Canada. A particularly interesting pair of impact craters can be found in Quebec near the eastern shore of Hudson Bay. The two impact craters, which have since filled in with water to form lakes, are believed to have been formed at the same time by the impact of a pair of asteroids approximately 290 million years ago. West Clearwater Lake (top left of Figure 2) has a diameter of 32 km, while East Clearwater Lake (bottom right of Figure 2) has a diameter of 22 km.
Like the Clearwater Lakes, the Mistastin Crater also contains a lake. Located in Labrador, the Mistastin Crater is the result of a meteorite crash around 36-38 million years ago. The original impact crater is believed to have been approximately 28 km wide. Since then, glaciers have greatly reduced its size. In the middle of the lake is a central island which is thought to be a central peak (which would make this a complex crater).
The Pingualuit Crater (French: Cratère des Pingualuit; Inuktitut word meaning where the land rises), is a young impact crater (approximately 1.4 million years old) located in the Ungava Peninsula of Québec. Like the other craters described above, a lake fills this crater. Pingualuit Lake is one of the deepest lakes in North America (400 m deep). The clearness of the lake inspired local Inuit to refer to it as the “Crystal Eye of Nunavik.” Pingualuit is a small crater (3.4 km in diameter) which you can see by visiting Pingualuit National Park.