Earth Impact Craters - Željko Lipanović

Earth Impact Craters

Until recently, impacts by extraterrestrial bodies were regarded as, perhaps, an interesting but certainly not an important phenomenon in the spectrum of geological process affecting the Earth. Our concept of the importance of impact processes, however, has been changed radically through planetary exploration, which has shown that virtually all planetary surfaces are cratered from the impact of interplanetary bodies. It is now clear from planetary bodies that have retained portions of their earliest surfaces that impact was a dominant geologic process throughout the early solar system. For example, the oldest lunar surfaces are literally saturated with impact craters, produced by an intense bombardment, at least a 100 times higher than present impact flux, which lasted from 4.6 to approximately 3.9 billion years ago. The Earth, as part of the solar system, experienced the same bombardment as the other planetary bodies.

On the Earth, a variety of possible effects have been ascribed to impacts. Currently, the best working hypothesis for the origin of the Earth's moon is the impact of a Mars-sized object with the proto-Earth. This resulted in the insertion into Earth orbit of vaporized material from the impactor and the Earth, which condensed and accreted to form the moon. Heat generated by early impacts may have led to outgassing of Earth's initial crust, thus, contributing to the primordial atmosphere and hydrosphere. Additionally, the impacting bodies themselves may have contributed to the Earth's budget of volatiles. This early bombardment would also have frustrated the development and evolution of early life, with the largest impacts having the capacity to effectively sterilize the surface of the globe. In more recent geologic time, there is evidence that at least one mass extinction event, notably that of the dinosaurs and many other species 65 million years ago, is linked to global effects caused by a major impact event. Impacts also have some economic significance. For example, the vast copper-nickel deposits at Sudbury, Canada are likely a related result of a large-scale impact 1850 million years ago and several impact structures in sedimentary rocks have provided suitable reservoirs for economic oil and gas deposits.

Most of the terrestrial impact craters that ever formed, however, have been obliterated by other terrestrial geological processes. Some examples however remain. To-date, approximately 150 impact craters have been identified on Earth. Almost all known craters have been recognized since 1950 and several new structures are found each year.

A very large number of meteoroids enter the Earth's atmosphere each day amounting to several hundred tons of material. But they are almost all very small, just a few milligrams each. Only the largest ones ever reach the surface to become meteorites. The largest found meteorite (Hoba, in Namibia) weighs 60 tons.

The average meteoroid enters the atmosphere at between 10 and 70 km/sec. But all but the very largest are quickly decelerated to a few hundred km/hour by atmospheric friction and hit the Earth's surface with very little fanfare. However meteoroids larger than a few hundred tons are slowed very little; only these large (and fortunately rare) ones make craters.

There are probably at least 1000 asteroids larger than 1 km in diameter that cross the orbit of Earth. One of these hits the Earth about once in 300,000 years on average. Larger ones are less numerous and impacts are less frequent, but they do sometimes happen and with disastrous consequences.

The impact of a comet or asteroid about the size of Hephaistos or SL9 hitting the Earth was probably responsible for the extinction of the dinosaurs 65 million years ago. It left a 180-km crater now buried below the jungle near Chicxulub in the Yucatan Peninsula.

Calculations based on the observed number of asteroids suggest that we should expect about 3 craters 10 km or more across to be formed on the Earth every million years. This is in good agreement with the geologic record. It is more difficult to compute the frequency of larger impacts like Chicxulub but once per 100 million years seems like a reasonable guess.

Here are educated guesses about the consequences of impacts of various sizes:

Impactor Diameter (meters)	Yield (megatons)	Interval (years)	Consequences
< 50	< 10	< 1	meteors in upper atmosphere most don't reach surface
75	10 - 100	1000	irons make craters like Meteor Crater; stones produce airbursts like Tunguska; land impacts destroy area size of city
160	100 - 1000	5000	irons, stones hit ground; comets produce airbursts; land impacts destroy area size of large urban area (New York, Tokyo)
350	1000 - 10,000	15,000	land impacts destroy area size of small state; ocean impact produces mild tsunamis
700	10,000 - 100,000	63,000	land impacts destroy area size of moderate state (Virginia) ocean impact makes big tsunamis
1700	100,000 - 1,000,000	250,000	land impact raises dust with global implication; destroys area size of large state (California, France)

Crater Images

	Aorounga - Chad The impact of an asteroid or comet several hundred million years ago left scars in the landscape that are still visible in this spaceborne radar image of an area in the Sahara Desert of northern Chad. The concentric ring structure is the Aorounga impact crater, with a diameter of about 17 kilometers. The original crater was buried by sediments, which were then partially eroded to reveal the current ring-like appearance.
The dark streaks are deposits of windblown sand that migrate along valleys cut by thousands of years of wind erosion. The dark band in the upper right of the image is a portion of a proposed second crater. Scientists are using radar images to investigate the possibility that Aorounga is one of a string of impact craters formed by multiple impacts. Radar imaging is a valuable tool for the study of desert regions because the radar waves can penetrate thin layers of dry sand to reveal details of geologic structure that are invisible to other sensors. The image was acquired by the Spaceborne Imaging Radar-C/X-band Synthetic Aperture Radar on April 18 and 19, 1994, onboard the space shuttle Endeavour. The area shown is 22 kilometers by 28 kilometers and is centered at 19.1 degrees north latitude, 19.3 degrees east longitude. North is toward the upper right.
	Ara Kul - Tajikistan

	Barringer Meteor Crater - Arizona The origin of this classic simple meteorite impact crater was long the subject of controversy. The discovery of fragments of the Canyon Diablo meteorite, including fragments within the breccia deposits that partially fill the structure, and a range of shock metamorphic features in the target sandstone proved its impact origin.
Target rocks include Paleozoic carbonates and sandstones; these rocks have been overturned just outside the rim during ejection. The hummocky deposits just beyond the rim are remnants of the ejecta blanket. This aerial view shows the dramatic expression of the crater in the arid landscape.
	Bosumtwi - Ghana

	Chicxulub - Mexico This three-dimensional map of local gravity and magnetic field variations shows a multiringed structure called Chicxulub named after a village located near its center. The impact basin is buried by several hundred meters of sediment, hiding it from view. This image shows the basin viewed obliquely from approximately 60° above the surface looking north, with artificial lighting from the south.
The image covers 88 to 90.5° west longitude and 19.5 to 22.5° north latitude. NASA scientists believe that an asteroid 10 to 20 kilometers in diameter produced this impact basin. The asteroid hit a geologically unique, sulfur-rich region of the Yucatan Peninsula and kicked up billions of tons of sulfur and other materials into the atmosphere. Darkness prevailed for about half a year after the collision. This caused global temperatures to plunge near freezing. Half of the species on Earth became extinct including the dinosaurs.
	Clearwater Lakes - Canada

	Deep Bay - Canada

	Gosses Bluff - Australia

	Manicouagan - Canada

	Mistastin Lake - Canada This shuttle image shows a winter view of the Mistastin Crater, a heavily eroded complex structure. Eastward moving glaciers have drastically reduced the surface expression of this structure, removing most of the impact melt sheet and breccias and exposing the crater floor. Glacial erosion has also imparted an eastward elongation to the crater that is particularly evident in the shape of the lake that occupies the central 10 kilometers of the structure.
Horseshoe Island, in the center of the lake, is part of the central uplift and contains shocked Precambrian crystalline target rocks. Just beyond the margins of the lake are vestiges of the impact melt sheet that contains evidence of meteoritic features in quartz, feldspar and diaplectic glasses.
	Roter Kamm - Namibia This space radar image shows the Roter Kamm impact crater. The crater rim is seen as a radar-bright, circular feature. Roter Kamm is a moderate sized impact crater, 2.5 kilometers in diameter, and is 130 meters deep. However, its original floor is covered by sand deposits at least 100 meters thick.
In a conventional aerial photograph, the brightly colored surfaces immediately surrounding the crater cannot be seen because they are covered by sand. The faint blue surfaces adjacent to the rim might indicate the presence of a layer of rocks ejected from the crater during the impact. The darkest areas are thick, windblown sand deposits which form dunes and sand sheets. The sand surface is smooth relative to the surrounding granite and limestone rock outcrops and appears dark in radar image. The green tones are related primarily to larger vegetation growing on sand soil, and the reddish tones are associated with thinly mantled limestone outcrops.
	Wolfe Creek - Australia Wolfe Creek is a relatively well-preserved crater that is partly buried under wind blown sand. The crater is situated in the flat desert plains of north-central Australia.
Its crater rim rises ~25 meters above the surrounding plains and the crater floor is ~50 meters below the rim. Oxidized remnants of iron meteoritic material as well as some impact glass have been found.

Last Updated: May 24, 2000.