I do not know what I seem to the world, but to myself I appear to have been like a boy playing upon the seashore and diverting myself by now and then finding a smoother pebble or prettier shell than ordinary, while the great ocean of truth lay before me all undiscovered. - Sir Isaac Newton
Impact craters are geologic structures formed when a large meteoroid, asteroid or comet smashes into a planet or a satellite. All the inner bodies in our solar system have been heavily bombarded by meteoroids throughout their history. The surfaces of the Moon, Mars and Mercury, where other geologic processes stopped millions of years ago, record this bombardment clearly. On the Earth, however, which has been even more heavily impacted than the Moon, craters are continually erased by erosion and redeposition as well as by volcanic resurfacing and tectonic activity. Thus only about 120 terrestrial impact craters have been recognized, the majority in geologically stable cratons of North America, Europe and Australia where most exploration has taken place. Spacecraft orbital imagery has helped to identify structures in more remote locations for further investigation.
Meteor Crater (also know as Barringer Crater) in Arizona was the first-recognized terrestrial impact crater. It was identified in the 1920s by workers who discovered fragments of the meteorite impactor within the crater itself. Several other relatively small craters were also found to contain impactor fragments; for many years, these remnants were the only accepted evidence for impact origin. However, scientists have come to realize that pieces of the impactor often do not survive the collision intact.
In massive events caused by a large impactor, tremendous pressures and temperatures are generated that can vaporize the meteorite altogether or can completely melt and mix it with melted target rocks. Over several thousand years, any detectable meteoritic component might erode away. In some cases, nonterrestrial relative abundance of siderophile elements can be detected in the impact melt rocks within large craters - a chemical signature of the meteorite impactor.
Since the 1960s, numerous studies have uncovered another physical marker of impact structures, shock metamorphism. Certain shock metamorphic effects have been shown to be uniquely and unambiguously associated with meteorite impact craters; no other earthly mechanism, including volcanism, produces the extremely high pressures that cause them. They include shatter cones, multiple sets of microscopic planar features in quartz and feldspar grains, diaplectic glass, and high-pressure mineral phases such as stishovite. All known terrestrial impact structures exhibit some or all of these shock effects.
Impact craters are divided into two groups based on morphology: simple craters and complex craters. Simple craters are relatively small with depth-to-diameter ratios of about 1:5 to 1:7 and a smooth bowl shape. In larger craters, however, gravity causes the initially steep crater walls to collapse downward and inward, forming a complex structure with a central peak or peak ring and a shallower depth compared to diameter (1:10 to 1:20). The diameter at which craters become complex depends on the surface gravity of the planet: The greater the gravity, the smaller the diameter that will produce a complex structure. On Earth, this transition diameter is 2 to 4 kilometers (1.2 to 2.5 miles) depending on target rock properties; on the Moon, at one-sixth Earth's gravity, the transition diameter is 15 to 20 kilometers (9 to 12 miles).
The central peak or peak ring of the complex crater is formed as the initial (transient) deep crater floor rebounds from the compressional shock of impact. Slumping of the rim further modifies and enlarges the final crater. Complex structures in crystalline rock targets will also contain coherent sheets of impact melt atop the shocked and fragmented rocks of the crater floor. On the geologically inactive lunar surface, this complex crater form will be preserved until subsequent impact events alter it. On Earth, weathering and erosion of the target rocks quickly alter the surface expression of the structure; despite the crater's initial morphology, crater rims and ejecta blankets are quickly eroded and concentric ring structures can be produced or enhanced as weaker rocks of the crater floor are removed. More resistant rocks of the melt sheet may be left as plateaus overlooking the surrounding structure.
Large terrestrial impacts are of greater importance for the geologic history of our planet than the number and size of preserved structures might suggest. For example, recent studies of the Cretaceous/Tertiary boundary, which marks the abrupt demise of a large number of biological species including dinosaurs, revealed unusual enrichments of siderophile elements and shock metamorphic features that are markers of meteorite impact events. Most researchers now believe that a large asteroid or comet hit the Earth at the end of the Cretaceous Period 66 million years ago. An environmental crisis triggered by the gigantic collision contributed to the extinctions. Based on apparent correspondences between periodic variations in the marine extinction record and the impact record, some scientists suggest that large meteorite impacts might be the metronome that sets the cadence of biological evolution on Earth - an unproven but intriguing hypotheses.
|Views of Terrestrial Craters|
Barringer Meteor Crater, Arizona
35°02'N, 111°01'W; diameter: 1.186 kilometers (.737 miles); age: 49,000 years
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
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.
(Courtesy of D. Roddy and LPI)
Chicxulub, Yucatan Peninsula, Mexico
21°20'N, 89°30'W; diameter: 170 km; age: 64.98 million years
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 (6 to 12 miles) 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.
(Image courtesy of V. L. Sharpton, LPI)
Aorounga, Chad, Africa
19°6'N, 19°15'E; diameter: 17 kilometers; age: 200 million years
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 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.
Wolfe Creek, Australia
19°18'S, 127°46'E; rim diameter: 0.875 kilometers (.544 miles); age: 300,000 years
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 (82 feet)
above the surrounding plains and the crater floor is ~50 meters (164
feet) below the rim. Oxidized remnants of iron meteoritic material as
well as some impact glass have been found a Wolf Creek. This photograph
is a south-looking, oblique aerial view of the crater.
(Courtesy of V. L. Sharpton, LPI)
Roter Kamm, South West Africa/Namibia
27°46'S, 16°18'E; rim diameter: 2.5 kilometers (1.55 miles); age: 5 +- 0.3 million years
Located in the Namibia Desert, the raised crater rim is clearly visible
against darker background vegetation. Target rocks include primarily
Precambrian crystalline rocks and
modest amounts of younger sedimentary rocks. Outcrops of impact melt
breccias are found exclusively on the crater rim. The crater floor is
covered by broad, shifting sand dunes. This image shows an oblique view
of the crater, from about 150 meters (492 feet) above ground looking
(Courtesy of W. U. Reimold and LPI)
Roter Kamm, SAR-C/X-SAR Image
This space radar image shows the Roter Kamm impact crater. The crater rim is seen as a radar-bright, circular feature. The bright white, irregular feature in the lower left corner is a small hill of exposed rock outcrop. Roter Kamm is a moderate sized impact crater, 2.5 kilometers (1.55 miles) in diameter, and is 130 meters (427 feet) deep. However, its original floor is covered by sand deposits at least 100 meters (328 feet) 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. (Courtesy NASA/JPL)
Mistastin Lake, Newfoundland and Labrador, Canada
55°53'N, 63°18'W; rim diameter: 28 kilometers (17.4 miles); age: 38 +- 4 million years
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 (6 miles)
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.
Manicouagan, Quebec, Canada
51°23'N, 68°42'W; rim diameter: ~100 kilometers (62 miles); age: 212 +- 1 million years
The Manicouagan impact structure is one of the largest impact craters
still preserved on the surface of the Earth. This shuttle view shows
the prominent 70 kilometers (43 miles) diameter, ice-covered annular
lake that fills a ring where impact-brecciated rock has been eroded by
glaciation. The lake surrounds the more erosion-resistant melt sheet
created by impact into metamorphic and
igneous rock types. Shock metamorphic
effects are abundant in the target rocks of the crater floor. Although
the original rim has been removed, the distribution of shock metamorphic
effects and morphological comparisons with other impact structures
indicates an original rim diameter of approximately 100 kilometers (62
Clearwater Lakes, Quebec, Canada
Clearwater Lake West: 56°13'N, 74°30'W; rim diameter: 32 kilometers (20 miles)
Clearwater Lake East: 56°05'N, 74°07'W; rim diameter: 22 kilometers (13.7 miles)
age: 290 +- 20 million years
Twin impact craters, which are formed simultaneously by two separate but
probably related meteorite impacts, are very rarely recognized on Earth.
This pair is situated in crystalline bedrocks of the Canadian
shield. The larger Clearwater Lake
West (left) shows a prominent ring of islands that has a diameter of
about 10 kilometers (6 miles). They constitute a central uplifted area
and are covered with impact melts. The central peak of the smaller
Clearwater Lake East (right) is submerged.
Deep Bay, Saskatchewan, Canada
56°24'N, 102°59'W; rim diameter: 13 kilometers (8 miles); age: 100 +- 50 million years
This crater consists of a near-circular bay, about 5 kilometers (3
miles) wide and 220 meters (720 feet) deep, in the otherwise shallow
Reindeer Lake. Such deep circular lakes are unusual in this region,
which is dominated by the shallow gouging of glacial erosion. The
circular shoreline, at a diameter of 11 kilometers (6.8 miles), is
partially surrounded by a ridge with heights to 100 meters (328 feet)
above the lake surface. The diameter of this ridge, ~13 kilometers (8
miles), is likely the outer rim of the impact structure. The structure
was formed in Precambrian metamorphic crystalline rocks with a
conspicuous northwest trending fabric. Although not obvious from the
surface, Deep Bay is a complex impact structure with a low, totally
submerged central uplift. Samples obtained in the 1960's from drilling
into the central structure revealed shocked and fractured metamorphic
rocks flanked by deposits of
allocthonous, mixed breccias.
06°32'N, 01°25'W; rim diameter: 10.5 kilometers (6.5 miles); age: 1.3 +- 0.2 million years
This crater is situated in crystalline bedrocks of the West African Shield
and is filled almost entirely by Lake Bosumtwi. Chemical, isotopic, and
age studies demonstrate that the crater is the most probable source for the
Ivory Coast tektites, which are found on
land in the Ivory Coast region of central Africa and as microtektites in
nearby ocean sediments. In this photo the crater lake is partly obscured
Gosses Bluff, Northern Territory, Australia
23°50'S, 132°19'E; rim diameter: 22 kilometers (13.7 miles); age: 142.5 +- 0.5 million years
This highly eroded structures is situated just south of the MacDonnell
Ranges (top of picture) in the arid Missionary Plain in the Northern
Territories, Australia. Although it could be mistaken for the crater rim,
the central ring of hills (5 kilometers or 3 miles diameter) results from differential
erosion of the central uplift within this large complex crater. The rim
itself has been eroded and is no longer visible, but the circular, grayish
colored drainage system outside the inner ring of hills probably marks the
original extent of the structure before erosion.
38°57'N, 73°24'E; rim diameter: 45 kilometers (28 miles); age: <10 million years
The spectacular Kara-Kul structure is readily apparent in this oblique view. Partly filled by the 25-kilometer (16-mile) diameter Kara-Kul Lake, it is located at almost 6,000 meters (20,000 feet) above sea level in the Pamir Mountain Range near the Afghan border. Only recently have impact shock features been found in local breccias and cataclastic rocks. (Courtesy NASA/LPI)
The information on this page was taken primarily from Koeberl and Sharpton.
Koeberl, Christian and Virgil L. Sharpton. Terrestrial Impact Craters Slide Set. Lunar and Planetary Institute.
Pilkington, M. and R. A. F. Grieve, "The Geophysical Signature of Terrestrial Impact Craters." Reviews of Geophysics, May 1992, vol. 30, pp. 161-181.
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