We look at more aerial and space images of impact craters in Europe, Africa, Asia, and Australia.
We move on now to craters elsewhere in the world. Lets begin in Europe - look at this map:
We have already learned about the most famous crater in Europe - the Ries, in Bavaria. The maps shows a cluster of craters in Scandinavia. This reflects the dominance of igneous and metamorphic rocks in the Fennoscandinavian shield, as these tend to preserve their craters better than sedimentary rocks or those in younger active mountain-building regions of the Earth's crust. There is a second cluster in eastern Europe. Here are two from Scandinavia:
A new impact site in northeast Spain was first reported in the 1980s. The current claim is that there are two craters (formed simultaneously), one called Azuara, the other Rubielos de la Cerida. The latter has an elliptical shape, possible if the angle of impact was low instead of near vertical. Each crater is in the 35-40 km size range. As seen by Landsat:
A wide variety of shock effects, from the lower pressure shatter cones to the high pressure shock melt occur at these craters.
A real oddity occurs in Austria. Near the hamlet of Kofels, in an alpine valley, are fractures in bedrock filled with dark gray glass. This is a typical sample:
The origin of this glass is still unsettled. Some consider it to fill veins beyond the true crater walls of a now almost totally eroded impact crater. There are no other convincing signs of impact (shatter cones, which might be expected, are absent); no indications of volcanism are present. The glass has been dated as less than 10000 years old. One hypothesis is that it was formed by friction during a huge landslide, deposits of which are present.
Space and aerial images of impact craters in Africa are much better represented in the remote sensing gallery. The deserts of northern Africa and the savannahs of southern Africa are favorable terrain since the heavy vegetation of central Africa is largely absent. Here is a map of African impact structures:
One of the most perfect young craters (9 km diameter) in the world is Tenoumer in Mauritania (west Africa); note its rim that rises 100 meters out of the desert surface:
Mauritania hosts another, much smaller (390 m) crater, Aouelloul:
Glass-rich ejecta from Aouelloul was one of the first such impactites studied in the early days of research on impact craters:
Another small (750 meters) impact crater in the Sahara of Mauritania is Temimichat:
The Sahara desert of southern Algeria is the site of the 1.75 km Talemzane crater:
A well-exposed multiringed impact crater, the Tin Bider structure, occurs in the Tin Rhert Plateau of southern Algeria. Its domal central peak contains sandstone units whose quartz shows definitive shock features. From space, this impressive structure, which is about 6 km in diameter, appears thusly:
A Radarsat image provides a color view of the newly-discovered Aorounga crater (about 17 km [10.5 miles] in rim diameter) found in inclined sandstone units and desert sand in the Sahara Desert of northern Chad.
The Libyan deserts has several structures that have been verified as impact in origin. They now are mainly rock exposures partly submerged by sand. One is the Arkenu pair, 10.8 and 6.8 km respectively, seen in this JERS-1 radar image.:
The BP structure (found during exploration by British Petroleum), also known as Gebel Dalma, is 2 km as exposed; it contains evidence of an impact origin:
This next example, reported in March, 2006, shows how Landsat has been used to pinpoint an impact structure whose existence was postulated from the presumption that one did exist that was responsible for some anomalous material first decades decades earlier. Within Libya a "mysterious" glass, yellow-greenish in color, had been found. Some thought it to be solidified melt from an impact crater. Two scientists at Boston University, Drs. Farouk El-Baz and Eman Ghoneim, searched through space imagery and finally found an outcrop area in southwestern Egypt they contend is the central peak region of a 31 km (19 mile) wide crater which they named Kebira (meaning "large"). The peak rocks seen in the second image are sandstone (this should contain abundant shock features, but the rocks have yet to be examined petrographically). Glass like that in Libya has been found near the crater.
Another well known African crater is Bosumtwi, in the jungle of Ghana, Africa. This 10.5 km (6.5 mile) crater may be the source of the Ivory Coast tektite strewnfield.
The Roter Kamm crater is 2.5 km in diameter. It is situated in the Namib Desert of Namibia.
The second largest impact crater (about 300 km [200 miles] wide) on Earth is the Vredefort Dome in South Africa southwest of Johannesburg. It once was considered to be an uparching structure caused by magma until shatter cones revealed its extraterrestrial origin as a true depression with unusual topographic expression. Later studies provided petrographic evidence of microscopic shock features. Here are two views as seen from space; compare with the accompanying map. In the upper image, the subdued expression of the largely eroded rim shows the subtlety of this old (Precambrian) impact structure that is a good example of a classic "astrobleme" (a term coined by R. Dietz to refer to much degraded impact "scars")
One of the earliest known African impact craters is about the size of Meteor Crater, but four times older. The Tswaing Crater is in South Africa, about 40 km north of Pretoria. It was long thought to have a cryptovolcanic origin and was known as the Praetoria Salt Pan (salt deposits from its dried-up lake). But studies since 1970 have found shock effects in its rocks. Here it is from the air and in a Landsat image:
There are a number of impact structures in Asia but images have been hard to find on the Internet.
The best known crater in India, Lonar Lake, was closely studied prior to the Apollo landings because it formed in basalt, the rock type expected in the lunar lowlands. About 1.8 km wide, it appears young (actual age estimated to be about 50000 year old when the impact struck the Deccan Plateau east of Bombay):
Here is an ASTER view of Lonar Lake.
Elsewhere in India is the Ranghar impact crater, recognized from space by a central uplift ring similar to Gosses Bluff (see below).
Some craters are obvious in space and aerial images. Their circularity, and possible rim, afford strong clues. Others, mainly those that have been eroded or were imposed on or involved in mountains, are typically irregular in outline and can easily be missed. Water often fills complex craters sufficiently intact to retain a central depression. A Seasat radar image of the Elgygytgyn Crater in Siberia (its diameter is 18 km [11 mi]) is a good example. The aerial oblique view offers a different perspective.
18-15: The Elgygytgyn Crater interior, defined by the lake, seems almost squarish rather than round. Speculate on the cause of this departure from the normal inner shape of impact craters. ANSWER
The Popigai structure is in central Siberia. It is at least 100 km in diameter but is not well expressed topographically:
Many impact structures have been moderately to severely eroded so that their crater rim morphology is no longer a strong clue to their presence and nature. Detection in space images is therefore difficult; breccias with associated shock metamorphic features are then the best indicators. Still, processed imagery can reveal signs of an astrobleme (sometimes drainage will adjust to the underlying structure, with a tendency towards circularity). A relatively young (~900,000 years) impact crater, the Zhamanshin structure (13 km; 8 miles) in Kazahkstan, is a case in point. Examine first this Landsat false color composite; you may be hard-pressed to find the actual crater, for, despite its youth, it has been severely eroded.
This next image of Zhamanshin was generated from all non-thermal Landsat TM bands regrouped into Principal Components. Shown above are Components 2, 3, and 4 in Red, Green, and Blue
18-16: Using the Principal Components image, locate (approximately) the apparent boundary (eroded rim segment) of the Zhamanshin impact structure (now an astrobleme). ANSWER
The Zhamanshin structure is a candidate source for tektite-like glass call irghizite, shown here:
Another discovery attributed directly to examination of space imagery is the Kara-Kul structure in the mountains of Tajikistan (central Asia). In this Landsat-7 image, look carefully around the two central lakes and you should see subtle evidence of a rim. The structure's size is about 45 km (28 miles) in diameter. Shock features have been found in surface rocks, verifying an impact identity.
A crater discovered in space imagery is Tabun Khara Obo, in the desert wilds of Mongolia. Here it is in an EO-1 image:
Australia has more impact craters than any area of comparable size (Canada is a close second). This is because of its diverse rock types, the large open spaces of sparse vegetation, limited areas of younger (obliterating) orogenies, and the piqued interest of Australian and other geoscientists in searching for these intriguing features. At last count there were 26 confirmed craters on the continent, plotted here (Bedout is not shown):
One of the most famed crater complex on Earth is the Henbury swarm found in the Northern Territories. An iron meteorite created 13 craters just 4 to 5 thousand years ago, ranging in size from 7 to 180 meters. This next group of images synopsizes the Henbury group story:
The next three craters, much older than Henbury, are also found in the Northern Territory. They are named and sized in the caption (point your mouse on each image):
Interior Australia is the home of perhaps the most dramatic exposure of the central peak of a complex crater anywhere in the world, seen below in this aerial oblique view of a ringed mountain at the Gosses Bluff Crater, found in the southern part of the Northern Territories, near the McDonnell Range.
With that view showing the surface expression of the central ring, you should be able to pick out the crater complex in this Landsat image:
A more detailed view of part of Gosses Bluff appears in this perspective view produced using topographic data and an ASTER image:
Another perspective rendition made from Landsat imagery is shown here:
The 6 km (4 mi) wide ring consists of layers of resistant sandstone, tilted at steep angles as the strata were driven upwards on end, during the rebound of the crater floor into the peak. They have since been breached so that the lower interior now exposes softer rocks being eroded. In the Large Format Camera photo below, taken from the Shuttle, this central peak stands out in sharp contrast to the folded rocks of the McDonnell Range to its north.
Erosion nearly obliterated the outer sections of the crater, but they are faintly expressed as just beyond a dark band in the photo. Field studies show the approximate diameter of the full crater is 22 km (14 mi).
Now, on to other Australian craters. First, try your acumen on this next image.
18-17: We challenge you to find the crater in this Landsat scene below. ANSWER
If you succeeded in the hunt, you will have pinpointed the 5 km (3 mile) wide Goat Paddock crater in Western Australia. It shows up much better in this photo taken from the Space Shuttle (STS-17):
A recently discovered crater, 30 km in diameter, has been found 110 km west of Wilana in Western Australia. At first called the Teague crater, it has been named Shoemaker crater in honor of Eugene Shoemaker, the famed astrogeologist who was tragically killed in central Australia (see bottom of page 19-23). Here is a Landsat image and a Principal Components image of this crater:
Gene Shoemaker is the only person who has an impact crater named after him on both the Earth and the Moon. The lunar crater is near the Moon's South Pole, and is shown here (B):
An intriguing crater in Western Australia is the Spider Crater, shown first in a vertical Landsat view (in the center left). It has a very strange central peak which gives it its name. An aerial photo shows the spiderlike ridges carved from that peak:
The ridges, made of sandstone, stand out in this space image, taken by Taiwan's Formost-1 high resolution satellite:
Western Australia has terrains favored to preserve impact structures. Here are three that are old (see their captions for name and size)
While most Australian impact craters are fairly old, several are relatively recent, as you saw with the Henbury swarm. One of the younger craters is Wolfe Creek (diameter about 800 meters), again in Western Australia, whose rim is scarcely eroded, suggesting a young age for this structure:
To the south in Western Australia, in the Carnarvon Basin east of Sharks Bay, another large impact structure, now buried by sedimentary rocks, was found using geophysical surveying. Below are a Bouguer anomaly map and a magnetic intensity map that show both circularity and a central peak on this, the Woodleigh impact structure (possibly up to 120 km [75 miles] in diameter).
Shocked quartz and other signs of impact metamorphism have been found in rocks recovered by drilling. This is the largest Australian crater actually on the land area of the Australian continent. The age of the crater is still uncertain but may be close to the end of the Permian. If that proves true, then Woodleigh might be contemporaneous with Bedout (see previous page), thus together these craters could really have delivered a "knockout blow" to life.
South Australia has a very large impact structure, Acraman, that does not show up well at the surface. The structure has an inner ring about 20 km in diameter and an intermediate ring at 90 km; some investigators maintain that there is still a third ring comprising the outer edge of a 150 km crater. The structure is much eroded and lies in part within the Gawler Range. Here are two Landsat views, and an aeromagnetic map of the impact region:
Lake Acraman is near the center of the structure. There, shatter cones and shocked quartz have been found. In the Flinders Range, some 300 km distant, breccia deposits have been correlated with the Acraman event. If this is correct, that age dates the impact at about 580 million years ago (the Proterozoic Vendean). Erosion has since nearly obliterated the crater outline, but its effect is hinted at in the aeromagnetic map.
The last Australian crater considered is Lawn Hill, 18 km wide, in Queensland:
All this coverage of a truly catastrophic natural phenomenon may leave you feeling uneasy. What are your chances of being killed from an impact event? Very small, but not zero. A small cometary body exploded (estimated between 10 and 100 megatons) over the Tunguska region in Siberia in 1908 and an iron meteorite made a 30 m [100 ft] crater in Siberia in 1947. Meteor Crater formed not long before North America was settled. Impacting bodies that form 20 km wide craters strike Earth at a frequency of only once every few million years (the Zhamanshin structure in southern Russia 13.4 km [7.5 mile] diameter is less than 900000 years old and an 8 km crater in Bolivia may be much younger). A Chicxulub-sized collision, capable of destroying much of life 65 m.y. ago through a "nuclear winter" type calamity and thus likely to be fatal to humans, is expected about once every 100 m.y. A strike on land would be devastating but if an asteroid hits in a ocean (70% chance), the number of people living along coastlines of that sea could die by the millions from the tsunami-like waves (which could be a mile high).
The now famous multiple impacts of the Shoemaker-Levy comet into Jupiter in 1993 (Section 19, page 19-23) proves convincingly that planets are targets of big hits that have occurred in the past, and will again, during the brief historical span (a few thousand years) when Man has recorded such dramatic events. And there are many thousands of larger asteroids and comets still out there, many not yet found and some destined to pass us nearby. (A paper given in May 1997 by Dr. Louis Frank of the University of Iowa reports on observations made by NASA's Polar satellite that comets in the range of 40 metric tons or less strike the Earth's atmosphere hundreds of times each day; these water-rich bodies may be responsible for significant original deposition and subsequent additions of water in the Earth's oceans.) At present there is no sure defense against these extraterrestrial invaders that would certainly wreak catastrophic havoc on Earth. Pleasant dreams! 18-18: Put your imagination in high gear and think of ways to avoid the potential catastrophe of an asteroid striking the Earth. Draw on your movie experience if you wish. ANSWER This Section, along with the last on Geomorphology, in which several of many scientific uses of space imagery have been demonstrated as adding valuable new information, are good prologues to another of the major applications of remote sensing from spacecraft: The exploration of the planets to be reviewed in the next Section, where we will see again that landforms analysis often plays a crucial role in characterizing planetary surfaces and is an integral part of interpretation procedures. And, even before we leave our nearest planetary neighbor, the Moon, we will appreciate even more the importance of impact cratering as a fundamental process in creating and modifying most planetary surfaces.
Primary Author: Nicholas M. Short, Sr.