http://rst.gsfc.nasa.gov/Sect18/Sect18_6.html
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We look at more aerial and space images of impact craters in Europe,
Africa, Asia, and Australia.
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We move on now to craters elsewhere in the world. Lets begin in Europe
- look at this map:
Impact Craters in Europe.
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:
Lake Siljan in Sweden, a 54 km wide structure;
The much smaller Jonisjarvi structure in northwest Russia near the
Finnish border.
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:
Kofelsite, a frothy glass.
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:
Impact craters in Africa.
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:
Tenoumer Crater.
Mauritania hosts another, much smaller (390 m) crater, Aouelloul:
The Aouelloul crater.
Glass-rich ejecta from Aouelloul was one of the first such impactites
studied in the early days of research on impact craters:
Aouelloul glass.
Another small (750 meters) impact crater in the Sahara of Mauritania
is Temimichat:
7
Ground view of Temimichat,
The Sahara desert of southern Algeria is the site of the 1.75 km
Talemzane crater:
The 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:
The Tin Bider structure, in a Landsat image.
A recently discovered small (less than a km) crater of probably impact
origin is Amguid, also in southern Algeria. It is less than 100,000
years old.
Amguid crater, aerial view.
Amguid crater from space.
Landsat and Radarsat images provide color views 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.
Landsat image of Aorounga crater.
Radarsat color composite showing the recently discovered Aorounga
impact structure in the Chad desert.
Aerial view of Aorounga.
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 Arkenu craters.
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:
The BP Structure in Libya.
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.
Libyian glass.
The Kebira crater, presumed to be impact in origin.
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 Bosumtwi crater.
The Roter Kamm crater is 2.5 km in diameter. It is situated in the
Namib Desert of Namibia.
The Roter Kamm crater.
Ground view of Roter Kamm.
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")
An ASTER image of Vredefort Dome, a very old impact structure.
A reprocessed image of the Vredefort impact crater; hills on its
northwest side indicate a more distinct rim structure.
Geologic map of the Vredefort Dome.
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:
Aerial view of the Tswaing Crater.
The Tswaing Crater, from Landsat.
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):
Ground view of Lonar Lake
Here is an ASTER view of Lonar Lake.
Lonar Lake, in India.
Elsewhere in India is the Ranghar impact crater, recognized from space
by a central uplift ring similar to Gosses Bluff (see below).
Ranghar crater, first identified from its central peak ring
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.
Seasat radar image of the Elgygytgyn Crater, Siberia.
Color aerial oblique photograph of the Elgygytgyn Crater, Siberia.
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:
The Popigai structure in Siberia.
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.
Landsat enhanced color composite of the Zhamanshin structure in
Kazakhstan, southern Asia; the crater, left of center, is eroded so
that the rim is gone and is hard to see in the image but there is a
crude circularity still as a remnant.
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
A Principal Components image of the Zhamanshin structure, based on
Landsat TM bands, improves the definition of the central crater.
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:
Irghizites.
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.
Landsat-7 image of the Kara-Kul impact structure in Tajikistan.
A crater discovered in space imagery is Tabun Khara Obo, in the desert
wilds of Mongolia. Here it is in an EO-1 image:
The impact crater Tabun Khara Obo, in Mongolia.
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):
Impact structures in Australia.
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:
A slice through a Henbury iron meteorite showing the characteristic
octahedrite structure.
Aerial view with many of the Henbury craters visible.
Aerial oblique photo showing two of the larger Henbury craters.
Ground view into Henbury crater 7.
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):
The 16 km wide Strangways crater.
Foelsche crater, 6 km wide; high altitude aerial photo.
Liverpool crater, 1.7 km wide; found in Arnhem Land.
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.
Aerial view of the ringed central peak of the Gosses Bluff complex
impact crater in the interior of Australia.
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:
Gosses Bluff impact structure seen in a Landsat image; the McDonnell
Ranges define the lowlands in which it is located.
A more detailed view of part of Gosses Bluff appears in this
perspective view produced using topographic data and an ASTER image:
Perspective look at Gosses Bluff, made from SWIR, NIR, and Green bands
of the ASTER sensor on Terra.
Another perspective rendition made from Landsat imagery is shown here:
Landsat-DEM perspective of the Gosses Bluff structure.
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.
Large Format Camera photo of the Gosses Bluff, Australia impact
structure.
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
Natural color Landsat scene of the Goat Paddock crater in Western
Australia.
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):
The Goat Paddock crater, from 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:
The Teague crater, now renamed the Shoemaker crater
PCA image of Shoemaker 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):
Shoemaker crater (B) on the Moon.
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:
Spider Crater, Western Australia.
Spider Crater, Western Australia, aerial perspective.
The ridges, made of sandstone, stand out in this space image, taken by
Taiwan's Formost-1 high resolution satellite:
Formost-1 view of the Spider Crater.
Western Australia has terrains favored to preserve impact structures.
Here are three that are old (see their captions for name and size)
The Liverpool Crater; 17 km.
The Piccaninny Crater; 7 km; ASTER image.
The Connolly Basin Crater; 9 km; ASTER image
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:
The Wolfe Creek impact 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).
Bouguer anomaly (gravity) map of the Woodleigh structure
Magnetic intensity map of Woodleigh crater
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:
the Acraman structure; Landsat image; Lake Acraman is number 1.
The Acraman structure, mosaic of several Landsat images.
Aeromagnetic map of the Acraman crater 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:
The Lawn Hill crater.
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.
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Primary Author: Nicholas M. Short, Sr.