http://rst.gsfc.nasa.gov/Sect18/Sect18_5.html
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Prior to satellites with high resolution, much of the searching for
impact craters in more desolate parts of the world used aerial
photography, if it existed. The Canadian group at the Dominion
Observatory found a number of craters that way. With Landsat, SPOT and
now much higher resolution IKONOS and other systems, and worldwide
coverage, the possibilities for finding new craters have notably
improved. To date, at least 10 more have been found with the aid of
space imagery. And, special processing such as the enhancements
described in Section 1 can bring out new information about a crater
imaged by a digital system. This page contain examples of impact
craters as recorded in both aerial and space imagery that are found in
North and South America. Particular attention is given to Meteor
Crater in Arizona.
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Remote Sensing of Craters
How has remote sensing played a role in the search for or verification
of a supposed impact crater? In two ways: 1) recognizing morphological
features that are compatible with an impact origin (but this does not
rule out certain volcanic craters) and 2) detecting shock-induced
changes as spectral changes (this is usually difficult to do). The
strategy is one of simply using remote sensing to identify landform
anomalies consistent with an impact origin and then confirming them by
on-site inspection and examination of rocks for shock metamorphic
effects. The absence of shock effects leads to ambiguities, making
volcanic craters an alternative hypothesis.
Impact-minded searchers have discovered at least ten new impact
structures through satellite remote sensing. Before Landsat, and to
some extent since 1973, observations from airplanes, and in particular
aerial photos, were a source of information about possible craters.
Many impact craters are fairly large, and a few are huge, so that in
favorable settings these survive various forms of erosion (terrain
containing such structures that becomes involved in major tectonic
activity, widespread volcanism, burial by thick sediments, or
glaciation may experience obliteration of distinctive crater
morphology, but even eroded ones [astroblemes] may have subtle
circular geometric expression). Thus, in large parts of the world the
more obvious craters have been found by conventional techniques. But
in parts that were poorly mappped or explored evidence from satellite
imagery may be the first overview type looking that discloses
heretofore unnoticed impact shapes or scars.
As seen from above, in photos and images, impact craters have several
hallmark signatures. Circularity is the prime sign. If young, a crater
will show a rim, and rarely an ejecta field. Most older craters have
been worn down by erosion, so that morphology may not disclose them.
One distinct feature, shown in several illustrations in this Section,
is a perturbation of stream drainage. Rivers may find weak rocks
related to the structural deformation caused by the impact, and thus
adjust their course, leading to an expression of partial circularity.
One such example is the Wembo Nyama structure in Africa, shown below,
as discovered in space imagery. But so far no verification of an
impact origin has been reported. This is a good example of how one
must be cautious in any claim of impact as the origin of a feature.
Other geologic situations can produce circular features.
The geologic expression of the Wembo Nyama feature in Africa.
Prior to Landsat, searches for impact craters using remote sensing
were confined to aerial surveys. However, discoveries were usually
serendipitous, since these surveys were expensive, were normally
confined to small areas, and were initiated for other reasons, such as
the hunt for oil or mineral deposits. Landsat afforded worldwide
coverage, and has been the prime imagery used in looking for craters.
As this and the next page will demonstrate, many of the larger
craters, whose locations were already known, do indeed show up well in
Landsat imagery. But systematic searches for new craters, most of
which would likely be small since larger one could typically have been
found by other means. Only a sharp, and suspicious, eye could spot new
ones. To substantiate this statement, we show here four craters (three
previously known) in rugged or bland terrain, that are hard to see (on
two images black horizontal lines [hard themselves to see] have been
drawn to pinpoint the craters; the caption may give further clues):
The Spider crater in Australia; note black lines near left center
Longchatka crater in Siberia; near upper right corner.
Tabun Kara crater, Mongolia; black lines near center.
Talemzane in Africa, black lines.
Tenoumer in the Saharan desert.
Sometimes "Mother Nature" helps out. Snowfall has emphasized this
unidentified crater:
A large crater in mountaineous terrain, topography emphasized by
snowfall.
On this and the next page, we will show many satellite-acquired images
of previously known, and a few directly discovered in those images,
craters. Those in North and South America are considered on this page;
those in Europe, Africa, Asia, and Australia are treated on the next
page.
We can gain a feel for what to look for at a crater site, especially
variations in morphologic expression due to differences in erosional
state, by switching again to the Geological Survey of Canada's Web
Page on Impact Cratering. Go to the list of individual craters and
check these especially interesting sites: North America:
Brent/Clearwater East and West/ Deep Bay/ New Quebec; South America:
Araguainha; Africa: Aorounga/ Bosumtwi/Rotor Kamm/Vredefort; Europe:
Ries; Asia: Bigach/Popigai; and Australia: Henbury/Wolfe Creek.
Another collection of air and space imagery focused on terrestrial
craters has been compiled by Koeberl and Sharpton as online slide sets
We begin with North America. This map shows the 57 that have been
found so far:
Impact Craters found in North America
The numbers on the map correspond to a list that makes up this Web
Page
Surprisingly, very few of the impact structures in the United States
have good surface expressions as craters with rims. Most are
astroblemes - eroded morphology but with the rocks still possessing
shock effects such as shatter cones or PDFs (the Middlesboro structure
in Tennessee is an example of this. Some have been discovered either
by geophysics or by drilling (for oil or water). Thus, remote sensing
has few U.S. examples to point to. The exception is probably the most
famous impact structure in the world - at least to Americans - Meteor
Crater (also called Barringer Crater) in Arizona.
This crater exists now as a 50000 year old depression cut into the
flat-lying sedimentary layers below the surface of the Colorado
Plateau some 73 km (45 miles) east of Flagstaff, Arizona and a lesser
distance west of Winslow. An aerial oblique view of this 1230 m (4000
ft) wide crater shows its freshness (pieces of the iron meteorite that
caused it can still be found in the ejecta); the road allows thousands
of tourists traveling along Interstate 40 to visit its overlook and
museum.
Color aerial oblique photograph of Meteor Crater, Arizona, looking
west.
One of hundreds of iron meteorites, collectively known as the Canyon
Diablo fall, distributed around Meteor Crater; this sawed specimen
shows Widmanstatten structure and brown nodules of Troilite (iron
sulfide mineral).
Modern field studies of Meteor Crater in the late 1950s by Eugene
Shoemaker and its shocked rocks shortly thereafter by Edward Chao led
to the first modern concepts of impact crater mechanics. (The SiO[2]
polymorph Coesite was first discovered in impact structures at this
crater.) Shoemaker also was the first to study nuclear explosion
craters at the Nevada Test Site (NTS); he and R. Eggleton presented
their results at a 1961 Cratering Symposium, with this illustration as
their centerpiece:
Illustration from Shoemaker and Eggleton comparing Meteor Crater and
Odessa Crater to the Teapot-Ess and Jangle-U nuclear craters.
Many craters are now found at NTS. Most of these result from collapse
of the alluvium above a short-lived spherical cavity after a nuclear
explosion. Here are two such collapse craters at NTS:
Collapse craters into nuclear explosion cavities in the Yucca Flats
desert basin at NTS.
The land around Meteor Crater is locally flat. This is how the crater
appears as one drives along the paved road leading to the overlook and
museum run by the Barringer family, who actually own the crater. Below
is a view looking in from the overlook:
The approach to Meteor Crater.
Looking into Meteor Crater.
The flat interior floor, without a central peak, is a characteristic
of simple craters; Meteor Crater's outline tends towards a square
shape - this departure from circularity is controlled by the dominant
set of two orthogonal joints (planar fractures) that run through the
layers; and the ejecta deposits outside the rim still retain a
hummocky (mound-like) topography. Another ground photo from its rim
185 m (600 ft) above the floor gives a sense of its grandiose size;
note the displaced (fault-bounded) blocks under the rim in both aerial
and ground photos.
Ground phototgraph from the rim of Meteor Crater looking down at the
floor of the crater.
The "squareness" of Meteor Crater is obvious in space imagery, such as
this subscene made by Landsat; note the white apron of ejecta
surrounding the crater (a hint of deposits still farther out is
evident in the brown surface beyond the apron):
Meteor Crater as seen from Landsat.
The ejecta apron stands out also in this IR photo:
Aerial oblique infrared photo of Meteor Crater.
Meteor Crater has been known for well over a century. G.K. Gilbert at
the turn of the 20th Century reported a purely terrestrial origin for
this crater, believing that it was probably a blow-out caused by
groundwater encountering hot rock below. He largely ignored the
presence of iron meteorites found in the apron near the crater.
A specially processed image made by the airborne Thematic Mapper
Simulator (TMS) shows that the ejecta blanket or apron (in reds and
yellows) around Meteor Crater is asymmetrically distributed with
maximum extension to the northeast. There is a notable tendency for
the ejecta deposits to appear elongated to the northeast; this may be
mainly an effect of wind-blown re-working rather than impact angle.
The ejecta contain fragments of the iron meteorite which caused Meteor
Crater, along with iron melt spherules. The red and blue lines are
power lines and roadways.
False color image of Meteor Crater, made by the airborne TMS sensor
(JPL).
18-12: Assuming the ejecta blanket pattern is not principally a wind
phenomenon and instead is the result of ejecta being tossed out
preferentially in one general direction owing to the meteorite coming
in at a low angle, from what direction did the bolide come? What is
peculiar about the crater outline? What might explain the tiny round
depression near the left bottom of the image? What could the long
straight red line be? ANSWER
A thermal multiband color image made (courtesy: Dr. James. Garvin)
from the airborne TIMS (Thermal Infrared Multispectral Scanner) sensor
divulges the expression of this ejecta, with reds and some yellow
corresponding largely to Moenkopi Siltstone and Coconino Sandstone
(whose spectral properties in the ejecta are influenced by their
particulate nature and, possibly, by shock effects) and the
blue-greens to the overlying Kaibab Limestone.
False color image of Meteor Crater, from thermal bands on JPL�s
airborne TIMS.
Much smaller, but possibly contemporaneous with Meteor Crater is
Odessa Crater in West Texas. The only sign that it might be impact or
even a crater as such is the disturbed limestones exposed at the
surface. Petrographic shock effects are sparse and inconclusive, and
shatter cones are absent, but iron meteorites around the site are
considered definitive evidence. Here is an aerial view:
The Odessa Crater.
A 13 km wide structure not far from Odessa, Sierra Madera, was first
identified as impact in origin from the presence of shatter cones.
Here is a space image which clearly shows the crater's outlines and a
hint of a central peak:
Sierra Madera; Landsat image.
The Crooked Creek structure in central Missouri was first visited by
the writer during a 1962 field trip to the so-called "cryptoexplosion"
structures in that state. At that time, despite the presence of
shatter cones, most geologists held these structures to have formed
from volcanic action (ascending lava caused some kind of blow out), to
which the name "cryptovolcanic" is given. The impact identity of this
7 km crater, seen in this aerial oblique view, has since been
confirmed:
Aerial view of the Crooked Creek structure.
These next images exemplify how craters that don't have good
expressions in space imagery can however be visualized in other ways.
The Weaubleau structure in northwest Missouri was identified as an
impact crater through field studies and recognition of shock
metamorphic effects. Its circular outline, although faint, is evident
in this shaded relief map;
The Weaubleau structure.
The Wetumpka structure is an eroded 7.6 km diameter structure in
Alabama, north of Montgomery. It shows up in both a Landsat image and
a DEM product made from SRTM radar data:
Landsat image of the Wetumpka impact crater
Attention was drawn to disturbed sedimentary rocks in coastal plains
units at the Wetumpka site. This led to discovery of shatter cones and
other shock features.
Disturbed sedimentary units within the Wetumpka impact structure.
A similar subtle expression of crater morphology is evident in this
colorized relief map of the 7 km Wells Creek structure in Tennessee,
which has a small central peak:
Shaded relief map of the Wells Creek structure, which has a small
central peak.
Possibly the largest impact crater in the continental U.S. is the
Beaverhead structure, astride the Idaho-Montana border (both states
claim it). The structure, which extends across at least 60 km (but may
be as wide as 150 km) has no circular outline since it predates the
Rocky Mountain orogeny so that tectonic activity has destroyed its
morphology. It does have an expression in the Bouguer gravity anomaly
map of the region:
Bouguer anomaly map showing the Beaverhead structure as a rough
circular blue area.
Large shatter cones and shocked rocks have been found in the
Beaverhead structure:
Shatter cones in rocks within the Beaverhead structure.
Shatter cones.
Canada has a large number of craters (as indicated on the map at
bottom of page 18-1) largely because much of that country is exposed
Precambrian Shield - made up mainly of hard, resistant igneous and
metamorphic crystalline rocks. Glaciation has carved into some of
these craters but glacial debris and boreal forests can mask smaller
structures. Typical is the Gow structure - a crater cut into ridges of
crystalline rock .
The Gow Structure
One of the first craters studied in detail (by Michael Dence) in terms
of its petrography is Brent in Ontario. Here is an aerial photo of
this 3.8 km wide structure, which is severely eroded:
The Brent Crater.
Also in Ontario is the Holleford crater. It was imposed on Precambrian
rocks that were later covered by sedimentary rocks. The trace of the
Holleford structure is evident in this aerial photo:
The Holleford crater.
East of the Nastapoka Arc are the two Clearwater Lakes structures,
formed simultaneously by the breakup of the incoming meteoroid into
two chunks. West Clearwater is a complex crater (32 km wide), with a
circular ridge as a remnant of the central peak; East Clearwater is a
simple crater (20 km diameter).
West and East Clearwater Lakes
The 13 km wide Deep Bay crater, in Saskatchewan, appears in this ESA
image:
The Deep Bay crater.
Carswell is a 39 km diameter eroded crater in Saskatchewan; it is
faintly visible in this Landsat image:
The Carswell crater.
A SIR-B radar image of southern Ontario highlights two juxtaposed but
unrelated craters that are very different in age, in size, and in
structural state.
SIR-B radar image of the Sudbury impact structure (elliptical because
of deformation by Grenville thrusting) and the nearby Wanapitei crater
(lake-filled) formed much later.
The partially circular lake-filled structure on the right (east) is
the 8 km (5 mi) wide Wanapitei crater, estimated to have formed 34
million years (m.y.) ago. The far larger Sudbury structure (second
largest on Earth) appears as a pronounced elliptical pattern, more
strongly expressed by the low hills to the north. This huge impact
crater, with its distinctive outline, was created about 1800 m.y. ago.
Some scientists argue that it was at least 245 km (152 mi) across when
it was circular. More than 900 m.y. later strong northwestward
thrusting of the Grenville Province terrane against the Superior
Province (containing Sudbury) subsequently deformed it into its
present elliptical shape (geologists will recognize this as a prime
example of the "strain ellipsoid" model). After Sudbury was initially
excavated, magmas from deep in the crust invaded the breccia filling,
mixing with it and forming a boundary layer against its walls. Some
investigators think that the resulting norite rocks are actually
melted target rocks. This igneous rock (called an "irruptive") is host
to vast deposits of nickel and copper, making this impact structure a
5 billion dollar source of ore minerals since mining began in the last
century.
18-13: Which part of the elliptical rim of Sudbury seems to have
better topographic expression? ANSWER
A geologic map shows the general geologic units present within and
around the main structure (just north of the town of Sudbury):
Map of the units composing the deformed Sudbury structure.
Part of this structure has been displayed in this Landsat image in
which classification procedures indicate many of the surface
constituents:
A classified Landsat image of surface units around Sudbury.
Because the Sudbury event led to invasion of magma that also brought
huge deposits of nickel minerals, this structure has been extensively
studied by many geologists. Here is a photo of typical Onaping breccia
(originally named a "tuff" by Howell Williams) and beneath it of some
of the melt rock at Sudbury:
Onaping tuff.
Sudbury melt.
On page 18-2 we first saw Canada's most conspicuous impact structure,
Manicouagan, in Quebec. Here is another version, made by Landsat-7, in
which the lakes that define the structure appear to be part of an
incomplete "8" with the upper half made (coincidentally) by smaller
lakes:
Landsat-7 image of the Manicouagan impact structure.
Also in the Canadian Shield of Quebec is the 8 km wide Couture crater,
seen in this aerial mosaic;
Lake Couture crater.
Radar can sharpen the appearance of an impact structure, as
demonstrated with this aerial radar image of the Haughton crater (24
km; 15 miles wide) on Devon Island in the Canadian Arctic. Although
about 23 m.y. old, much of the crater's morphology has survived
erosion.
Aerial radar image of the Haughton crater in Canada.
Compare the radar image with this one made by Landsat-7:
The Haughton crater.
18-14: Where does the crater rim of Haughton appear to be? ANSWER
The Charlevoix crater is truncated by the St. Lawrence River in
Quebec. This 54 km wide structure is imaged in this aerial radar
composite:
The Charlevoix crater.
Large craters are not always evident morphologically, especially where
modified by erosion. The 38-km wide Mistastin impact crater in
Labrador, seen in this wintertime astronaut photo, has a lake and
central peak but glaciation has obscured its rim boundary:
The Mistastin structure
Several South American impact structures have a tie-in with Landsat
and other imaging systems. A crater in Brazil named Araguainha had
earlier been studied and classified as a dome. But when visited by Dr.
Robert S. Dietz - reknown for his ability to find new craters -
evidence (shatter cones and breccias) was found that pointed towards
an impact origin. Samples were sent to Dr. Bevan M. French, a
colleague, to search for shock metamorphic features. On the very same
day he confirmed their presence, the writer (NMS) phoned him to say
that I had found the following Landsat image, shown here as a
subscene:
Landsat MSS subscene in which the circularity of the eroded Araguainha
impact structure in the Brazilian Pampas is evident.
What we learned from the image was that the crater structure was about
twice as wide (40 km; 25 miles) as field studies had suggested.The
several tonal bands are due to differences in vegetation in this
pampas grass country.
The hunt pressed on to find other craters in vegetated Brazil. Landsat
was instrumental in finding this 13 km (8 mile) structure, Serra da
Cangalha, with its central rim and inner depression.
Serra da Cangalha crater in the Brazilian savannan/forest.
Then more looking turned up a smaller crater (4 km), Riachao, about 50
km to the northwest. It is shown as the inset in the above image. An
aerial photo taken of it later gives details about its appearance:
Aerial photo of the Riochao structure in Brazil.
After being visited and sampled, both structures yielded evidence of
shock metamorphism, putting them squarely in the impact camp.
Still another impact structure, the Ituralde crater (8 km; 5 mile
diameter) was discovered from space just within the rain forest in
eastern Bolivia:
The Ituralde crater in Bolivia; photo from the International Space
Station.
Of probably impact origin, the Vargeao Dome, in the Brazilian rain
forest, shows up in a topographic image made from SRTM (radar) data:
The Vargeao Dome.
Lets move on now to impact craters in the eastern hemisphere.
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Primary Author: Nicholas M. Short, Sr.