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.
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):
Sometimes "Mother Nature" helps out. Snowfall has emphasized this unidentified crater:
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:
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.
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 SiO2 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:
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:
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 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.
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):
The ejecta apron stands out also in this IR photo:
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.
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.
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:
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:
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:
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 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:
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.
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:
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:
Large shatter cones and shocked rocks have been found in the Beaverhead structure:
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 .
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:
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:
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).
The 13 km wide Deep Bay crater, in Saskatchewan, appears in this ESA image:
Carswell is a 39 km diameter eroded crater in Saskatchewan; it is faintly visible in this Landsat image:
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.
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.
A geologic map shows the general geologic units present within and around the main structure (just north of the town of Sudbury):
Part of this structure has been displayed in this Landsat image in which classification procedures indicate many of the surface constituents:
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:
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:
Also in the Canadian Shield of Quebec is the 8 km wide Couture crater, seen in this aerial mosaic;
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.
Compare the radar image with this one made by Landsat-7:
The Charlevoix crater is truncated by the St. Lawrence River in Quebec. This 54 km wide structure is imaged in this aerial radar composite:
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:
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:
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.
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:
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:
Of probably impact origin, the Vargeao Dome, in the Brazilian rain forest, shows up in a topographic image made from SRTM (radar) data:
Lets move on now to impact craters in the eastern hemisphere.