Craters come in three shape/size categories: 1) Simple; 2) Complex; 3) Basins. The difference between the first two is evident in this diagram (Complex craters have a central peak and terraces along their walls, resulting from slumping along concentric fractures.)
Generally, simple craters are less than 20 km in diameter and complex ones are wider in diameter. Basins, which are several hundred kilometers or more in diameter, may also have central peaks but these can be completely submerged because these big craters can induce widespread crustal or mantle melting that fills them up. In the upper diagram, the top of the red area (the lens of the fall back and slump breccia fill) is known as the surface of the apparent crater (what one sees looking into the crater. i.e., the top of the ejecta and/or some solidified melt [caused by shock and/or by subsequent invasion by activated lava from below]); the bottom of the red area is the boundary of the true crater (the limit of excavation by the cratering process, below which the country rock is intact although fractured).
We will use three lunar impact structures to illustrate the three types - Simple, on top; Complex, in middle; Basin, at bottom:
When craters are exposed at the surface, the younger, usually less eroded ones are recognized by their morphology or external form. They are approximately circular (unless later distorted by regional deformation), have raised rims, show structural displacements in their wall rocks, and may have a central peak, consisting of rocks raised from deep original positions. We can emphasize the morphology of these craters in 3-D perspectives (commonly using Digital Elevation Map data) of their contours, exaggerating the elevations and applying shading or artificial illumination (computer-controlled). An example of how this makes craters more obvious, when today they often have low relief, is the Flynn Creek structure (3.5 km [2.2 mi] wide) in Tennessee:
The structural deformation in the affected rock immediately beyond the crater boundary is usually intense and distinctive. Initially flat-lying layers of sedimentary rocks near the surface, beyond the rim, are commonly deformed by upward bending (layers inclined downward [dip] away from the crater walls). Anticlines may be formed or in the extreme the layers are completely overturned producing a flap in which the top layers are upside down, flipped over on top of layers farther out. Modes of layer deformation at two impact craters and one nuclear explosion crater are shown in this diagram (the term "authigenic breccia" refers to fragments (clasts) formed in place with little or no transport; "allogenic breccia" refers to fragments that were broken up elsewhere and then transported to their present sites.
The term "overturned flap" warrants an example. Mapping at the Sedan nuclear crater was particularly revealing about this unusual deformation. Look at this cross-section of the area just beyond the crater:
There were several distinct layers in the Sedan alluvium that could be used as markers. The cross-section reveals that as the crater deformation proceeded, these layers were completely folded back on themselves (overturned) for nearly 100 meters. When the writer (NMS) visited the mapped area, he was shown where an asphalt road leading up to the pre-detonation drill hole that emplaced the nuclear device was flipped over onto itself. Amazing!
Deformation involving bending and overturning is well exposed in the layered limestones exposed by quarrying at the Kentland, Indiana impact structure:
This structure involves deformation of an area up to 13 km wide in a region where all other rocks are still horizontal. Shatter cones, coesite, and other shock feature provide the proof that this is the remnant of an impact crater.
One of the most famous, and best studied, large complex craters is the the 24 km (15 mi) wide Ries Kessel (also referred as the Nordlinger Ries or just plain Ries) in Bavaria. (An Internet site [in German] that provides more information and field photos is sponsored by the Ries Museum of Nordlingen). Here is a photo montage (made by piecing together several wide-angle lens photos) of part of this structure. (On one of its rim units, thick largely evergreen forests develop, whose dark appearance helps to outline the structure; the occurrence of clouds over this unit appears to result from local evapotranspiration from trees.)
And here is a DEM reconstruction of its generalized subsurface structure:
In satellite images, the Ries structure is not easy to spot. Its interior depression has been backfilled and its rim is now notably eroded. But analysis of its topography using elevation data extracted from ASTER data on Terra brought out the still surviving circularity of the crater, as seen in this bottom view (the top view is an ASTER pseudo-natural color image in which the circularity is obscured but to some degree hinted at):
The geologic nature of the present, somewhat eroded Ries structure is encapsulized in this cross-section
The Ries is young enough for much of the ejecta that deposited in thick units (when consolidated the general term "breccias" applies; at the Ries the special name "Suevite" is given to this rock) to still be preserved. Here is some field outcrops:
The Ries lies astride "Das Romantische Weg" - The Romantic Way - made up of towns and cities that have preserved much of their medieval buildings. Within the Ries is a remarkable small town, Nordlingen, surrounded by a protective wall. Here is an aerial view of this marvelous throw-back to another era:
Since medieval times, the local residents in Nordlingen quarried some of the breccia deposits that had hardened into rock. This was used as building stone. The Catholic Church near the center of this walled city is made up of this Suevite rock; unfortunately, the rock is easily weathered (because it contains much glass that is unstable over time), so that the Church today is in constant need of repair. Here is this Church:
Some of the ejecta "clasts" at the Ries are as big as a house. Megabreccias, similar to those found on the lunar highlands (Section 19), are not uncommon at terrestrial impact structures. A striking example is exposed along a cliff next to a lake inside the Popigai impact crater in Siberia:
Simple craters (and some larger ones) often have depressions that fill with water. On the top below is an aerial view of the 3.5 km (2.2 mi) wide New Quebec crater (renamed Pinqualuit crater) in granitic shield rock, exposed in Northern Quebec; at the bottom is a view from space.
The West Hawk Lake structure (2.5 km [1.6 mi] diameter) formed in metamorphic rocks in westernmost Ontario near the line with Manitoba was the first impact crater studied in detail by the writer [NMS], in 1965; the details were published in 1966 in the Bulletin of the Geological Society of America.
In Canada, and other northern latitude countries, lakes filling impact structures, such as at West Hawk Lake, freeze in winter, allowing support for drill rigs, so that scientists can explore the crater infill materials by recovering core. One observation from this study was that the distribution of shock effects (mainly planar features and the degree of isotropization) varied rather nonsystematically within the breccia deposits. To quantify this, the writer developed the concept of a "shock log" which plots variations in shock damage as a function of depth; the shock metamorphic features (see next page) used to determine the level of shock damage are given in the key below the plot:
A spectacular complex crater is Manicouagan, a 100 km (62 mile) structure in southern Quebec, Canada, which has a great central peak area of igneous and metamorphic rocks, among which are feldspar-rich rocks. In these rocks, much of the feldspar has transformed by shock into glass, known as Maskelynite. Similar to Manson, there is a depression or moat between the peak and rim (now eroded) that formed annular valleys, which filled with water when a hydroelectric power dam blocked the draining rivers. Because of this contrasting surface expression, astronauts journeying back from the Moon could see this crater from well out in space.
A great deal of impact-produced melt is found along the lake at Manicouagan, as shown here:
Deposits of fragmental rock surround most younger craters. An example (top, below) of such rock , from an outcrop at the Ries crater, illustrates these ejecta deposits (Suevite breccias). A second example (bottom) seen in core from a drilling that penetrated the Manson central peak, shows the diverse nature of the rock types making up these breccia fragments (called clasts).
Most ejecta blocks found around younger craters consist of fragmented bedrock derived from subsurface units. There can be exceptions if the surface material is unconsolidated. The writer (NMS) discovered a fabulous example of this, which at first was discounted by other specialists in this field. The crater is the small Wabar structure (there are 3 craters - 2 are small - there). in the sandy desert of southern Saudi Arabia. Around the rim are small fragments of white quartz sand, many coated with a black glass. Here is two views:
A few years earlier the writer had been given small pieces of "sandstone" around chemical explosion craters formed during an experimental program at the Nevada Test Site (NTS), where white loose quartz sand had been used to backfill the access hole through which the explosives were loaded. He postulated that the fragments were made up of this sand that had been driven together and compressed (a process he named "shock lithification", calling the fragments "instant rock"). He proposed the same origin for the Wabar fragments, namely, that they were desert sand shock-liithified by shock waves from the impact (and many were then covered by shock melt that overtook them). This pair of photomicrographs shows the texture of the NTS instant sandstone on the left and the Wabar lithified fragments on the right.
Below is a second photomicrograph of the NTS instant sandstone, showing more details.
The paper on this interpretation was rejected by Science Magazine because the reviewer had been there and thought he had noted thin sandstone layers in the rim. Through a stroke of luck, the writer, telling a colleague at Shell Oil in Houston of the discrepancy in interpretation, was surprised to receive a call later from that friend who reported the loose sand at Wabar was more than 200 meters thick (he had asked a Shell field geophysical crew to run a seismic line next to the site; they determined an accurate thickness). With this new "proof" the paper was resubmitted to Science and was published.
Wabar not only has abundant examples of shock-lithified "sandstone", but it also has numerous samples of shock-melted sand as black glass:
The writer obtained several Wabar samples which were "most peculiar". These consisted of the instant rock but with a thin coating on the exterior of this black glass (the bottom row above may show examples). Odd indeed! My speculation: the instant rock fragments were expelled as ejecta that passed through a "cloud" of the melted sand that made up the black glass. This is speculation but seems reasonable - I can't think of a better explanation. Strange things happen during impact events.
Eroded craters lack definitive external shapes, although the initial circularity may have a persistent effect on drainage, keeping streams in roughly circular courses. Such craters are often hard to detect but the presence of anomalous structural deformation and of brecciated rocks give clues. In rocks that were just outside the original wall boundaries (the true crater), a peculiar configuration, known as shatter cones, commonly develops.
These "striated" conical structures (described as "horsetail"-like in shape) can be very small or can reach six feet or more in length, as seen above in quartzites at the Sudbury, Canada, impact structure. When the folded rocks containing the cones are restored to their original positions in an orientation graph, the cone apices invariably point toward an interior location that lies above the central crater floor. In effect, this denotes that the position where the energy was released was above the floor, a situation incompatible with a deep volcanic source, as once advocated by skeptics. The cones, which also sometimes form in rocks subjected to nuclear explosions, occur in lower (peripheral) shock pressure zones, as the rarefaction phase of the shock waves, spreading outward, places the rock into tensional stress. Many cones appear to originate from point discontinuities (e.g., a pebble) as though the waves were diffracted.
Shatter cones come in a range of sizes from a few centimeters to the 9 m cone at the Slate Island (Canada) impact structure, shown here:
Still, the best evidence for a extraterrestrial origin of a crater is the survival of the incoming bolide as pieces of meteorites or asteroid/cometary material. This is relatively rare, although abnormal chemistry (such as iridium and other unusual concentrations of trace elements) in rocks and melt from older structures often can point to the intermixing of the bolide with the target. Iron meteorites are found in and around Meteor Crater, Arizona (see page 18-5). Iron meteorites are present in the small, relatively recent Campo del Cielo craters in South America and also at the Wabar crater discussed above. Eight small craters formed less than 10000 years ago in Poland near Poznan, with the largest being about 100 meters wide, were identified quite by accident as caused by large fragments of an iron meteorite, pieces of which were discovered by troops digging fortifications in World War I. The depressions are well-preserved, as seen in this photo:
A word of caution: Lest one assume that every circular structure is impact in origin (we've already pointed out circular volcanic craters), here is a case where circularity on a grand scale does not mean a great impact event occurred. Consider the Nastapoka Arc on the eastern shore of Hudson's Bay in Canada, as seen in this Landsat mosaic:
Its circularity is imposing. Many hoped this was the largest impact structure on Earth. Possibly it may someday prove just that. But all the field evidence so far has not yield a single positive indicator of impact. Granted the surface rocks include some younger sedimentary cover. But no shatter cones have been found. The Belcher islands to the west show no shock features, expected if they were part of a central uplift. Limited deep drilling has not encountered shocked rocks. The circularity may simply be a fortuitous configuration of a sedimentary basin. But note the two circular features, side by side, to the east. These are the Clearwater Lakes impact structures, shown on the next page.
The Nastapoca Arc ambiguity calls attention to the fact that there are still other circular structures whose origins - impact, volcanic, tectonic - have been questioned. A case in point is the Upheaval Dome in Utah, seen here in this annotated astronaut photograph; for a long while this dome was considered to be the surface manifestation of some type of intrusion (perhaps a salt diapir):
It was this structure that was studied by the Dean of Astrogeologists - Dr. Eugene Shoemaker - when he first worked for the USGS. He concluded that it could (?) be impact in origin. This aroused his interest in impact mechanics that eventually led him into planetary studies. For decades, the consensus disputed this origin. But recently several diagnostic shock features at the microscopic level have been discovered in its rocks, tipping the opinion balance in favor of a meteoritic collision.