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Impact craters almost always start out as circular structures bounded by a raised rim and bottomed by a depression which may have a central uplift or peak (exception to roundness is the elliptical form that occur when a crater strikes at a very low incidence angle). As crater diameter increases, the ratio of depth to diameter decreases. Crater morphology is altered with time as erosion (mainly by water on Earth and by repeated subsequent impacts and buried by ejecta on the Moon) tends to subdue its topographic expression. As craters wear down to scars (astroblemes) in the bedrock, their initial circularity may still have an effect on drainage. Buried craters are sometimes identifiable by their patterns in seismic or gravity surveys. Marks in the field of craters include shatter cones and breccias. This page also describes several very large craters whose aftereffects may have altered conditions for life on Earth.


Crater Morphology; Some Major Impact Structures

In view of the tremendous energies involved, it is no wonder then that we classify the Chicxulub impact in the Yucatan Peninsula as one of the biggest short-term natural events known in the geologic record (of nuclear-equivalent magnitude in excess of 100 trillion tons of TNT equivalent). It occurred 65 million years ago and led to a 200-300 km (>150 mi) wide (there’s still some uncertainty regarding the location of the outer rim) and perhaps 16 km (10 mi) deep depression.

This huge structure has no evident surface expression, being covered by younger sedimentary rocks, but does appear subsurface as a strong gravity anomaly, as shown below. It was discovered almost incidentally through oil drilling, in which core samples, containing so-called volcanic rocks (now known to be shock-melted rock), showed distinct shock effects. The samples languished for years in the basement of the University of New Orleans' Geology Building, before someone re-examined them and discovered their origin.

The buried Chicxulub crater shows a suggestive circular depression pattern in this gravity map in which different values are shown in different colors.

The Chicxulub impact into shallow waters of the Gulf of Mexico generated huge waves and, even more destructive to the planet, tossed enormous amounts of hot rock and water/stream into the atmosphere. An immediate result was to set forests and grasslands over much of the globe on fire, in the biggest firestorm in history. These materials, in turn, caused a worldwide "cloud deck" of aerosols, gases and particulates leading to temperature fluctuations, general darkening, an anoxic (oxygen-poor) atmosphere and reduced photosynthesis that wiped out much of the food chain and provided the "coup de grace" to the few dinosaur families still living then on Earth. Up to 50% of angiosperm (flowering plants) species were destroyed along with many animal families in the sea and on land. Some have estimated that it took thousands to a million or more years for ecosystems to recover. Mammanls, inconspicuous before this event, were able to flourish in these new systems and gradually gain ascendancy during the Cenozoic.

The resulting debris from Chicxulub that ejected into high altitudes spread around the globe and settled as a thin layer of material that marks the precise K/T boundary between the last rocks of the Cretaceous (symbol K) Period and the first sediments formed in the younger (overlying) Tertiary Period (symbol T). The deposits contain iridium, a metallic element present in some meteorites, and mineral grains that bear evidence of intense shock (including quartz crystals with planar features; see page 18-4).

The deposits at the K-T boundary are usually very thin. They represent the fallout layer that may have been worldwide in distribution. Here is an example of this now-famous layer, which also includes soot particles from the after-impact fires.

The clay deposit making up the Cretaceous-Tertiary (K/T) Boundary.

For the last 10 years or so it was thought that Chicxulub could not be recognized in space imagery because of the post-impact sedimentary rocks covering the structure and also because of dense vegetation cover. However, radar data from the SRTM program have been specially processed to bring out otherwise subtle suggestions of the buried crater rim showing its presence by some manifestation at the surface (probably induced by differential subsidence). Here is that SRTM image of much of the Yucatan Peninsula. See if you can locate the rim trace:

SRTM enhanced topography image of the Yucatan Peninsula of Mexico.

In case you missed this trace, here are a pair of images derived as an enlargement of the part of the Yucatan containing the Chicxulub crater. On the lower one, the rim boundary has been drawn in and the location of sink holes that seem to relate to subsurface control by the fallback material beyond the rim is marked.

Enlargement of the section of the Yucatan Peninsula in which the buried Chicxulub structure is located.

A report in November, 2003 has presented evidence from rocks in Europe and China of a very large impact, about 251 million years ago, that coincides with the end of the Paleozoic (Permian-Triassic boundary), a time in which about 90% of species living then are estimated to have vanished - a time now referred to as the "Great Dying". This is the greatest mass extinction in Earth history - 90% of marine life and 70% of land life; it set the stage for a new burst of life in the Mesozoic. The hunt for this super crater has been a top priority in the last five years.

Most destruction of life was in the oceans. But reptiles that had occupied the land also were largely (but not completely or there would have been no dinosaurs or mammals in the Mesozoic) were also severely affected. Best known of these is Dimetrodon, a reptile with a saillike dorsal fin, which disappeared completely at the Permian's end.

Dimetrodon, pictured in a parched world; from the <i>Economist</i>.

In May 2004 announcement was made that the killer crater may indeed have been found. Marine geophysics surveys off the northwest coast of Australia turned up a distinct anomaly buried under shallow seas that was promising enough to drill two deep holes- in search for oil (several impact craters have served as petroleum traps). Here is a seismic refraction survey map that shows the buried structure including its central peak (40 km [25 miles] diameter}.

Seismic multichannel profile across late Permian structure (blue line).

The structure has been identified by Professors Luann Becker (UCSB) and Robert Poreda and Asish Basu (University of Rochester) as an impact structure, and named the Bedout (pronounced "bidowe"; French word) crater. Because of the extreme importance of this find, we strongly recommend that you access a web site containing (in .PDF format) their most recent paper at this online site. Most of the images shown on this page were extracted from that paper. The figure below shows the paleogeography at the close of the Paleozoic, in which most continents were grouped together in Pangaea. Bedout lies just off the soon to become Australian block, in shallow waters of the PaleoTethys Ocean.

See text next paragraph.

The yellow-centered circle is the Fraser Park, Australia locality containing what they (and others) interpret to be fallout debris - small fragments of rock and glass. The red dots are other localities - each containing a layer of highly probable impact ejecta containing such evidence as shocked meteoritic chips, glass spherules, Fe-Ni-Si grains, and fullerenes (so-called "buckyballs" made up of carbon arranged in a "geodesic" structure. Note the occurrence of extensive volcanism, the "Siberian Traps". Basaltic volcanism also covered the Bedout structure on the sea floor; afterwards about 3 km (2 miles) of mid-Triassic through Cretaceous, and then Tertiary-Quaternary sediments were formed.

This montage of photos shows a land outcrop traceable to the Bedout event, a guide fossil, Glossopteris, which disappeared immediately afterward, and a grain of quartz which shows planar features - a key indicator of impact as discussed below and on the next page.

A field outcrop in Australia (left) that contains debris showing some evidence of a shock event; the Glossopteris guide fossil to the close of the Permian (upper right), and a shocked quartz fragment (lower right); all associated with the Bedout structure.

In the May 2004 paper, the above mentioned investigators claim to have now found actual pieces of the asteroidal impactor (estimated the size of Mt. Everest) itself. They note too that at the close of the Permian, Australia was part of Pangaea, so the asteroid might have struck on land but the resulting crater has since detached and become buried by marine sediments and ocean water.

Core recovered from the structure, which presently has the dimensions of 200 km (125 miles) but may be even larger, produced breccias in a zone more than 300 meters thick. This was initially interpreted as a volcanic breccia One core segment shows the recovered breccia with a Chicxulub example along side.

Core from the Bedout and Chicxulub buried craters

Another group of Bedout core and core from Chicxulub (bottom) at first glance looks almost like gray sediment of clay or fine-crystalline limestone nature:

Core from the Bedout structure (top) and Chicxulub structure bottom.

When that core was examined in detail, the breccia clasts were mostly made up of largely devitrified glass, containing plagioclase, iron oxides, iron-titanium oxide, and recrystallized chorite. This is a typical view of clast petrography, which resembles impact melt the writer has seen at various accepted impact structures:

Photomicrograph of microcrystalline texture in a once glassy clast; yellow areas are glass not recrystallize; the larger light areas are plagioclase-rich; a calcite vein cuts across the chlorite-rich matrix; width of image equivalent to about 3 mm.

Very strong proof of impact origin for Bedout is the shock metamorphic phenomenon of conversion of plagioclase crystals (as laths) into the glass known as maskelynite. This is very evident in this pair of images showing photomicrographs of a sample in which (in the top view) laths of plagioclase showing a brown tone are set in a crystalline albite (white) and titanite (black) matrix. In the bottom, the laths are now revealed to be totally isotropic (remain dark in crossed-polarized light when the microscope stage is rotated) as bespeaks of a glassy state.

A breccia clast seen in plane polarized light in a microscope view (top); same field of view under crossed-Nicols, showing that the plagioclase laths have converted to thetomorphs (glass but the crystal retains its shape, i.e., did not melt and flow.

Magnesium Carbonate material in the core has in some instances been converted to glass (this is very rare but was first produced by the writer in an implosion tube experiment). Another indicator of shock at the Bedout site is the "toasted" quartz grains with single to multiple sets of planar deformation lamellae found rarely in the core but more commonly in the fallout layer tied to this event. Here are two examples from the Fraser Park, Sydney basin site:

Quartz fragments containing sets of PDFs; Fraser Park locality.

The Becker team and others have done age dating on the core materials from the crater and from fallout layers elsewhere. The Ar40/A39 dating yields ages of 250.1 +/- 1 million years, almost exactly the time assigned to the Permian/Triassic boundary (end of the Paleozoic).

At the moment, several impact specialists have disagreed with the interpretation of the evidence cited so far as proof of an impact origin for Bedout, so the inevitable dispute from multiple interpretations has commenced (the concept of "multiple hypotheses" was first promulgated by a geologist). (As a sidebar opinion, the writer is convinced of the impact hypothesis, since the petrographic features for Bedout are familiar to him from his shock metamorphism studies.) Others believe this to be but one manifestation of intense volcanic activity at the close of the Paleozoic that so saturated the Earth's atmosphere as to cause the die-out. But the study is just beginning. If proved to be a huge impact, Bedout probably is at least one, and maybe the exclusive, factor in the Great Dying.

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, thus together these craters really delivered a "knockout block" to life.

But as if usually the case in science, hypotheses as "outrageous" as killer impacts have their challengers. Despite models that show how an impact crater hundreds of kilometers in diameter could affect the atmosphere and surfaces worldwide, there are those still skeptical of this mechanism. These scientists propose other reasons for mass extinction, chief of which is intense regional to worldwide volcanism which at its height could saturate the atmosphere with light-blocking ash and moisture. Michael Payne has plotted the size-geologic age distribution of impact crater together with times of major volcanism and major extinctions. Examine the graph below and set up your own hypothesis:

Diagram of impact crater ages and sizes, times of major volcanism, and post-Cambrian mass extinctions.

A buried crater under the Chesapeake Bay may have been large enough to kill off some life during the Tertiary. This structure has a diameter of at least 90 km (56 miles), with a central peak. It is located as shown below:

The Chesapeake Bay crater, located near the mouth of that estuary; inner white line outlines the central uplift; outer line the crater rim; USGS image.

Like some others, the structure was first found during a geophyical survey. It lies buried beneath both coastal and estuarine sedimentary materials. It has been drilled; another drill is underway. Recovered core shows extensive breccia units. Age dating of these deposits places an age of 35 million years for the breccia matrix. Proof of an impact origin includes telltale planar deformation features in quartz within breccia clasts, as shown here:

PDFs in quartz grains within the Chesapeake Bay crater breccia; USGS source.

Chicxulub, Chesapeake and Bedout are among a growing number of impact structures that are buried and have been discovered during geophysical surveys. Another is the Ames structure (~13 km diameter) in northern Oklahoma, found when it was picked up by gravity and magnetic surveys and then drilled in search for oil. Here is a cross-section prepared by Prof. Judson Ahern showing the crater, its distribution of materials of different densities, and survey results:

The Ames Structure: magnetic and gravity profiles and materials density distribution.

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). Two examples illustrate how we can make craters more obvious, when today they have moderated and often have low relief. First is the Flynn Creek structure (3.5 km [2.2 mi] wide) in Tennessee:

DEM shaded relief map of the Flynn Creek impact structure in Tennessee.

The structural deformation in the excavated rock 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 into 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.

Cross-sections through the Meteor Crater and Odessa impact structures and the Teapot-Ess chemical explosion crater at the Nevada Test Site, showing the modes of layer deformation at each.

Deformation involving bending and overturning is well exposed in the layered limestones exposed by quarrying at the Kentland, Indiana impact structure:

Strongly deformed limestone beds seen along a quarry face at the Kentland, IN structure.

One of the most famous, and best studied, large complex craters is the the 24 km (15 mi) wide Ries Kessel 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, helping to outline the structure; the occurrence of clouds over this unit appears to result from local evapotranspiration.)

Photomontage of the Ries structure in Bavaria.

And here is a reconstruction of its generalized subsurface structure; topography exaggerated.

DEM shaded relief map of the Ries Kessel impact structure in Bavaria.

 

18-9: In the Ries Kessel perspective view, the crater appears surrounded by mountains. But in reality, the actual landscape is hilly but not mountainous. Explain the illusion. ANSWER

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 a field exposure:

Part of the ejecta blanket around the Ries impact crater; breccia clasts can range from almost microscopic to as big as houses (seen elsewhere).

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:

Aerial oblique view of Nordlingen in Bavaria, a town within the impact structure named the Ries Kessel.

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:

The Church at Nordlingen, one of only two buildings on Earth that is constructed of impact breccias.

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:

Megabreccias at the Popigai crater, Siberia.
Courtesy: Phillipe Claesp

In general, craters smaller than 3-5 km (1.8-3.1 mi) in diameter lack central peaks, i.e, they have bowl-shaped interiors, and we call them simple. Most larger craters have central peaks, in which the rocks below the true crater boundary have "rebounded" upward from the collision, further aided by centripetal forces associated with crater wall slumping. We call these complex craters, but erosion and infill may subdue the peaks. Flynn Creek, similar to most other craters in the U.S. that cut into carbonate rocks, just barely received a central peak, which still shows topographically. The Ries does not retain a morphological peak, but the depth to the crater boundary, as determined by drilling, is less in the interior.

Simple craters (and some larger ones) often have depressions that fill with water. On the top, below, is the 3.5 km (2.2 mi) wide New Quebec crater in granitic shield rock, exposed in Northern Quebec. On the bottom is the much older, West Hawk Lake structure (2.5 km [1.6 mi] diameter) formed in metamorphic rocks in westernmost Ontario near the line with Manitoba (rock core from which was first studied in detail by the writer [NMS] in 1966; published in the Bulletin of the Geological Society of America).

The New Quebec impact crater in Northern Quebec.

Aerial photograph of West Hawk Lake in western Ontario.

In Canada, and other northern latitude countries, these lakes freeze in winter, allowing support for drill rigs, so that we can explore the crater infill materials by recovering core.

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).


Suevite breccias in the field at the Ries structure.

Core segment from drill hole into central peak of the Manson (Iowa) impact structure, showing breccias of impact origin.

18-10:Suppose a continuous length of drill core consists of first an interval of breccia much like that shown in this figures, then a 10 meter interval of a single rock type, say granite, and followed by more small fragmental breccia. What explains this? ANSWER

Most ejecta blocks found around younger craters consists 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 in the aeolian desert of southern Arabia. All around the rim are small fragments of white quartz sand, many coated with a black glass. Here is a view:

The Wabar crater, with sandstone-like ejecta, often coated with glass.

A few years earlier the writer had "discovered" small pieces of "sandstone" around chemical explosion craters formed in 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.

Two examples of shock-lithified sandstones: on the left, produced in a cratering experiment at the NTS; on the right, a fragment of a quartz sandstone-like rock collected at the Wabar Crater in Arabia, known to have been caused by an impact because of iron meteorite pieces scattered around the site.

Below is a second photomicrograph of the NTS instant sandstone, showing more details.

Another look at NTS instant sandstone.

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.

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, a peculiar configuration, known as shatter cones, commonly develops.

Large shatter cones in an outcrop at the Sudbury structure.

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 (as an aside: the writer's "favorite" geological outcrop, anywhere, is the low bank partly around the parking lot of the MacDonalds fast food restaurant in downtown Sudbury, where excavators exposed a continuous cluster of shatter cones.). When we plot the original positions of the folded rocks containing the cones, the cone apexes invariably point toward an interior location that lies above the central crater floor. In effect, they denote 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 shock waves, spreading outward, place the rock into tensional stress. Many cones appear to originate from point discontinuities (e.g., a pebble) as though the waves were diffracted.

18-11: Try to explain what happens to cause the apex of a shatter cone to point towards the upper center of the crater near the point of impact. ANSWER

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 18-6). Iron meteorites are present in the small, relatively recent Campo del Cielo craters in South America. Eight small craters formed about 10000 years ago in Poland, with the largest being about 100 meters wide, were identified as caused by large fragments of an iron meteorite quite by accident, as pieces were discovered by troops digging trenches in World War I. The depressions are well-preserved, as seen in this photo:

One of the Morasko swarm of small meteor craters, in Poland.

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 Nastapoca Arc on the eastern shore of Hudson's Bay in Canada, as seen in this Landsat mosaic:

The Nastapoca Arc.

Its circularity is imposing. Many hoped this was indeed 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.

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Primary Author: Nicholas M. Short, Sr. email: [email protected]