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.

### Crater Morphology

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 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.

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:

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.

(Note: A report in November, 2003 has presented evidence from rocks in Europe and China of an even larger impact event, about 261 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. The crater causing this has yet to be found but the "hunt is on". Likewise, a buried crater under the Chesapeake Bay may have been large enough to kill off some life during the Tertirary.)

Chicxulub is one of 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:

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:

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.

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

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

18-6: 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:

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:

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).

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).

18-7: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:

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.

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.

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.

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.

18-8: 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

Primary Author: Nicholas M. Short, Sr. email: nmshort@ptd.net