Now on to a very relevant subject: Can impact craters, which represent huge releases of explosive energy, affect and even threaten life on Earth? Indeed, large impact craters - big enough to produce "basins" - are, of themselves, interesting to search for and study. But they also have important implications for the history of life on Earth. To determine whether impact plays such a role we start first with a quick overview of what is known about mass extinctions since life in its macroscopic form (large plants and animals) became abundant at the beginning of the Paleozoic. Consider this diagram:
The diagram indicates that there have been many small extinction events during this time span, and a few larger ones. One of these is the famed demise of the dinosaurs at the Cretaceous-Tertiary boundary. Many geologists now believe that the biggest extinction of all ended the Paleozoic at the close of the Permian. Other mass extinctions affected large fractions of animal and plant life at the end of the Ordovician, the Devonian, the Triassic, and possibly the Paleocene.
These extinctions are best defined at the family level, as shown in the graph below, and the accompanying explanation beneath it:
* The late Ordovician period (about 438 million years ago) - 100 families extinct - more than half of the bryozoan and brachiopod species extinct.
* The late Devonian (about 360 mya) - 30% of animal families extinct.
* At the end of the Permian period (about 245 mya) - Trilobites go extinct. 50% of all animal families, 95% of all marine species, and many trees die out.
* The late Triassic (208 mya) - 35% of all animal families die out. Most early dinosaur families went extinct, and most synapsids died out (except for the mammals).
* At the Cretaceous-Tertiary (K-T) boundary (about 65 mya) - about half of all life forms died out, including the dinosaurs , pterosaurs, plesiosaurs, mosasaurs, ammonites, many families of fishes, clams, snails, sponges, sea urchins and many others.
However, the above diagram tends to obscure the fact that extinctions are very common in the geologic record. At various times, the extent of extinction has been less than those associated with "mass extinctions" but in notable numbers nevertheless. Some researchers attribute at least one or more of these destructive occurrences to impact - although other mechanisms have been proposed (see bottom of page).In the context of these known extinction events, can impact be demonstrated as the cause of at least one or more of these? Consider this diagram, which shows most of the extinction events, along with the time estimates of both major impact crater formation and several spans of major volcanic activity, from which you can draw your own conclusion:
With this background re: mass extinction, let us examine the evidence that includes both impact cratering and volcanism as the most likely causes of the disappearance of life forms on a global scale. The discovery of a highly probable huge impact at the end of the Mesozoic Cretaceous period is now believed to have destroyed worldwide a large percentage of living forms (about 70%, mostly on the lands) including the dinosaurs.
Evidence for an impact catastrophe is best demonstrated by the K/T boundary (time horizon between the Cretaceous [K] and Tertiary [T]). The first important report of unusual features in deposits at the boundary was made by the father and son scientists Luiz and Walter Alvarez, based on studies they made of deposits at Gubbio, Italy and elsewhere:
The Alvarez's deduced the nature of these deposits, and their significance, from the presence of Iridium, an element related to Platinum in the Periodic Table. Iridium is rare on Earth, but is found in minute amounts in some basaltic rocks. It is calculated to be much more abundant in the Earth's iron core. Iridium is a notable constituent in meteorites, mainly those composed primarily of iron or with iron inclusions. The Alvarez pair proposed that the Iridium in the K-T boundary layer deposits was part of the fallout of debris injected into the high atmosphere by a huge impact. They, and others, began to hunt for Iridium in deposits considered to be at the K-T boundary as recognized in various parts of the world. Many localities with Ir-enriched K-T layers have been found, as shown in this map:
The resulting debris from the reputed impact event was 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 Period and the first sediments formed in the younger (overlying) Tertiary Period. The deposits contain Iridium, a metallic element related to Platinum present in many iron meteorites, in amounts far greater than can be accounted for by volcanic sources or other terrestrial rocks.
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 two examples of this now-famous layer, which also includes soot particles from the after-impact fires that consumed much of Earth's forests.
The immediate stratigraphy above and below the K-T time plane is quite distinctive. This cross-section for the Hell Creek locality in Montana is typical:
Below the K-T layer, sedimentary rocks are rich in fauna, both macroscopic (such as animals like the dinosaurs) and microscopic (such as plankton and other tiny marine life forms). The Tertiary rocks above the layer, for a few meters at least (representing several million years of deposition time), are missing most of these fauna, although some do carry over from the Cretaceous. This plot shows some of these characteristics:
The nature of this abrupt change in faunal speciation and distribution has proved complicated. Suffice to say that some paleontologist dispute the claim of an abrupt disappearance of many fauna just above the boundary. Consult this Wikipedia web site for more details.
Soon after the Alvarez announcement, they and others presented evidence that shock features occur within the clay layers right at the boundary. The deposits usually contain small glass spherules, some similar to the tektites that have been generally accepted as evidence of impact. Here is an example.
Some mineral grains found in the boundary layer bear evidence of intense shock (including quartz crystals with planar features; see page 18-4). Here are two example, the first in quartz and the second in a feldspar grain:
Although Iridium is sometimes spread into sediments by erosion of volcanic rocks that show somewhat larger concentrations, the very large amounts in the K-T layers, together with the shock features just shown (which are never produced from volcanic processes), have proved definitive in the association of the layers with fallout from an impact. The worldwide distribution of the Iridium implies a huge impact event. The question at the time was simply WHERE?
The killer impact site has now been found. 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 largest short-term natural events known in the geologic record (of nuclear-comparative 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. Here is its location:
Satellite images of the Yucatan fail to disclose any sign of surface expression of the crater, mainly because the surface is almost completely masked in thick jungle vegetation. However, it does
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.
When the effects of vegetation are compensated for, this huge structure has no evident surface expression, being covered by younger sedimentary rocks, but does appear subsurface as a strong gravity anomaly, with a definite circular pattern, as shown below.
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:
Evidence in the impact-related rocks (bedrock below the crater floor and ejecta within) was then discovered almost incidentally in core samples obtained through earlier exploration drilling for oil. The samples languished for years in the basement of the University of New Orleans' Geology Building, before someone re-examined them and deduced the origin of the anomalous materials beneath the post-impact sedimentary rocks. Intervals with these core samples, containing so-called volcanic rocks (now known to be shock-melted rock), showed distinct shock effects. These two stratigraphic sections, from two drill holes, present this interpretation.
Quenched melt (glass), consistent with an impact event, has been recovered from drill core into Chicxulub; it contains fragments showing other evidence of shock damage:
As interest in Chicxulub grew because of its apparent connection with the K-T boundary extinction, new evidence from its immediate vicinity and from deposits found at various distances in the U.S., Mexico, and Central America helped to confirm the impact hypothesis. This map shows some of the localities where deposits that could be tied to Chicxulub were found:
Ejecta deposits in Belize are definitely at the K-T boundary and show kinship with Chicxulub.
One of the consequences of the crater hypothesis is that the distribution of cenotes (Spanish for "sinkhole") in carbonate rocks could now be explained. This is evident in this map:
Some of the sinkholes were used by the Aztecs as ceremonial sites for killing rituals. This included several cenotes in the semi-circular ring that overlies the crater boundary, which had an effect on locating sinkholes outside the range where most were produced.
To summarize the Chicxulub event in terms of its almost certain relationship with the K-T boundary layers: The Chicxulub impact into shallow waters of the Gulf of Mexico generated huge waves (tsunamis) 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 (including SO2) 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 reduced number of 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. Mammals, inconspicuous before this event, were able to flourish in these restored systems and gradually gain ascendancy during the Cenozoic; this led the way for the eventual appearance of humans. For even more information on the now famous Chicxulub crater, go to this Wikipedia web site.
One could almost predict that once a strong candidate for the impact crater that killed the dinosaurs had been found, others would propose another crater that seems to fit the bill. The Shiva structure off the west coast of India in the Indian Ocean has been offered up as either an alternative to Chicxulub or in the view of some a second crater formed either at the same time or as a separate unrelated event very close in time to the Mexican crater and the K-T event. The structure is huge: some 650 by 400 km in dimension (this asymmetry suggest a glancing or low angle impact). It is still known only from geophysical evidence since it is under water and is topped by younger sedimentary material. But drill core (justified as exploration for oil) has returned brecciated rocks that date close to the K-T event. Its chief advocate is Prof. S. Chatterjee of Texas Tech University, who claims that rocks recovered from the site contain both shock features and high concentrations of Iridium. He notes the association of the event with the Deccan Trap basalt flows and suggests that outpourings of that lava were accelerated by the impact. You can learn more about Shiva crater at this Wikipedia web site.
However, many impact advocates have questioned the Chatterjee observations and some, including Christian Koeberl, declare the conclusions to be unproven and possibly erroneous. One problem currently complicating the postulate that Shiva is the main culprit in the K/T extinction is that age dates for it may be different from that for Chicxulub by as much as 30000 years. If this holds up, and Shiva is indeed the largest impact crater on Earth, this would mean that the two biggest such events took place almost (but not quite) simultaneously - a remarkable (but plausible) coincidence.
Now to other craters that could trigger extinction consequences. A report in November, 2003 has presented evidence from rock deposits 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 marine 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 - it set the stage for a new burst of now different 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 occupying the land also were largely compromised (but not completely wiped out or there would have been no dinosaurs or mammals in the Mesozoic). Among these was Dimetrodon, a reptile with a sail-like dorsal fin, who disappeared completely at the Permian's end.