http://rst.gsfc.nasa.gov/Sect18/Sect18_4.html
navigation image map
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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. In situ
features of craters include shatter cones and breccias. This page also
describes several very large craters whose after effects may have
altered conditions for life on Earth.
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Crater Morphology; Some Characteristic Impact Structures; Impacts as causes
of mass extinction of life
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.)
Simple (a) vs Complex (b) Craters.
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:
A simple lunar crater.
A complex lunar crater.
The Orientale basin on the Moon.
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:
DEM shaded relief map of the Flynn Creek impact structure 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.
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.
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:
Cross-section through the area just outside of the apparent crater at
Sedan.
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:
Strongly deformed limestone beds seen along a quarry face at the
Kentland, IN 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.)
Photomontage of the Ries structure in Bavaria.
And here is a DEM reconstruction of its generalized subsurface
structure:
DEM shaded relief map of the Ries Kessel impact structure in Bavaria.
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
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):
ASTER color image of the Ries Kessel structure (top) and a generalized
elevation map (bottom).
The geologic nature of the present, somewhat eroded Ries structure is
encapsulized in this cross-section
Cross-section through the Ries crater.
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:
Basal suevite overlying the local top of the Bunte Breccia.
Part of the ejecta blanket around the Ries impact crater; breccia
clasts can range from almost microscopic to as big as houses (seen
elsewhere).
Close-up of the Bunte Breccia.
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; photo taken by NMS.
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
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 New Quebec impact crater in Northern Quebec.
The Pinqualuit (New Quebec) crater.
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.
Aerial photograph of West Hawk Lake in western Ontario.
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:
Planar features in quartz and feldspar in West Hawk Lake breccia (rock
type was a gneiss)
Shock log for the main drill hole at West Hawk Lake.
Scale of increasing shock effects used to determine the shock log for
West Hawk Lake.
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.
Landsat image of the Manicouagan structure in southern Quebec, Canada.
A great deal of impact-produced melt is found along the lake at
Manicouagan, as shown here:
A cliff exposing a thick unit of shock melt at Manicouagan.
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.
18-7: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 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:
Aerial view of the Wabar crater; note the longitudinal sand dunes.
Ground view of the Wabar crater, with sandstone-like ejecta, often
coated with black glass.
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.
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.
Wabar not only has abundant examples of shock-lithified "sandstone",
but it also has numerous samples of shock-melted sand as black glass:
Black glass from around the Wabar crater.
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.
Shatter cones in limestone.
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.
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:
Shatter cone at a Slate Island outcrop
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
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:
One of the Morasko swarm of small meteor craters, in Poland.
One of the Morasko meteorites.
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:
The Nastapoca Arc.
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):
The Upheaval Dome.
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.
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. 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. 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. Many geologists now believe that
the biggest extinction of all ended the Paleozoic at the close of the
Permian. Some researchers attribute at least one or more of these
destructive occurrences to impact - although other mechanisms have
been proposed (see bottom of page).
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:
Luiz and Walter Alvarez at the Gubbio, Italy locality.
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:
Global distribution of Iridium-bearing deposits associated with the
K-T boundary.
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 Iridium spike within the K/T clay layer.
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 K-T boundary layer deposits at Raton, New Mexico.
Close-up of the K-T boundary deposits
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:
Stratigraphy of the K-T boundary at Hell Creek.
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:
Faunal extinctions at the K-T layer sequence.
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.
Glass spherules from K-T boundary layer.
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:
Planar features in a quartz grain found in the K/T boundary clay
layer.
Shocked feldspar from Chicxulub crater.
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:
Location map of the Chicxulub crater.
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
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.
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.
Gravity map of the Chicxulub structure.
The buried Chicxulub crater shows a suggestive circular depression
pattern in this 3-D gravity map in which different values are shown in
different colors.
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:
Shuttle radar image of part of the Chicxulub structure
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.
Stratigraphic sequence from two Chicxulub drill core sites.
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:
Chicxulub glass.
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:
Locations of K-T deposits in southern North America.
Ejecta deposits in Belize are definitely at the K-T boundary and show
kinship with Chicxulub.
Ejecta deposits in Belize.
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:
Distribution of cenotes in the Yucatan.
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 SO[2]) 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.
Reconstruction of the Shiva crater using geophysical data.
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.
Dimetrodon, pictured in a parched world; from the Economist.
In May 2004 announcement was made that the Permian 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. Most of the Bedout images shown on this page were
extracted from their online paper (no longer available). 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". (This outpouring of basaltic lava
supports an alternative hypothesis for the demise of so much life,
namely the accumulation of harmful gases and particulates in the
atmosphere may have created adverse conditions [such as acidic
seawater] that diminished life.) 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 (a plant), 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 this supercontinent but the land containing 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 fallback
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 cores 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 the Bedout core was examined in detail, the breccia clasts were
determined to be mostly made up of largely devitrified glass,
containing plagioclase, iron oxides, iron-titanium oxide, and
recrystallized chlorite. This is a typical petrographic microscope
view of a Bedout clast, which resembles impact melts 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 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.
Some Calcium-Magnesium Carbonate material in the core has 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 Ar^40/A^39
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). 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.
As so often happens in Science, just when a definitive answer seems to
have been found, something new is discovered that clouds the issue. A
competing event, in some ways superior because of size, has been
announced as a leading candidate of the Killer Crater. This - as yet
unnamed - lies under 1.5 km of ice in Wilkes Land in the Antarctic.
The buried structure may be as wide as 500 km (300 miles); indirect
evidence suggests it may be about the same age as Bedout (but drilling
to sample its rocks will be needed to confirm this). It shows up as a
gravity anomaly (mascon; positive mass caused by upward movement of
denser subcrustal rocks) and has a crude circular outline. Inspect
these three maps:
Gravity map showing a Bouguer anomaly (circled) in Wilkes Land.
Radar map indicating a possible surface expression under the ice in
the Antarctic that may be an indication of a buried impact structure.
More detailed gravity map of the Wilkes crater.
But as is 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 craters 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 geophysical
survey. It lies buried beneath both coastal and estuarine sedimentary
materials. It has been drilled; another drilling 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, Bedout and Wilkes Land are among a growing
number of impact structures that are buried and have been discovered
during geophysical surveys. Another - almost a type case - 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.
The above proposed impact events extend into geologic time, well
before humans entered the evolutionary picture. But the threat of
world-level catastrophes owing to impacts has now been accepted as
real and concerning. Has there ever been such an event? The answer
seems to be 'yes'.
Some 12,900 years before the present, there was a sudden, abrupt
disappearance from North America of at least 35 species of large
mammals including the Wooly Mammoth, the Saber-toothed Tiger, and the
Giant Sloth. Primitive Indians known as the Clovis people also seem to
have vanished as a culture. The climate too became colder (about an
average of 8 degrees Celsius) and glaciation, already covering most of
Canada, expanded. What could have caused this?
Although several explanations have been advanced, the best fit seems
to be a comet impact. The Clovis comet, as it has now been named, is
proposed as the culprit. There is abundant evidence for a major
explosion, either in the atmosphere or in the Canadian ice (no crater
resulting from impact has yet been found). A layer, named the Younger
Dryan, of black soot, about 4 cm. thick, is found widespread across
North America and even into Europe. Here is an outcrop in Arizona with
this layer:
The layer contains several hallmarks of impact, including iridium
anomalies, nanometer diamonds, and fullarenes. The iridium spikes are
not as pronounced as those associated with the K-T boundary /layers.
Iridium spikes associated with the black (soot) mat at six localities
The spherules are distinctive, as shown here:
The diamonds are typical from 10 to 100 nanometers in width. Their
crystal structure is hexagonal, not the isometric diamonds found in
intrusive pipes on Earth; hexagonal diamonds are known only from
settings that involve shock pressures, such as impacts.
Nanodiamonds.
These diamonds have unique signatures when examined by transmission
electron microscopy:
Signatures of nanodiamonds.
So, where does the scientific community stand on the general topic of
mass extinctions, both from the standpoint of impacts and the other
possible causes of disappearance of entire classes of animals and
plants. Comments above indicate that there is a difference of opinions
about the actual reason why the dinosaurs did not survive past the K-T
boundary. Whether this was due to one short-lived event or was the
result of a set of circumstances spread over a gradual period of time
is still being debated. Multiple explanations have been proposed. This
chart is the plot of a survey among involved scientists as to the
primary cause as well as possible secondary or multiple causes:
A pie chart showing various causes or factors responsible for the K-T
extinction.
The impact hypothesis remains the currently favored cause of the
extinction that prevailed at and after the K-T boundary layer was
deposited. But, the mechanisms responsible for the extinction are
still being debated. Some scientists claim they have found
contradictory evidence pertaining to proposed causes. For instance, if
global fires were instigated by the fallout from the impact, the
atmosphere would have held huge quantities of soot that would lead to
deposits of charcoal in the boundary layer. In fact, only traces of
charcoal are present. Therefore, such extensive fires seem unlikely.
Again, atmospheric chemistry would have been affected by the impact,
with some models leading to acid rain, which would have severely
stressed vegetation. But evidence for this is inconclusive. Various
models result in the possibility of overall cooling and/or warming (in
succession) of the atmosphere. This is to be expected from the vast
amounts of dust particulates that an impact would eject into the
atmosphere. So, once more the evidence is frought with uncertainty for
either modification of temperatures. But, the presence of fern
remnants just above the boundary layer is consistent with the
postulate that vegetation was severely diminished by whatever was
happening to the environment after the K-T event. Needless to say,
much more research must be done to resolve these anomalies.
Another "bone of contention" is whether a Chicxulub-sized crater
tosses enough material into the atmosphere to have had a global
influence. To go global would require efficient transportation and
distribution by the atmospheric circulation system operating at the
time. In today's world, the present circulation would likely not carry
particulates across the globe; transportation into the southern
hemisphere from a northern hemisphere event would be difficult. In the
writer's opinion, this is a major stumbling block to the idea that the
Chicxulub impact (for which the evidence is very solid) was the sole
cause of the general extinction that did happen. It seems more likely
that there were other contributing factors. One, often cited, is the
likelihood of extensive atmospheric contamination by chemicals
(including sulphur compounds) introduced from the vast outpourings of
lava that formed the Deccan Traps (basaltic flows) of India. These
lava deposits extend over much of the Indian subcontinent and the sea
floor to the west, as depicted in this diagram:
The Deccan Traps of India.
Another problem with the Chicxulub model may have merged. Examine this
diagram.
Plots of four data sets.
The bottom chart is our "talking point". A group of scientists who are
studying the Chicxulub material obtained through drilling have
acquired radioactive dates from the volcanic rocks that indicate the
rocks are 300000 years older than the K-T boundary layer age. If this
is valid (but the error range for the dating might overlap the two
ages), then the implication is that there were two separate large
impacts closely spaced in geologic time - not impossible, but not of
high likelihood.
There is another, perhaps more imposing, objection to impact as the
main reason for dinosaur (and other animals) extinction. This is the
mounting evidence that the dinosaurs were faced with adverse
conditions over millions of years prior to the K-T event which were
making their environment increasingly unstable. Evidence in upper
Cretaceous rocks show a general trend of aridity (desertlike
conditions) across much of the world. If this indeed did happen, it
could account for the decrease in the number of dinosaur species
observed in Cretaceous rocks prior to the K-T event. So, the dinosaurs
were being stressed and heading for extinction anyway, according to
some views. The impact(s) at the close of the Cretaceous would have
some (probably serious) influence on the dinosaur demise - they would
have administered the "coup de grace", but were not the sole cause.
Near the top of this page I alluded to other mass extinctions
deciphered from the geologic record of the last 500 million years.
These are shown in the graph below, and the accompanying explanation
beneath it:
Plot of the major mass extinctions since the beginning of the
Paleozoic.
* 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. This is shown in the
next two illustration, made by Prof. Norman MacLeod, with the first
showing in bar graph format extinctions averaged for discrete time
intervals. In the second, this graph is shown again, and on the right
are bars indicating 1) impacts that have been dated; 2) periods of
volcanism, and 3) rises in sea level. There is evidently some positive
correlation (but not necessarily causative) between extinction and the
three factors on the right.
The mass extinctions and other extinctions since the start of the
Paleozoic
Plot of the variations in species extinction through the
post-Precambrian.
Like the K-T extinction, the cause(s) of the other extinctions is
still being debated. Large impacts certainly can be detrimental to the
animal and plant species of the time. But volcanism, climate change,
and other factors must be considered. However, the mechanism that is
most likely to force the various environments at any given time into
"catastrophe mode" is worldwide atmospheric contamination that brings
about temperature and precipitaton modifications so drastic that a
multitude of species cannot adapt. Impact and subcontinental scale
volcanism are the two causitive leaders.
A good treatment of the K-T event, with links to web sites that
consider mass extinctions is presented on the reliable Wikipedia K-T
Extinction and Mass Extinction Internet connections.
Now that you are familiarized with the general appearance of impact
craters and their possible role in influencing life on Earth, we next
will consider the best evidence for proof that a circular structure
has been caused by a cratering event - shock metamorphism.
navigation image map
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Primary Author: Nicholas M. Short, Sr.