40 SIGNIFICANT CRATER IMPACTS
 |
| Arizona's Barringer Meteor Crater |
Editor’s note:
We’ve all scene meteor craters on the Moon and Mars. The craters are everywhere. Scientists on earth, however, say they’ve
only been able to discover 160 craters—to date.
Why does the moon have so many craters and the earth so few?
If
you don’t know the answer to this trick question then read on.
If you go to
the following address you will come across a 40-slide presentation of the
crater activity on Earth:
GUEST
BLOG--by Christian Koeberl and Virgil L. Sharpton writing for the Lunar and
Planetary Institute--Impact cratering research has gained
attention throughout the world following the suggestion that a large impact
event caused the extinction of about 50% of all living species, including the
dinosaurs, approximately 65 million years ago.
The evidence that a large asteroid or comet struck the Earth at
that time came from detailed studies of the thin clay layer that globally marks
the stratigraphic boundary between the Cretaceous and Tertiary (K-T) geological
periods. This layer is enriched in the siderophile elements (such as iridium),
indicating that the clay represents a mixture of normal crustal rocks, which
typically have low siderophile-element abundances, and a small percentage of
extraterrestrial material.
The worldwide integrated volume of the extraterrestrial material
in the K-T boundary layer is equivalent to an asteroid approximately 10 kilometers
in diameter — large enough to have produced a 200-kilometer-diameter crater. In
the early 1990s, the subsurface Chicxulub structure in Mexico (slide #37) was
confirmed as the long-sought Cretaceous-Tertiary boundary impact crater. An
environmental crisis, triggered by the gigantic collision, contributed to the
extinctions.
Based on apparent correspondences between periodicities observed
in the marine extinction record and in the terrestrial impact record, some
scientists have suggested that large meteorite impacts might be the metronome
that sets the cadence of biological evolution on Earth — an unproved but
intriguing hypothesis. Nevertheless, the study of the K-T extinction and its
association with one of the largest impact structures known on Earth led to
renewed and widespread interest in impacts.
_____________________
Tomorrow
in PillartoPost.org
CHICXULUB:
THE MOTHER
OF
ALL CRATERS
_____________________
Impact craters are formed when a large meteoroid (asteroid or
comet) crashes into a larger planetary body that has a solid surface. All the
bodies in our solar system have been heavily bombarded by meteoroids throughout
their history. The landscapes of the Moon, Mars, and Mercury have conspicuously
preserved this bombardment record because the surfaces of these relatively
small planetary bodies have remained unchanged over hundreds of millions of
years.
Compared with the Moon, the Earth has been even more heavily
bombarded over the course of its history due to its stronger gravitational
attraction. However, impact craters are not immediately obvious on the surface
of Earth because our planet is geologically active; the surface is in a
constant state of change from erosion, infilling, volcanism, and tectonic
activity.
These processes have led to the rapid removal or burial of
Earth's impact structures. Thus, only about 160 terrestrial impact craters have
been recognized to date. The majority of them are located within the
geologically stable cratons of North America, Europe, southern Africa, and
Australia; this is also where most of the crater searches have taken place.
Spacecraft orbital imagery and geophysical surveys for resource exploration
have helped to identify structures in more remote locations.
 |
| Daniel Moreau Barringer, early U.S. explorer of Meteor Crater |
Meteor Crater (also known as Barringer Crater), Arizona, with a
diameter of approximately 1.2 kilometers, was the first terrestrial impact
crater (slides #10 and #11) to be recognized as such. Its impact origin was
first suspected late in the nineteenth century, when abundant iron meteorite
fragments were discovered in the immediate vicinity of the crater.
This finding led the mining engineer Daniel Moreau Barringer to
embark, between about 1905 and 1928, on a drilling project to find a suspected
large iron meteorite body underneath the crater floor.
At this time, however, researchers did not yet have a clear
understanding of the immense energy that is liberated when an extraterrestrial
body hits the surface of the Earth with cosmic velocity. It was only in the
1920s that the first quantitative studies revealed the explosive nature of
meteorite impact. Under impact conditions, tremendous amounts of energy are
released instantaneously, completely destroying the cosmic projectile and
generating a crater that is many times larger than the original meteoroid.
In the case of Meteor Crater, an iron meteorite body only about
30–50 meters in diameter was sufficient to create a crater 1.2 kilometers in
diameter.
The key to understanding the explosive nature of an impact event
is the high velocity with which a meteoroid hits the Earth. These velocities
range between 11.2 kilometers per second (the escape velocity of the Earth-Moon
system) and 72 kilometers per second (the orbital velocity of the Earth plus
the escape velocity of the solar system at the distance of the Earth from the
Sun). Because the kinetic energy liberated on impact of an object is
proportional to the square of its velocity, these high-speed meteoroids can be,
gram for gram, more than 100 times as explosive as TNT!
After the first studies on Meteor Crater, several other
relatively small craters were also found to contain impactor fragments, and for
many years these remnants were the only accepted evidence for impact origin.
But because the projectile does not survive intact in large impact events,
scientists have developed more sophisticated means of detecting the signatures
of meteorite impact.
In some cases, nonterrestrial relative abundance of siderophile
elements can be detected in the impact melt rocks within large craters (or in
impact ejecta, as at the K-T boundary sediments mentioned above); this provides
a chemical signature of the meteorite impactor.
The most commonly used chemical elements for such studies are
the platinum group elements (e.g., iridium, osmium, and platinum). This is
based on the fact that almost all meteorites have abundances of these elements
that are higher by factors of 20,000 to 100,000 than those of average
terrestrial crustal rocks. The addition of even a small meteoritic component
(less than 1%) results in distinctly elevated platinum group element contents in
the impact breccias or melt rocks.
Since the 1960s, numerous studies have documented another
physical marker of meteorite impact: shock metamorphism. This refers to
metastable or irreversible effects produced in various target rocks and
minerals as the strong shock wave passes through them. As hypervelocity impact
is the only naturally occurring process capable of generating strong shocks in
crustal rocks, certain shock-metamorphic effects are unambiguous signatures of
meteorite impact.
Diagnostic shock effects include shatter cones, multiple sets of
microscopic planar deformation features (PDFs) in quartz, feldspar, and most
other rock-forming and accessory minerals, diaplectic glass, and high-pressure
mineral phases, such as stishovite (a high-pressure form of quartz). Even
diamonds are formed by high-pressure conversion of graphite in target rocks.
Researchers have recognized that the presence of
shock-metamorphic effects is a much better indicator of the impact origin of a
geologic structure than the presence of meteorite fragments (which are rapidly
destroyed by erosion anyway). Experimental studies over the past three to four
decades have provided a good database that shows which types of shock features
form at which pressures. It was also recognized that the effects resulting from
shock (nonequilibrium processes) are different from those resulting from static
high pressures (an equilibrium process).
Today, terrestrial impact structures are confirmed based on the
presence of some or all of these shock effects.
Impact craters are divided into two main groups, based on their
morphology: simple craters and complex craters. Simple craters are relatively
small, with depth-to-diameter ratios of about 1:5 to 1:7 and a smooth bowl
shape (slide #1, top). In larger craters, however, gravity causes the initially
steep crater walls to collapse downward and inward, forming a complex structure
with a central peak or peak ring and a shallower depth compared to diameter
(1:10 to 1:20) (slide #1, bottom). The diameter at which craters become complex
depends on the surface gravity of the planet: The greater the gravity, the
smaller the diameter that will produce a complex structure. On Earth, this
transition diameter is 2–4 kilometers (depending on target rock properties); on
the Moon, at one-sixth Earth's gravity, the transition diameter is 15–20
kilometers.
The cratering process is traditionally divided into three
stages: The contact and compression stage begins when the impactor hits the
ground and initiates a shock wave that travels into the target and into the
impactor, compressing the target and generating shock metamorphic effects. This
is followed by the excavation stage, wherein the release of the shock
compression leads to mass flow that opens up the crater and ends with a
relatively deep transient cavity.
Finally, the modification stage involves collapse of the steep
walls of the transient crater and infilling of the crater by fall-back debris.
The complete crater-forming sequence takes less time than it would to free fall
from a height equivalent to the final crater diameter. In the case of Meteor
Crater, the compression and excavation phases of the crater formation were over
in a few seconds. Thus, impact cratering has the distinction of being the
geologic process that releases the greatest amount of energy in the shortest
amount of time.
The final crater expression depends on the magnitude of the
event. The central peak or peak ring of the complex crater is formed as the
initial (transient) deep crater floor rebounds from the compressional shock of
impact. Slumping of the rim further modifies and enlarges the final crater.
Complex structures in crystalline rock targets may also contain coherent sheets
of impact melt overlying the shocked and fragmented rocks of the crater floor.
On the geologically inactive lunar surface, this complex crater
form will be preserved until subsequent impact events alter it. On Earth,
weathering and erosion of the target rocks quickly alter the surface expression
of the structure; despite the crater's initial morphology, crater rims and
ejecta blankets are quickly eroded and concentric ring structures can be
produced or enhanced as weaker rocks of the crater floor are removed. More
resistant rocks of the melt sheet may be left as plateaus overlooking the
surrounding structure.
Large terrestrial impacts are of greater importance for the
geologic history of the Earth than the number and size of preserved structures
might suggest. Currently, about 160 structures of impact origin have been
confirmed on the Earth's surface (slide #2). Precise ages are known for only
about one-third of these structures. Crater ages can be determined by a variety
of methods.
The more precise ones involve radiometric dating of impact melt
rocks or impact glasses or biostratigraphic dating of related impact ejecta
within a well-defined stratigraphic sequence. The paucity of age data reflects
not only the lack of detailed studies, but, in many cases, the lack of datable
material, especially for deeply eroded or subsurface structures.
This collection of slides in this report presents orbital and
aerial photographic views of a selection of proven or suspected terrestrial
impact structures that represent the variety in appearance of impact structures
at different erosional stages on Earth. Several examples of impact structures
on Earth's planetary neighbors are included to show the fundamental role impact
plays in shaping planetary surfaces. These relatively well-preserved
extraterrestrial craters provide an important reference for understanding the
more eroded impact features on Earth.
The terrestrial structures we have chosen represent a compromise
between those with the best surface expression and those that represent a
diversity of age, size, and appearance as the craters are reworked by geologic
processes. The freshest craters are presented first, followed by those that
exhibit more ambiguity and complexity. We have included two examples of
features that have the appearance of impact structures, but have not yet been
shown to contain any of the diagnostic chemicals or physical markers of impact
events.
However, we would like to caution against just using images or
remote sensing to identify impact craters on Earth without corroborating
petrographic and geochemical studies on crater rocks. Only such studies can
provide confirming evidence that a geological structure is of impact origin.
NOTE:
In the orbital photographs of this 40-slide presentation of terrestrial craters,
north is up, unless otherwise noted. The Lunar and Planetary Institute is
operated by the Universities Space Research Association under contract number
NASW-4574 with
the
National Aeronautics and Space Administration.
SPACE
CADETS / METEOR CHARGED WITH MURDERING MILLIONS OF DINOSAURS
GUILTY
AS CHARGED / CHICXULUB METEOR CAUSES EXTINCTION EVENT
GUEST
BLOG—By Wikipedia--The Chicxulub crater is an impact crater buried
underneath the Yucatán Peninsula in Mexico. Its center is located near the town
of Chicxulub, after which the crater is named. The age of the Chicxulub
asteroid impact and the Cretaceous–Paleogene geological boundary (K–Pg boundary)
coincide precisely. The crater is more than 110 miles in diameter and 12 miles
in depth, making the feature one of the largest confirmed impact structures on
Earth; the impacting bolide (a large meteor that explodes in the atmosphere) that
formed the crater was at least six miles in diameter.
Antonio Camargo and Glen Penfield, geophysicists who had been
looking for petroleum in the Yucatán during the late 1970s, discovered the
crater. Penfield was initially unable to obtain evidence that the geological
feature was a crater, and gave up his search. Through contact with Alan
Hildebrand, Penfield obtained samples that suggested it was an impact feature.
Evidence for the impact origin of the crater includes shocked quartz, a gravity
anomaly, and tektites in surrounding areas.
The age of the rocks marked by the impact shows that this impact
structure dates from roughly 66 million years ago, the end of the Cretaceous
period, and the start of the Paleogene period. It coincides with the K-Pg
boundary, the geological boundary between the Cretaceous and Paleogene.
The impact associated with the crater is thus implicated in the
Cretaceous–Paleogene extinction event, including the worldwide extinction of
non-avian dinosaurs. This conclusion has been the source of controversy. In
March 2010, 41 experts from many countries reviewed the available evidence: 20
years' worth of data spanning a variety of fields. They concluded that the
impact at Chicxulub triggered the mass extinctions at the K–Pg boundary.
OIL
EXPLORER’S BIG DISCOVERY.
In 1978, geophysicists Antonio Camargo and Glen Penfield were
working for the Mexican state-owned oil company Petróleos Mexicanos, or Pemex,
as part of an airborne magnetic survey of the Gulf of Mexico north of the
Yucatán peninsula. Penfield's job was to use geophysical data to scout possible
locations for oil drilling. In the data, Penfield found a huge underwater arc
with "extraordinary symmetry" in a ring 40 miles across.
He then obtained a gravity map of the Yucatán made in the 1960s.
A decade earlier, the same map suggested an impact feature to contractor Robert
Baltosser, but he was forbidden to publicize his conclusion by Pemex corporate
policy of the time. Penfield found another arc on the peninsula itself, the
ends of which pointed northward. Comparing the two maps, he found the separate
arcs formed a circle, 111 miles wide, centered near the Yucatán village
Chicxulub; he felt certain the shape had been created by a cataclysmic event in
geologic history.
Pemex disallowed release of specific data but let Penfield and
company official Antonio Camargo presented their results at the 1981 Society of
Exploration Geophysicists conference. That year's conference was underattended
and their report attracted scant attention. Coincidentally, many experts in
impact craters and the K–Pg boundary were attending a separate conference on
Earth impacts. Although Penfield had plenty of geophysical data sets, he had no
rock cores or other physical evidence of an impact.
He knew Pemex had drilled exploratory wells in the region. In
1951, one bored into what was described as a thick layer of andesite about 4,200
ft. down. This layer could have resulted from the intense heat and pressure of
an Earth impact, but at the time of the borings it was dismissed as a lava dome
— a feature uncharacteristic of the region's geology. Penfield tried to secure
site samples, but was told such samples had been lost or destroyed. When attempts
at returning to the drill sites and looking for rocks proved fruitless,
Penfield abandoned his search, published his findings and returned to his Pemex
work.
At the same time, scientist Luis Walter Alvarez put forth his
hypothesis that a large extraterrestrial body had struck Earth and, unaware of
Penfield's discovery, in 1981 University of Arizona graduate student Alan R.
Hildebrand and faculty adviser William V. Boynton published a draft
Earth-impact theory and sought a candidate crater. Their evidence included
greenish-brown clay with surplus iridium containing shocked quartz grains and
small weathered glass beads that looked to be tektites.
Thick, jumbled deposits of coarse rock fragments were also
present, thought to have been scoured from one place and deposited elsewhere by
a kilometres-high tsunami resulting from an Earth impact. Such deposits occur
in many locations but seem concentrated in the Caribbean basin at the K–Pg
boundary.
So when Haitian professor Florentine Morás discovered what he
thought to be evidence of an ancient volcano on Haiti, Hildebrand suggested it
could be a telltale feature of a nearby impact. Tests on samples retrieved from
the K–Pg boundary revealed more tektite glass, formed only in the heat of
asteroid impacts and high-yield nuclear detonations.
In 1990, Houston Chronicle
reporter Carlos Byars told Hildebrand of Penfield's earlier discovery of a
possible impact crater. Hildebrand contacted Penfield in April 1990 and the
pair soon secured two drill samples from the Pemex wells, stored in New
Orleans. Hildebrand's team tested the samples, which clearly showed
shock-metamorphic materials.
A team of California researchers including Kevin Pope, Adriana
Ocampo, and Charles Duller, surveying regional satellite images in 1996, found
a sinkhole (cenote) ring centered on Chicxulub that matched the one Penfield
saw earlier; the sinkholes were thought to be caused by subsidence of the
impact crater wall. More recent evidence suggests the actual crater is 190
miles wide, and the previous ring is in fact an inner wall of it.
The Chicxulub impactor had an estimated diameter of 10 km (6.2
mi) and delivered an estimated energy equivalent of 100 teratons of TNT
(4.2×1023 J).[21] By contrast, the most powerful man-made explosive device ever
detonated, the Tsar Bomba, had a yield of only 50 megatons of TNT (2.1×1017
J),[22] making the Chicxulub impact 2 million times more powerful. Even the
most energetic known volcanic eruption, which released an estimated energy
equivalent of approximately 240 gigatons of TNT (1.0×1021 J) and created the La
Garita Caldera,[23] delivered only 0.24% of the energy of the Chicxulub impact.
IMPACT
SCENARIO.
The impact would have caused some of the largest megatsunamis in
Earth's history. A cloud of super-heated dust, ash and steam would have spread
from the crater as the impactor (meteor) burrowed underground in less than a
second. Excavated material along with pieces of the impactor, ejected out of
the atmosphere by the blast, would have been heated to incandescence upon
re-entry, boiling the Earth's surface and possibly igniting wildfires;
meanwhile, colossal shock waves would have triggered global earthquakes and volcanic
eruptions.
The emission of dust and particles could have covered the entire
surface of the Earth for several years, possibly a decade, creating a harsh
environment for living things. The shock production of carbon dioxide caused by
the destruction of carbonate rocks would have led to a sudden greenhouse effect.
Over a longer period, sunlight would have been blocked from reaching the
surface of the Earth by the dust particles in the atmosphere, cooling the
surface dramatically. Photosynthesis by plants would also have been interrupted,
affecting the entire food chain. A model of the event developed by Lomax et al.
(2001) suggests that net primary productivity (NPP) rates may have increased to
higher than pre-impact levels over the long term because of the high carbon
dioxide concentrations.
In February 2008, a team of researchers led by Sean Gulick at
the University of Texas at Austin's Jackson School of Geosciences used seismic
images of the crater to determine that the impactor landed in deeper water than
was previously assumed. They argued that this would have resulted in increased
sulfate aerosols in the atmosphere. According to the press release, that
"could have made the impact deadlier in two ways: by altering climate
(sulfate aerosols in the upper atmosphere can have a cooling effect) and by
generating acid rain (water vapor can help to flush the lower atmosphere of
sulfate aerosols, causing acid rain)."
In their 1991 paper, Hildebrand, Penfield, and company described
the geology and composition of the impact feature. The rocks above the impact
feature are layers of marl and limestone reaching to a depth of almost 3,300 ft.
These rocks date back as far as the Paleocene. Below these layers lie more than
1,600 ft of andesite glass and breccia. These andesitic igneous rocks were only
found within the supposed impact feature, as is shocked quartz. The K–Pg
boundary inside the feature is depressed to 2,000 to 3,600 ft. compared with
the normal depth of about 1,600 ft. measured 3.1 miles away from the impact
feature.
Along the edge of the crater are clusters of cenotes or
sinkholes, which suggest that there was a water basin inside the feature during
the Neogene period, after the impact. The groundwater of such a basin would
have dissolved the limestone and created the caves and cenotes beneath the
surface. The paper also noted that the crater seemed to be a good candidate
source for the tektites reported at Haiti.
OTHER
SUSPECTS.
In September 2007, a report published in Nature proposed an origin for the asteroid that created Chicxulub
Crater. The authors, William F. Bottke, David Vokrouhlický, and David Nesvorný,
argued that a collision in the asteroid belt 160 million years ago resulted in
the Baptistina family of asteroids, the largest surviving member of which is
298 Baptistina. They proposed that the "Chicxulub asteroid" was also
a member of this group. The connection between Chicxulub and Baptistina is
supported by the large amount of carbonaceous material present in microscopic
fragments of the impactor, suggesting the impactor was a member of a rare class
of asteroids called carbonaceous chondrites, like Baptistina.
According to Bottke, the Chicxulub impactor was a fragment of a
much larger parent body about 110 miles across, with the other impacting body
being around 40 miles in diameter.
In 2011, new data from the Wide-field Infrared Survey Explorer
revised the date of the collision which created the Baptistina family to about
80 million years ago. This makes an asteroid from this family highly improbable
to be the asteroid that created the Chicxulub Crater, as typically the process
of resonance and collision of an asteroid takes many tens of millions of years.
In 2010, another hypothesis was offered which implicated the
newly discovered asteroid P/2010 A2, a member of the Flora family of asteroids,
as a possible remnant cohort of the K/Pg impactor.
The Chicxulub Crater lends support to the theory postulated by
the late physicist Luis Alvarez and his son, geologist Walter Alvarez, that the
extinction of numerous animal and plant groups, including dinosaurs, may have
resulted from a bolide impact (the Cretaceous–Paleogene extinction event). Luis
and Walter Alvarez, at the time both faculty members at the University of
California, Berkeley, postulated that this enormous extinction event, which was
roughly contemporaneous with the postulated date of formation for the Chicxulub
crater, could have been caused by just such a large impact.
This theory is now widely accepted by the scientific community.
Some critics, including paleontologist Robert Bakker, argue that such an impact
would have killed frogs as well as dinosaurs, yet the frogs survived the
extinction event.
Gerta Keller of Princeton University argues that recent core
samples from Chicxulub prove the impact occurred about 300,000 years before the
mass extinction, and thus could not have been the causal factor.
The main evidence of such an impact, besides the crater itself,
is contained in a thin layer of clay present in the K–Pg boundary across the
world. In the late 1970s, the Alvarezes and colleagues reported that it
contained an abnormally high concentration of iridium.
Iridium levels in this layer reached 6 parts per billion by
weight or more compared to 0.4 for the Earth's crust as a whole; in comparison,
meteorites can contain around 470 parts per billion of this element. It was
hypothesized that the iridium was spread into the atmosphere when the impactor
was vaporized and settled across the Earth's surface amongst other material
thrown up by the impact, producing the layer of iridium-enriched clay.
In recent years, several other craters of around the same age as
Chicxulub have been discovered, all between latitudes 20°N and 70°N. Examples
include the disputed Silverpit crater in the North Sea and the Boltysh crater
in Ukraine. Both are much smaller than Chicxulub, but are likely to have been
caused by objects many tens of metres across striking the Earth. This has led
to the hypothesis that the Chicxulub impact may have been only one of several
impacts that happened nearly at the same time. Another possible crater thought
to have been formed at the same time is the larger Shiva crater, though the
structure's status as a crater is contested.
The collision of Comet Shoemaker–Levy 9 with Jupiter in 1994
demonstrated that gravitational interactions can fragment a comet, giving rise
to many impacts over a period of a few days if the comet should collide with a
planet. Comets undergo gravitational interactions with the gas giants, and
similar disruptions and collisions are very likely to have occurred in the
past. This scenario may have occurred on Earth at the end of the Cretaceous, though
Shiva and the Chicxulub craters might have been formed 300,000 years apart.
In late 2006, Ken MacLeod, a geology professor from the
University of Missouri, completed an analysis of sediment below the ocean's
surface, bolstering the single-impact theory. MacLeod conducted his analysis
approximately 2,800 miles from the Chicxulub Crater to control for possible
changes in soil composition at the impact site, while still close enough to be
affected by the impact. The analysis revealed there was only one layer of
impact debris in the sediment, which indicated there was only one impact. Multiple-impact
proponents such as Gerta Keller regard the results as "rather
hyper-inflated" and do not agree with the conclusion of MacLeod's
analysis, arguing that there might only be gaps of hours to days between
impacts in a multiple impact scenario (cf. Shoemaker-Levy 9) which would not
leave a detectable gap in deposits.
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