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Catastrophes and Prehistory

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« Reply #15 on: March 18, 2007, 09:18:11 pm »

the Gulf of Mexico
owes its doubly arced shape to the K-T impact shockwave. ...



... there appears to be a gravitational anomaly (party dress pink) arc ... on the floor of the Atlantic Ocean off Florida ... which centers on the impact crater, as do several arcs in the Gulf of México. In fact, features such as the Alacrán Reef and Florida appear to be parts of the extended structure of the complex crater. The Blake Nose drill sites for cores of the K-T boundary are on the pink arc on the Atlantic side of Florida. ... This

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« Reply #16 on: March 18, 2007, 09:19:44 pm »



is a hi res rendering of the gravity anomaly satellite image (Scripps Institute of Oceanography) that I mapped onto a sphere (MetaCreations's Bryce 4). Some concentric arcs centered on the Chicxulub crater are clear in the bright lavender tones of the image. At high resolution on the images downloaded from the Scripps Institution of Oceanography Geodesy site, there is radial and concentric cracking of the Pacific Plate centered on the impact site. ...".
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« Reply #17 on: March 18, 2007, 09:29:55 pm »

Duncan Steel hypothesizes that the "... giant impact in Mexico apparently induced seismic waves which were focused on western India, causing fracturing which then led to the widespread Deccan eruptions. ...". Since India was then south of its present position, the Deccan basalt traps of India were then roughly antipodal to the Chicxlub Yucatan crater.

According to an islandone.org web article released by Don Savage, and Diane Ainsworth of JPL, dated 28 December 1994:

"... it was the sulfur-rich atmosphere created in the aftermath of an immense asteroid collision with Earth 65 million years ago that brought about a global freeze and the demise of the dinosaurs. The impact of this large asteroid hit a geologically unique, sulfur-rich region of the Yucatan Peninsula in Mexico ... the impact kicked up billions of tons of sulfur and other materials and was between 10,000 to 50,000 times more powerful than the comet Shoemaker-Levy 9 impact on Jupiter last July. ... this asteroid was between 10 to 20 kilometers (6 to 12 miles) in diameter and its collision on Earth brought about total darkness around the world for about half a year ... But more importantly, persistent clouds generated by the impact on this geologically distinct region of sulfur-rich materials caused temperatures to plunge globally to near freezing. ... These environmental changes lasted for a decade and subjected organisms all over the world to long-term stresses to which they could not adapt in such a brief time span ... Half of the species on Earth became extinct as a result. ...
... it was the specific geological location of the impact in a region that is rich in sulfur materials that created catastrophic climate changes and led to the downfall of the dinosaurs. If this asteroid had struck almost any other place on Earth, it wouldn't have generated the tremendous amount of sulfur ... On impact, the asteroid hurled some 35 billion to 770 billion tons of sulfur high into the atmosphere, along with other materials.

The NASA team ... recently discovered rocks in Belize -- some the size of a small car -- that were blown out of the crater and landed south of the Chicxulub site. The boulder deposit in Belize also contained fragments of glass ... known as "tektites," ... The tektites have been found in other regions near the crater, such as Haiti, Mexico, Texas and Alabama, but never in association with large boulders. Another important find at the Belize rock quarry was limestone with fossils dating to the early part of the Cretaceous. ... Early Cretaceous fossils have been found deep below the surface near the crater during drilling by the Mexican Petroleum Company. We think the limestone found in Belize was excavated by the impact, which probably blew a hole more than 15 kilometers (nine miles) deep in the Yucatan Peninsula. ...

... The researchers used sophisticated atmospheric models of the sulfur-rich atmosphere of Venus to model their impact scenario. ... Initially, thick sulfur clouds, combined with soot and dust generated by this impact, would have spread worldwide and blocked out the Sun ... Night-like conditions probably existed all over Earth for at least six months essentially bringing photosynthesis to a halt.

Unlike the aftermath of typical impacts, the skies remained murky for at least a decade, due to chemically generated clouds of sulfuric acid high in the stratosphere. ... The reflection of sunlight back into space from these high-altitude clouds caused surface temperatures to drop to nearly freezing for many years all over the planet. ... These atmospheric conditions occur in Venus' perpetually cloudy atmosphere ... where ultraviolet sunlight and water in the high atmosphere can convert sulfur dioxide into sulfuric acid clouds. Sulfuric acid clouds like those that cover Venus may have continued to blanket the Earth for more than a decade after the initial impact of the asteroid, causing a secondary and more long-lasting effect which killed much of life on Earth. ...".

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« Reply #18 on: March 18, 2007, 09:30:33 pm »

According to a Fossil Cephalopods FAQ by Neale Monks:

"... Cephalopods ... evolved from primitive molluscs during the Late Cambrian, approximately 500 million years ago. Unlike the other molluscan groups, cephalopod evolution has concentrated not on a energetically connservative, defensive lifestyle but on mobility, dynamism, and intelligence. ...
... Cephalopods today are divided into the nautiluses, which reproduces many times, and the coleoids which reproduce only once (usually) ... The basic fact is that no ammonites are known beyond the end of the Cretaceous, while the other cephalopod groups, the coleoids and nautiloids, survive to the present day. This has been tied with other extinctions which occurred at the end of the Cretaceous (known as the Cretaceous-Tertiary, or K/T, boundary). ... How the squids and octopuses, which lived in the Mesozoic seas along with the ammonites, belemnites and nautiloids, survived is unclear. ...".

 



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A Comet-Earth collision about 250 million years ago caused the Permo-Triassic Extinction.
 



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A Mars-sized body colliding with Earth formed the Moon about 4,400 million years ago, perhaps leaving the Pacific Ocean as an impact crater and possibly forming the initial continental crust of the Earth.
 

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http://www.valdostamuseum.org/hamsmith/EarthCollisions.html

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« Reply #19 on: March 18, 2007, 09:33:17 pm »

Jul 26, 2004


  Shield memorizing impact craters: Labrador Peninsula, Canada



Fig. 1 Northeast Canada
(*Click boxed areas A-C to see a larger picture.)

Figure 1 depicts Labrador Peninsula in Northeast Canada in early spring of last year. The nothern area of around 50 degrees north was still covered with snow and ice . This image covers Hudson Bay in the upper left, a part of the Great Lakes in the lower left, the Gulf of Saint Laurence, Nova Scotia Peninsula and Prince Edward Island (made famous in the novel and the movie "Anne of Green Gables") in the lower right, and Newfoundland Island and the Atlantic Ocean on the right.
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« Reply #20 on: March 18, 2007, 09:35:50 pm »

Labrador ****ula is a part of the Canadian Shield (*1), a plateau of granite made 600 million years ago (around the end of Precambrian Era), and is characterized geographically by many lakes formed by erosion from wind, rain and glaciers over a long time. Many gigantic impact craters also remain in this area (*2), and you can find the following craters in this GLI image. (Larger pictures will appear when you click boxed areas A to C.) Some craters became lakes and some were covered with vegetation, depending on when they were created, its geology, subsequent weathering and erosion, etc. The following table shows the craters in the order of created era.

   Crater Diameter Created Era Remarks
A.  Charlevoix  54 km 342 million years ago (the former half of the Carboniferous) Only northwest half can be seen in the image. Southeast half is in the bottom of the Saint Laurence River.
B.  Clearwater West & East  36 km & 26 km  290 million years ago (border between the Carboniferous and the Permian) Crater chain
C.  Manicouagan 100 km 214 million years ago (border between the Triassic and the Jurassic) The largest crater in North America. A part of crater chain.

The Manicouagan crater (C above) consists a crater chain together with Rochechouart crater (25 km in diameter) in France and Saint Martin crater (40 km in diameter) in Manitoba, Canada (*3). "Crater chain" means a chain of craters made by sequential impacts of the pieces broken up of original one terrestrial body like the impact of Comet Shoemaker-Levy 9 on Jupiter in July 1994. There are some crater chains on the Moon and Ganymede, Jupiter's satellite.

"Tableland" in Gros Morne National Park on the west coast of Newfoundland Island is a 600 m high plateau made of p e ridot that was once a part of the upper mantle exposed on the ground surface after breaking through the crust of the ocean floor when the North American Plate and Eurasian/African Plate collided 450 million years ago (the Ordovician). This is one of the main reasons why Gros Morne National Park has been designated a World Heritage Site by UNESCO (the United Nations Education, Scientific, and Cultural Organization).

At L'Anse aux Meadows (meaning cove at meadows) National Historic Site, located on the northen end of Newfoundland Island and declared a UNESCO World Heritage Site, there are archaeological remains of activities of native people around 6,000 years ago and of a settlement of Viking people around 1,000 years ago, or about 500 years before Columbus reached the West Indian Islands.


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(*1) The Shield is a flat, extensive area composed primarily of precambrian rocks. If viewed as a whole, the center is a bit higher, and the height gradually decreases in the radial direction. It thus resembles the shape of a shield used by knights in medieval times and is called "Shield." In addition to the Canadian Shield, there is also the Baltic Shield and others.

Related sites
(*2) Earth Impact Database operated by the University of New Brunswick in Canada
(*3)News release of the University of Chicago




Explanation of the images
(Fig. 1 and Frames A to C)
Satellite: Advanced Earth Observing Satellite - II (Midori - II)
Sensor: Global Imager (GLI) 
Date: April 7 to 22, 2003 
   
These are color composite images generated from GLI spectral channel 26 (1,240 nm) in the medium infrared band (red), channel 24 (1,050 nm) in the medium infrared band (green), and channel 19 (865 nm) in the near infrared band (blue). In these images, snow and ice are white or light blue, soil or sparse vegetation is brown, and water surfaces are black. The original resolution is 1 km.

It is difficult to acqure cloud-free images of this area, even if observed by GLI with 1,500 km wide observation swath because of the climate of this area. We produced the cloud-free seamless image in Fig. 1 by selecting fair parts pixel by pixel from data acquired during the above-mensioned 16 days and composing them by computer processing. This kind of image is called a cloud-free composite image.

Frames A to C exhibit somewhat different coloring because they were partly expanded and image processed to clarify the craters.

Appendix: Tales told of Newfoundland Island
It seems that English fishermen fished off Newfoundland Island in summer as early as around 1500. On the Island shore, dogs unrivaled for hardiness and stamina helped the fishermen. They were carried to England in the beginning of the 19th century and were bred as retrievers. The "Labrador Retiever" was subsequently recognized by the Kennel Club in England in 1903, and they have become familiar as pets, police dogs and seeing-eye dogs.

The Titanic tragedy occured about 650 km southeast off Newfoundland Island from midnight of April 12, 1912 to just before daybreak of the next day (45th year in Meiji Era in Japan); that is 92 years ago. Over 1,500 persons died due to the sinkng after collision with an iceberg. Halifax, Nova Scotia, was the base for rescue activities at that time. Today, you can find a cemetery for the victims, and some articles left by the Titanic are displayed at the Atlantic Maritime Musium there.

 http://www.eorc.jaxa.jp/en/imgdata/topics/2004/tp040726.html
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« Reply #21 on: March 18, 2007, 09:37:28 pm »

The Collapse of Ancient Societies By Great Earthquakes
Amos Nur

Department of Geophysics, Stanford University, Stanford, CA 94305, USA
e- mail: nur@pangea.stanford.edu


Although earthquakes have often been associated with inexplicable past societal disasters, their impact has thought to be only secondary for two reasons: Inconclusive archaeological interpretation of excavated destruction, and misconceptions about patterns of seismicity. However, a better understanding of the irregularities of the time-space patterns of large earthquakes suggest that earthquakes (and associated tsunamis) have probably been responsible for some of the great and enigmatic catastrophes in ancient times. The most relevant aspect of seismicity is the episodic time-space clustering of earthquakes such as during the eastern Mediterranean seismic crisis in the 4th century AD and the seismicity of the north Anatolian fault during our century. During these earthquake crises plate boundaries rupture by a series of large earthquakes that occur over a period of only 50 to 100 years or so, followed by hundreds or even thousands of years of relative inactivity. The extent of the destruction by such rare but powerful earthquake clusters must have been far greater than similar modem events due to poorer construction and the lack of any earthquake preparedness in ancient times. The destruction by very big earthquakes also made ancient societies so vulnerable because so much of the wealth and power w as concentrated and protected by so few. Thus the breaching by an earthquake of the elite's fortified cities must have often led to attacks by (1) external enemies during ongoing wars (e.g., Joshua and Jericho, Arab attack on Herod's Jerusalem in 31 BCE); (2) neighbours during ongoing conflicts (e.g., Mycenea's fall in 1200 BCE, Saul's battle ~1020 BCE); and (3) uprising of poor and often enslaved indigenous populations (e.g., Sparta and the Helots in 465 BCE, Hattusas ~1200 BCE?, Teotihuacan ~700 AD?). When the devastation was by a local earthquake, during a modest conflict, damage was probably limited and may have required a few tens of years to rebuild. But when severe ground shaking is widespread, and when it happened during a major military conflict, the devastation may have been so great that it took hundreds of years for a society to recover - going through a dark age period during which many of the technical skills (e.g., writing) are abandoned (e.g., the cessation of linear B), construction and repairs of monumental buildings ceased, and looting of building materials by surviving squatters was common. In contrast, we can imagine the pastoral countryside, especially away from the tsunami prone coastal areas, to have been much less affected (and perhaps even flourished a little as their tax burden to the ruling elite is reduced). During a regional seismic crisis an entire region must have been subjected to a series of devastations by earthquakes over a short period of time. The catastrophic collapse of the main Eastern Mediterranean civilizations at the end of the Bronze age may be a case in point, with the Sea People being mostly squatters and refugees.


--------------------------------------------------------------------------------

AMOS NUR is the Wayne Loel Professor of Earth Sciences and Professor of Geophysics at Stanford University. Amos specializes in earthquake physics. For over twenty years, he has been investigating the temporal and spatial patterns of earthquakes throughout history to find clues useful for earthquake prediction. The longest and most complete record is in the Holy Land, where the Dead Sea seismic fault defines the Arabia - Africa plate boundary, as the San Andreas defines the N. American/Pacific plate boundary. Together with colleagues in archaeology, history, geology and geophysics at Stanford and Israel, Amos has organized an expedition to search for, and excavate and recover skeleton/s, artifacts, and Dead Sea scrolls buried 2000 years ago in the "Cave of Letters. in Israel's Judean desert by the devastating Dead Sea Earthquake of 31 BC. The first part of this expedition took place in March of this year. Amos is a winner of the Silver Apple Award for physical sciences at the National Educational Film Festival, 1991 for producing and directing a video on Earthquakes in the Holy Land. This has been shown extensively on a number of PBS stations around the country. Publications: Over 180 papers in refereed journals; and 3 books.

http://www.knowledge.co.uk/sis/abstract/nur.htm

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« Reply #22 on: March 18, 2007, 10:04:55 pm »

meteorites and extinctions
Meteroites


based on the lecture notes of Stephen A. Nelson, Tulane University

Definitions

A Meteorite is a piece of rock from outer space that strikes the surface of the Earth.

A Meteoroid is a meteorite before it hits the surface of the Earth.

Meteors are glowing fragments of rock matter from outside the Earth's atmosphere that burn and glow upon entering the Earth's atmosphere.  They are more commonly known as shooting stars.  Some meteors, particularly larger ones, may survive passage through the atmosphere to become meteorites, but most are small objects that burn up completely in the atmosphere.  They are not, in reality, shooting stars.

Fireballs  are very bright meteors.

Meteor Showers - During certain times of the year, the Earth's orbit passes through a belt of high concentration of cosmic dust and other particles, and many meteors are observed.  The Perseid Shower, results from passage through one of these belts every year in mid-August.


Throughout history there have been reports of stones falling from the sky, but the scientific community did not recognize the extraterrestrial origin of meteorites until the 1700s.  Within recent history meteorites have even hit humans-

1938 - a small meteorite crashed through the roof of a garage in Illinois


1954 - A 5kg meteorite fell through the roof of a house in Alabama.


1992 - A small meteorite demolished a car near New York City.


Meteorite fragments have been found all over the surface of the Earth, although most have been found in Antarctica.  In Antarctica they are easily seen on the snow covered surface or embedded in ice.

The fall of meteorites to the Earth's surface is part of the continuing process of accretion of the Earth from the dust and rock of space.  When these rock fragments come close enough to the Earth to be attracted by its gravity they may fall to the Earth to become part of it.  As we will see the evolution of life on the Earth has likely been affected by collisions with these space objects, and collisions could affect the Earth in the future as well.

 

Composition and Classification of Meteorites

Meteorites can be classified generally into three types:

Stones - Stony meteorites resemble rocks found on and within the Earth. They are the most common type of meteorite, although because they resemble Earth rocks they are not commonly recognized as meteorites unless someone actually witnesses their fall.  Stony meteorites are composed mainly of the minerals olivine, and pyroxene.  Some have a composition that is roughly equivalent to the Earth's mantle.  Two types are recognized:


Chondrites - Chondrites are the most common type of stony meteorite.  They are composed of small round glassy looking spheres, called chondrules, that likely formed from condensation from the gaseous solar nebula early in the history of the formation of the solar system.  Most chondrites have radiometric age dates of about 4.6 billion years. 


Achondrites - Achondrites are composed of the same minerals as chondrites, but lack the chondrules. They appear to have been heated, melted, and recrystallized so that the chondrules are no longer present.  Most resemble volcanic rocks found on the Earth's surface.


Irons - Iron meteorites are composed of alloys of iron and nickel. They are easily recognized because they have a much higher density than normal crustal rocks. Thus, most meteorites found by the general populace are iron meteorites. When cut and polished, iron meteorites show a distinct texture called a Widmanstätten pattern (see figure 10.5, p. 254 in your text).  This pattern results from slow cooling of a once hot solid material.  Most researchers suggest that such slow cooling occurred in the core of much larger body that has since been fragmented. Iron meteorites give us a clue to the composition of the Earth's core.

Stony Irons - Stony iron meteorites consist of a mixture of stony silicate material and iron.  Some show the silicates embedded in a matrix of iron-nickel alloy. Others occur as a breccia, where fragments of stony and iron material have been cemented together by either heat or chemical reactions.

 

Origin of Meteorites

Most meteorites appear to be fragments of larger bodies called parent bodies.  These could have been small planets or large asteroids that were part of the original solar system.  There are several possibilities as to where these parent bodies, or their fragments, originated.

The Asteroid Belt
The asteroid belt is located between the orbits of Mars and Jupiter.  It consists of a swarm of about 100,000 objects called asteroids.  Asteroids are small rocky bodies with irregular shapes that have a cratered surface.  About 4,000 of these asteroids have been officially classified and their orbital paths are known.  Once they are so classified they are given a name. 

 
The asteroids are either remnants of a planet that formed in the region between Mars and Jupiter but was later broken up by a collision with another planetary body, or are fragments that failed to accrete into a planet.  The latter possibility is more likely because the total mass of the asteroids is not even equal to our moon.  It does appear that some of the asteroids are large enough to have undergone internal differentiation.  Differentiation is a process that forms layering in a planetary body (i.e. the Earth has differentiated into a core, mantle, and crust). If these larger asteroids did in fact undergo differentiation, then this could explain the origin of the different types of meteorites.  Because of the shapes of the asteroids it also appears that some of them have undergone fragmentation resulting from collisions with other asteroids.  Such collisions could have caused the larger bodies to be broken up into the smaller objects we observe as meteorites.

The Asteroids as Parent Bodies of Meteorites

Much evidence suggests that the asteroids could be the parent bodies of meteorites. The larger ones could have differentiated into a core, mantle, and crust.  Fragmentation of these large bodies would then have done two things:  First the fragments would explain the various types of meteorites found on Earth - the stones representing the mantle and crust of the original parent body, the irons representing the cores, and the stony irons the boundary between the core and mantle of the parent bodies. Second, the collisions that caused the fragmentation could send the fragments into Earth-crossing orbits.

Some of the asteroids have orbits that bring them close to Earth.  These are called Amor objects.  Some have orbital paths that cross the orbital path of the Earth.  These are called Earth-crossing asteroids or Apollo objects.  All objects that have a close approach to the Earth are often referred to as Near Earth Objects or NEOs.  About 150 NEOs with diameters between 1 and 8 km are known, but this is only a fraction of the total number.  Many NEOs will eventually collide with the Earth.  These objects have unstable orbits because they are under the gravitational influence of both the Earth and Mars.  The source of these objects is likely the asteroid belt.
 
Comets as Parent Bodies of Meteorites
A Comet is a body that orbits around the Sun with an eccentric orbit. These orbits are not circular like those of the planets and are not necessarily within the same plane as the planets.  Most comets have elliptical orbits which send them to the far outer reaches of the solar system and back toward a closer approach to the sun. As a comet approaches the sun, solar radiation generates gases from evaporation of the comet's surface. These gases are pushed away from the comet and glow in the sun light, thus giving the comet its tail. While the outer surface of comets appear to composed of icy material like water and carbon dioxide solids, they likely contain a more rocky nucleus.  Because of their eccentric orbits, many comets eventually cross the orbit of the Earth.  Many meteor showers may be caused by the Earth crossing an orbit of a fragmented comet.



The collision of a cometary fragment is thought to have occurred in the Tunguska region of Siberia in 1908.  The blast was about the size of a 15 megaton nuclear bomb.  It knocked down trees in an area about 850 square miles, but did not leave a crater.  The consensus among scientists is that a cometary fragment about 20 to 60 meters in diameter exploded in the Earth's atmosphere just above the Earth's surface. Only small amounts of material similar to meteorites were found embedded in trees at the site.   
Other Sources
While the asteroid belt seems like the most likely source of meteorites, some meteorites appear to have come from other places.  Some meteorites have chemical compositions similar to samples brought back from the moon. Others are thought to have originated on Mars. These types of meteorites could have been ejected from the Moon or Mars by collisions with other asteroids, or from Mars by volcanic eruptions.
 
 


Impact Events

When a large object impacts the surface of the Earth, the rock at the site of the impact is deformed and some of it is ejected into the atmosphere to eventually fall back to the surface.  This results in a bowl shaped depression with a raised rim, called an Impact Crater.  The size of the impact crater depends on such factors as the size and velocity of the impacting object and the angle at which it strikes the surface of the Earth.

Meteorite Flux and Size
Meteorite flux is the total mass of extraterrestrial objects that strike the Earth.  This is currently about 107 to 109 kg/year. Much of this material is dust-sized objects called micrometeorites.   The frequency at which meteorites of different sizes strike the Earth depends on the size of the objects, as shown in the graph below.  Note the similarity between this graph and the flood recurrence interval graphs we looked at in our discussion of flooding. 
 

Tons of micrometeorites strike the Earth each day. Because of their small size, they do not usually burn up when entering the Earth's atmosphere, but instead settle slowly to the surface. Meteorites with diameters of about 1 mm strike the Earth about once every 30 seconds.  Upon entering the Earth's atmosphere the friction of passage through the atmosphere generates enough heat to melt or vaporize the objects, resulting in so called shooting stars.  Meteorites of larger sizes strike the Earth less frequently.  If they have a size greater than about 2 or 3 cm, they only partially melt or vaporize on passage through the atmosphere, and thus strike the surface of the Earth. 
Objects with sizes greater than 1 km are considered to produce effects that would be catastrophic, because an impact of such an object would produce global effects.  Such meteorites strike the Earth relatively infrequently -  a 1 km sized object strikes the Earth about once every million years, and 10 km sized objects about once every 100 million years.
Velocity and Energy Release of Incoming Objects
The velocities at which small meteorites have impacted the Earth range from 4 to 40 km/sec.  Larger objects would not be slowed down much by the friction associated with passage through the atmosphere, and thus would impact the Earth with high velocity. Calculations show that a meteorite with a diameter of 30 m, weighing about 300,000 tons, traveling at a velocity of 15 km/sec (33,500 miles/hour) would release energy equivalent to about 20 million tons of TNT.   
 

Such a meteorite struck at Meteor Crater, Arizona (the Barringer Crater) about 49,000 years ago leaving a crater 1200 m in diameter and 200 m deep.  The amount of energy released by an impact depends on the size of the impacting body and its velocity.  An impact like the one that struck the Yucatan Peninsula, in Mexico about 65 million years ago, thought responsible for the extinction of the dinosaurs and numerous other species, created the Chicxulub Crater, 180 km in diameter and released energy equivalent to about 100 million megatons of TNT.   
For comparison, the amount of energy needed to create a nuclear winter on the Earth as a result of nuclear war is about 8,000 megatons, and the energy equivalent of the world's nuclear arsenal is about 60,000 megatons.

Cratered Surfaces

Looking at the surface of the Moon, one is impressed by the fact that most of the surface features of the moon are shaped by impact craters.  The Earth is subject to more than twice the amount of impacting events than the moon because of its larger size and higher gravitational attraction. Yet, the Earth does not show a cratered surface like the moon.  The reason for this is that the surface of the Earth is continually changing due to processes like erosion, weathering, tectonism, sedimentation, and volcanism.  Thus, the only craters that are evident on the Earth are either very young, very large, or occurred on stable continental areas that have not been subject to intense surface modification processes.  Currently, approximately 200 terrestrial impact structures have been identified, with the discovery rate of new structures in the range of 3-5 per year (see figure 10.15, page 262 in your text).

The Mechanics of Impact Cratering
When a large extraterrestrial object enters the Earth's atmosphere the initial impact with the atmosphere will compress the atmosphere, sending a shock wave through the air.  Frictional heating will cause the object to heat and glow.  Melting and even vaporization of the outer parts of the object will begin, but if the object is large enough, solid will remain when it impacts the surface of the Earth. 
 


Impact of large meteorites have never been observed by humans.  Much of our knowledge about what happens next must come from scaled experiments.  As the solid object plows into the Earth, it will compress the rocks to form a depression and cause a jet of fragmented rock and dust to be expelled into the atmosphere. This material   is called ejecta.  The impact will send a shock wave into the rocks below, and the rocks will be crushed into small fragments to form a breccia.  Some of the ejecta will be hot enough to vaporize, and the heat generated by the impact could be high enough to actually melt the rock at the site of the impact.   The shock wave entering the Earth will first move in as a compressional wave (P-wave), but after passage of the compressional wave an expansion wave (rarefaction wave) will move back toward the surface.  This will cause the floor of the crater to be uplifted and may also cause the rock around the rim of the crater to bent upward.  Faulting may also occur in the rocks around the crater, causing the crater to become enlarged, and have a concentric set of rings. 
The ejecta will eventually settle back to the Earth's surface forming an ejecta blanket that is thick near the crater rim and thins outward from the crater.   Rocks below the crater that were not melted by the impact will be intensely fractured.  All of this would happen in a matter of 1 to 2 minutes.



Meteorite Impacts and Mass Extinctions

The impact of a space object with a size greater than about 1 km would be expected to be felt over the entire surface of the Earth.  Smaller objects would certainly destroy the ecosystem in the vicinity of the impact, similar to the effects of a volcanic eruption, but larger impacts could have a worldwide effect on life on the Earth. We will here first consider the possible effects of an impact, and then discuss how impacts may have resulted in mass extinction of species on the Earth in the past.

Regional and Global Effects
Again, we as humans have no firsthand knowledge of what the effects of an impact of a large meteorite or comet would be.  Still, calculations can be made and scaled experiments can be conducted to estimate the effects. The general consensus is summarized here.

Massive earthquake - up to Richter Magnitude 13, and numerous large magnitude aftershocks would result from the impact of a large object with the Earth.


The large quantities of dust put into the atmosphere would block incoming solar radiation. The dust could take months to settle back to the surface.  Meanwhile, the Earth would be in a state of continual darkness, and temperatures would drop throughout the world, generating global winter like conditions. A similar effect has been postulated for the aftermath of a nuclear war (termed a nuclear winter).  Blockage of solar radiation would also diminish the ability of photosynthetic organisms, like plants, to photosynthesize. Since photosynthetic organisms are the base of the food chain, this would seriously disrupt all ecosystems.


Widespread wildfires ignited by radiation from the fireball as the object passed through the atmosphere would be generated.  Smoke from these fires would further block solar radiation to enhance the cooling effect and further disrupt photosynthesis.


If the impact occurred in the oceans, a large steam cloud would be produced by the sudden evaporation of the seawater.  This water vapor and CO2 would remain in the atmosphere long after the dust settles.  Both of these gases are greenhouse gases which scatter solar radiation and create a warming effect.  Thus, after the initial global cooling, the atmosphere would undergo global warming for many years after the impact.


If the impact occurred in the oceans, giant tsunamis would be generated.  For a 10 km-diameter object the leading edge would hit the seafloor of the deep ocean basins before the top of the object had reached sea level.  The tsunami from such an impact is estimated to produce waves from 1 to 3 km high.  These could easily flood the interior of continents.


Large amounts of nitrogen oxides would result from combining Nitrogen and Oxygen in the atmosphere due to the shock produced by the impact.  These nitrogen oxides would combine with water in the atmosphere to produce nitric acid which would fall back to the surface as acid rain, resulting in the acidification of surface waters.
The Geologic Record of Mass Extinction

It has long been known that extinction of large percentages families or species of organisms have occurred at specific times in the history of our planet.  Among the mechanisms that have been suggested to have caused these mass extinctions have been large volcanic eruptions, changes in climatic conditions, changes in sea level, and, more recently, meteorite impacts.  While the meteorite impact theory of mass extinctions has become accepted by many scientists for particular extinction events, there is still considerable controversy among scientists. In this course we will accept the possibility that an impact with a large object could have caused at least some of the mass extinction events, as it would certainly seem possible given the effects that an impact could have, as discussed above.  Still, because of their are many other possibilities for the cause of mass extinctions, please read your book  for the arguments against the impact theory.
 
Major extinction events occurred at
the end of the Tertiary Period, 1.6 million years (m.y.) ago.



the end of the Cretaceous Period, marking the boundary between the Cretaceous and Tertiary periods 65 m.y. ago. (Geologists use the letter K to stand for Cretaceous Period and the letter T for the Tertiary Period. Thus this boundary is commonly called the K-T boundary).

the end of the Triassic, 208 m.y. ago.

the end of the Permian, 245 m.y. ago (estimated that over 96% of the species alive at the time became extinct).

the end of the Devonian, 360 m.y. ago

the end of Ordovician, 438 m.y. ago

the end of the Cambrian period, 505 m.y. ago
 
The mass extinction at the end of the Mesozoic Era, that is the Cretaceous - Tertiary boundary (often called the K-T boundary) 65 million years ago, shows much evidence that it was related to an impact with an extraterrestrial object. This event resulted in the extinction of over 50% of the species living at the time, including the dinosaurs. In 1978 a group of scientist led by Walter Alvarez of the University of California, Berkeley, were able to locate the K-T boundary very precisely in layers of limestones near Gubbio, Italy. At the boundary they found a thin clay layer.  Chemical analysis of the clay revealed that it contains an anomalously high concentration of the rare element Iridium (Ir).  Ir has extremely low concentrations in most crustal rocks, however it reaches very high concentrations in meteorites.  The only other possible source of high concentrations of Ir is basaltic magmas.  Over the next several years, the K-T boundary was located at several other sites throughout the world, and also found to have a thin clay layer with high concentrations of Ir.  Although a large eruption of basaltic magma could not immediately be ruled out as the source of the high concentration of Ir, other evidence began to accumulate that the fallout of impact ejecta had been responsible for both the thin clay layers and the high concentrations of Ir.  Among the evidence found at different localities where the K-T boundary is exposed is:
Clay layers at some localities have a high proportion of black carbon that could have originated as soot produced by wildfires set off by an impact.


Some of the clay layers contain grains of quartz with a crystal structure that shows evidence that the quartz was severely strained by a large shock.


In some clay layers tiny grains of  the mineral stishovite is found. Stishovite is a high pressure form of SiO2 that is not found at the Earth's surface except around known meteorite impact sites.  The mineral can only be produced as a result of extremely deep burial in the Earth, or by high pressure generated by an impact.


Other clay layers contain tiny spherical droplets of glass, called spherules.  The glass is not basaltic in composition, but could represent droplets of melt formed during an impact event.
 


At the time of these discoveries, there was no known impact structure on the Earth with an age of 65 million years.  This is not unexpected, since 71% of the Earth's surface is covered by water, and is largely unexplored.  But, in the late 1980s attention started to be focused on a buried impact site near the tip of the Yucatan Peninsula, in Mexico.  Here oil geologists had drilled through layers of brecciated rock and found  impact melt rock.  Further geophysical studies revealed a circular structure about 180 km in diameter.  Radiometric dating reveals that the structure, called the Chicxulub Crater,  formed about 65 million years ago.   
Although the crater itself is now filled and buried by younger rocks, drilling throughout the Gulf of Mexico has revealed the presence of shocked quartz, glass spherules, and soot in deposits the same age as the crater.  In addition, geologists have found deposits from the tsunami that was generated by the impact all along the Gulf of Mexico coast extending considerable distance inland from the current shoreline. The size of the crater suggest that the object that produced it was about 10 km in diameter. 
While there is still some debate among geologists and paloebiologists as to whether or not the extinctions that occurred at the K-T boundary were caused by the impact that formed Chicxulub Crater, it is clear that an impact did occur about 65 million years ago, and that it likely had effects that were global in scale. What would happen if another such event occurred while we humans dominate the surface of the Earth, and what could we as humans do, if anything to prevent such a catastrophic disaster?
 

Human Hazards

It should be clear that even if an impact of a large space object did not cause the extinction of humans, the effects would cause a natural disaster of proportions never witnessed by the human race.  Here we first look at the chances that such an impact could occur, then look at how we can predict or provide warning of such an event, and finally discuss ways that we might be able to protect ourselves from such an event.
Risk - It is estimated that in any given year the odds that you will die from an impact of an asteroid or comet are about 1 in 20,000.  The table below shows the odds of dying in the U.S. from various other causes.  Although 1 in 20,000 seem like long odds, you have about the same odds of dying in an airplane crash, and somewhat less risk of dying from other natural disasters likes floods and tornadoes.  In fact the odds of dying from an impact event are much better than the odds of winning the lottery.
 

Odds of Dying in the U.S. from Selected Causes
Cause Odds
Motor Vehicle Accident 1 in 100
Murder 1 in 300
Fire 1 in 800
Firearms Accident 1 in 2,500
Electrocution 1 in 5,000
Asteroid or Comet Impact 1 in 20,000
Airplane Crash 1 in 20,000
Flood 1 in 30,000
Tornado 1 in 60,000
Venomous Bite or Sting 1 in 100,000
Food Poisoning by Botulism 1 in 3,000,000
Odds of winning the Lottery 1 in 7,000,000

In March, 1989 an asteroid named 1989 FC passed within 700,000 km of the Earth, crossing the orbit of the Earth.  It was not discovered until after it had passed through the orbit of the Earth.  Its size was estimated to be about 0.5 km.   Such a body is expected to hit the Earth about once every million years or so, and would release energy equivalent to about 10,000 megatons of TNT, a little greater than the energy released in a nuclear war, and enough to cause nuclear winter event (see graph above).  Although 700,000 km seems like a long distance, it translates to a miss of the Earth by only a few hours at orbital velocities.
 


The Torino Scale - In order to develop a better means of communicating the potential hazards of a possible impact with a space object, scientists have developed a scale that describes the potential (see - http://neo.jpl.nasa.gov/torino_scale.html).  The scale is called the Torino Scale, and is shown below.
 
 
Events Having No Likely Consequences
(White Zone)  0  The likelihood of a collision is zero, or well below the chance that a random object of the same size will strike the Earth within the next few decades. This designation also applies to any small object that, in the event of a collision, is unlikely to reach the Earth's surface intact. 
Events Meriting Careful Monitoring
(Green Zone)  1  The chance of collision is extremely unlikely, about the same as a random object of the same size striking the Earth within the next few decades. 
Events Meriting Concern
(Yellow Zone)  2  A somewhat close, but not unusual encounter. Collision is very unlikely. 
3  A close encounter, with 1% or greater chance of a collision capable of causing localized destruction. 
4  A close encounter, with 1% or greater chance of a collision capable of causing regional devastation. 
Threatening Events
(Orange Zone)  5  A close encounter, with a significant threat of a collision capable of causing regional devastation. 
6  A close encounter, with a significant threat of a collision capable of causing a global catastrophe. 
7  A close encounter, with an extremely significant threat of a collision capable of causing a global catastrophe. 
Certain Collisions
(Red Zone)  8 A collision capable of causing localized destruction. Such events occur somewhere on Earth between once per 50 years and once per 1000 years. 
9 A collision capable of causing regional devastation. Such events occur between once per 1000 years and once per 100,000 years. 
10 A collision capable of causing a global climatic catastrophe. Such events occur once per 100,000 years, or less often. 






For an object making a close approach to Earth, its categorization on the Torino Scale is dependent upon its placement within this plot showing kinetic energy versus collision probability. (One MT = 4.3 x 10^15 J.) The left-hand scale also indicates approximate sizes for asteroidal objects having typical encounter velocities. For an object that makes multiple close approaches over a set of dates, a Torino Scale value should be determined for each approach. It may be convenient to summarize such an object by the greatest Torino Scale value within the set.

Prediction and Warning - It is estimated that over 90% of NEOs have not yet been discovered.  Because of this, with our present knowledge, there is a good chance that the only warning we would have is the flash of light from the fireball as one of these objects entered the Earth's atmosphere.  Scientists have proposed the "Spaceguard Survey" to find and track all of the large NEOs.  If such a survey is carried out, we could predict the paths of all NEOs and have years to decades to prepare for an NEO that could impact the Earth.
Mitigation - Impacts are the only natural hazard that we can prevent from happening by either deflecting the incoming object or destroying it.  Of course, we must first know about such objects and their paths in order to give us sufficient warning to prepare a defense.  Sufficient time is usually thought to be about 10 years.  This would likely give us enough time to prepare a space mission to intercept the object and deflect its path by setting off a nuclear explosion.  Currently, however, there are no detailed plans.  But, even if we did not have the ability to destroy or deflect such an object, 10 years warning would provide sufficient time to store food and supplies, and maybe even evacuate the area immediately surrounding the expected impact site.
 
 http://earthsci.org/education/teacher/basicgeol/meteorn_add/met_extinct.html
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« Reply #23 on: March 18, 2007, 10:20:23 pm »

Asteroid/Comet Impact Craters and Mass Extinctions
and Shiva Hypothesis of Periodic Mass Extinctions.
 
by Michael Paine


[Craters by age] [Volcano and climate change links] [Updates] [Chicxulub debate][More on extinctions] [Falklands] [Impacts and vulcanism] [Shiva Hypothesis - periodic extinctions]

Craters by age
Since writing my Space.com/Explorezone article  "How an asteroid impact causes extinction" in 1999, I have gathered some  more information about the possible links between asteroid impacts and mass extinctions. There also appears to be a link between large impacts and volcanic eruptions.
The following graph shows impact craters on Earth by age and diameter. Also shown are the main geologic boundaries involving mass extinctions (tall, bold lines), minor boundaries (thin, short lines - fewer extinctions) and the approximate timing of "flood basalt eruptions". Originally the graph only showed craters which aligned with major extinction events but it was considered better to show all craters 20km diameter or more to avoid "counting the hits and ignoring the misses". Those which appear to align with a geologic boundary are shown as dark blue diamonds. The most notable is Chicxulub at the Cretaceous/Tertiary boundary - the event that saw the extinction of the dinosaurs.

Since multiple impacts appear to be very common throughout the solar system it is expected that some of the smaller craters are associated with other major impacts, evidence of which has not been discovered or has vanished over time. For example, the Triassic/Jurassic and Jurassic/Cretaceous boundaries appear to involve multiple impacts. Craters 40km diameter or more are likely to be caused by 2km diameter asteorids or comets. Such impacts would probably result in severe global climate disruption but it takes an asteroid/comet 10km or larger to cause mass extinctions. It is estimated that such impacts occur, on average, once every 50 to 100 million years.



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« Reply #24 on: March 18, 2007, 10:21:19 pm »

Graph best viewed 1024x768

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« Reply #25 on: March 18, 2007, 10:23:07 pm »

Here is some of the data used for the graph. (Ma = Million Years).
  CRATER NAME DIAMETER AGE
Ma ERROR
MARGIN
Ma LOCATION
Siljan 52 368
 Sweden
Charlevoix 54 357 15 Quebec, Canada
Araguainha Dome 40 247 5.5 Brazil
Rochechouart 25 214 8 France  (see Nature 395, p126, 1998)
Red Wing 9 200 25 North Dakota, U.S.A.
Obolon 15 215 25 Ukraine
St Martin 40 219 32 Manitoba, Canada  (see Nature 395, p126, 1998)
Manicouagan 100 214 1 Quebec, Canada (see Nature 395, p126, 1998)
Puchezh-Katunki 80 175 3 Russia
Gosses Bluff 24 142.5 0.8 Australia
Mjolnir 40 142 2.6 Norway
Morokweng 70 145 3 South Africa
Tookoonooka 55 128 5 Queensland, Australi
Kara 65 73 3 Russia
Chicxulub 170 64.98 0.05 Yucatan, Mexico
Chesapeake Bay 90 35.2 0.3 Virginia, U.S.A. (see Nature 388, p365,1997)
Popigai 100 35.7 0.8 Russia (see Nature 388, p365,1997)
Kara-Kul 52 5
 Tajikistan
Eltanin 30? 2.14
 South Pacific - ocean impact
(see Nature 390, p357,1997)
SPECULATIVE CRATERS
 
 
 
 
Bedout 180 250?
 Western Australia
Woodleigh  120 250-360?
 Western Australia (PDF) Note about age.
Ewing Structure 55-150 11
 Western Pacific?


Several craters between 20km and 80km are missing from this table but shown in the graph. See NRC (updated URL) for a full list of craters. "Eltanin" was an ocean impact and did not leave a crater. Bedout and Woodleigh are speculative - see below. Ewing is a possible oceanic crater. Woodleigh is now in the NRC database and Bedout is looking promising.

NRC also has an excellent series of maps of the continents over geologic time.

Uni Arizona: Interactive global map of impact craters. Australia.

Uni Tennessee: Suspected Earth Impact Sites - new (2006) online database.

"Impacts - no crater" are cases where there is evidence of an impact, such as tektites, but no crater has been found. Eltanin (see above) is an example. The other cases are described by Dallas Abbott in a pending EPSL paper - stay tuned for an online copy.



--------------------------------------------------------------------------------

  GEOLOGIC BOUNDARIES
 
Period or Epoch  Ma
Precambriam/Cambrian 570
Cambrian/Ordovician 505
Ordovician/Silurian 438
Silurian/Devonian 408
Frasnian/Famennian (Trilobites) 367
Devonian/Carboniferous 350
Carboniferous/Permian 286
Permian/Triassic 250
Triassic/Jurassic ? 208
Jurassic/Cretaceous 144
*Cretaceous/Tertiary (Dinosaurs) 65
* KT stands for Cretaceous Tertiary, Kriede is the German word for Cretaceous.
More information
  ERUPTIONS Ma
Ethiopean Plateau 35
Deccan Traps, India 65
Emperor-Hawaii Chain 65
Sudan Volcanics 144
Central Atlantic Volcanics 213
Siberian Traps 250
Antrim Plateau 511
Flood basalts and mass extinctions - some "other" eruptions and "minor" geologic boundaries shown in the graph are from this website. This website indicates the Triassic/Jurassic boundary occurred 208 Ma. There may have been two or more extinction episodes at this time.
Volcano and climate change links:
Alan Robock's publications
Volcano World including Dead Dinosaurs and Gases
Time Magazine When life nearly died
Dinosaur Volcano Greenhouse - an alternative view of the K/T extinctions. Information about the Deccan Traps.
Large Igneous Provinces - Scientific American, October 1993.
3 Feb 2000 BBC: Supervolcanoes could trigger global freeze.
21 Nov 2000 SpaceDaily: Massive Lava Flows Triggered Apocalyptic Climate Changes. Also University of Buffalo press release.
Scientific American: Volcanic Accomplice. See clarification.
Abstracts of GSA 2000 (CCNet item 13) refer to impacts and eruptions.
9 Mar 01 BBC: 'Quick' demise for the dinosaurs. The work lends support to the idea that a single, giant impact of an asteroid or comet was responsible for the mass extinction of life that occurred 65 million years ago.  In so doing, the research also undermines the popular, alternative theory for the demise of the dinosaurs: climate change brought on by huge volcanic eruptions.
13 Mar 01: K–T TRANSITION GREENHOUSE AND EMBRYOGENESIS DYSFUNCTION IN THE
DINOSAURIAN EXTINCTIONS. Virginia Polytechnic Institute and State University
30 Apr 01 SciAm: Scientists Shake Up Theory of Plate Tectonics.
BBC: Supervolcanoes.
American Museum of Natural History: Climate Effects of Historic Volcanoes.
13 Jun 02 Buffalo Times: Study of Dust in Ice Cores Shows [recent] Volcanic Eruptions Interfere with the
19 Oct 03 EPSL (abstract):Rapid eruption of Siberian flood-volcanic rocks and evidence for coincidence with the Permian-Triassic boundary and mass extinction at 251 Ma
Effect of Sunspots on Global Climate (CC)
MantlePlumes.org - This website discusses the origin of "hotspot" volcanism.
CCNet: LARGE IMPACTS AND VOLCANISM AND MASS EXTINCTIONS.
Jones A. P. and others (2002), Impact induced melting and the development of large igneous provinces. Earth Planet. Sci. Lett., 202, 551-561.(PDF)
Jones A.Pand others (2003) Impact decompression melting: a possible trigger for impact induced volcanism and mantle hotspots? In: Impact markers in the Stratigraphic Record (eds. C. Koeberl and F. Martinez-Ruiz), Springer, Berlin, p 91-120.(PDF)
22 Dec 04 SciAm: How do volcanoes affect world climate?
10 Mar 05 BBC: Experts weigh super-volcano risks + Live Science: Super Volcano Will Challenge Civilization, Geologists Warn. See also Nature: Super-eruptions might not be as environmentally devastating as we thought. (CC)
6 Dec 05 BBC: Poison [volcanic] gas 'caused' great [P/T] dying.

2 Dec 05 EPSL: Absence of extraterrestrial 3He in Permian–Triassic age sedimentary rocks
27 May 06 SciAm: The Secrets of Supervolcanoes - A supervolcano eruption packs the devastating force of a small asteroid colliding with the earth and occurs 10 times more often...
24 Jul 06 EPSL: Volatile fluxes during flood basalt eruptions and potential effects on the global environment: A Deccan perspective - atmospheric perturbations associated with SO2 emissions from just one of these long-lasting eruptions were likely to have been severe, and constantly augmented over a decade or longer. By contrast, the amounts of CO2 released would have been small compared with the mass already present in the atmosphere, and thus much more limited in effect [global warming]. (more at CCNet)
1 Oct 06 SciAm: Impact from the Deep - Strangling heat and gases emanating from the earth and sea, not asteroids, most likely caused several ancient mass extinctions.
15 Mar 07 EPSL: Contemporaneous massive subaerial volcanism and late cretaceous Oceanic Anoxic Event 2.
More on Extinctions and Impacts + Updates (at end of list)
Explorezone Did asteroid-induced firestorm kill the dinosaurs?
New Scientist article What really killed the dinosaurs?
How Dinsoaurs became extinct
Extinction lab - University of Arizona - more links on the K/T event.
DINOSAURS, COMETS, AND ASTEROIDS -- RESOURCES - NASA education site
Earth Science Resources: Geology, Oceanography, Astronomy & Ecology - Miami University
BBC The Extinction Files
"The March Towards Extinction" National Geographic, June 1989 (paper version only). This was written before Chicxulub was discovered.
Interview with David Raup about mass extinctions.
BBC: Ill wind 'killed dinosaurs' - one of the more extreme ideas.
8 Sep 2000 Space.com: Rocks Reveal Details of Mass Extinction - possible evidence of a NEO impact associated with the Permian extinction 251 million years ago.
27 Oct 2000 CCNet: 'UPPER EOCENE IMPACTS NOT RELATED TO LATE EOCENE MASS EXTINCTION '
Global Climate Change (and impacts) - Tri-College University
National Geographic, Sep 2000: When Life Nearly Came to an End (Permian extinction)
Dinosaurs, Meteorites, and Extinctions by Wendy Wolbach.
23 Feb 01 Space.com: Mass Extinction & Rise of Dinosaurs Tied to Cosmic Collision. Also Washington Uni press release. Scientific American: E.T. Molecules Explain a Mass Extinction. Science@NASA: Apocalypse Then.
23 Feb 01 University of Washington: Asteroid or comet triggered death of most species 250 million years ago

15 Apr 01 SciAm: Deeper Impact (updated URL) - Was yet another mass extinction (PT) the work of an asteroid?
14 May 01 Science: SUDDEN PRODUCTIVITY COLLAPSE ASSOCIATED WITH THE TRIASSIC-JURASSIC BOUNDARY MASS EXTINCTION (subs reqd). CNN, Excite.
31 May 01: Left Hand Network - many extinctions links
1 Jun 01: PSRD Hot Idea: Impact at the end of the Permian
6 Jun 01 Uni Oregon: Lecture course on astronomy covers extinctions.
28 Aug 2001: GSA Permian Extraterrestrial Impact Caused Largest Mass Extinction on Earth.
 EXTINCTIONS Fossil Company
8 Dec 01 Abstracts of Annual AGU meeeting: "Ewing structure: a possible abyssal impact crater" by Dallas Abbott - about 150km in diameter...about the age of the late/middle Miocene boundary, a prominent mass extinction event" [not in my list but see this New Zealand article - an ocean impact might have started off the Antarctic ice sheet due to the water released and the global cooling]. GSA 2002: MICROFOSSIL MELTING BY THE EWING IMPACT
29 Jan 02 NASA Science News: The Great Dying 250 Million Years Ago - telltale signs of a collision between our planet and an asteroid 6 to 12 km across...
March 02 Scientific American: Repeated Blows (UCSB copy plus other papers) - by Luann Becker. Extraterrestrial impacts ended the age of the dinosaurs. New  research shows that they could have been the culprits behind many mass extinctions as well. This article uses the above graph.

31 Mar 02 Nature: Two cheers for extinction + Determinants of extinction in the fossil record
17 May 02 BBC: Impact led to dino rule. Also New Scientist: Giant dinosuars arrived with a bang. From Science.
17 May 02 NAI: The Cambrian Explosion: Tooth and Claw + Evolution’s Slow Recovery.
7 Jun 02 BBC: Volcanic 'flood' linked to [PT] extinction. (larger than previously thought). From Science:
7 Jun 02 Nature: FORGET NEO IMPACTS: "VOLCANOES KILLED OFF PERMIAN SPECIES"
8 Jun 02 Geology.about: Extinction links.
12 Jun 02 BBC: Dino heatwave recorded in leaves + New Scientist: Fossils point to asteroid causing dinosaurs' demise.
11 Feb 03 NAI: Great Impact Debate I: Benefits of Hard Bodies (mass extinctions)
19 Mar 03 CCNet: THE BULLET THEORY: A NEW LINK BETWEEN MASS EXTINCTIONS AND ITS IMPLICATIONS FOR HUMANKIND.
6 May 03 ACA: Asteroid impact puts heat on Snowball Earth theory of key evolutionary jump
8 May 03 Astrobiology: Comparing the Evidence Relevant to Impact and Flood Basalt at Times of Major Mass Extinctions - (the) Walter Alvarez
28 May 03 EPSL: A case for a comet impact trigger for the Paleocene/Eocene (55My) thermal maximum and carbon isotope excursion
13 Jun 03 LSU: Evidence for meteor in early mass extinction found - not sure if this is referring to the Frasnian/Famennian which could be associated with the Woodleigh impact structure in Western Australia
Age and implications of the 120 km-diameter Woodleigh impact structure, Carnarvon Basin, Western Australia
Killer Crater Found (Woodleigh)

Progress in meteoritic impact and crustal evolution research - ANU report
23 Aug 03 Nature: Boiling seas linked to mass extinction (CC) - see also Rocks from Space - impacts may trigger methane releases.
11 Sep 03 SpaceDaily: Did Earth Blow Up The Dinosaurs
26 Sep 03 Princton: Princeton paleontologist produces evidence for new theory [?] on dinosaur extinction (NNN)
4 Nov 03 CCNet: K/T MASS EXTINCTION DEBATE KICKED WIDE OPEN AGAIN + GSL Forum
20 Nov 03: PhD student Peter Schulte has several publications about the Chicxulub impact.
4 Dec 03 Astrobiology Magazine: Repeated Blows: The Great Dying (possible PT impact)
1 Feb 04 Science Direct: Causes and consequences of extreme Permo-Triassic warming to globally equable climate and relation to the Permo-Triassic extinction and recovery (abstract). No mention of possible post-impact Greenhouse effect (assuming a major impact is associated with PT).
4 Feb 04 Astronomy & Geophysics v45.1 Feb04: A COMET IMPACT IN AD 536? (Abstract - full text at CCNet?)
3 May 04: Multiple impacts at the KT boundary and the death of the dinosaurs 1997 paper by Dr.Chatterjee, Texas Tech Uni plus Animations of Plate techonics at Uni California, Berkeley (thanks Richard Lazzara).
8 May 04: Mass Extinctions of Life: An Update on Astrophysical Causes by Charles A. Breiterman
11 May 04 NewSci: FOUR DAYS THAT SHOOK THE WORLD (speculative alternative to impacts)
14 May 04 Bedout Structure and the P/T Mass Extinction
BBC: Boost to asteroid wipe-out theory
NASA Announces Site Of "Great Dying" Meteor Crater (CC)

Times Despatch: Scientists will reveal today where a major space rock hit Earth (CC)
GRA: GLOBAL EVIDENCE FOR AN END-PERMIAN EXTINCTION EVENT (CC)

Science: Evidence of Huge, Deadly Impact Found Off Australian Coast? + Bedout: A Possible End-Permian Impact Crater Offshore of Northwestern Australia.
The Bedout structure has been shown as a speculative impact structure associated with the P/T extinction for several years on the above graph - this latest work by Luann Becker and her team confirms earlier work, particularly by John Gorter.

15 May 04 GSA 2003: THE SHIVA CRATER: IMPLICATIONS FOR DECCAN VOLCANISM, INDIA-SEYCHELLES RIFTING, DINOSAUR EXTINCTION, AND PETROLEUM ENTRAPMENT AT THE KT BOUNDARY.
13 Oct 04 ABC: Asteroid did not end dinosaurs: NZ scientists +  Trends in Ecology & Evolution: The rise of birds and mammals: are microevolutionary processes sufficient for macroevolution? See also: How to kill (almost) all life: the end-Permian extinction event

The Mass-Extinction Debates - How Science Works in a Crisis. Edited by William Glen. This (1994) book examines the arguments and behavior of the scientists who have been locked in conflict over two competing theories to explain why, 65 million years ago, most life on earth—including the dinosaurs—perished.
1 Dec 04 SpaceDaily: New Evidence Supports Terrestrial Cause Of End-Permian Mass Extinction - "Our geochemical analyses of these two famous end-Permian sections in Austria and Italy reveal no tangible evidence of extraterrestrial impact," said Koeberl.
9 Dec 04 Kansas City Star: Debate still rages on demise of dinosaurs
6 Mar 05 Space.com: Huge Space Clouds May Have Caused Mass Extinctions.
15 May 05 EPSL (abs): Basaltic volcanism and mass extinction at the Permo-Triassic boundary: Environmental impact and modeling of the global carbon cycle.
2 Sep 05 EPSL (abs): Geophysical evaluation of the enigmatic Bedout basement high, offshore northwestern Australia [Evidence suggests no major impact] (CC)
21 Sep 05 NYT (regn): Fossils Offer Support for Meteor's [sic] Role in Dinosaur Extinction + Geology (abs): Cretaceous-Paleogene boundary deposits at Loma Capiro, central Cuba: Evidence for the Chicxulub impact (CC)
29 Oct 05 EPSL (abs): Giant meteoroid impacts can cause volcanism. Our model demonstrates that a giant impactor could cause a flood basalt, and this process may have been significant early in Earth history...
30 Dec 05 EPSL (abs): The 3.26-3.24 Ga Barberton asteroid impact cluster: Tests of tectonic and
magmatic consequences, Pilbara Craton, Western Australia - ...imply impact-triggered reactivation of mantle convection, crustal anatexis, faulting and strong vertical movements in Archaean granite–greenstone terrains associated with large asteroid impacts.
16 Mar 06 LPSC: Extraterrestrial Chromium at the Graphite Peak P/Tr boundary and in the Bedout Impact Melt Breccia.
8 Jun 06 ABC Science: Killer crater may have spawned Australia [PT extinction?] + PPP: Close-up of the end-Permian mass extinction horizon recorded in the Meishan section, South China: Sedimentary, elemental, and biotic characterization and a negative shift of sulfate sulfur isotope ratio (thanks CCNet)
2 Oct 06 EPSL (abs): Chicxulub impact event is Cretaceous/Paleogene boundary in age: New micropaleontological evidence.
30 Nov 06 Universe Today: Just a Single Asteroid Strike Wiped out the Dinosaurs (see EPSL item above)
10 Mar 07 EPSL ($): Chicxulub impact predates K–T boundary: New evidence from Brazos, Texas


See also the bibliography for Rocks in Space.



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« Reply #26 on: March 18, 2007, 10:24:02 pm »

Chicxulub Debate


"MORTAL BLOW TO K/T-CHICXULUB THEORY?" I DON'T THINK SO!

CCNet 2 June 2005

(Smit) Dear Benny,

Gerta Keller and colleagues again think they have dealt "... the mortal wound for the
Chicxulub theory" on the basis of new cores taken at Brazos River, Texas.
See http://www.nature.com/news/2005/050523/full/050523-2.html

Hermann Burchard, not a geologist, says he believes her now (CCNet 2 June 2005).

Yet the only evidence we have seen is an abstract at the AGU New Orleans 2005 meeting,
where is stated that between the event sandstone beds (marked by coarse Chicxulub ejecta)
and the K/T boundary (marked by an iridium anomaly) a normal claystone layer would exists
(labelled as interval F and G by Thor Hansen, who established the classic stratigraphy of
the Brazos K/T section). The cornerstone of Keller's arguments is that deposition of this
F/G claystone would have lasted 300kyr.

I have analysed the grainsize distribution of the same F/G claystone interval several
times, and every time I got the same results: the entire claystone layer is size graded,
with the largest grainsize at the bottom and the finest at the top near the iridium anomaly
(Smit et al 1996). This stands in stark contrast with the normal claystones above the
iridium anomaly and below the event beds, that are not graded at all.

Grading is due to settling of silt and clay-sized material out of a suspended sediment cloud
in the Sea, in our view stirred by the mega-tsunamis that resulted from the Chicxulub impact.
Such settling takes days to perhaps week, not 300.000 years.

Other arguments, same as used last year in the GeolSoc debate (http://www.geolsoc.org.uk/template.cfm?name=NSG2349857238495) include burrowed layers,
magnetostratigraphy and Cretaceous foraminifers. These arguments are easily countered.

Burrows can originate between the individual tsunami waves surges, that easily last 1 hour,
plenty of time for oprooted organisms to dig in the seafloor again (to be uprooted again by
the next wave!)

Foraminifers in F/G are all of Cretaceous affinity, and like the silt and mud stirred by
the tsunamis, will slowly settle to the seafloor in interval F/G. Thus, no evidence for
indigenous foraminifers!

Magnetochron 29Reversed straddles the KT boundary from roughly 350 kyrs before to 350 kyr
after the K/T boundary. So anything happening in that 700Kyr interval has a reversed
magnetic signature, including the Chicxulub impact, K/T boundary, and claystone F/G.
Thus nothing argues for either pro or contra Chicxulub being the KT crater.

J. Smit et al., in The Cretaceous-Tertiary Event and Other Catastrophes in Earth History G. Ryder, D. Fastovski, S. Gartner, Eds. (Geol. Soc. of Amer., Boulder, 1996), vol. Sp. Pap. 307, pp. 151-182.

So, once again, this is not a "remakable discovery", but a rehash of the same, erroneous arguments.

Sincerely,

Prof. Dr. J. Smit
Department of Sedimentology
Faculty of Earth and Life Sciences
Vrije Universiteit, de Boelelaan 1085
1081HV Amsterdam, the Netherlands

(Geller) Dear Benny

There is nothing better in science than to have the weight of empirical
evidence proof or disproof a popular theory.

But the hardest thing in science is to convince the true believers and
main proponents of a popular theory that the evidence doesn't support it.
Frequently, there is gut reaction denial of the existence of the evidence,
expressions of disbelief and some go as far as discrediting the messenger.
But every once in a while some scientists realize that the weight
of the evidence is lined up against the popular theory that the
Chicxulub impact is the KT killer that caused the mass extinction.
Hermann Burchard is such a scientist and it took some courage to publicly
retract his former doubts on CCNet.

Keller, Adatte, Stinnesbeck and others (2003, 2004) have shown that the
Chicxulub impact predates the KT boundary by about 300,000 years based on
the Chicxulub impact crater core Yaxcopoil-1, KT sections throughout NE Mexico
and now also in Texas along the Brazos River (AGU, 2005, see write-up by Rex
Dalton in NATURE NEWS.
http://www.nature.com/news/2005/050523/full/050523-2.html

New drilling by DOSECC this spring and investigations of new outcrops
along tributaries of the Brazos River by Thierry Adatte, Tom Yancey,
Jerry Baum and myself have uncovered outcrops that detail the KT boundary,
storm event beds (formerly called KT impact tsunami deposits), and the original
Chicxulub impact ejecta layer. The three events are separated by laminated
fossiliferous shales with the KT boundary and Ir anomaly up to 1.6 m
above the top of the storm event beds and the original Chicxulub impact
ejecta layer at least 45 cm below the base of the storm event beds. The storm
event beds have been studied extensively by Tom Yancey (l996) and Andy Gale
(2005); both came to the conclusion that these bioturbated sandstone
layers were deposited in separate storm events over an extended time
period and not related to the KT boundary impact.

The evidence indicates that two major impacts occurred about 300,000
years apart: the one at the KT boundary is marked by a global Ir anomaly.
The earlier Chicxulub impact is never associated with an Ir anomaly, but is
known by its breccia in the crater and glass shards and spherule ejecta
throughout the Caribbean, Central America and southern US. There is a precedent
for multiple impacts in the late Eocene (originally discovered by Keller et
al.,1983) with large craters known from Popigai and Chesapeak Bay. The
end-Cretaceous appears to have been a time of multiple impacts and massive
volcanism.

Gerta Keller

References:
Gale, A.S., 2005 Proceedings of the Geologists Association (in press)
Keller et al., l983, Science 221, 150-152.
Keller et al., 2003, ESR 62, 327-363.
Keller et al., 2004, PNAS 101(11), 3753-3758.
Yancey, l996, Gulf Coast Assoc. of Geol. Societies 46, 433-442.

Gerta Keller
Department of Geosciences
Princeton University
Princeton NJ 08544, USA


Website:
http://geoweb.princeton.edu/research/Paleontology/paleontology.html

The Chicxulub Debate:



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« Reply #27 on: March 18, 2007, 10:24:44 pm »

Update on the possible Falkland craters.

In his book "Rogue asteroids and doomsday comets" Duncan Steel refers to investigations by Michael Rampino (then at the Goddard Institute of Space Studies): "The appropriate antipodal point to Siberia (Siberian Traps 250Ma) is
the Falkland plateau off Tierra del Fuego... on that plateau Rampino has identified two sub-oceanic circular basins with diameters of about 300km and 200km respectively. Dating the rocks indicates the same age as the Late Permian Extinction." He cites a Rampino paper in the book "Hazards due to Comets and Asteroids" called "Extraterrestrial impacts and mass extinctions" - this may contain more information.
I cannot find any recent references to this subject. It seems that the same issues that delayed discovery of Chicxulub are occurring in the case of the Falkland Plateau - commercial and political sensitivities. The area has potential oil reserves and is also very sensitive politically. Altimetry map of the Falkland Plateau from Delft Institute for Earth-Oriented Space Research.
CCNet 21 Oct 1999
(1)With reference to Michael Paine's communication (CCNet 20.10.1999), several indicators exist for extraterrestrial impacts contemporaneous with the Permian-Triassic boundary - which saw the largest extinction recorded in Earth history - including:
 1.  Araguainha impact structure, Brazil - ~247 +/-5.5 Ma; D=40 km (see R.A.F. Grieve's crater listing).
2.  Lorne Basin - New South Wales - a candidate P-T boundary impact  structure, 35x30 km large (Tonkin, P.C., 1998, Aust. J. Earth Sci.  45, 669-671).
3.  shock features in quartz (PDF) along the P-T boundary in Antarctic  and New South Wales (Retallack G.J., Geology, Jan. 1999; for other  references re-P-T boundary conditions refer to Retallack and Krull, 1999, Aust. J. Earth Sci. 46:785-812.
4.  Weak Ir anomalies reported from China and Japan, remaining  unconfirmed due to possible analytical problems.
On the basis of known stratigraphic constraints, more than one impact structures may prove to be of a P-T boundary age by future isotopic age studies. The Falkland structure (M.R. Rampino) and Bedout structure (off
NW Australia, J.D. Gorter) are only candidate P-T impact structures inferred from geophysical and in the latter case drilling data, as yet unconfirmed and undated. As yet the magnitude of the confirmed impact/s is not large enough to link them to the P-T boundary extinction and/or as triggers of the Siberian volcanic traps (248.4+/-2.4 Ma), although it
is definitely possible further crater/s identification and isotopic dating may shed light on these questions.

Andrew Glikson, Research School of Earth Science,Institute of Advanced Studies, Australian National University

(2) POSSIBLE FALKLAND IMPACT STRUCTURE
There is a large, circular gravity anomaly on the Falkland Plateau that resembles anomalies associated with large impact craters.  It is quite large; greater than 200 km in diameter.

The basin that is indicated could be Late Paleozoic or Early Mesozoic in age, but not much more is known about it. Recent papers have suggested that it is of tectonic origin, but more study is needed.

I suggested that it might be an impact structure, and should be more closely studied back in 1992.

Dr. Michael R. Rampino


See also Duncan Steel's book "Rogue asteroids and doomsday comets"



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« Reply #28 on: March 18, 2007, 10:25:31 pm »

Impacts and vulcanism
CCNet 15 Feb 2001
P/T BOUNDARY: NO IRIDIUM

From Hermann Burchard <burchar@mail.math.okstate.edu>


Dear Benny,

In Permian/Triassic boundary strata in South China, the element iridium is not present or at most only in trace amounts, according to Doug Erwin, who kindly responded to my e-mail question. This can be understood, as I would like to suggest, by noting certain connections with the iridium-rich Hawai'i hotspot, which has been moving in a SE direction across the Pacific for >100Ma, probably 225Ma, starting off from Sibiria.

As mentioned by Victor Clube and Bill Napier in their book "Cosmic Winter", magmas from the great Hawai'i volcanoes are rich in iridium. They discuss this, because it's an argument against cometary impact as a cause of the abundance of the element in extinction layers, such as the famous K/T-boundary.

There is a clear trace on the floor of the Pacific ocean beginning with the Emperor Seamount chain from the Kamchatka Peninsula to Midway Island, then angling off in a slight left turn along the Hawai'ian island chain. Although the trace possibly is now partly subducted in the Kamchatka - Aleutian trench, it seems clear enough that the hotspot was originally positioned in Eastern Sibiria.

Underlying the hotspot is a mantle plume which presumably was created when a cosmic body hit Sibiria and created the vast flood basalts of Yakutia (Sakha).  See the article by Renne et al. in "Science", 1995, 269:1314, for a map of the conjectured extent of the original lava beds, which may not have been fully explored.  These cover Yakutia (Sakha), bordering directly on the Sea of Okhotsk near Magadan, immediately adjacent to the present day NW-terminus of the Emperor Seamount chain. From my less than adequate maps, the basalt beds seem to abut on or even include the Kolyma gold and diamond fields; diamonds have been studied in connection with impact sites e.g. by Christian Koeberl.)

Therefore, little doubt can exist concerning the essential identity of the following events:

          1.  Inception of Hawai'i hotspot in Sibiria.
          2.  Sibirian flood basalt eruption.
          3.  Cause of P/T mass extinction.

We owe the identity of 2. and 3. to the work of paleobiologists like Doug Erwin. Here, we wish to explain that event 1. probably was a cosmic body impacting in Sibiria - more precisely a spot in Gondwana-land which became present-day Eastern Sibiria.

Much of the meteoritic material from the comet or asteroid, that struck Earth at the P/T transition, appears to remain still in the hole punched in the upper mantle by the cosmic impact body, the Hawai'i hotspot (I sincerely doubt that this will seem like a very novel idea in the minds of many geologists).

Hence we may conclude:

   [A] Iridium continues to be pumped upward with deep mantle material in Hawai'i volcanoes to this day.
    Little of the cosmic material was thrown into orbit at impact time, because of uniquely deep penetration of the giant P/T impactor.
   [C] Iridium cannot be traced in the layers separating Paleozoic and   Mesozoic rocks, never having been dispersed to a great extent.
   [D] Rather than refute it, as Clube-Mapier feared, abundant ir in the  magmas from Hawai'i confirms the impact theory of mass exinctions.

The relationship between impacts and hotspots is perhaps still somewhat controversial, so I will attempt to elaborate on this. Hotspot physics and geology is probably not a perfect science.  If I understand it correctly, the main mechanism is the same as in spreading or rift zones:
Pressure on the upper mantle is relieved as the minerals rise with reduced overburden, causing a phase transition which we see as melting. The causes of pressure release are somewhat different in the two arrangements of a) impact related hotspot and b) rift zone.
In case b) of a rift zone one possible initial cause of reduced pressure seems to be thinning of continental crust due to erosion of a stable craton over many 100Ma. The African rift valley is a case in point. In North America, at the beginning of the Jurassic era, the Atlantic ocean first began as a rift, with the margin seen today e.g. in the New York palisades
rock facade. At present, the New Madrid fault along the middle Mississippi may exemplify the same phenomena at an early stage. However, a rift may begin with a hotspot, as one other interpretation of the Great African Rift suggests! One hotspot is in the Afar region.

In case a) of an impact-induced hotspot the initial step is that the impact events destroy the phase equilibrium of the upper mantle in a narrow region underlying the crater.  This may be due partly to the shock wave of impact upsetting crystalline structures, or partly because surface rocks are excavated and removed by the impact explosion.  A massive melt results in the form of flood basalts from the suddenly relieved pressure, and/or from impact shock wave induced phase transition. The effect is a snowballing phase transition and melting.

Again, to avoid misconceptions, and because this does seem to remain controversial in some circles, it should be emphasized that:
       The melt in the plume after impact is _NOT_ caused by the initial  energy yield of impact, but rather by the reduced pressure which  forces a phase transition to take place that ends up in a phase  equilibrium at a lower Gibbs energy.

(Other views [as in Renne et al.] present a picture of a spontaneous rise of mantle, liquifying over a huge area, for which no account of origin can be given.  This can be considered for basalt floods, but could not explain narrow-bounded hotspots.)

Once in operation, lower mantle material appears to be resupplied continually from the sides to the punch hole, which maintains a pore where the pressure remains lower than in the surrounding mantle.  Thus the plume can rise indefinitely, as we see happen today in Hawai'i.

Any computations of effect of impact on the mantle not modeling phase equilibria and transitions should be treated with suspicion. Above description of mantle plumes, to make a disclaimer, is conjectural, not substantiated by actual computation.  My limited understanding of these things is based on a study of stable computation of phase equilibria, working with a petroleum engineer, on computing "flash" crude oil separation.

Although apparently still controversial, years ago already I have heard mention made by geologists of the connection of hotspots and impacts, as in the example of the Yellowstone hotspot, now in Wyoming, that has travelled East along the Snake river plateau for > 10Ma, and that is implicated in the flood basalts in Western Idaho and probably Washington State (?).

(Unfortunately, I was unable to attend the February 9 RAS conference on impacts, where Christian Koeberl was keynote speaker. I missed talks by Adrian Jones and Simon Kelly on impact, flood basalt, & hotspot related topics, that might have led me to improve this account).

Best regards,

Hermann G.W. Burchard
burchard@math.okstate.edu

See a response by Andrew Glikson, Item 12, CCNet 23 Feb 01. He questions the P/T link.

IMPACT VOLCANISM CLARIFICATION

>From Mark Boslough <mbboslo@sandia.gov> CCNet 1 Mar 01

A few inaccuracies crept into the March Scientific American article.

1) Our seismic focusing calculations showed that the peak in seismic energy
dissipation is in the asthenosphere both antipodal and directly beneath the
point of impact. We suggested that for a sufficiently large impact the
increased melting in the asthenosphere would be a significant contributor to
any impact-induced volcanism, but we did not speculate about effects on
pre-existing plumes or extinctions (although these ideas are worth
considering). Our idea was that a narrow column of hotter mantle could
create an instability that *looks* like a plume (as opposed to a classic
fluid plume that pushes its way up from the CM boundary).

2) I'm not sure where the "may not have been antipodal" phrase came from.
The impact antipode was clearly something like 30 degrees from the Deccan
Traps at the time of the K/T boundary.  If the Deccan Traps are
impact-induced it was not the Chicxulub impact (which came too late and in
the wrong place!) but an earlier impact either into the east Pacific or into
India.

3) We suggested that an impact might generate the same surface
manifestations normally associated with mantle plumes (i.e. flood basalts
and long-lived hotspots). We did not connect them to superplumes which is
what Dallas Abbott proposed. It was Jon Hagstrum of the USGS who suggested
the connection to sea level, weathering, ocean chemistry, sediments, etc.
The ideas of Abbott and Hagstrum are also interesting worth considering--but
they're not mine as the article implies.

Mark Boslough
Sandia National Laboratories
 
 

Can Impacts Induce Volcanic Eruptions?
 Authors: Melosh, H. J.
 Journal: International Conference on Catastrophic Events and Mass Extinctions: Impacts and Beyond, 9-12                   July 2000, Vienna, Austria, abstract no.3144
Available from ADS Abstract service.
Conclusions: The bottom line of this discussion is that there is not a
single clear instance of volcanism induced by impacts, either in the
near vicinity of an impact or at the antipodes of the planet. This
accords well with theoretical expectation from our current understanding
of the impact cratering process. The possibility of impact-induced
volcanism must thus be regarded with extreme skepticism.


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« Reply #29 on: March 18, 2007, 10:26:37 pm »

The Shiva Hypothesis - Periodic Mass Extinctions



The Jan/Feb 98 Issue of Planetary Report has an article by Michael Rampino "The Shiva Hypothesis". This describes a 30 million year cycle of mass extinctions over the past 540 million years (see diagram). One hypothesis is that this corresponds the the solar system oscillating through the galactic plane as it orbits the Milky Way.  Rampino notes that the last crossing of the galactic plane occurred a few million years ago and it has been suggested that this led to a disturbance of comets in the Oort Cloud, some of which could now be approaching the inner solar system.

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