Visions of the Cosmos

Life in the Universe

Mass Extinctions

Despite it's overall advancement, life has suffered several severe setbacks, in which huge numbers of species have suddenly died out.   These events are called Extinctions.   Five of them are described as MASS EXTINCTIONS because of their severity 

They were:-

The Precambrian extinction during the 'Snowball Earth Glaciation' about 600 million years ago

The Vendian Extinction, which is problematical, about 543 million years ago

The Cambrian Extinction around 510 million years ago –called the first mass extinction

The Ordovician Extinction around 438 million years ago –called the second mass extinction

The Devonian Extinction around 360 million years ago – called the third mass extinction 

The Permian extinction around 286 million years ago –called the fourth mass extinction

The Cretaceous Extinction around 65 million years ago – called the fifth mass extinction

         The Toba Volcano occurred about 75,000 years ago. Is of importance to the human species.

         The Holocene Extinction that is happening now – called the sixth extinction by some biologists

 

Geologists and astronomers have puzzled about the reasons for Extinctions and have advanced many theories to account for them.

They can be divided into two possible major causes

 Catastrophic agents occurring from space outside the Earth such as impacts by comets or asteroids, solar flares or coronal mass ejections, nearby supernova or nearby neutron star collisions, and molecular clouds.

Agents within the planet itself such as volcanism, glaciation, global climate change, variations in sea level, variations in the oxygen content of the sea water and changes in the salinity of sea water and, at the present day, by the actions of the human species on the global environment.  Another possibility is a global pandemic affecting a large number of species.

 Catastrophic Causes from Outside Space

 The following theories will be discussed:-

The Impact Theory

Massive Solar Flares and Coronal Mass Ejections                                  

Switches in the Earth's Magnetic Field

The Supernova Theory

Gamma Ray bursts.

Impact Theory 

The extinction which has been most studied is that which took place at the end of the CRETACEOUS PERIOD.  Many theories have been advanced in the past for the reason why the dinosaurs and hosts of other orders of animals were wiped out at the end of this period. Although there appear to be conflicting views about exactly what caused the demise of the dinosaurs it now seems certain that there was a massive impact by either a comet or an asteroid at the end of the Period.  It is also clear that around this time there was an enormous outpouring of lava in the Indian Tectonic Plate.   Whilst the impact theory is now almost universally accepted, it is possible that huge volcanic activity was taking place at the same time and could have played a large contributory factor in the Cretaceous Extinction.

The Cretaceous Extinction - The Death of the Dinosaurs

 Sixty five and a half million years ago at the end of the Cretaceous period, a dramatic change occurred.    Not only did the dinosaurs die out, but also whole groups of other animals such as the pterosaurs, elasmosaurs and pliosaurs (the ichthyosaurs and plesiosaurs died out earlier in the Cretaceous) vanished from the face of the Earth.  Large groups of marine creatures such as the ammonites also disappeared at the end of the Cretaceous.

Within a very short time a completely new flora and fauna appeared and the ecology of the planet was changed forever. The land mammals replaced the dinosaurs and aquatic mammals such as the whales and dolphins took the place of aquatic reptiles.  The birds, who are now regarded as very close relatives of the dinosaurs, filled the ecological niche left by the extinct pterodactyls. The death of the old age ushered in the birth of the new.

Ten million years after the demise of the dinosaurs the world had changed enormously.  Things were beginning to look a lot more like they do at the present time. It is ironic that the cataclysmic events that occurred at the end of the Cretaceous allowed the acceleration of the evolution of the mammals.  It is a salutary thought that if the dinosaurs had not been annihilated, the higher mammals, including the human species might never have evolved to the extent that they have.

In 1979 Luis Alvarez, his son Walter and their colleagues, Frank Ashore and Helen Michel were working on a series of rock strata separating the Cretaceous and Tertiary deposits at a site in Gobi, a small village near Perugia  in Italy. They studied a stratum that was between the Cretaceous and the Tertiary.  It has now become famous throughout the scientific world and is known as the K/T boundary layer.  The team discovered that in the K/T layer there were extremely large amounts of IRIDIUM and other rare elements compared to that normally found on the Earth's surface. 

The initial discovery by the Alvarez team was followed by the investigation of many other sites throughout the world where the K/T Boundary occurs.  Some of them are listed below    

GUBBIO in the Apennines near Perugia Italy

Clear Creek North in Colorado U.S.A.

HØJERUP CHURCH at STEVNS KLINT 50 kilometres south of Copenhagen

WOODSIDE CREEK 40 kilometres northeast of Wellington New Zealand

CARAVACA in Northern Spain

EL KEF in Northern Tunisia

        CAVES NEAR MAARSTRICHT in the Netherlands

        PACIFIC DEEP-SEA CORES  Hole 465 in the Pacific Ocean between Yokohama and San Francisco.

        MAARSTRICHT caves in the Netherlands

A spectacular discovery of the K/T boundary layer was made in 1992 near Maarstricht in the Netherlands   An entry to the site at the Geulhhemmerberg Caves (actually artificial caves or underground galleries) was made by an expedition of members of a the First international Conference on Impact Cratering in the Solar System.  It was the 40th ESLAB Conference and was jointly organised by ESTEC of the European Space Agency and was led by Professor Jan Smit of the University of Amsterdam. The illustrations below were taken by the author (Ray Goodwin).   The middle photograph shows the boundary layer clearly

Entering the caves near Maarstricht.  Field trip May 2006 Organised by Jan Smit University of Amsterdam for delegates to ESLAB/ European Space Agency Conference on Cratering in the Solar System

The K/T Boundary Layer in the Caves

Leaving the Caves after the field trip May 2006 Organised by Jan Smit University of Amsterdam for delegates to ESLAB/ European Space Agency Conference on Cratering in the Solar System

Studies of the boundary layer between the Cretaceous and the Tertiary have led to the conclusion that  an asteroid  travelling at 50 times the speed of sound 15 kilometres in length or a comet travelling at 170 times the speed of sound hit the Earth 65.5 million years ago. It has been found that a layer of clay rich in iridium and certain other chemical elements settled all over the planet following the impact of the asteroid or comet in the Gulf of Mexico near the Yucatan Peninsular.  It is usually referred to as the IRIDIUM LAYER.  Concentrations of Iridium Osmium, Arsenic, Selenium and Tin were found to be extremely far above the average values for the Earth's Crust.  It has not yet been established whether the event at the K/T Boundary was caused by an asteroid or a comet.   The Shoemaker-Levy comet, which hit Jupiter in 1995, broke up into 22 pieces before impact.  It is possible that the crater in the Gulf of Mexico may not have been the only impact site.  The disaster, which destroyed the dinosaurs, could have been accompanied by more than one hit if the projectile broke up into several pieces before impact in a similar way to the Jupiter comet

The study of the iridium layer illustrates the vital importance of chemistry in understanding both the geology and the history of the Solar System.  The chemical element IRIDIUM is rare in the Universe as a whole.  However, percentage-wise it is far more common in meteorites than on the Earth's surface.   Also its percentage as a ratio of other elements in the photosphere of the Sun and many other stars is far higher than in the surface layer of the Earth. Not only iridium but also osmium, arsenic, selenium and tin were far higher in the K/T boundary layer than in the normal rocks of the crust.  This is explained by the fact that they are strongly siderophilic ('iron loving') elements - that is to say they dissolves in iron and tends to migrate with iron and nickel to the core of planetary bodies. Their concentrations are believed to increase with depth through the mantle and finally to the core. To test for the iridium and the other rare elements, highly sensitive analytical techniques were used such as Neutron Activation Analysis (NAA) and  Laser ablation, Inductively coupled plasma, Quadrupole mass spectrometry (LA-ICP-QMS).   To give some idea of the meaning of rarity from a geochemist's point of view - an average granite rock may contain only 0.006 parts per million of iridium which means that it would be necessary to concentrate 6,000,000 kilograms of rock to obtain 1 gram of iridium.

The blast resulting from the impact would have been hundreds of millions of times more devastating than the most powerful nuclear weapon ever detonated.   It would have created a hurricane of unimaginable fury and certainly would have thrown massive amounts of dust and vapour into the upper atmosphere and even into interplanetary space.

Huge tsunami waves swept over the Yukatan Peninsular, the Caribbean and the coast of North America carrying rocks huge distances from their place of origin.

Gary Patterson of the University of Memphis, Tennessee recently studied  large xenolithic boulders, 120 kilometres northeast of Little Rock in Arkansas.   They were up to seven metres in diameter and perched up on a 76 metre high hill.  Patterson observes that the SANDSTONE boulders contain a mineral called GLAUCONITE, which is typical of coastal deposits, and that the rocks had their origins many hundreds of kilometres to the south.   He suggests that they were washed there by the immense tsunami waves, which must have spread outwards in all directions from the impact site in the Gulf of Mexico. (Reported in The New Scientist 7 Nov 98) 

A massive heat pulse accompanied the blast waves from the impact and firestorms raged all over the planet.

        In 1984 Wendy Wolbach and her co-workers at the University of Chicago showed that deposits collected at K/T boundary sites contained large amounts of soot.  It was most likely formed by immense coniferous forest fires, which ignited following the impact.  The nature of the soot from the New Zealand and European sites is very similar and bears out the theory that immediately after the impact global fires enveloped the planet.  

       Analyses of fluid inclusions in ancient amber suggest that the oxygen content of the atmosphere was very high (30-35%) during the late Cretaceous.  This high O2 level would have supported intense combustion. The level of atmospheric O2 plummeted in the early Tertiary Period.

Evidence for changes in Atmospheric Oxygen Concentrations  is admirably summarised in the United States Geological Survey website http://minerals.cr.usgs.gov/gips/na/aii-indx.htm# space

 Minute bubbles of ancient air trapped by successive flows of tree resin during the life of the tree are preserved in amber. Analyses of the gases in these bubbles show that the Earth’s atmosphere, 67 million years ago, contained nearly 35 percent oxygen compared to present levels of 21 percent. Results are based upon more than 300 analyses by USGS scientists of Cretaceous, Tertiary, and Quaternary amber from 16 world sites. The oldest amber in this study is about 130 million years old.

This chart shows a major decrease in oxygen content in the atmosphere from 35 percent to the present day level of 21 percent. This decrease occurred about the same time that the dinosaurs disappeared 65 million years ago. 

From the section 11c Environment -- air bubbles, amber and dinosaurs by Gary Landis in the USGS website compiled by Joseph Taggart entitled Understanding our Planet through Chemistry

From the section 11a Environment – Disaster from Space by Glen Izett in the USGS website compiled by Joseph Taggart entitled Understanding our Planet through Chemistry

Website http://minerals.cr.usgs.gov/gips/na/aii-indx.htm#space

The worldwide cloud would have blocked sunlight for months, decreasing photosynthesis and thus depleting food resources. This period of reduced sunlight, would have resulted in an ''impact winter' and may also have been a factor in the extinctions. Gradually skies cleared, but greenhouse gases from the impact probably raised temperatures for many years.

From the section 11a Environment – Disaster from Space by Glen Izett in the USGS website compiled by Joseph Taggart entitled Understanding our Planet through Chemistry

Website http://minerals.cr.usgs.gov/gips/na/aii-indx.htm#space

A worldwide cloud of dust reducing the sunlight, would have resulted in an ''impact winter' and may also have been a factor in the extinctions. Gradually skies cleared, but greenhouse gases from the impact probably raised temperatures for many years.

In 1984 further parts of the jig-saw puzzle were filled in by Bruce Bohor and his colleagues at the U.S. Geological Survey in Denver, Colorado. 

In investigating the deposits at the K/T Boundary, Glen Isett and his team found 'shocked quartz crystals' at Raton Basin Colorado/New Mexico.  Atoms in the quartz were disturbed under the enormous pressure waves created by the comet or asteroid.   Shocked quartz has only been found at impact sites and at the sites of nuclear explosions. 

Shocked Quartz

Shocked quartz crystal from the K/T boundary layer of the Raton Basin, Colorado/New Mexico.
Photo of petrological microscope specimen by Glen Isett, US Geological Survey  from Geology Volume 14 No 3 Quoted in website
http://www.lawrence.edu/dept/environmental_studies/breccias....  

 

In 1991, Haraldur Sigurdsson of the Institute of Oceanography in the University of Rhode Island discovered the presence of silica/calcium rich micro-tektites in the K/T Boundary in Haiti.  They consisted of tiny glass spheres around 0.2 cm in diameter.  It is believed that a loose rock called MARL containing clay and calcium carbonate was melted and ejected into the atmosphere resulting in a world-wide rain of these small glass objects called tektites.

The Chicxulub Crater

In 1978, before the discovery of the special features associated with the K/T Boundary Layer, a number of oil survey scientists searching for the presence of oil in Mexico discovered signs of a large impact crater hidden beneath the sea in the Gulf of Mexico. 

Magnetic and gravity studies were carried out and it was found by Glenn Penfield and co-workers that a massive crater did in fact exist in the Gulf of Mexico near the Yucatan Peninsular.

It was finally established from core samples taken from the sea-bed that andesite containing shocked quartz existed in the assumed crater.  Radioactive dating using the 40K/40Ar system showed it to be the same age as the K/T boundary (65 million years old). The Crater was named the Chicxulub Crater translated from the Mayan language as 'the tail of the devil'!   

 Other Impacts

It has been suggested that several other disasters may have been caused by impacts.  Evidence for a possible crater at the Jurassic/Cretaceous Boundary has been advanced.  The end of the Eocene saw a degree of extinction and evidence for two large impacts around that time have been advanced.  In looking for possible impacts, Rettalak and Krinsley have put forward suggestions of an impact at the end of the Permian.

Some authorities have suggested that the Earth is in more danger at certain times than at others and have advanced various theories to back this up.  Since this has direct relevance to the position of the Sun with respect to other stars and to gas clouds in the galaxy these ideas are well worth considering.

‘Star of Ill Omen’

 Since the dawn of history people have distinguished between the planets or 'wanderers' and the much larger number of 'fixed stars'; so called because they did not appear to change their positions in the sky.  Even over historical time, they do not seem to have altered and we still recognise the constellations described by our ancient forebears in Greece over 2,000 years ago.  In fact, they do change with time as the Sun and all the other stars pursue their separate journeys round the Galaxy.  This change has actually been measured by very sensitive astrometric techniques by instruments carried by the Hipparcos satellite. It is only the gigantic distances between the stars that make it appear that they are fixed in the same positions forever.  If we could have viewed the heavens as seen by the dinosaurs at the time of their demise we would have beheld a completely different picture of the sky to the one we see at the present time.

A vast number of comets are believed to orbit the Sun at a distance of about 0.5-1.0 light-years in THE OORT CLOUD.   From time to time other stars may come close enough to disturb the comets in the Oort Cloud.  As an unwanted visitor brushes past the frontiers of the Solar System, it will disturb the comets.  Some will change their orbits and be slung outwards into interstellar space.  Others will be catapulted towards the Sun.  Those, which pass the gravitational barriers of Jupiter and the other giants worlds, will enter the Inner Solar System, thus placing the Earth and the other terrestrial planets in danger of a massive impact. 

Another possibility is the 'Star of Ill Omen'.   It has been suggested that a very small star, a brown dwarf or a massive gas giant far larger than the planet Jupiter orbits the Sun at between a half and one light year and that this disturbs the Oort Cloud periodically every 26 –30 million years – the so called ‘death star’ theory.

Galactic Tidal Effects

John Maltese and Daniel Whitmore of the University of South Eastern Louisiana, Lafayette, have proposed another version of disaster caused by comets.  They pointed out that during its journey through the Galaxy the Solar System bobs up and down through the plane of the Milky Way about once every 30 million years.   They suggest that tidal effects are created which disturb the comets in the Oort Cloud.  This would have the result of catapulting many more comets than usual towards the Inner Solar System, which will mean that there is a much greater statistical chance of the Earth being hit by one.  So the theory suggests that every 30 million years or so there is a greater chance than usual of disaster.

The Danger of Future Impacts

Even allowing for its typical Hollywood approach, the film 'Deep Impact' brought home to people at large that there is always a possibility that a giant impact could occur at any time. As prophesied in the film, the time should soon come when we shall have the technology to divert any near-Earth objects and prevent us going the way of the dinosaurs.

Operation Planet Watch

The object that hit the Earth at the end of the Cretaceous Period is believed to have had a mass of about 1020 tonnes and to have been travelling at around 45,000 miles per hour when it hit the planet.  It landed in the shallow sea in the Gulf of Mexico.  The Tsunami waves rose to heights of around 30 metres (a hundred feet approximately) and carried huge boulders from the coast  many kilometres inland.   The huge heat generated set the forests alight all over the planet (evidence from Italy, Denmark, New Zealand and North America show that this effect was world wide).  This terrible fire was followed by the deposition of dust over many years and the light of the Sun was cut off causing the equivalent of a 'nuclear winter'.  The recovery took several thousand years and the flora and fauna of the planet changed.  After ten million years at the end of the Palaeocene Period the planet began to look like modern times and the name given to the next period is the Eocene which means 'The Dawn of the Recent'.  Between then and now there have been several large impacts which have caused damage on a continental scale but nothing compared to the one which occurred at the end of dinosaur times.

The world is now populated by so many human beings that the developed countries of North America, Western Europe, Australasia, Japan and Russia are joining together in Project Planet Watch to scan the skies for asteroids or comets which may threaten disaster on a scale large enough to wipe out cities as well as far larger ones which could cause global devastation.

         Even if the Cretaceous disaster was one of the biggest, evidence has been found of more recent events.  On 30 June 1908 something exploded in the atmosphere above the Tunguska River Valley in Siberia.  The Tunguska explosion levelled over 2,000 square kilometres of Siberian forest, an area equivalent to Greater London.

     Illustration London and the M25

An expedition led by Professor Leonid Kulik of the Soviet Academy of Science travelled to the impact site in 1930.  Even then the devastation was immense.  Kulik surmised that a meteorite had been the cause of the destruction but could find no trace of a crater. Scientists now believe that the object was reduced to dust by its terminal explosion. This dust was ejected high into the atmosphere and spread around the globe by stratospheric winds. More recent expeditions to the impact area have recovered minute particles of dust from the meteor. This dust has been compared with similar material found in Antarctica (deposited there as the material settled to earth) and was found to be a close match. Both samples contain relatively large amounts of iridium.

The blast is now thought to have been caused by a stony body some 50 meters in diameter, that exploded with the force of 15-30 megatons of TNT, 6 km above the ground, at a spot 60o 55’ North, 101o 57’ East.

Summary of Factors supporting the Impact Theory for the Cretaceous Cataclysm

The Iridium Layer indicates that a body containing relatively large amounts of Iridium and other elements hit the Earth

 

The Soot Layer indicates that trees all over the planet caught alight

Shocked Quartz can only be caused by a massive impact or a nuclear explosion

The evidence for the presence of a Tsunami. Xenoliths have been found in the form of huge boulders, which could have only been deposited by a tsunami (tidal wave or seismic wave).  Xenoliths are boulders that have been carried a  long distance from their original place of formation.

Very sudden changes of fauna between the uppermost Cretaceous and lowermost Tertiary indicate a sudden cataclysmic cause for the extinction rather than a slow process.

The Existence of the Crater  There is now clear evidence that a large crater exists beneath the sea in the Gulf of Mexico which was formed at the time of the deposition of the K/T Boundary layer (potassium/argon dating)

Magnetic Field Reversals

Every so often the Earth's magnetic field reverses.  For brief periods of time during the change the field is very weak.  If a massive Solar Flare occurred during such a time, living creatures could be subjected to lethal doses of fierce radiation from the Solar Wind. 

The question of solar flares was discussed at a meeting of the American Astronomical Society in Austin Texas in January 1999.  It was pointed out that many apparently sun-like stars are subject to much more erratic behaviour than our Sun and exhibit violent flare activity.  The famous flare activity of the Sun in 1989, which knocked out a power grid in northern Quebec and disrupted communication satellites was mild by comparison with many of our Sun's sister stars.   A survey of single stars in our galaxy with roughly the same composition and mass of our Sun has shown that many of them are subject to violent stellar superflares about once per century on the average and it has been suggested that our Sun may be the exception rather than the rule.  If such flares occurred on the Sun some of them would be violent enough to melt the ice on the moons of Jupiter and destroy half the ozone layer and most of the communication satellites.

 The Supernova Theory

It has been suggested that, in the event of a supernova explosion very close (about 30 light years) to the Solar System, the Earth could be drenched with harmful radiation. Great damage could be caused to living organisms to the extent that large numbers would die out after certain vital parts of the food chain had been virtually destroyed.  This could account for the 100% mortality rate of certain groups and the survival of others.  Some evidence could perhaps remain in the form of relatively large amounts of rare elements such as iridium occurring in strata laid down at the time that the ejecta from the supernova hit the Earth.   That this was not the cause in the case of the Cretaceous event is almost conclusively proved by isotopic evidence.  In the layer of the clay which occurs at the Cretaceous/Tertiary Boundary there are relatively high concentrations of the very rare metals, iridium and osmium.  Measurements of the ratios of the pairs of stable isotopes of the two elements 191Ir/193Ir and 184Os/190Os show them to be the same as that found in chondrites and in the photosphere of the Sun.  In simple language, this means that they are of Solar System origin, which would be true if a Solar System asteroid or comet hit the Earth.  It would however be highly unlikely to be so if the four isotopes were the product of a rain of high-energy particles from a source outside the Solar System such as a supernova or a collision of a pair of neutron stars.  This is simply because all such events are likely to produce an individual mix of isotopes which will be different to the mix in our Solar System.

It was reported in the New Scientist of 10 July 1999 that metallic sediments laid down during the last 13 million years were dredged up from the seabed close to the island of Mona Pihoa in the South Pacific. The sediments were analysed using a specially designed accelerator mass spectrometer and found to contain significant amounts of iron 60 a radioactive isotope with a half-life of about 1.5 million years.   Gunther Korschinek of the Technical University of Munich has advanced the hypothesis that the iron 60 may have been derived from a relatively recent supernova event.  He has suggested that he and his colleagues should examine the sediments for plutonium 244 and other isotopes that may be derived from supernovae.

 Gamma Ray Bursts

Even more violent than ordinary supernova are gamma ray bursts.  They could probably wipe out life if they went off at a distance of 3000 light years (twice the distance of the Orion Star Nursery)

There have been some suggestions that the Ordovician Extinction may have been caused by a gamma ray burst.

A gamma ray burst could have caused the Ordovician extinction, killing 60% of marine invertebrates at a time when life was largely confined to the sea.
These cosmic blasts are the most powerful explosions in the Universe.
The scientists think a 10-second burst near Earth could deplete up to half of the planet's ozone layer.
   We don't know exactly when one came, but we're rather sure it did come - and left its mark’   Quote from Dr Adrian Melott, University of Kansas
With the ozone layer devastated, the Sun's ultraviolet radiation could have killed off much of the life on land and near the surface of oceans and lakes.
Gamma ray bursts can be caused by the formation of a black hole by the collapse of a supermassive star or by the collision of two neutron stars  The gamma ray emission is highly directional and  could have easily caused the Ordovician Extinction by intense radiation damage from a distance of 6,000 light years.  Illustration Courtesy BBC News 11 April 2005

 

 
Colliding Neutron Stars

Somewhere in the depths of interstellar space, a rare event occurs from time to time. It is one of the most violent phenomenon of which we are aware.  Imagine two massive stars both over ten solar masses that started their lives together in a binary system.     First one and then the other would suffer a supernova explosion and each would leave behind as a remnant a neutron star.   A deadly time bomb would be bequeathed to the future - two neutron stars spinning round each other and getting ever closer.  For millions of years, they would orbit around each other, spinning ever faster.   It has been calculated that when they were at a distance of 800 kilometres from one another their speed would approach a third of that of light.  At this point, they would be close to their final stupendous nemesis and to the moment of their merger, which would make the brilliance of a supernova explosion look like a firefly at noon.  Slowly and inexorably, they would draw closer and closer to one another until their edges were almost touching.

In one catastrophic moment they would coalesce and become one - their masses merging as they disappeared into a black hole. In the moment of their incredible metamorphosis, they would eject as much energy into the interstellar medium in a single instant as all the stars in the universe. A gamma flash of unbelievable intensity would leave the merging neutron stars as a jet accompanied by a gigantic outpouring of life destroying sub-atomic particles and atomic nuclei travelling close to the speed of light. 

There is general agreement that if the Earth were between 1,500 and 3,000 light years in the path of the jet of such an event the intense radiation would extinguish all forms of life on the planet.  The immense burst of radiation would rip away the ozone layer and drench the surface with massive quantities of energy.   Such an event might spell the death of millions of life forms on countless worlds and injure countless more a little further out from the epicentre of the stupendous explosion.

Perhaps life on planets further out would suffer severe damage leading to a massive but only partial extinction.  Some have suggested that such an event could have killed the dinosaurs and the other creatures, who died out at the end of the Cretaceous.  However the same argument concerning the isotopic ratios for a nearby supernova applies equally to a neutron star collision.  The isotopic ratios of iridium and osmium in the K/T Boundary layer were 'solar system ratios'. which indicates that such an event did not cause the demise of the dinosaurs

The Death of a Massive Star

The other cause of a gamma ray burst is when a very massive star forms a'hypernova' (like a supernova but far more violent).  It forms a BLACK HOLE.   Many of the early stars were 30-100 times as massive as the Sun.  This was because they began with hydrogen and helium with almost no other elements present.  In astronomers language they had a very low metallicity.   It is usually only in early stars and in stars born in very small galaxies that massive stars of very low metallicity form.   The subject of gamma ray bursts formed the subject of a discussion group organised by Sir Martin Rees the Astronomer Royal in early September at the Royal Society   

Deterioration of the Environment.

 The greatest extinction of all time took place at the end of the PERMIAN PERIOD, 250 million years ago. Many authorities that accept the impact theory for the Death of the Dinosaurs argue that the Permian Extinction, may have had quite different causes.  Because of the convincing evidence for an impact at the end of the Cretaceous there is a tendency to assume that all great extinctions are brought about in the same way.  However, it does not follow that the Permian Extinction necessarily had the same cause as the Cretaceous.  So far, no clear evidence has been found to support the view that the Permian was ended by a major impact but strong evidence has recently been advanced that there was a massive impact at the end of the Permian Period.   However, there have been attempts to attribute an impact cause for the events that occurred at the end of this period. At a conference of the American Geological Society at Denver, Colorado, in November 1996 Gregory Retallack of the University of Oregon claimed that he and his colleague David Krinsley had discovered some evidence that an impact had occurred at the time of the extinction.

Towards the end of the Permian Period, the continental tectonic plates came together to form one vast continent called Pangea.  There was violent volcanic activity along the fault lines where the plates collided.  It is believed that the carbon dioxide content of the air and of the marine environment increased and that large deserts were formed.

From a complex mathematical analysis of the fossil record Richard Sole of the Polytechnic University of Barcelona and Per Bake of the Niels Bohr Institute in Copenhagen have concluded that the extinction was due to the internal dynamics of the global ecosystem.  A small extinction of a few species led to a kind of domino effect that resulted in the catastrophic annihilation of vast numbers of animals in the food chain.   Over 90% of the hard-shelled marine animals died out. The number of brachiopoda genera dropped from 225 to 10. The trilobites who had reigned supreme since the beginning of the Cambrian completely disappeared.

Despite the appalling conditions at the end of the Permian, which seemed to threaten all life on Earth, sufficient numbers of creatures survived to evolve into the next age.   During the Permian, some interesting evolutionary developments occurred in the land animals. It was during the Permian that the thecodonts evolved and divided into two branches, which gave rise to the dinosaurs.   A highly successful group of animals, the dinosaurs evolved after the Permian and dominated higher animal life for 175 million years.    Some authorities attribute the appalling conditions at the end of the Permian to violent volcanic activity or drops in the oxygen content and increased levels of carbon dioxide in the atmosphere.

 The 'Pandemic' Theory

One of the early theories for mass extinctions such as the annihilation of the dinosaurs was that whole orders of animals died out as a result of pandemic diseases.  One idea was surprisingly prophetic.  It was suggested that the dinosaurs and other orders of animals could have contracted a sexually transmitted virus, which destroyed their immune systems.  This was long before the coming of AIDS

 The Sixth Extinction

 We have seen how many hazards there are in space.  How grateful we should be to live on Earth in such a benign environment.  Let us learn more about the universe around us but do not let us be deluded into believing that our species is above nature.    If we work with it and learn from it we can advance to a bright and happy future.  We can learn how to overcome the threat of celestial bombardments and violent bursts of radiation.   We can learn to tame the violence of earthquakes, volcanoes, tsunamis, tornadoes and hurricanes on our own world and colonise the other planets of the solar system. Above all we must learn to live together in peace, harmony and co-operation. The appalling evils of ethnic and racial conflict must disappear forever.  Men and women of all creeds and races must learn to live together for the common good.  The evils of greed, hatred and delusion must be replaced by generosity, love and enlightenment if we are to avoid the sixth extinction. Humankind has the power to progress to undreamed of heights of achievement or to be the cause of the greatest extinction of life this world has ever seen. We stand even now upon the brink of what Richard Leakey calls the Sixth Extinction in his famous book.   Already our species is responsible for the mass slaughter of tigers, elephants, whales and many other creatures that share with us this world.   The great forests of the Earth are rapidly disappearing.   Our chemical and radioactive toxins pollute our planet.    The threat of global warming and the destruction of the ozone layer are direct results of humankind's misuse of resources.    If we allow the worst elements of our nature to control our actions, we ourselves shall be destroyed in the sixth great extinction of life in the history of our planet.    The agent of nemesis will not be an asteroid; or a pair of colliding neutron stars.  It will be our species that, by its greed and stupidity, will have turned our beautiful planet into a wasteland.

The Volcanic Theory

This will be dealt with under plate tectonics

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