|
There
have been several mass extinctions throughout
geologic time on Earth. Many have been regional, and
a few have been global in scope. The “Big Five”
global extinction events are shown below:
|
Geologic Period |
Time – Millions of years
ago |
Percent of extinct genera |
Primary Victims |
|
End of Ordovician |
440 |
57 |
Marine Invertebrates |
|
Late Devonian |
365 |
50 |
Marine Invertebrates
Plankton (Trilobytes)
Primitive Fish |
|
End of Permian |
250 |
83 |
Marine Invertebrates
Mammal-like reptiles |
|
End of Triassic |
210 |
48 |
Marine Invertebrates
Mammal-like reptiles
Large amphibians |
|
Cretaceous – Tertiary
(K-T) |
65 |
50 |
Marine Invertebrates
Dinosaurs
Plankton
Marine reptiles |
What
caused these mass extinctions?
According to the BBC Mass Extinction web site (http://www.bbc.co.uk/education/darwin/exfiles/permian.htm)
the following information is noted:
The Late Cambrian Extinction Event:
The cause(s) of the late Cambrian extinction are not
well known, but it seems almost certain that a
change in sea level occurred at this time, changing
the habitat to which many of these early animals
were adapted, and driving them to extinction.
The
Late Devonian Extinction Event:
Although it is clear that there was a massive loss
of biodiversity towards the end of the Devonian
Period, it is not clear over how long a period these
extinctions took place, with estimates varying from
500 thousand to 15 million years. Neither is it
clear whether it was a single mass extinction or a
series of several smaller extinctions one after the
other. Indeed it has been written that the late
Devonian extinction is one in which not even the
major facts are agreed on yet. However, the balance
of evidence suggests that the extinctions took place
over a period of some 3 million years, about 365
million years ago. As many as 70% of all species
vanished from the Earth during the late Devonian
extinction. Marine species were more severely
affected than those in freshwater - Brachiopods,
ammonites and many other invertebrate groups
suffered heavily, as did Agnathan and Placoderm
fish. On land, where plants were diversifying and
amphibians were beginning their evolution, there
seem to be have been far fewer losses. The causes of
the Devonian extinction(s) are far from clear. the
disproportionate losses amongst warm water species
suggest that climate change, in this case a global
cooling, was an important factor and it has been
suggested that this was associated with (or may even
have caused) a drop in the oxygen levels of these
shallower waters.
The
Late Ordovician Extinction Event:
The Ordovician was a period of relative stability in
the Earth's history, which may have been an
important factor in the substantial growth of
biological diversity which took place. However, as
the period drew to a close, 440 million years ago,
there was a huge extinction event- with some animal
groups losing more than half their species. The Late
Ordovician extinction seems to have been the result
of a period of glaciation - an ice age. The seas
retreated as more and more water was taken up into
ice sheets, and the marine habitats (which harbored
the vast majority life in the Ordovician) changed
drastically, destroying habitats and reducing the
number of ecological niches. There seem to have been
two peak periods of extinction - one at the
beginning of the glaciation, and another at the end
of the extinction, between 500 thousand and 1
million years later, when sea levels rose rapidly.
Echinoderms, trilobites, nautiloids and many other
groups suffered significant losses, although the
overall effect was less drastic than that of most
other major extinctions.
The Permian Extinction Event:
Specifically, what caused the major extinction at
the end of the Permian? This is harder to define.
The end of the Permian (245 million years ago) saw
the largest extinction event in the Earth's history
- far more devastating than the much more famous
Cretaceous extinction, when the dinosaurs died out.
It has been estimated that as many as 96% of all
marine species were lost, while on land more than 3
quarters of all vertebrate families became extinct.
Many causes have been proposed for the Permian
extinctions - including fluctuations in sea-level, a
change in the salinity of the ocean, and volcanic
activity. The most important factor seems, once
again, to be climate change.
The
Triassic Extinction Event:
Whether the extinction was a single event or a
cluster of smaller events, there is no doubt that
sponges, cephalopods, brachiopods, insects and many
vertebrate groups lost many of their families as the
Triassic drew to a close, some 208 million years
ago. Conodonts disappeared completely, as did the
Labyrinthodonts. While the losses may not have been
as drastic as in other extinction events, they were
important for a relatively new group of animals. As
the loss of species opened up new opportunities, the
dinosaurs were ready to take advantage. The cause(s)
of the Triassic extinction are poorly known, perhaps
because it has attracted relatively little study,
but climate change seems to be important and, in
particular, an increase in rainfall.
The
End of the Cretaceous Extinction Event:
The extinction at the boundary of the Cretaceous and
the Tertiary periods, 65 million years ago, is the
most famous of all mass extinctions. Its fame comes
not from its magnitude (the Permian extinction was
far larger) but from the victims of the extinction -
the dinosaurs. The Cretaceous-Tertiary (or 'K-T')
extinction wiped out around 85% of all species. the
dinosaurs were not the only victims -
pterosaurs(flying reptiles), mosasaurs and other
marine reptiles, fish, brachiopods, plankton and
many plants either died out completely or suffered
heavy losses. Even the ammonites, who had survived 4
previous extinctions, finally disappeared. For some
reason though, some groups seem to have been almost
entirely unaffected by the K-T extinction -
crocodiles, turtles and lizards, mammals and birds
all made it through relatively unscathed. The cause
of the K-T extinction event has been the subject of
intensive research, with many hundreds of research
papers published since 1980, when Luis Alvarez
reawakened interest in the subject, almost
overnight. He suggested that the death of the
dinosaurs, and all the other victims of the K-T
extinction, was due to a giant meteorite crashing
into the earth, severely disrupting the earth's
ecosystem. While other theories suggest volcanic
activity, climate change, environmental pollution or
even cosmic radiation as causes, the meteorite
impact theory remains the most probable - at least
for the moment.
Could
these mass extinction events be related to meteorite
impacts on Earth? According to Michael Paine, as
written on the Asteroid/Comet Impact Craters and
Mass Extinctions and Shiva Hypothesis of Periodic
Mass Extinctions web site:
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 asteroids 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.
It’s
pretty well accepted that the end of the Cretaceous
extinction was due to a meteorite or series of
meteorite impact events. The impact or impacts had a
dramatic affect on the global climate and ecosystem.
Small changes in climate affect sea levels,
rainfall, temperature, ocean currents and
atmospheric circulation. Any one of these can impact
both plants and animals regionally or globally.
Human
history has been recorded over the last 10,000
years, which has generally been a very stable period
on Earth. During the Jurassic and Triassic, in which
the dinosaurs ruled the planet, the Earth’s climate
was also stable. It is also similar to the
Ordovician period of climate stability, during which
growth in biological diversity took place. However,
each of these geologic periods were ended with mass
extinctions due in part to climatic upheaval.
Dinosaurs lasted approximately 200 million years on
Earth. Will humans last that long? Is there a mass
extinction event in our future? How likely is a
major meteorite impact event or series of events?
Only time will tell.
H. Court Young
Geologist, author and publisher
Promoting awareness through the written word
http://www.hcourtyoung.com
http://www.tmcco.com
(303) 726 8320
©August, 2007 |
The
Orphan Boy mine was started in Colorado's glory days. It was reopened in
1948 by one man, Herbert T. Young, who had a passion for mining. He was
a member of the "greatest generation."