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Mass Extinctions
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Mass Extinctions
Cases in which many species become extinct within a geologically short interval
of
time
are
called
mass
extinctions.
There
was
one
such
event
at
the
end
of
the
Cretaceous
period
(around
70
million
years
ago).
There
was
another,
even
larger,
mass extinction at the end of the Permian period (around 250 million years ago). The
Permian event has attracted much less attention than other mass extinctions because
mostly unfamiliar species perished at that time.
The fossil record shows at least five mass extinctions in which many families of
marine organisms died out. The rates of extinction happening today are as great as the
rates during these mass extinctions. Many scientists have therefore concluded that a
sixth great mass extinction is currently in progress.
What
could
cause
such
high
rates
of
extinction?
There
are
several
hypotheses,
including
warming
or
cooling
of
Earth,
changes
in
seasonal
fluctuations
or
ocean
currents,
and
changing
positions
of
the
continents.
Biological
hypotheses
include
ecological changes brought about by the evolution of cooperation between insects and
flowering plants or of bottom-feeding predators in the oceans. Some of the proposed
mechanisms required a very brief period during which all extinctions suddenly took
place;
other mechanisms
would be more likely to have taken place more gradually,
over
an
extended
period,
or
at
different
times
on
different
continents.
Some
hypotheses fail to account for simultaneous extinctions on land and in the seas. Each
mass
extinction
may
have
had
a
different
cause.
Evidence
points
to
hunting
by
humans and habitat destruction as the likely causes for the current mass extinction.
American
paleontologists
David
Raup
and
John
Sepkoski,
who
have
studied
extinction
rates
in
a
number
of
fossil
groups,
suggest
that
episodes
of
increased
extinction have recurred periodically, approximately every 26 million years since the
mid-Cretaceous
period.
The
late
Cretaceous
extinction
of
the
dinosaurs
and
ammonoids
was
just
one
of
the
more
drastic
in
a
whole
series
of
such
recurrent
extinction episodes. The possibility that mass extinctions may recur periodically has
given
rise
to
such
hypotheses
as
that
of
a
companion
star
with
a
long-period
orbit
deflecting other bodies from their normal orbits, making some of them fall to Earth as
meteors and causing widespread devastation upon impact.
Of
the
various
hypotheses
attempting
to
account
for
the
late
Cretaceous
extinctions,
the
one
that
has
attracted
the
most
attention
in
recent
years
is
the
asteroid-impact hypothesis first suggested by Luis and Walter Alvarez. According to
this
hypothesis,
Earth
collided
with
an
asteroid
with
an
estimated
diameter
of
10
kilometers, or with several asteroids, the combined mass of which was comparable.
The force of collision spewed large amounts of debris into the atmosphere, darkening
the
skies
for
several
years
before
the
finer
particles
settled.
The
reduced
level
of
photosynthesis
led
to
a
massive
decline
in
plant
life
of
all
kinds,
and
this
caused
massive
starvation
first
of
herbivores
and
subsequently
of
carnivores.
The
mass
extinction would have occurred very suddenly under this hypothesis.
One
interesting
test
of
the
Alvarez
hypothesis
is
based
on
the
presence
of
the
rare-
earth element iridium (Ir). Earth’s crust contains very little of this element, but
most asteroids contain a lot more. Debris thrown into the atmosphere by an asteroid
collision
would
presumably
contain
large
amounts
of
iridium,
and
atmospheric
currents would carry this material all over the globe. A search of sedimentary deposits
that span the boundary between theCretaceous and Tertiary periods shows that there is
a dramatic increase in the abundance of iridium briefly and precisely at this boundary.
This iridium anomaly offers strong support for the Alvarez hypothesis even though no
asteroid itself has ever been recovered.
An asteroid of this size would be expected to leave an immense crater, even if
the
asteroid
itself
was
disintegrated
by
the
impact.
The
intense
heat
of
the
impact
would produce heat-shocked quartz in many types of rock. Also, large blocks thrown
aside
by
the
impact
would
form
secondary
craters
surrounding
the
main
crater.
To
date,
several
such
secondary
craters
have
been
found
along
Mexico’s
Yucatan
Peninsula,
and
heat- shocked
quartz
has
been
found
both
in
Mexico
and
in
Haiti.
A
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