Throughout the 1980s, the view that one or more giant meteorites hit
the Earth and caused the extinction of dinosaurs has gained ground while
more established geological theories waned. Now two geologists in the US
claim to have found evidence that may swing the arguments over extinctions
back in favour of the geological theories
Two American Earth scientists have found a way in which processes operating
within our planet could have caused the death of the dinosaurs and other
mass extinctions of life on Earth. They suggest that 鈥榖lobs鈥� of molten rock
rising from near the Earth鈥檚 core are responsible for mass extinctions.
This idea challenges the currently popular theory that such catastrophes
were caused by the impact of a huge meteorite or comet.
Until 1980, most scientists believed that normal geological processes
had caused the demise of the dinosaurs some 66 million years ago, as well
as other mass extinctions that appear in the record of fossils in the Earth鈥檚
rocks. Geologists had found that the sea level changed dramatically at such
times. These changes could have been directly responsible for the extermination
of many species in the sea. A fall in the sea level would also have made
winters colder and summers hotter, affecting creatures and plants on land.
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However, work by an American team led by the late Luis Alverez, a physicist,
and his son, Walter, a geologist, challenged the geological version of events.
The researchers found an enormous amount of the element iridium in a layer
of clay deposited at the same time as the death of the dinousaurs. This
layer was at the boundary of the Cretaceous and Tertiary periods.
Later research showed that this layer of clay occurs all over the world.
It also includes other unusual materials 鈥� in particular, small crystals
of quartz showing fine lines which indicate that the quartz has been subjected
to intense shocks. This material is known as 鈥榮hocked quartz鈥�.
The evidence gave rise to the theory that the impact of a meteorite
or comet caused the mass extinction 66 million years ago. The theory suggests
that the impact changed the Earth鈥檚 climate so dramatically that many species
died out. The proponents of this theory have two pieces of evidence that
seemed to be conclusive.
The first piece of evidence is that the iridium could not have come
from the rocks on the Earth鈥檚 surface, where the element is extremely rare,
but it could easily have come from a cosmic impact, because iridium is very
common in meteorites.
Secondly, the only places on Earth where geologists have found shocked
quartz are near craters that have undoubtedly been caused by meteorites.
Such impacts are smaller than the large one that supposedly killed the
dinosaurs. It seems that the pressures required to affect the quartz are
far higher than any that occur naturally on the Earth.
Other evidence, however, points to widespread volcanic activity, or
volcanism, at this time. Some scientists have suggested a hybrid version
of the theory, in which an impact triggered an outburst of volcanism (鈥楧inosaurs,
comets and volcanoes鈥�, 快猫短视频, 18 February).
Some researchers, such as Jack Sepkoski of the University of Chicago,
have also produced evidence that mass extinctions on Earth tend to recur
regularly, with a period of about 26 million years. Others found a similar
period, 30 million years, in the ages of craters from impacts.
Astronomers have suggested that the Earth could encounter 鈥榮howers鈥�
of comets every 30 million years or so, as comets far from the Sun are shaken
loose by the gravity of either a companion star or dense clouds of gas in
the Galaxy.
The idea that an impact triggered the death of the dinosaurs has become
so popular that most researchers at recent conferences on mass extinctions
have taken it as fact.
They have been arguing over the details, such as whether the object
fell on land or into the sea (鈥楨volving theories for old extinctions鈥�, New
快猫短视频, 12 November 1988). But is the idea really correct? Two researchers
at Florida State University have developed a model that they believe is
equally viable 鈥� and which does not require an impact to make sense. Kevin
McCartney is a geologist and David Loper, now at the University of Maine,
has been studying the motion of the semi-molten rocks in the Earth鈥檚 mantle.
In 1985, Dewey McLean of Virginia Polytechnic Institute suggested that
a vast outpouring of lava in India known as the Deccan Traps occurred at
the same time as the extinctions of the Cretaceous-Tertiary boundary. He
suggested that carbon dioxide from this episode of volcanism caused the
climatic change that could have been responsible for the extinctions.
McLean telephoned Loper to ask if there was any way that processes inside
the Earth could give rise to a period of about 30 million years in the mass
extinctions.
鈥楳y first response was 鈥榥o鈥�,鈥� Loper recalls, 鈥榖ut then I realised it
was possible.鈥� Loper concentrated his attention on the lowest part of the
mantle, the so-called D鈥漧ayer, which rests directly on the iron core.
Radioactive elements in the core keep the core itself hot, so that it
heats the D鈥� layer from below. The heat reduces both the density and the
viscosity of the D鈥� layer. Eventually, the layer becomes unstable as this
low-density rock tries to 鈥榝loat鈥� above the denser rock of the mantle above.
Because the material of the D鈥� layer has a low viscosity, it can flow relatively
easily and eventually it escapes upwards in a blob of molten rock.
With his colleagues, Frank Stacey and Ibrahim Eltayeb, Loper has calculated
that it takes between 20 and 30 million years for the D鈥� layer to become
unstable. So researchers would expect an outbreak of blobs, known as 鈥榙iapirs鈥�,
rock which pierces an upper layer of rock from below, to recur with roughly
this period. In this way, such instability can provide an internal 鈥榗lock鈥�
within the Earth, that causes regular bouts of diapirs. But how would these
diapirs affect the Earth鈥檚 surface? And, could they cause mass extinctions?
Loper found that the effects depend on how the diapir rises. He modelled
the passage of a diapir in a container 15 centimetres high. He put into
the container a thick, viscous layer of corn syrup lying on top of a thin
layer of water which he dyed black to make it easily identifiable. The water
represented the low-viscosity, low-density D鈥� layer and the corn syrup represented
the mantle. This system contains the necessary instability to model the
theory, even without heating the container. 鈥楧iapirs鈥� of water rise up through
the corn syrup of their own accord.
Loper found that a diapir will sometimes take the path of a previous
diapir, which has left an easy route 鈥� a plume 鈥� open to the surface. This
produces volcanism at an existing volcanic site, and the volcanism will
be comparatively gentle 鈥� outpourings of lava, but no great explosions.
鈥業f a diapir is outside an established plume it rises at a much slower
rate,鈥� Loper says. Over the millions of years that it takes to reach the
surface, the diapir becomes much bigger, because molten rock flows up into
it from below.
When it reaches the surface, the diapir may become trapped under a thick
layer of crust. The diapir then has to force its way to the surface.
At this stage, the cooler environment at the surface means that the
outer part of the diapir begins to solidify, concentrating the gases into
the central molten regions, and raising the pressure there. 鈥楾his increase
in pressure could set the stage for a very explosive event,鈥� Loper suggests.
In some of his experiments, Loper found that a large, slowly ascending
diapir expands sideways enough to encounter an existing plume. Then its
molten material would rush up the plume conduit, reaching the surface over
a very short period of time 鈥� with 鈥榮pectacular, if not explosive, surface
别蹿蹿别肠迟蝉鈥�.
Loper and McCartney believe that such eruptions can explain most of
the geological evidence dating from the boundary of the Cretaceous and the
Tertiary. Most significantly, it could account for the large amount of iridium.
Although the Earth鈥檚 surface is poor in this element, its lower regions
have roughly the same composition as a meteorite, with plenty of iridium.
So an eruption of material from the D鈥� layer could certainly spread iridium
over the world. Indeed, the element would probably be emitted in the form
of iridium hexafluoride, which is a gas. Iridium hexafluoride would disperse
more uniformly than matter thrown out from a meteorite impact.
鈥極ur candidate model does have some serious problems 鈥� just as the impact
scenario has serious problems,鈥� Loper admits. 鈥極urs concerns the shocked
quartz.鈥� No known volcanic eruption has been powerful enough to produce
these.
There is no direct test of whether one of Loper鈥檚 鈥榮uper-explosions鈥�
would produce a sufficiently high pressure, because the actual site of the
eruption would subsequently become buried under several kilometres of lava,
as has happened at the Deccan Traps.
Loper suggests that geologists should look carefully for shocked quartz
near to much smaller explosive volcanic 鈥榥ecks鈥�, known as kimberlites, which
have strewn their surroundings with material, including diamonds, from the
mantle.
Conversely, however, McCartney and Loper can explain several facts that
the impact theory cannot. Another researcher, Tony Hallam, of the University
of Birmingham, has found that the end of the Cretaceous period was not marked
solely by a sudden extinction.
A detailed study of the rocks and fossils shows that 鈥榯he last few hundred
thousand years of the Cretaceous were marked by environmental changes more
dramatic than experienced for a long time previously or subsequently鈥�. During
this time, the level of the sea was falling, and many marine organisms gradually
died out.
Hallam argues that the new model can explain the geological record very
neatly. The gradual change in sea level could be a result of the diapirs
moving up towards the Earth鈥檚 crust and distorting its surface. The final
extinctions would result from the explosive volcanism as the material in
the diapir broke through.
The new theory can also explain why the Earth鈥檚 magnetism seems to have
a cycle that ties in with the cycle of extinctions. The Earth鈥檚 magnetic
field can change direction, as the flow of liquid metal changes in its iron
core. According to some geophysicists, the Earth suffers periods every 30
million years when these reversals occur much more frequently. According
to Loper, these periods would occur naturally at times when the D鈥� layer
is thin.
This theory would tie in changes in magnetism with ascending diapirs
that would cause mass extinctions.
Geophysicists are generally enthusiastic about the new model, although
some think that it may need some modification in its details. Ken Creer,
of the University of Edinburgh, has studied the Earth鈥檚 magnetism and believes
that the periods of frequent reversal occur every 15 million years, and
that this length of time is too short a period to be associated with growing
instabilities in the D鈥� layer. He thinks, indeed, that disintegration of
the D鈥� layer should take place over a longer time scale than 30 million
years.
Creer would invoke the new mechanism not to account for extinctions
every 30 million years, but for much greater extinctions that occur more
rarely. He says 鈥榯he last two of these would be the (Cretaceous-Tertiary)
event and the Permian-Triassic extinctions鈥�, which occurred 160 million
years earlier.