
Environmental Encyclopedia 3
Mass extinction
in Siberia, which catapulted huge quantities of sunlight-
blocking dust and
aerosol
droplets into the
atmosphere
.
This, in turn, cooled the climate abruptly, expanding the
polar icecaps, shrinking the oceans, and causing a new
ice
age
. The rapid fluctuations in sea levels decimated the ma-
rine creatures of this period. To make matters worse, volcanic
explosions could also have turned sulfate minerals into sulfu-
ric
acid
and
sulfur dioxide
gas, which would have produced
a ruinous rain of acidic precipitation. Over a 600,000 year
period, these eruptions spewed a flood of molten basalt that
created rock formations 870 miles (1400 km) in diameter
next to
Lake Baikal
, an area called the Siberian Traps.
Another mass extinction occurred about 198 million
years ago, during the late Triassic period, when a quarter of
marine families and half or more of marine genera disap-
peared. Cephalopods,
gastropods
, bivalves, brachiopods,
and reptiles, were destroyed, as were the conondonts, the
fish from which vertebrates may have descended.
The most recent and best known mass extinction oc-
curred a mere 65 million years ago, at the end of the Creta-
ceous period. It brought about the end of many creatures,
including the dinosaurs and ammonites (shellfish). Sixteen
percent of marine families and up to 46% of marine genera
were lost at this time.
Scientists are still debating the causes of this cata-
strophic event. It could have coincided with (and may have
been caused or accelerated by) a massive volcanic eruption
and lava flood in what is now India, where it created a rock
formation called the Deccan Traps. Some scientists suggest
that a catastrophic climate change and a drastic cooling of
the atmosphere made survival impossible for the dinosaurs,
since they had no fur, feathers, or hibernating dens to shield
them from the cold, nor could they migrate to a warmer
climate. Other experts believe that the explosion of a nearby
star cooled the atmosphere and emitted deadly radiation for
thousands of years. Or perhaps herbivorous dinosaurs starved
because they could not adapt to the new types of vegetation
that developed. Their extinction would have killed off the
carnivorous dinosaurs that preyed on the plant-eaters. Maybe
the dinosaurs could not compete for food with the newly-
emerging mammals, which ultimately replaced them as the
dominant creatures on the planet.
In truth, much is unknown about the causes of the
past mass extinctions. A theory with the weight of some
evidence behind it raises the possibility that one or more
comets or asteroids may have hit the earth, flinging billions
of tons of dust and other debris—or ice crystals, if the object
impacted in the oceans—into the atmosphere. By blocking
out the sunlight for several months, and plunging the earth
into darkness, the collision could have lowered temperatures
below freezing, killing off the dinosaurs and the plants on
which they fed. This hypothesis is based in part on the
881
1979 discovery, by Dr. Walter Alvarez of the University of
California at Berkeley, of the unusual presence of the pre-
cious metal iridium in sediments from the time of the dino-
saurs’ extinction. Iridium, which belongs to the platinum
family of elements, is much more plentiful in meteorites
than planetary rocks, but it is also frequently found in strata
from volcanic eruptions. Other scientists contend that virtu-
ally all mass extinctions could have been caused by volcanic
eruptions and the huge floods of basalt resulting from such
explosions. Evidence supports both theories. University of
Chicago paleobiologist Dr. David M. Raup may have put
it best when he wrote, “The disturbing reality is that for
none of the thousands of well-documented extinctions in
the geologic past do we have a solid explanation of why the
extinction occurred.”
Mass extinctions are not just a phenomenon of ancient
geologic history; the last few thousand years have witnessed
several such events, albeit on a far smaller geographic scale
than those previously discussed. For example, the extinction
of the giant animals of North America appears to have
coincided with the arrival of humans on the continent, with
the changing climate at the end of the last ice age also a
possible factor. Crossing the Bering land bridge linking Sibe-
ria and Alaska about 12,000 years ago, primitive tribespeople,
possibly from Siberia, found a population of large mammals,
including giant beavers,
bison
, camels, mammoths, mast-
odons, and even lions in North America. These prehistoric
hunters, moving south from Alaska towards South America,
may have wiped out most of these species within 1,000 years.
Fossil remains of these Pleistocene mammals, some
with spear points imbedded in them, reveal such creatures
as the elephant-like mastodon; the huge-tusked, shaggy red-
haired mammoth; a giant beaver weighing over 400 pounds
(181 kg); the fabled saber toothed “tiger”; a 7 foot (2.1 m)
wide, long-necked camel; 20-foot (6 m) long ground sloths;
a bison 7 feet (2.1 m) high at the hump, with a 6 foot (1.8
m) spread between its horns; and huge
wolves
, lions, bears,
and horses.
As the twenty-first century approaches, the earth faces
another threat of mass extinctions which may occur over a
few years or decades, instead of thousands or millions of
years. Particularly endangered are those species that are less
well known, and in many cases not yet identified by science.
Many of these are plants, animals, and invertebrates found
in tropical rain forests, which are rapidly being cut and
destroyed.
For example, the 1980 Global 2000 Report to the Presi-
dent, compiled by the President’s
Council on Environmen-
tal Quality
and the State Department, with the help of other
federal agencies, projected that “...between half a million and
two million species—15–20 percent of all species on earth—
could be extinguished by the year 2000.” And a report issued