- What killed the dinosaurs? For years,
scientists have debated the cause - was it a change in diet? A loss of
habitat? A global epidemic? The great extinction event at the end of the
Cretaceous - the Age of the Dinosaurs - remains one of the most fascinating
topics in the scientific community.
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- One theory has gained prominence over
the past decade, particularly after scientists witnessed the results of
the spectacular impact of Comet Shoemaker-Levy 9 into the planet Jupiter
in 1994. That theory, which Princeton geologist Walter Alvarez first proposed
in the late 1970s, suggests the dinosaur extinction was the result of a
catastrophic impact with an asteroid - which may have resulted in some
of the most rapid, dramatic, and deadly weather changes the planet has
ever seen.
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- Impact Minus Six Hours
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- It was a tropical, humid day in what
is now downtown Denver. Billowing clouds hung low on the eastern horizon,
hugging a shallow sea. A weak, salty breeze blew onshore. A North American
Tyrannosaurus Rex raised his head from his meal, distracted by the scurrying
of a small mouse-like rodent. After a moment's hesitation, he returned
to his breakfast, ripping the meat from the carcass of a Utah raptor. If
his eyesight had been more acute, the T-Rex might have seen a bright trail
of milky light that had become increasingly conspicuous in the sky over
the past few days.
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- The next few hours would prove fatal
to the T-Rex and perhaps 80 percent of life on Earth. They also would herald
a sudden change in the weather. Indeed, climatologist Alan Hildebrand has
characterized the days following the impact as "a living hell, a dark,
burning, sulfurous world where all the rules governing survival of the
fittest changed in minutes. The dinosaurs never had a chance."
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- Recent scientific studies and computer
simulations of that terrible time some 65 million years ago give us a glimpse
of a great global catastrophe.
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- Impact Minus Three Hours
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- Oblivious to impending disaster, the
T-Rex had begun a search for more food. The humid sea air was filled with
buoyant cumulus clouds, probably precursors of a late afternoon thunderstorm,
a common event in the area.
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- Scientists estimate global temperatures
were perhaps 12 [degrees] F warmer than now. These estimates are based
in part on fossil deposits of warm-climate species (such as crocodiles
and other prehistoric animals) found in northern latitudes. These animals
could not have survived at the northerly locations the fossils suggest
they did - unless the temperatures there were much warmer than today. In
fact, the dinosaurs likely roamed far north of the Arctic Circle, and warm
water coral reefs extended a full 15 [degrees] both north and south of
their present tropical positions.
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- "Continued hot and muggy" would
have been the predominant local forecast in the late Cretaceous period.
Why so warm? Climatologists have developed elaborate computer model simulations
that suggest there was perhaps four times as much greenhouse gas (such
as carbon dioxide) in the atmosphere as today.
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- Why "muggy"? During the late
Cretaceous, shallow seas flooded large parts of many continents. North
America itself was split in half by a huge inland sea extending northward
from the Gulf of Mexico along the lee of the youthful Rocky Mountains.
Much more of North America was coastline than today.
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- Impact!
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- A brilliant fireball flashed across the
southeastern sky, startling a flock of small bird-like dinosaurs into flight.
At the sudden motion, the T-Rex, blood dripping from his huge teeth, raised
his head from the carcass of an old Triceratops he had found and roared
a challenge to the heavens.
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- The six-mile-wide asteroid - a rock the
size of Mount Everest - slammed into the Gulf of Mexico off the coast of
the Yucatan Peninsula. As it did so, a backshock (like the recoil of a
rifle) shot back through it, instantly altering its probable oval shape
and ripping its comet-like tail of debris to shreds. The rock itself melted
a huge hole through the ocean floor at incredible speed. As it did, most
of the asteroid vaporized. Within seconds the impact crater reached its
maximum depth, likely about 30 miles. Then the center of the crater quickly
began to rise - like the liquid that leaps back into the air when you splash
milk into a glass. Giant landslides cascaded off the rapidly rising center,
which soon grew so high it collapsed under its own weight, creating a ringlike
set of ridges resembling a bull's-eye.
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- Impact Plus One Minute
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- A monstrous earthquake rumbled over the
Colorado landscape. T-Rex stumbled over the Triceratops carcass, surprised
but not terribly alarmed; the earth had shaken under his feet before.
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- Interestingly, a direct earthquake from
the asteroid itself would have been one of the later occurrences in the
sequence of events of an impact with the Earth. This is because the shock
waves associated with an earthquake travel fairly slowly through the Earth.
So what did our T-Rex feel? It is likely that any large asteroid would
have several chunks of rock associated with it - much like the Shoemaker-Levy
strike on Jupiter four years ago. A smaller, earlier strike could have
caused an earthquake that rattled the prehistoric Earth at the time of
the primary strike.
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- Current scientific thought suggests the
impact of a six-mile-wide asteroid would have generated about ONE BILLION
times the energy released by the atom bomb that destroyed Nagasaki and
ended World War II. The incredible sledgehammer force of the impact crushed
rock to the point of destroying even the crystal structure of minerals,
creating what scientists call "shocked quartz." This was a big
explosion.
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- Impact Plus Four Minutes
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- T-Rex glanced towards the southeast as
the ground continued to shake. Even his poor eyes could detect the huge
orange flare of light along the southeastern horizon, with a brilliant,
almost incandescent tail stretching into the sky. The light grew brighter
and closer with alarming speed.
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- The asteroid's passage bored a tunnel
through the atmosphere extending from the impact crater back into space.
This created a near-vacuum that sucked in extremely hot, low-density plasma
from the impact site, as well as surrounding air, and jetted the combination
out into space. The rush of air into the plasma column at speeds beyond
the speed of sound created a huge sonic boom.
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- The speed of the burning rock, minerals,
and gases ejected from the impact crater also would have been incredible;
current research suggests the first effects of the asteroids impact off
the Yucatan coast took only ten minutes to reach present-day New Jersey.
The atmosphere of North America was in a state of deadly flux, with shock
waves and burning ejecta filling the air.
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- Impact Plus Six Minutes
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- T-Rex looked up, startled, as a thunderous
roar pounded his ears. Suddenly an unimaginable blast of wind knocked him
down while ripping huge trees and boulders out of the ground like matchsticks.
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- The shock wave created by an asteroid
hitting at a conservatively estimated speed of over 40,000 m.p.h. would
have had a devastating impact on large plants and animals over a very great
area. Indeed, pollen records in parts of western America and Japan indicate
large plants were completely absent from the landscape for a long period
after impact. Apparently almost everything standing even inches above the
ground was smashed flat and likely killed.
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- Impact Plus Six-And-One-Half
Minutes
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- His thick hide bleeding Sore numerous
cuts, the huge dinosaur picked himself up and emitted a piercing bleat
of pain. Confused, he peered towards the southeast, where his weak eyes
detected the movement of another solid line of dirt and debris racing towards
him at an unimaginable speed. The T-Rex lowered his head in pain as the
second line of superhurricane winds crashed into him, lifting him up, then
pummeling him into a bloody heap on the ground.
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- The second shock wave was caused by the
type of rock off the Yucatan coast. The underlying rock is limestone, which
is rich in carbon. The tremendous energy released by the impact instantly
converted this stored carbon to carbon dioxide gas - which exploded outward
into the atmosphere like spray erupting from a shaken bottle of soda, creating
a second shock wave. Scientists theorize the slightly different origins
and speeds of the two shock waves could have produced the mixed, rather
than stratified, layers of shattered rock in North America dating to this
event that had long puzzled geologists.
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- Impact Plus Seven Minutes
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- Battered and dying, the stricken T-Rex
had one final sight - a horizon-to-horizon wall of flaring fire raging
up from the southeast - a conflagration that would consume him and almost
all other life forms.
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- The fireball, together with burning ejecta
from the impact crater, would have ignited most of the plants, animals,
and surface fossil fuels in the Western Hemisphere and perhaps large parts
of the rest of the world and started great wildfires. Indeed, a layer of
charcoal is a key geologic indicator of this time.
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- In some areas the fires could have burned
for weeks, filling the air with smoke and gas and temporarily blotting
out the sun. The fires also would have caused atmospheric carbon monoxide
levels to skyrocket ... perhaps even to reach toxic levels.
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- Impact Plus Ten Minutes
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- As the T-Rex's charred bones lay smoldering
in the aftermath of the fireball inferno, another line - this one a deep
ocean blue tipped with white - appeared on the southeastern horizon, rapidly
growing larger. A half-mile-high wall of water was about to crash down
on the burning rubble of the ancient Colorado coastline.
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- Tsunami! The asteroid's ocean impact
likely would have created one of the largest tsunamis the planet has ever
seen. This ring of watery destruction would have rushed outward from the
impact site at perhaps more than 600 m.p.h., growing into a monstrous wave
as it crashed onto surrounding lands with such force it washed away anything
still standing on the margins of North and South America. It also pulled
up massive amounts of submarine sediments which covered the 120-mile-wide
impact crater, hiding it from scientists' eyes until it was finally found
by gravity anomaly analysis in the mid-1980s.
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- The Krakatoa volcanic eruption of 1883
produced a tsunami estimated at perhaps 135 feet tall when it hit Java
and Sumatra. "Like a high mountain," one sea captain reported,
"the monstrous wave precipitated its journey towards the land. And
before our eyes this terrifying upheaval of the sea ... consumed in one
instant the ruin of the town ... all was finished. There where a few: moments
ago lived the town of Telok Betong was nothing but the open sea."
The tsunami produced by the meteorites impact may have been ten times higher
than the Krakatoa wave.
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- Impart Plus Five Days
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- Safe from tsunami inundation in the highlands
west of the remains of the T-Rex, a small mouse-like rodent scurried around
the charred and broken landscape, stopping occasionally to nibble at the
scorched meat of larger, less fortunate animals. She and her mate had survived
the terrible shock waves and inferno safe and snug in their tiny burrow
- one of very few species on the planet to do so. Although she was extremely
uncomfortable in the mind-numbing heat, hunger forced her out of her coot
burrow to hunt for food.
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- The fireball is thought to have raised
global air temperatures by at least 50 [degrees] F. Many places would have
been substantially warmer than that. This temperature increase was from
two sources: the extreme amount of heat energy the asteroid impact had
dissipated into the atmosphere; and the re-entry of ejecta the impact had
thrown back into space. Some estimates suggest the heat from these two
sources may have been as much as 50 to 150 times the present solar output.
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- And while there is still debate, some
scientists believe this terrible heat may have lasted for weeks. This temperature
increase alone probably was intolerable to many species, particularly large
vertebrates and cold-blooded animals, who could survive only in rather
limited temperature ranges. Such a dramatic and rapid temperature increase
- combined with changes in the food chain - would have led to massive death
totals.
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- The underground temperature of the earth,
however, where small burrowing animals lived, would not have changed much.
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- The heat would have also radically altered
atmospheric gases. The furnace-like atmosphere could have even caused basic
chemical reactions. Atmospheric oxygen, nitrogen, and water vapor, for
example, may have combined to create toxic nitric acid. Some extremely
acid rain may have fallen for a month or two after the impact. The amount
of acid rain would have depended on the composition of the asteroid. If
it was ice-rich, global pH values might have plummeted to 0 to 1.5, similar
to the values of pure hydrochloric or sulfuric acid. Perhaps the best comparison
is the atmosphere of Venus - a hostile world of dark clouds, acidic rains,
and toxic air.
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- Impact Plus Two Months
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- The small rodent-like creature stuck
her head out of her underground burrow and surveyed the dismal scene. The
'day,' like its recent predecessors, was dark - dry, cold gloom untouched
by precipitation. The few ferns and other small plants that had survived
the initial catastrophe now were withering into brown death. Each day,
she was forced to roam farther afield for food - and water was also becoming
increasingly scarce.
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- The great quantity of dust shot into
the stratosphere by the meteorite's impact soon would have circled the
globe and blocked out significant sunlight over a large portion of the
Earth. This brought an end to the intense heat, and began a period of bone-chilling
cold. Photosynthesis would have been impossible for many plant species.
Meanwhile, according to current computer simulations, after the acid rain
ended, global precipitation would have been completely shut down due to
a marked increase in atmospheric stability caused by the massive dust loading;
warm temperatures in the upper troposphere and cold temperatures at the
surface would have severely inhibited convection.
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- Both the global cooling and the global
drought may have contributed to the extinction of the dinosaurs and other
species. Even large ocean dinosaurs such as the mossaur and plesiosaur
died - perhaps because of breaks in their food chain.
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- Impact Plus One Year
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- The little rodent-like mammal had survived
the shock waves, the firestorm, the tsunami, the global drought, and the
icy cold. Now, a year later, she felt warm as Earth's weather shifted back
towards conditions prior to impact. Once again, as skies slowly cleared,
clouds built up during the afternoon to bring wonderful soaking rains,
and tiny green shoots of plants worked their way up through the burnt,
crusty soil. The little mammal watched her litter of tiny offspring scurry
around her mate - life had survived on Planet Earth.
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- Recent computer models suggest the cooling
caused by the dust cloud would not have been as severe or prolonged as
models had projected back in the early 1980s. This "nuclear autumn"
likely would only have lasted for months, rather than years. Then, as the
dust slowly settled out of the sky, sunlight once again would have begun
to warm the surface. Eventually, the Earth turned even warmer than before
the asteroid strike. This was because the global fires that followed the
impact had added tons of carbon dioxide to the atmosphere, amplifying the
greenhouse effect.
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- Over the next 64 million years the ghastly
scars of that terrible day were slowly covered up or hidden as the continents
continued grinding their ways to their present positions. Gradually, the
climate began to cool, until about a million years ago great ice sheets
began to form and dissipate cyclically over much of North America and northern
Europe, further camouflaging the planetary wounds. Only recently have scientists
managed to piece together the geologic and climatological clues to retell
the story. Meanwhile, the small mammal survivors of the planetary holocaust
have flourished and diversified greatly in kind and size to fill the
biological niche left vacant by the destruction of the dinosaurs - and
perhaps to await the next cosmic visitor.
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- Impact Minus? Days
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- It is a warm, humid day in downtown Denver.
The morning forecast calls for cumulus clouds to build along the Front
Range, perhaps leading to late afternoon thunderstorms. Meanwhile, millions
of miles out in the blackness of space, a tumbling rock hurtles towards
Earth, promising a dramatic change in the weather.
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- __________________
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- RANDY CERVENY is a professor of geography
at Arizona State University and a Weatherwise contributing editor.
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- COPYRIGHT 1998. Helen Dwight Reid Educational
Foundation
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