TO SCIENTISTS trying to understand the risks of natural catastrophes, the devastating Colombian volcanic eruption coming less than two months after the deadly Mexico City earthquake are grim reminders of the planet-changing forces nature seems to unleash capriciously.
Only recently has there been a growing awareness that even bigger catastrophes have occurred and are likely to happen again.
In the last 5 to 10 years scientists have started looking at these and other disasters with an eye toward not only predicting them but eventually preventing them.
The Earth, it turns out, faces three basic risks that could wipe out much of the planet`s life.
— Volcanos erupt all the time, but big ones on the scale of Mt. St. Helens or El Chichon go off about every 5 to 10 years. Really big atmosphere- smothering volcanos that are 1,000 times as powerful as Mt. St. Helens explode about every few thousand years.
— Major earthquakes happen every few years. Disastrous earthquakes in prime fault zones such as around San Francisco, Los Angeles and Mexico City can be expected every 100 years. A quarter-million Chinese are thought to have died in a devastating earthquake near Tangshan in northeastern China in 1976. — Meteoroids, which can be either asteroids or comets, can wreak the greatest destruction. A comet the size of the one believed to have knocked down a forest in the remote Tunguska River area of Russia in 1908 can be expected every 300 years. The risk of a comet this size hitting a city is about every 30,000 years. The chance that a mammoth meteoroid will strike the Earth and wipe out most life, which many scientists believe happened 65 million years ago when the dinosaurs became extinct, is about every 30 million years.
”We`ve been blissfully ignorant of many of these major hazards that could be far worse than a nuclear war,” said Joseph V. Smith, a University of Chicago geophysicist.
The basic problem has been that geologists and other scientists have been slow in recognizing the global threats from these truly big catastrophes and their potential for disastrous climatic changes.
Generations of geologists were taught that geological changes occur gradually and smoothly, said Eugene M. Shoemaker, of the U.S. Geological Survey in Flagstaff, Ariz.
Growing evidence, however, indicates that catastrophic events, especially meteoroid bombardments, played a major role in the Earth`s evolution, he said. Another problem is that scientists did not see the signs of earlier catastrophes. Most of the evidence of huge craters from giant impacts millions of years ago, for instance, have been erased by weathering, volcanic activity and continental movement.
The space program made believers out of a lot of geologists when the probes revealed that the Moon, Mars, Mercury, and the satellites of Jupiter and other planetary bodies were covered with impact craters, Shoemaker said.
Tremendous volcanic explosions have occurred so far in the past that there is little if any recorded history describing them. Only now are scientists discovering evidence of some of these mighty blasts.
Smithsonian Institution scientists recently found ash from an explosion of the Santorini volcano in the southern Agean Sea of Greece nearly 500 miles away in Egypt. The gigantic eruption is thought to have occurred 3,500 years ago and probably destroyed the Minoan civilization on Crete.
”There are more than 1,300 volcanos around the world that are potentially catastrophic,” Smith said. ”We have not seen a really big volcano in recorded history. We`re talking about something 1,000 times bigger than Mt. St. Helens.”
Big earthquakes also occurred long ago or in remote areas. The biggest one on record occurred along the New Madrid fault in the Mississippi Valley in 1811. Since the area was sparsely settled, damage was moderate. That same quake today would destroy most of St. Louis and cause extensive damage in Chicago. Geologists speculate that another big New Madrid quake is overdue.
”Although we justifiably worry about the danger of nuclear war to civilization, and perhaps even to survival of the human race, we tend to consider natural hazards (comets, asteroids, volcanos, earthquakes) as unavoidable acts of God,” Smith reported in the journal Geology.
Obviously the things that people worry about the most are cancer, heart attacks, accidents and other things that pose a more immediate risk, he said. In any human lifetime, a truly catastrophic natural event is very unlikely, but ultimately one will occur, he said.
People often do not understand what the chance of something occurring really means, Smith said. If something has a chance of happening once in 10,000 years, that means it could happen tomorrow or it could happen 20,000 or 30,000 years from now, he said.
”We know that Mexico City is a dangerous place to live because of the earthquake risk,” said Smith. ”But the people there didn`t have a clue that an earthquake was going to occur when they went to bed that night. Yet there is a risk of a major earthquake in Mexico City about every 100 years, which is roughly within their lifetimes.”
Smith said that two years ago when he started looking at the potential deadly after-effects of a nuclear war he began to realize that the human race may not be helpless in the face of natural disasters.
New technology is improving the ability of scientists to monitor volcanos and earthquake faults, he said. Telescope-equipped satellites can be used to look for celestial objects headed for a collision with the Earth and rockets can be sent to rendezvous with them to blow them up with nuclear bombs or divert them in other ways, he said.
”For the first time in human history we have sufficient technical skills to begin protection of the Earth from some natural hazards,” said Smith.
A major effort probably could be mounted in an attempt to destroy an oncoming meteoroid if it were spotted far enough away, he said. Controlling volcanic eruptions and earthquakes may take 100 years or more but steps should begin immediately to start monitoring all active volcanos and fault zones, he said.
The project would require the cooperation of the world`s nations and diverting only a small fraction of their defense budgets could provide financial support, Smith said.
”Worldwide cooperation to mitigate natural hazards might help psychologically to lead us away from the divisive bickering that triggers wars,” he said. ”Future generations could hail us as pioneers of peace and safety rather than curse us as agents of death and destruction.”
The marked advances in predicting the location and severity of earthquakes in the last 10 years will be useful in constructing ”quake-proof” buildings and in taking other measures to reduce future losses from earthquakes, said Joseph I. Ziony of the U.S. Geological Survey, Reston, Va.
Intensive monitoring is underway in the Los Angeles area because the highly populated region has had more than 40 damaging earthquakes snce 1800 and inevitably will be shaken by future major earthquakes of possibly catastrophic impact, he said.
”The probability that a large earthquake will occur sometime during the next 30 years along the San Andreas Fault near Los Angeles is currently estimated to be 40 percent or greater,” Ziony said.
A catastrophic earthquake in the Los Angeles area would produce losses of billions of dollars and tens of thousands of casualties and would surpass the effects of any previous natural disaster in the U.S., he said.
The eruption of Mt. St. Helens in Washington state in 1980 put volcanology on a new emergency footing. The renewed interest in volcanos, however, has uncovered fears of more eruptions.
Scientists monitoring the Cascade Mountains in Washington, Oregon and California have detected new activity that leads many of them to believe that the Cascades and possibly other western U.S. volcanos are on the threshold of another outburst of volcanic activity similiar to that of the mid-1800s.
Some of these volcanos, including Mono-Inyo Craters, Lassen Peak, Mt. Shasta and Medicine Lake in California and Three Sisters and Newberry in Oregon, are approaching a stage of chemical evolution similar to that attained by Mt. Mazama in Oregon just before it cataclysmically erupted to form Crater Lake about 6,600 years ago.
”A similar eruption today from any one of these volcanos would be a disaster of previously unexperienced proportions,” said R.A. Bailey in a recent U.S. Geological Survey report.
An eruption on the scale of Mt. Mazama would totally devastate life and property within 50 to 100 miles of the volcano, and would cover a large part of the western U.S. with many inches of volcanic ash, he said. The economic and social impact would be felt throughout the country, he added.
Some volcanic eruptions are large enough to compete with comet and asteroid impacts as a mechanism for changing the climate and causing at least partial extinctions.
Of major concern is a really big blowout that could occur in Wyoming`s Yellowstone Park, which has produced the largest volcanic eruptions known anywhere in the world.
These eruptions, about 6,000 times bigger than Mt. St. Helens, shattered the area in cycles that tend to appear every 700,000 years. The last big eruption was 630,000 years ago and some scientists believe another gigantic blast could be in the works.
Scientists were surprised recently to find that the ground in Yellowstone, which lies over a huge chamber of molten material, is rising at the rate of 0.7 inches a year and that the area is seismically very active.
Other areas that are capable of producing exceedingly destructive eruptions include the Long Valley caldera, Clear Lake volcanos and Coso volcanos in California; the San Francisco Peak in Arizona; and Socorro in New Mexico.
The biggest danger facing the Earth, and fortunately the rarest, is an impact with a big celestial object. There are two types: asteroids, which orbit the sun between Mars and Jupiter; and comets, which circle at the outer edge of the solar system.
Comets can be knocked out of their icy orbits and sent on a collision course with the Earth by gravitational tugs from passing stars. Asteroids are jarred out of their safe orbits by the influence of passing planets.
About 100 asteroids up to six miles in diameter are known to cross the Earth`s orbit and there are probably hundreds more that have not yet been discovered, Shoemaker said.
An Earth-crosser means that an asteroid theoretically could hit the Earth when their paths cross. An asteroid between a half mile and 1.2 miles across is estimated to hit the Earth about every 330,000 years. Smaller, but still very destructive bodies may hit the Earth every 1,000 to 10,000 years.
Comets are harder to detect but they are being discovered now at the rate of about six new ones a year. Some scientists think that the Tunguska explosion was caused by a piece of comet Encke that broke off.
Only in the last five years have scientists begun to track asteroids and comets to look for those headed toward Earth. A giant asteroid 12 miles across called Eros has been found to have a 20 percent chance of colliding with the Earth in the next 400 million years.
An object half that size is thought by many scientists to have collided 65 million years ago. The explosion would have sent tremendous amounts of dust and smoke into the atmosphere, blocking out sunlight, turning the climate cold, and, according to some theories, eventually leading to the death of the dinosaurs that depended on the vegetation that perished.
Traveling at a speed of about 56,000 miles an hour, the asteroid`s impact force would have been equivalent to about 100 million tons of TNT, a destructive power 10,000 times greater than a full-scale nuclear war.
Scientists are beginning to look more earnestly for the remnants of old impact craters. More than 100 craters have been found so far ranging in diameter from half a mile to 60 miles.
The immediate catastrophic hazards to mankind in the next few centuries are earthquakes and volcanos, said Shoemaker. Impacts are lower on the list. However, in the long run a very large impact could be more threatening on a global scale, he said.
