Continuous monitoring complements geological mapping of potentially active volcanoes, which is generally the best way of determining their past eruptive habits. History often repeats itself in the natural world, so this type of geological assessment can provide very useful data for long-range forecasting of the activity of an individual volcano. A good example is the Nevado del Ruiz volcano in Colombia. An eruption and mudflow in 1845 killed 1,000 field hands at a tobacco plantation near its base. The summit of Nevado del Ruiz is covered by a large snow and ice cap, and even a small eruption can generate massive floods of meltwater and debris that course down the canyons on the volcano's flanks. Sweeping up soil and debris, the floods become destructive mudflows. When earthquake swarms and small eruptions began at Nevado del Ruiz in 1985, geologists warned that mudflows similar to those in 1845 were likely to occur again. They did—killing 25,000 people. The reasons for this tragedy are complex, but one was that people in Armero—the principal town that was destroyed—did not comprehend what a massive mudflow was. Nothing like this had happened for 140 years—long enough for everyone but a few alarmed geologists to completely forget what such a threat could mean.

Only about 10 percent of the world's 1,300 potentially active volcanoes have been geologically mapped to assess their past eruptive habits. This leaves 90 percent yet to be studied. Efforts to monitor and map these threats using field geologists can be accomplished with a high degree of return for a low hazard-assessment budget investment.

Understanding how a bomb works does not eliminate its danger, but if something is known about its size and the way in which it is detonated, the danger can assuredly be reduced or avoided. The same holds true for geological hazards—the better a phenomenon is understood, the more likely it is that its threat can be mitigated. At the same time, urging the establishment of simple but effective educational programs to inform governing officials and the public at risk remains the responsibility of the scientist studying the hazard. How might such an education program work? As in advertising, keep the message simple, and present it over and over again.

HAZARDS OF EXTRATERRESTRIAL ORIGIN

Within the past dozen years the idea of catastrophic terrestrial impact has been revived. From being a topic that was hardly considered respectable, it has become accepted as something that certainly happens occasionally and that may have had global consequences at various times in the past. Recent theories attribute both the origin of the Moon and the extinction of the dinosaurs to impacts of extraterrestrial objects.

The historical record, which goes back less than 3,000 years, contains no reference to anyone killed by a meteorite fall—although injury from a meteorite that penetrated a house is reported, and early in the century the Naklha meteorite that fell in Egypt may have hit a dog. On the time scale of current societal interest, the danger from impacts is insignificant. However, if a large extraterrestrial object did collide with Earth, the consequences could be devastating.

A distinction should be made between the near-field consequences of impact, dominated by the crater and its surrounding debris field, and the far-field consequences, which for larger impacts would be catastrophic. Research related to the former includes the need for better understanding of the flux of impactors on the Earth. Astronomical observations are essential, while the search for and study of impact craters on Earth yield complementary information about the flux in the past. Astronomical monitoring has produced estimates of impact flux that indicate there must be many unrecognized impact structures waiting to be discovered. At present, attention is focused on the search for evidence of an impact that occurred 66-million-years ago and was large enough to have caused the extinction of the dinosaurs by means of the far-field effects. The Chicxulub structure in the north of the Yucatan Peninsula is considered a strong candidate, as discussed in Chapter 3.

Far-field effects become important if the impactor is large enough to make a crater more than a few tens of kilometers in diameter. Dust and aerosols propelled to great heights would induce fluctuations in weather patterns as well as produce acid rain. Wildfires might ignite over huge areas, and part of the atmosphere could be blasted away. In such catastrophic circumstances, many organisms would perish and many species would face extinction. The geological record should contain evidence of such drastic events. The challenge of impact theory is twofold: to seek evidence of past impacts in the geological record and to model patterns of atmospheric and oceanic systems that could produce distributions matching the evidence.

Giant terrestrial impacts are capable of perturbing the earth system rapidly and to an extent that few other phenomena can rival. Within the past half



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