Big rocks falling from space have been popular in the past few years. We've had movies and books ("Armageddon," "Deep Impact," "Moonfall") in which giant meteorites wipe out life on earth, or fail to do so only because of heroic and remotely improbable human action. We have an active space-watch program, looking for asteroids that may some day collide with our planet; and we have Air Force studies and Congressional hearings, asking what we can do to protect ourselves should such a thing happen.

The short answer is, not a thing, but the interest and the official reports continue. The impact of a meteorite 65 million years ago is believed to have done in the dinosaurs, and another one close to 200 million years earlier caused the Permian Extinction, when more than 90 percent of all forms of life vanished from the earth. What was good enough for the dinosaurs should surely be good enough for us.

I certainly think we should keep watching the skies. However, falling rocks are only one potential source of planetary disaster, and maybe not the worst. As recently as 200 years ago it was possible to deny the existence of meteorites. (Thomas Jefferson said, "I could more easily believe that two Yankee professors would lie than that stones would fall from heaven.") Earthquakes and volcanic eruptions, on the other hand, have been known and feared for all of recorded history.

Historical records of earthquakes, inevitably, are largely descriptive. Most people experiencing an earthquake have other things than exact measurement on their minds. However, the development of seismometers, delicate instruments able to register, locate, and measure an earthquake half the world away, has changed nervous anecdotes to precise science.

Today two scales are used to describe the strength of an earthquake. The better known, called the Richter Scale, was developed by C.F. Richter in 1935, and it is now routinely reported for earthquakes all around the world. It is actually an energy scale, and a logarithmic one at that, so a Richter rating of, say, 7.5, releases ten times as much energy as one with a rating of 6.5, and a hundred times as much energy as one of 5.5.

As a rule-of-thumb, property damage begins with a Richter Scale magnitude of about 5. The largest recorded earthquakes rate 8.6 on the Richter Scale. There have been only two of them since the scale came into use: in India, on August 15, 1950; and in Alaska, on March 27, 1964. Judging from the reported effects, the Alaska earthquake in 1899 also rated 8.6, as did the Colombia earthquake of 1906. The Lisbon earthquake on November 1, 1755 probably had a magnitude between 8.7 and 9.0. The great Chinese earthquake of July 28, 1976, which killed half a million people in Tangshan, registered 8.2 on the Richter Scale. The 1906 earthquake in San Francisco is estimated as 8.3.

The other scale for earthquakes is called the Mercalli Scale, and it is less precise. It defines "degrees of intensity" between I and XII. An intensity II earthquake is barely perceptible to humans; intensity IX damages buildings and cracks the ground, and intensity XI shatters masonry buildings, bends railroad tracks, and destroys most freestanding structures. I should point out that the Mercalli scale concerns itself only with effects on manmade structures. This sounds rather like the old question, "If a tree falls in the forest and no one hears it, does it make a sound?" Is it an earthquake if there are no manmade objects to be affected by it? Not according to the Mercalli scale.

Locating the point of maximum intensity, or epicenter, of an earthquake, can be tricky. The location of a volcanic eruption, on the other hand, is usually in no doubt at all. We lack the equivalent of the Richter Scale for volcanoes, but since the total amount of energy release in a large eruption and a large earthquake appear to be comparable, it seems reasonable to use such a measure for either one.

The situation is made a bit more complicated because volcanic eruptions are of two distinct types, labeled without much imagination as Type A and Type B. Type B eruptions are in many ways more interesting and spectacular, because they are accompanied by gigantic explosions and produce large volumes of ejecta - lava, dust and ash thrown high into the air. Krakatoa, in 1883, and Mount St. Helens, in 1980, both were Type B events. Type B eruptions send dust high into the stratosphere, to produce colorful sunsets all around the world. Type A eruptions are often just as energetic, but they are less noisy and they don't get the same publicity. They produce thermal energy, heating the environment but not causing major explosions. They may involve huge lava flows, and even the creation of new volcanic islands. A famous modern example is the island of Surtsey, created by a volcanic eruption off southwest Iceland, in 1963. That event produced about one-tenth the energy of the Krakatoan eruption.

All these, however, are nothing compared with events farther back history. Tambora, in 1815, on the Indonesian island of Sumbawa, is estimated to have been 80 times as energetic an eruption as Krakatoa. The following year, 1816, was known as "the year without a summer," when crops failed to ripen throughout Europe. The most likely cause was the stratospheric layer of reflective dust from Tambora. Vesuvius in 79 AD and Mount St. Helens in 1980 are puny by comparison, with energy releases only one-tenth and one-thirtieth that of Krakatoa, though they are far better known to most people than Tambora (location, location, location).

Biggest of all blow-ups during historical times, but one for which no eyewitness or contemporary records exist, was the destruction of the island of Thira (formerly Santorini) in the Aegean Sea north of Crete. From archeological evidence and the examination of the shattered remnant of Thira, this eruption is estimated to have had the energy of maybe a hundred Krakatoas. It happened about 1,470 BC and produced a monstrous tidal wave which possibly destroyed the Cretan Minoan civilization.

And before historical times? That's when things become scary. We find evidence in the geological record of lava flows spanning half a continent, and of volcanic eruptions taking place close to simultaneously across much of the planet. Such events, just like the impact of monster meteorites, can cause species' extinctions.

We are a species. And we - writers, moviemakers, astronomers, Air Force and Congress - have been busy in the past few years looking up at the sky. Perhaps too busy. Maybe it's time we gave equal attention to looking down.

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