by Isaac Asimov
Figure 4.2. Earthquake epicenters 1963-1977. Courtesy National Geophysical Data Center, National Oceanic and Atmospheric Administration.
VOLCANOES
Volcanoes are a natural phenomenon as frightening as earthquakes and longer-lasting, although in most cases their effects are confined to a smaller area. About 500 volcanoes are known to have been active in historical times, two-thirds of them along the rim of the Pacific.
On rare occasions, when a volcano traps and overheats huge quantities of water, appalling catastrophes can take place. On 26-27 August 1883, the small East Indian volcanic island Krakatoa, situated in the strait between Sumatra and Java, exploded with a roar that has been described as the loudest sound ever formed on earth during historic times. The sound was heard by human ears as far away as 3,000 miles and could be picked up by instruments all over the globe. The sound waves traveled several times completely about the planet. Five cubic miles of rock were fragmented, hurled into the air, and fell over an area of 300,000 square miles. Ashes darkened the sky over hundreds of square miles, leaving in the stratosphere dust that brightened sunsets for years. Tsunamis 100 feet in height killed 36,000 people on the shores of Java and Sumatra, and their waves could be detected easily in all parts of the world.
A similar event, with even greater consequences, may have taken place over 3,000 years before in the Mediterranean Sea. In 1967, American archaeologists discovered the ash-covered remains of a city on the small island of Thera, 80 miles north of Crete. About 1400 B.C., apparently it exploded as Krakatoa did but with still greater force, a possibly louder sound, and even more disastrous consequences. The tsunami that resulted struck the island of Crete, then the home of a long-developed and admirable civilization, a crippling blow from which that civilization never recovered. The Cretan control of the seas vanished, and a period of turmoil and darkness eventually followed; recovery took many centuries. The dramatic disappearance of Thera lived on in the minds of survivors, and its tale passed down the line of generations with embellishments. It may very well have given rise to Plato’s tale of Atlantis, which was told about eleven centuries after the death of Thera and of Cretan civilization.
Perhaps the most famous single volcanic eruption in the history of the world was minute compared with Krakatoa or Thera. It was the eruption of Vesuvius in 79 A.D. (up to that time it had been considered a dead volcano), which buried the Roman resort cities of Pompeii and Herculaneum. The famous encyclopedist Gaius Plinius Secundus (better known as Pliny) died in that catastrophe, which was described by his nephew, Pliny the Younger, an eyewitness.
Excavations of the buried cities began in serious fashion after 1763. These offered an unusual opportunity to study relatively complete remains of a city that had existed during the most prosperous period of ancient times.
Another unusual phenomenon is the actual birth of a new volcano. Such an awesome event was witnessed in Mexico on 20 February 1943, when in the village of Paricutin, 200 miles west of Mexico City, a volcano began to appear in what had been a quiet cornfield. In eight months, it had built itself up to an ashy cone 1,500 feet high. The village had to be abandoned, of course.
On the whole, Americans have not been very conscious of volcanic eruptions, which seem, for the most part, to take place in foreign lands. To be sure, the largest active volcano is on the island of Hawaii, which has been an American possession for over eighty years, and an American state for over thirty. Kilauea has a crater with an area of 4 square miles and is frequently in eruption. The eruptions are never explosive, however; and while the lava overflows periodically, it moves slowly enough to ensure little loss of life, even though there is sometimes destruction of property. It was unusually active in 1983.
The Cascade range, which follows the Pacific coast line (about 100 to 150 miles inland) from northern California to southern British Columbia, has numerous famous peaks, such as Mount Hood and Mount Rainier, which are known to be extinct volcanoes. Because they are extinct, they are given little thought, and yet a volcano can lie dormant for centuries and then come roaring back to life.
This fact was brought home to Americans in connection with Mount Saint Helens in south-central Washington State. Between 1831 and 1854, it had been active, but not many people lived there then, and the details are vague. For a century and a third, it had certainly been absolutely quiet, but then 011 18 May 1980, after some preliminary rumbling and quaking, it erupted suddenly. Twenty people, who had not taken the elementary precaution of leaving the region, were killed, and over one hundred were reported missing. It has been active ever since—not much as volcanic eruptions go, but it was the firsl such eruption in the forty-eight contiguous states in a long time.
There is more to volcanic eruptions than immediate loss of life. In giant eruptions, vast quantities of dust are thrown high into the atmosphere, and years may pass before the dust settles. After the Krakatoa eruption, there were gorgeous sunsets as the dust scattered the light of the setting sun for a long period. A less benign effect is that the dust can reflect sunlight so that less of the sun’s warmth reaches the earth’s surface for a time.
Sometimes the delayed effect is relatively local but catastrophic. In 1783, the volcano of Laki in south-central Iceland erupted. Lava eventually covered 220 square miles during a two-year eruption but did little direct damage. Ash and sulfur dioxide, however, spewed out over almost all of Iceland and even reached Scotland. The ash darkened the sky, so that the crops, unable to get sunlight, died. The sulfur dioxide fumes killed three-quarters of the domestic animals on the island. With crops gone and animals dead, 10,000 Icelanders, one-fifth of the whole population of the island, died of starvation and disease.
On 7 April 1815, Mount Tambora, on a small island east of Java, exploded. Thirty-six cubic miles of rock and dust were hurled into the upper atmosphere. For that reason, sunlight was reflected to a greater extent than usual, and temperatures on Earth were lower than usual for a year or so. In New England, for instance, 1816 was unusually cold, and there were freezing spells in every month of that year, even July and August. It was called “the year without a summer.”
Sometimes volcanoes kill immediately but not necessarily through lava or even ash. On 8 May 1902, Mount Pelee on the island of Martinique in the West Indies erupted. The explosion produced a thick cloud of red-hot gases and fumes. These gases poured quickly down the side of the mountain and headed straight for Saint Pierre, the chief town on the island. In 3 minutes, 38,000 people in the city were dead by asphyxiation. The only survivor was a criminal in an underground prison who would have been hanged that very day, if everyone else had not died.
FORMATION OF EARTH’S CRUST
Modern research in volcanoes and their role in forming much of the earth’s crust began with the French geologist Jean Etienne Guettard in the mid-eighteenth century. For a while, in the late eighteenth century, the singlehanded efforts of the German geologist Abraham Gottlob Werner popularized the false notion that most rocks were of sedimentary origin, from an ocean that had once been world-wide (neptunism). The weight of the evidence, particularly that presented by Hutton, made it quite certain, however, that most rocks were formed through volcanic action (plutonism). Both volcanoes and earthquakes would seem the expression of the earth’s internal energy, originating for the most part from radioactivity (see chapter 7).
Once seismographs allowed the detailed study of earthquake waves, it was found that those most easily studied came in two general varieties: surface waves and bodily waves. The surface waves follow the curve of the earth; the bodily waves go through the interior—and, by virtue of this short cut, usually are the first to arrive at the seismograph. These bodily waves, in turn, are of two types: primary (P waves) and secondary (S waves) (figure 4.3). The primary waves, like sound waves, travel by alternate compression and expansion of the medium (to visualize them, think of the pushing together and pulling apart of an accordion). Such waves can pass through any medium—solid or fluid. The secondar
y waves, on the other hand, have the familiar form of snakelike wiggles at right angles to the direction of travel and cannot travel through liquids or gases.
Figure 4.3. Earthquake waves’ routes in the earth. Surface waves travel along the crust. The earth’s liquid core refracts the P-type bodily waves. S waves cannot travel through the core.
The primary waves move faster than secondary waves and consequently reach a seismograph station sooner. From the time lag of the secondaries, it is possible to estimate the distance of the earthquake. And its location or epicenter (the spot on the earth’s surface directly above the rock disturbance) can be pinpointed by getting distance bearings at three or more stations: the three radii trace out three circles that will intersect at a single point.
The speed of both the P and the S types of wave is affected by the kind of rock, the temperature, and the pressure, as laboratory studies have shown. Therefore earthquake waves can be used as probes to investigate conditions deep under the earth’s surface.
A primary wave near the surface travels at 5 miles per second; 1,000 miles below the surface, judging from the arrival times, its velocity must be nearly 8 miles per second. Similarly, a secondary wave has a velocity of less than 3 miles per second near the surface and of 4 miles per second at a depth of 1,000 miles. Since increase in velocity is a measure of increase in density, we can estimate the density of the rock beneath the surface. At the surface of the earth, as I have mentioned, the average density is 2.8 grams per cubic centimeter; 1,000 miles down, it amounts to 5 grams per cubic centimeter; 1,800 miles down, nearly 6 grams per cubic centimeter.
At the depth of 1,800 miles, there is an abrupt change. Secondary waves are stopped cold. The British geologist Richard Dixon Oldham maintained, in 1906, that therefore the region below is liquid: the waves have reached the boundary of the earth’s liquid core. And primary waves, on reaching this level, change direction sharply; apparently they are refracted by entering the liquid core.
The boundary of the liquid core is called the Gutenberg discontinuity, after the American geologist Beno Gutenberg, who in 1914 defined the boundary and showed that the core extended 2,160 miles from the earth’s center. The density “Ofthe various deep layers of the earth were worked out in 1936 from earthquake data by the Australian mathematician Keith Edward Bullen. His results were confirmed by the data yielded by the huge Chilean earthquake of 1960. We can therefore say that at the Gutenberg discontinuity, the density of the material jumps from 6 to 9 and, therefore, increases smoothly to 11.5 grams per cubic centimeter at the center.
THE LIQUID CORE
What is the nature of the liquid core? It must be composed of a substance that has a density of from 9 to 11.5 grams per cubic centimeter under the conditions of temperature and pressure in the core. The pressure is estimated to range from 10,000 tons per square inch at the top of the liquid core to 25,000 tons per square inch at the center of the earth. The temperature is less certain. On the basis of the rate at which temperature is known to increase with depth in deep mines and of the rate at which rocks can conduct heat, geologists estimate (rather roughly) that temperatures in the liquid core must be as high as 5,000° C. (The center of the much larger planet Jupiter may be as high as 50,000° C.)
The substance of the core must be some common element–common enough to be able to make up a sphere half the diameter of the earth and one-third its mass. The only heavy element that is at all common in the universe is iron. At the earth’s surface, its density is only 7.86 grams per cubic centimeter; but under the enormous pressures of the core, it would have a density in the correct range—9 to 12 grams per cubic centimeter. What is more, under center-of-the-earth conditions it would be liquid.
If more evidence is needed, meteorites supply it. These fall into two broad classes: stony meteorites, composed chiefly of silicates, and iron meteorites, made up of about 90 percent iron, 9 percent nickel, and I percent other elements. Many scientists believe that the meteorites are remnants of shattered asteroids, some of which may have been large enough to separate out into metallic and stony portions. In that case, the metallic portions must have been nickel-iron, and so might be the earth’s metallic core. (Indeed, in 1866, long before seismologists had probed the earth’s core, the composition of the iron meteorites suggested to the French geologist Gabriel Auguste Daubree that the core of our planet was made of iron.)
Today most geologists accept the liquid nickel-iron core as one of the facts of life as far as the earth’s structure is concerned. One major refinement, however, has been introduced. In 1936, the Danish geologist Inge Lehmann, seeking to explain the puzzling fact that some primary waves show up in a shadow zone on the surface from which most such waves are excluded, proposed that a discontinuity within the core about 800 miles from the center introduced another bend in the waves and sent a few careening into the shadow zone. Gutenberg supported this view, and now many geologists differentiate between an outer core that is liquid nickel-iron, and an inner core that differs from the outer core in some way, perhaps in being solid or slightly different chemically. As a result of the great Chilean earthquakes of 1960, the entire globe was set into slow vibrations at rates matching those predicted by taking the inner core into account. This is strong evidence in favor of its existence.
EARTH’S MANTLE
The portion of the earth surrounding the nickel-iron core is called the mantle. It seems to be composed of silicates, but judging from the velocity of earthquake waves passing through them, these silicates are different from the typical rocks of the earth’s surface—as was first shown in 1919 by the American physical chemist Leason Heberling Adams. Their properties suggest that they are rocks of the so-called olivine type (olive-green in color), which are comparatively rich in magnesium and iron and poor in aluminum.
The mantle does not quite extend to the surface of the earth. A Croatian geologist named Andrija Mohorovicic, while studying the waves produced by a Balkan earthquake in 1909, decided that there was a sharp increase in wave velocity at a point about 20 miles beneath the surface. This Mohorovicic discontinuity (known as Moho for short) is now accepted to be the boundary of the earth’s crust.
The nature of the crust and of the upper mantle is best explored by means of the surface waves I mentioned earlier. Like the bodily waves, the surface waves come in two varieties: Love waves (named for their discoverer Augustus Edward Hough Love) are horizontal ripples, like the shape of a snake moving over the ground; Rayleigh waves (named after the English physicist John William Strutt, Lord Rayleigh) are vertical, like the path of the mythical sea serpent moving through the water.
Analysis of these surface waves (notably by Maurice Ewing of Columbia University) shows that the crust is of varying thickness. It is thinnest under the ocean basins, where the Moho discontinuity in some places is only 8 to 10 miles below sea level. Since the oceans themselves are 5 to 7 miles deep in spots, the solid crust may be as thin as 3 miles under the ocean deeps. Under the continents, on the other hand, the Moho discontinuity lies at an average depth of about 20 miles below sea level (it is about 22 miles under New York City, for instance), and it plunges to a depth of nearly 40 miles beneath mountain ranges. This fact, combined with evidence from gravity measurements, shows that the rock in mountain ranges is less dense than the average.
The general picture of the crust is of a structure composed of two main types of rock—basalt and granite—with the less dense granite riding buoyantly on the basalt, forming continents and, in places where the granite is particularly thick, mountains (just as a large iceberg rises higher out of the water than a small one). Young mountains thrust their granite roots deep into the basalt, but, as the mountains are worn down by erosion, they adjust by floating slowly upward (to maintain the equilibrium of mass called isostasy, a name suggested in 1889 by the American geologist Clarence Edward Dutton). In the Appalachians, a very ancient mountain chain, the root is about gone.
The basalt beneath the oceans is covered with on
e-quarter to one-half mile of sedimentary rock, but little or no granite—the Pacific basin is completely free of granite. The thinness of the crust under the oceans has suggested a dramatic project: Why not drill a hole through the crust down to the Moho discontinuity and tap the mantle to see what it is made of? It would not be an easy task, for it would mean anchoring a ship over an abyssal section of the ocean, lowering drilling gear through miles of water, and then drilling through a greater thickness of rock than anyone has yet drilled. Early enthusiasm for the project evaporated, and the matter now lies in abeyance.
The “floating” of the granite in the basalt inevitably suggests the possibility of continental drift. In 1912, the German geologist Alfred Lothar Wegener suggested that the continents were originally a single piece of granite, which he called Pangaea (“all-Earth”). At some early stage of the earth’s history, this fractured, and the continents drifted apart. He argued that they were still drifting—Greenland, for instance, moving away from Europe at the rate of a yard a year. What gave him (and others, dating back to Francis Bacon about 1620) the idea was mainly the fact that the eastern coastline of South America seemed to fit like a jigsaw piece into the shape of the western coast of Africa.
For a half-century, Wegener’s theory was looked upon with hard disfavor. As late as 1960, when the first edition of this book was published, I felt justified, in view of the state of geophysical opinion at that time, in categorically dismissing it. The most telling argument against it was that the basalt underlying both oceans and continents was simply too stiff to allow the continental granite to drift through it, even in the millions of years allowed for it to do so.
