The Story of the Earth in 25 Rocks

Tales of Important Geological Puzzles and the People Who Solved Them

Donald R. Prothero

• Publisher: Columbia University Press
• Year: 2018
• ISBN:
  9780231544276
• Topic: Physical Sciences
• Subtopic: Geology & Earth Sciences

Every rock is a tangible trace of the earth's past. The Story of the Earth in 25 Rocks tells the fascinating stories behind the discoveries that shook the foundations of geology. In twenty-five chapters—each about a particular rock, outcrop, or geologic phenomenon—Donald R. Prothero recounts the scientific detective work that shaped our understanding of geology, from the unearthing of exemplary specimens to tectonic shifts in how we view the inner workings of our planet.

Prothero follows in the footsteps of the scientists who asked—and answered—geology's biggest questions: How do we know how old the earth is? What happened to the supercontinent Pangea? How did ocean rocks end up at the top of Mount Everest? What can we learn about our planet from meteorites and moon rocks? He answers these questions through expertly chosen case studies, such as Pliny the Younger's firsthand account of the eruption of Vesuvius; the granite outcrops that led a Scottish scientist to theorize that the landscapes he witnessed were far older than Noah's Flood; the salt and gypsum deposits under the Mediterranean Sea that indicate that it was once a desert; and how trying to date the age of meteorites revealed the dangers of lead poisoning. Each of these breakthroughs filled in a piece of the greater puzzle that is the earth, with scientific discoveries dovetailing with each other to offer an increasingly coherent image of the geologic past. Summarizing a wealth of information in an entertaining, approachable style, The Story of the Earth in 25 Rocks is essential reading for the armchair geologist, the rock hound, and all who are curious about the earth beneath their feet.

01	VULCAN’S WRATH: THE ERUPTION OF VESUVIUS

VOLCANIC TUFF

Live in danger. Build your cities on the slopes of Vesuvius.

—FRIEDRICH NIETZSCHE

FIRE OF THE GODS

The eruption of a volcano can be a terrifying event. In ancient times, and even today in many different cultures, volcanoes were seen as a sign of the wrath of the gods or a punishment for transgressing divine decrees. Their enormous power, noise, and potential for destruction made volcanoes more feared than any other geologic event except earthquakes. The Romans thought that the fires of the Mount Etna volcano in Sicily were due to the forges of the god of fire, Vulcan (Hephaistos to the Greeks). He used the heat of the underworld to hammer out armor, metalwork, and weapons for the gods (including the thunderbolts thrown by Jupiter/Zeus). When eruptions occurred, it was said that Vulcan was angry because his wife Venus (Aphrodite to the Greeks) had cheated on him. The Romans considered Mount Vesuvius on the Bay of Naples sacred to Hercules (Herakles to the Greeks), and some scholars think that the name “Vesuvius” is derived from the Greek for “son of Zeus” (as Herakles was Zeus’s son).

Yet one of the first truly scientific descriptions and insights into what a volcano is and how it erupts came from ancient times. In some senses, the eruption of Vesuvius in 79 C.E. can be considered the beginning of our modern understanding of Earth and the event that led to the birth of geology as a science.

At the time, the towns around Mount Vesuvius were prosperous and growing. The Bay of Naples supported a large fishing industry, and wine grapes were grown in many places, including the slopes of Vesuvius itself. Then, as now, the volcanic soils around Vesuvius were too rich to be ignored, and they were valued for growing crops and vines of wine grapes. The Roman emperors had a large villa out on the nearby island of Capri, and many other powerful Romans had homes in the region. Pompeii was a large city with a population of over 20,000, and there were many smaller communities in the region.

Vesuvius had not erupted since 217 B.C.E., so most Romans thought it was extinct. Yet there had been many earthquakes in the 17 years since the great earthquake of 62 C.E. that had destroyed much of Pompeii, Herculaneum, and Neapolis (Naples). As early as 30 B.C.E., the Greek historian Diodorus Siculus described the Campanian plain as “fiery” (Phlegrean) because Vesuvius showed signs of the fires that had burned long ago.

By August of 79 C.E., earthquakes had become more frequent, and many of the wells and springs in the area had dried up. August 23 was the Vulcanalia, the feast day of Vulcan, which the Romans celebrated every year. Ironically, the next day Vulcan replied to his worshippers with a huge explosion that darkened the skies with ash and a rain of pumice that lasted 20 hours (figure 1.1). Some of the people of Herculaneum and Pompeii evacuated immediately, but there were many left behind, unwilling to leave or unable to do so because there were not enough boats in the harbor and the roads were clogged with traffic and almost 2.8 meters (9 feet) of fallen ash and pumice. Not only was it hard to escape, but it was even hard to breathe with the ash, which choked the lungs of people and animals. But this was only the warm-up. A day later, Vesuvius spewed out many nuées ardentes (“glowing clouds” in French) or pyroclastic flows. These superheated (up to 850°C, or 1,560°F) mixtures of volcanic gases and ash roared down the mountain slope at 160 kilometers per hour (100 miles per hour), incinerating everything in their path. They buried Herculaneum under tens of meters of volcanic deposits known as tuff.

Figure 1.1

The 1944 eruption of Mount Vesuvius. (Courtesy of Wikimedia Commons)

A HISTORIAN OF DISASTER

The eyewitnesses of the eruption mostly died or left no written records, so their thoughts have been lost in the mists of history. Fortunately, we do have one excellent eyewitness account, written by the historian Pliny the Younger. He was 17 at the time, fleeing with his family in a boat to the town of Misenum, across the bay 35 kilometers (22 miles) from the volcano. In a letter to his friend, the famous historian Cornelius Tacitus, the younger man described how his 56-year-old uncle Pliny the Elder, one of Rome’s leading admirals, scholars, and naturalists, decided to take a boat closer to the mountain to rescue his friends. It has been one of my favorite accounts of any eruption ever since I first read it in the original in my high school Latin class:

My dear Tacitus,

You ask me to write you something about the death of my uncle so that the account you transmit to posterity is as reliable as possible. I am grateful to you, for I see that his death will be remembered forever if you treat it [in your Histories]. He perished in a devastation of the loveliest of lands, in a memorable disaster shared by peoples and cities, but this will be a kind of eternal life for him. Although he wrote a great number of enduring works himself, the imperishable nature of your writings will add a great deal to his survival. Happy are they, in my opinion, to whom it is given either to do something worth writing about, or to write something worth reading; most happy, of course, those who do both. With his own books and yours, my uncle will be counted among the latter. It is therefore with great pleasure that I take up, or rather take upon myself the task you have set me.

He was at Misenum in his capacity as commander of the fleet on the 24th of August [79 C.E.], when between two and three in the afternoon my mother drew his attention to a cloud of unusual size and appearance. He had had a sunbath, then a cold bath, and was reclining after dinner with his books. He called for his shoes and climbed up to where he could get the best view of the phenomenon. The cloud was rising from a mountain—at such a distance we couldn’t tell which, but afterward learned that it was Vesuvius. I can best describe its shape by likening it to a pine tree [today we would compare it to a “mushroom cloud”]. It rose into the sky on a very long “trunk” from which spread some “branches.” I imagine it had been raised by a sudden blast, which then weakened, leaving the cloud unsupported so that its own weight caused it to spread sideways. Some of the cloud was white, in other parts there were dark patches of dirt and ash. The sight of it made the scientist in my uncle determined to see it from closer at hand. [This style of explosive mushroom cloud of ash and pumice is now called a “Plinian eruption” in his honor.]

He ordered a boat made ready. He offered me the opportunity of going along, but I preferred to study—he himself happened to have set me a writing exercise. As he was leaving the house he was brought a letter from Tascius’s wife Rectina, who was terrified by the looming danger. Her villa lay at the foot of Vesuvius, and there was no way out except by boat. She begged him to get her away. He changed his plans. The expedition that started out as a quest for knowledge now called for courage. He launched the quadriremes and embarked himself, a source of aid for more people than just Rectina, for that delightful shore was a populous one. He hurried to a place from which others were fleeing, and held his course directly into danger. Was he afraid? It seems not, as he kept up a continuous observation of the various movements and shapes of that evil cloud, dictating what he saw.

Ash was falling onto the ships now, darker and denser the closer they went. Now it was bits of pumice, and rocks that were blackened and burned and shattered by the fire. Now the sea is shoal; debris from the mountain blocks the shore. He paused for a moment wondering whether to turn back as the helmsman urged him. “Fortune helps the brave,” he said. “Head for Pomponianus.”

At Stabiae, on the other side of the bay formed by the gradually curving shore, Pomponianus had loaded up his ships even before the danger arrived, though it was visible and indeed extremely close, once it intensified. He planned to put out as soon as the contrary wind let up. That very wind carried my uncle right in, and he embraced the frightened man and gave him comfort and courage. In order to lessen the other’s fear by showing his own unconcern he asked to be taken to the baths. He bathed and dined, carefree or at least appearing so (which is equally impressive). Meanwhile, broad sheets of flame were lighting up many parts of Vesuvius; their light and brightness were the more vivid for the darkness of the night. To alleviate people’s fears my uncle claimed that the flames came from the deserted homes of farmers who had left in a panic with the hearth fires still alight. Then he rested, and gave every indication of actually sleeping; people who passed by his door heard his snores, which were rather resonant since he was a heavy man. The ground outside his room rose so high with the mixture of ash and stones that if he had spent any more time there escape would have been impossible. He got up and came out, restoring himself to Pomponianus and the others who had been unable to sleep. They discussed what to do, whether to remain under cover or to try the open air. The buildings were being rocked by a series of strong tremors, and appeared to have come loose from their foundations and to be sliding this way and that. Outside, however, there was danger from the rocks that were coming down, light and fire-consumed as these bits of pumice were. Weighing the relative dangers they chose the outdoors; in my uncle’s case it was a rational decision, others just chose the alternative that frightened them the least.

They tied pillows on top of their heads as protection against the shower of rock. It was daylight now elsewhere in the world, but there the darkness was darker and thicker than any night. But they had torches and other lights. They decided to go down to the shore, to see from close up if anything was possible by sea. But it remained as rough and uncooperative as before. Resting in the shade of a sail he drank once or twice from the cold water he had asked for. Then came a smell of sulfur, announcing the flames, and the flames themselves, sending others into flight but reviving him. Supported by two small slaves he stood up, and immediately collapsed. As I understand it, his breathing was obstructed by the dust-laden air, and his innards, which were never strong and often blocked or upset, simply shut down. When daylight came again two days after he died, his body was found untouched, unharmed, in the clothing that he had had on. He looked more asleep than dead.

In a second letter to Tacitus a few days later, Pliny wrote:

By now it was dawn, but the light was still dim and faint. The buildings ’round us were already tottering, and the open space we were in was too small for us not to be in real and imminent danger if the house collapsed. This finally decided us to leave the town. We were followed by a panic-stricken mob of people wanting to act on someone else’s decision in preference to their own (a point at which fear looks like prudence), who hurried us on our way by pressing hard behind in a dense crowd. Once beyond the buildings we stopped, and there we had some extraordinary experiences which thoroughly alarmed us. The carriages we had ordered to be brought out began to run in different directions though the ground was quite level, and would not remain stationary even when wedged with stones. We also saw the sea sucked away and apparently forced back by the earthquake: at any rate it receded from the shore so that quantities of sea creatures were left stranded on dry sand. On the landward side a fearful black cloud was rent by forked and quivering bursts of flame, and parted to reveal great tongues of fire, like flashes of lightning magnified in size.

At this point my uncle’s friend from Spain spoke up still more urgently: “If your brother, if your uncle is still alive, he will want you both to be saved; if he is dead, he would want you to survive him—why put off your escape?” We replied that we would not think of considering our own safety as long as we were uncertain of his. Without waiting any longer, our friend rushed off and hurried out of danger as fast as he could.

Soon afterward the cloud sank down to earth and covered the sea; it had already blotted out Capri and hidden the promontory of Misenum from sight. Then my mother implored, entreated, and commanded me to escape the best I could—a young man might escape, whereas she was old and slow and could die in peace as long as she had not been the cause of my death too. I refused to save myself without her, and grasping her hand forced her to quicken her pace. She gave in reluctantly, blaming herself for delaying me. Ashes were already falling, not as yet very thickly. I looked round: a dense black cloud was coming up behind us, spreading over the earth like a flood. “Let us leave the road while we can still see,” I said, “or we shall be knocked down and trampled underfoot in the dark by the crowd behind.” We had scarcely sat down to rest when darkness fell, not the dark of a moonless or cloudy night, but as if the lamp had been put out in a closed room. You could hear the shrieks of women, the wailing of infants, and the shouting of men; some were calling their parents, others their children or their wives, trying to recognize them by their voices. People bewailed their own fate or that of their relatives, and there were some who prayed for death in their terror of dying. Many besought the aid of the gods, but still more imagined there were no gods left, and that the universe was plunged into eternal darkness for evermore. There were people, too, who added to the real perils by inventing fictitious dangers: some reported that part of Misenum had collapsed or another part was on fire, and though their tales were false they found others to believe them. A gleam of light returned, but we took th...