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Earth Explorer
A volcanologist finds new history buried in ash.

This article originally appeared in Archaeology, V. 53, No. 4, July/August 2000.

Haraldur Sigurdsson strides across the volcanic cone Raudakula, crunching over its brick red pumice and the tufts of green moss and grass that cling to its windswept slopes. It is early July on the Snaefellsnes Peninsula, the spine of volcanoes that juts into the Atlantic on the west coast of Iceland. The sun is now up 21 hours a day, melting the last of the winter snows from the smaller peaks. The day is dazzling, the visibility infinite, but Sigurdsson's eyes are focussed closely, scanning the ground just a few tens of feet ahead and to either side. From time to time he darts off, pounces on a rock, examines it intently, and then swings his two-pound steel hammer to smash it open on the ground. Each time he only looks briefly at the pieces and then, disappointed, tosses them aside.
Sigurdsson is searching for xenoliths, nodules of pyroxene that have been coughed to the surface by erupting volcanoes. He calls these unaltered chunks of the earth's subterranean mantle “the Rosetta Stones of Icelandic geology.” Not only do they carry the secrets of the source material of which Iceland is made, but they are also extremely rare - over the last four decades, Sigurdsson has found no intact xenoliths, but he keeps searching. “You have to break a lot of rocks,” he says, adding wryly, “we Icelanders are very stubborn.”
The 60-year-old volcanologist has spent his life picking through the stones spewed by volcanoes, discovering facts about Earth's workings, and revealing clues to human history hidden in ash. His findings have changed our ideas about Minoan Crete, Herculaneum, and Pompeii, and his research into the climatic impact of eruptions has helped archaeologists gain insights into the distant past. “He studies volcanoes with a really broad brush, and that's useful for us,” says archaeologist Payson Sheets of the University of Colorado at Boulder. Sheets is excavating an ash-buried settlement in El Salvador and has been researching volcanic impacts on Mesoamerican societies for more than two decades. “So much of his work has the human dimension in it, what happens with people and how volcanoes effect human society.”
Sigurdsson is a lean, articulate man with sandy hair, an easy manner, and the resume of a scientist from a Jules Verne novel. He first fell under the spell of volcanoes when he was growing up in Iceland. In the years since, he has hacked through jungles, paddled across a boiling lake, camped out for days on the sweltering floor of an active crater, unearthed artifacts, discovered long-lost villages, ridden in submarines to the bottom of the sea, and studied traces of titanic eruptions etched in polar ice.  
Sigurdsson lives in Jamestown, Rhode Island, and every day kayaks nearly two miles across Narragansett Bay to his office at the University of Rhode Island's School of Oceanography in Kingston. He is editor-in-chief of Academic Press's forthcoming Encyclopedia of Volcanoes, and in his spare time, he collects volcano art and operates a tour company that sends travelers on expeditions to volcanoes in Iceland, Costa Rica, and Italy. But Sigurdsson's passion is unraveling the secrets of explosive eruptions - probing the mysteries of their subterranean births, charting the passage of their violent lives, gauging how their powerful blasts and chemical vapors affect the planet and humankind.
When he studies volcanoes, Sigurdsson likens himself to a detective picking through crime scenes, reconstructing the history of eruptions through clues they have left behind. Along the way he has made discoveries that shed light not only on geophysical processes, but on the course of ancient cultures. Increasingly archaeologists are using his studies, and the work of other volcanologists, to reevaluate the evolution of ancient societies.
“Volcanologists are becoming more versed in the significance of archeological aspects, and similarly archeologists are becoming more interested in the volcanic process,” says Sigurdsson. “It's a classical merging of two cultures, of two schools of research.”
Eruptions provide archaeologists with dates for buried cities and explanations for changes in trade routes and settlement patterns; interred remains give volcanologists clues that quantify a volcano's energy - its output of lava, the temperature of its gases, and the chronology of its eruption. When a volcano pummels a human settlement, says Sigurdsson, the remains show how “the volcanic process interacts with common everyday objects - melting glass, carbonizing furniture, cooking people, blowing down walls, taking off roofs, coating roofs with ash, collapsing them - so these are parameters that you can calibrate.”
In the past 200 years alone volcanoes have killed more than a quarter of a million people; countless others have died in the millennia outside the reach of the historical record. By studying volcanoes on both a local and global scale, Sigurdsson illuminates the human experience far into the past.
Iceland, Sigurdsson's birthplace, lies athwart the Mid-Atlantic Ridge, the spreading seam between the North American and Eurasian tectonic plates. Over millions of years the lava that has gushed from this rift has built Iceland into the world's largest volcanic island, a mountainous, snowcapped geologic wonder that is pocked with craters and ruptured by fissures that regularly pave the countryside with red-hot rock. Icelandic eruptions can be catastrophic disasters: the six-month Laki eruption in 1783, the largest lava eruption in historic times, killed off most of the island's pastureland and grazing animals with lava, ash, and poisonous gas. As a result, 10,000 people-almost one quarter of the country's population-starved to death.
Although Sigurdsson moved to the United States in 1974 and recently became an American citizen, Iceland still is home to his heart. He recently bought a small summer house in Stykkisholmur, the small fishing village where he grew up; the desk of his office faces a giant map of Iceland that covers one wall; and every day, in front of his house in Rhode Island, he displays the Icelandic flag.
Iceland is also still central to Sigurdsson's science. Its volcanoes were his inspiration, and he is quick to show a visitor a treasured volume from his office shelves, the Ph.D. thesis of the Icelandic volcanologist Sigurdur Thorarinsson. “He was a real pioneer in integrating the volcanological and historical record,” Sigurdsson says. “He subverted the whole field. He mastered Latin, German, and Greek to study Iceland's historical literature, he studied meteorology and climatology, he studied the volcanic history of Iceland. He worked very closely with the archeologists, marking finds and making outlines.”
In the late 1930s and early 1940s, Thorarinsson invented a new science he called “tephrochronology” - after tephra, the Greek word for ash. When a volcano erupts it shoots bits of molten magma into the air. The magma cools into pieces of rock, or tephra, that range from pinhead- to Volkswagen-size. The biggest blasts shoot huge boulders tens of miles and launch blistering columns of tephra into the stratosphere, where winds carry it around the globe.
Each volcano's tephra has a unique chemical composition, a tell-tale signature that pinpoints its source. It also forms a blanket which can cover tens of thousands of square miles; this mantle, if found in an archaeological excavation, forms a definitive time-marker when linked to a dated eruption. Thorarinsson used volcanic ash layers to date Viking excavations, and Viking history to date the eruptions. This synthesis of science, history, art, and archaeology, has been a model for Sigurdsson's own work.
In 1932, Greek archaeologist Spyridon Marinatos was excavating a Minoan villa at Amnissos, a harbor town on the northern coast of Crete. He was puzzled by the astonishing amount of force that destroyed the ancient villa - walls and corners had collapsed, huge stone blocks were knocked off kilter, and stone plinths were charred. Other towns and palaces on the island had also been thoroughly destroyed. The collective disaster seemed the product of violence beyond the power of even the most vicious invaders, but he soon unearthed a clue to a possible culprit: the lower levels of a seafront building were filled with pumice.
Few things are more engaging to a romantic mind than the link between a volcano's awesome power and an ancient civilization that has mysteriously died. A society of seafaring merchants who traded extensively in the eastern Mediterranean, the Minoans of Crete flowered and quickly declined in the space of a few hundred years. At around 1450 B.C., Minoan culture came to a sudden end, although Knossos, the center of Cretan power, survived for a time under Mycenaen domination.  
Marinatos knew that 62 miles to the north, across the Aegean's cobalt blue water, lay a cluster of five islands, the shattered remnant of an enormous volcano called Santorini. A massive blast at Santorini would have sent a tsunami crashing against Crete's rocky coast, dashing the Minoan fleet to bits and leveling shoreline palaces. In 1939 Marinatos published “The Volcanic Destruction of Minoan Crete.” He wrote:
If the explosion took place during the day, the day was surely turned into night and much damage was caused by the tremendous vibration of the air. The thunderous roar, too, must have deafened and terrified the Cretans, who had, of course, no means of knowing what was its cause. Then must have come the rain of mud and ashes, some cold, some ablaze and burning. Worst of all, however, were the waves which broke over the island, much higher and more rapid than those at Krakatau.[Krakatoa, Indonesia].
Scholars began citing the eruption as the cause of the Biblical plague of Exodus when “there was a thick darkness in all the land of Egypt for three days.” They also believed they found its traces in Greek mythology, Homer's Odyssey, China's Shang Dynasty Annals, and on an ancient Egyptian stele which tells of a catastrophic storm with earsplitting noise, unnatural darkness, torrential rain, floods, and houses afloat like papyrus skiffs.
In 1967, on Thera, Santorini's largest island, Miranatos discovered Akrotiri, an ancient city remarkably preserved in twenty feet of volcanic ash. Akrotiri was an Aegean Pompeii, with stone buildings featuring frescoes of dynamic beauty.
The findings captured the world's attention but also raised questions. The style of pottery buried at Akrotiri had been out of favor on Crete for a generation. Some archaeologists said this proved that the eruption had destroyed Akrotiri well before Crete met its end, but others countered that the eruption had occurred in phases over half a century, first knocking out Akrotiri, then destroying Minoan settlements on Crete itself.
Sigurdsson entered the debate in 1975 as one of a team of scientists on the research ship Trident that sampled the seafloor for Santorini's tephra. While the ship crisscrossed the southeastern Mediterranean downwind of Santorini, from the caldera to the Nile Delta, Sigurdsson and his colleagues extracted 31 separate cores of layered seafloor mud. They then correlated the ash layers from core to core to create the most detailed map of the tephra's fall.
Their findings refuted Marinatos's thesis that ash and flaming stones rained destruction upon Crete. The eruption had, indeed, been enormous, one of the most massive eruptions in the last 10,000 years. But the seafloor records showed the wind had spread its tephra primarily to the east. No more than four inches of tephra had fallen on Crete, enough to cause some inconvenience but not evacuations. On the island of Rhodes and Turkey's south coast, the ash fall would have been far more severe, but probably insufficient to tip local societies into decline.
In 1989, Sigurdsson and his colleagues Steve Carey and Joe Devine announced that additional research had revealed that Santorini had spewed out three times as much ash over the Aegean, near East and Central Asia than had been previously thought, enough to cover the state of Ohio under a layer more than ten feet deep.
As for Marinatos's theory that tsunamis swept Crete, Sigurdsson believes the answer lies in the unexplored deposits on Santorini's underwater flanks. Tens of billions of tons of tephra, falling suddenly into the sea, could easily have sent a colossal surge of water over Crete. Indonesia's Krakatoa, after all, produced tsunamis over 110 feet high, and Santorini's eruption was twice as large. Sigurdsson is hoping to study the deposits in the near future. “I have no doubt there was a tsunami,” says Sigurdsson. “The question is how big it was.”
Sigurdsson and British volcanologist Steve Sparks were standing on the deck of the Trident as it pulled to the dock in Naples in 1975, on the final day of the Santorini research cruise. The ship's hold was full of seafloor cores, and the two volcanologists were looking forward to a little relaxation. Around them unfolded a vista familiar from countless paintings and postcards - the arcing coastline enveloping the Bay of Naples, and the silent cone of Vesuvius standing sentinel over all. “The ship came into Naples, and Steve and I said, `Oh, let's go on a little holiday to Pompeii and Herculaneum, on a little busman's holiday.'”  
Naples lies in the heart of the most active geologic zone in Europe. In the last 10,000 years the Neapolitan area has experienced 100 explosive eruptions as Vesuvius, Ischia, and the volcanoes of the Campei Flegrei have blown their tops. When Vesuvius last erupted in 1944, fountains of lava shot more than half a mile into the air, and enough ash fell to destroy crops and crush 26 victims under collapsing roofs.
Archaeologists and volcanologists long believed that falling ash had buried Pompeii, a city of 20,000, and that Herculaneum, home to 4,500, had been inundated in massive mud flows - but one look at the volcanic debris told Sigurdsson and Sparks that two centuries of accepted history were wrong.
The cities had fallen victim to a volcanic one-two punch - first, a rain of falling rock and ash, then a fierce battering by what volcanologists call pyroclastic flows, hurricane-force blasts of blistering hot gas and ash strong enough to level buildings. “We were immediately struck by how beautiful the evidence was for fallout and pyroclastic flows and surges,” remembers Sigurdsson. “We started to putter around with our rock hammers, and of course the guards weren't too happy about that. So we had to have one guy on watch while the other was digging.”
Almost everywhere they looked was evidence of hot, turbulent blasts: some of the strata were curved like wind-blown dunes, others were extremely thin and made of fine-grained ash. The conventional view that only falling ash and pumice had destroyed Pompeii was obviously far too simple. And the conclusion that mud had buried Herculaneum was simply impossible: embedded in the strata were plentiful chunks of charcoal, and mud cannot burn wood.
Eventually Sigurdsson received research permits from the authorities, and over the next ten years he and his colleagues would perform the eruption's first truly comprehensive study. By the time they were done they had rewritten history and produced two landmark papers that are certain to become classics of volcanology.
“The Eruption of Vesuvius in A.D. 79: Reconstruction from Historical and Volcanological Evidence,” and “The Eruption of Vesuvius in A.D. 79”mix numbers and human details to chronicle the cities' last agonized hours. They cite sources as diverse as Tacticus, Seneca, The New England Journal of Medicine, wall paintings, Strabo, and Ships and Seamanship in the Ancient World. And they dissect the chaos with an array of chilling data: the terminal velocity of falling fist-sized rocks (112 miles per hour), the maximum possible speed of a Roman warship en route to rescue victims (eight knots), the rate of raining rocks (84 per minute per square yard), the density of the dead in a beachfront shelter (three per square meter), Vesuvius's output at the eruption's height (165,000 tons of debris per second), the temperature required to singe human hair (248° F).
Sigurdsson had advantages over his predecessors. He had studied some of the century's most violent eruptions - Mt. St. Helens in 1980, Mexico's El Chichón in 1982, and Martinique's 1902 eruption of Mt. Pelée - and had read scientific analyses of the ground-hugging clouds spawned by nuclear bombs, but he also turned to perhaps the eruption's most insightful expert - an eyewitness who has been dead for almost two thousand years.
Pliny the Younger (c. A.D. 61-113) was a prominent Roman lawyer and administrator who portrayed Roman society in 368 published letters. He was seventeen years old at the time of the catastrophe, and two of the letters recount his experience and form the first historical observations of an eruption:
We had scarcely set 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 the sound of their voices…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.
-- Book VI, Letter 20
Volcanologists have honored Pliny by adopting the adjective “Plinian” to describe the most powerful, devastating eruptions and the rising tephra column that Pliny saw above Vesuvius. These gigantic clouds, launched on a jet of hot gas and buoyed by convection high into the sky, inevitably cool enough so that their lower portions collapse. This plummeting tephra, still hot enough to glow, cascades down the sides of the volcano, riding on an entrapped cloud of scorching gas. This pyroclastic flow obliterates everything in its path.
Sigurdsson united his own field observations with Pliny's descriptions to create a detailed chronology of the 36-hour eruption. Vesuvius awoke early on August 24, 79 A.D., with a steam explosion that showered the volcano's eastern flank with fine, gray ash. By noon, the Plinian eruption had begun with the birth of a 20-mile high plume that dumped a 13-hour rain of white and gray pumice to the east and southeast. At around 1:00 AM on August 25, a series of pyroclastic surges began to sweep down the mountain, each more destructive than the last. The first roared through Herculaneum; the fourth, at about 7:30 AM, charged through Pompeii; the sixth, at 8:00 AM, crossed the Bay of Naples to Capo Misenum, 18 miles away, forcing Pliny to flee his home. When the eruption was complete, Vesuvius had smothered its surroundings in up to 75 feet of ash.
Sigurdsson was also able to take advantage of a momentous discovery. In the summer of 1982, Herculaneum archaeologists were excavating the city, digging towards the Mediterranean shoreline five hundred yards away. Suddenly, there were no more buildings to unearth. The town stopped. Curious, the archaeologists dug downward. Finding nothing, they dug some more, ultimately hitting the water table after sinking a seventy-foot shaft through dense volcanic debris. When they began a secondary shaft for drainage, they made a discovery that totally changed their concept of the city and its end. Says Sigurdsson, “All of a sudden they struck beach sand, and that beach sand contained human remains. Bones.”
At the time of the eruption, the city had been prime real estate, a prosperous resort town perched on a beautiful bluff, overlooking the ocean and a small beach below. Dug into the bluff were storage chambers where fishermen hung their nets or stowed their boats. Inside the chambers the archaeologists discovered hundreds of huddled skeletons, killed as they writhed in the volcano's fury.
“A large part of the population had sought refuge or ran towards the boats, away from the eruption in order to try anything to escape this pyroclastic flow that was sweeping through the town around midnight on the first day of the eruption,” says Sigurdsson. “Prior to that there had only been ten human remains found in Herculaneum. It had been assumed that everyone had been evacuated, except some babies in cradles and some sick or old people lying in bed. But the population had indeed not escaped but had only come to the beach. And that's where they were overwhelmed.”
Sigurdsson painstakingly dissected the scene and found clues to the eruption in the condition of the victims. The bodies lay in the first surge, which roared down the mountain at approximately 1:00 AM on the eruption's second day, propelling tiles, planks, and timber and filling the far end of the chamber with five feet of ash. This surge, however, was relatively cool, for the portions of the victims' bodies buried by it were not charred; it is likely they choked on ash and asphyxiated, rather than succumbing to thermal shock. However, the heels, knees, elbows, and heads that poked above this layer were charred an hour later when the much hotter second surge thundered into the chamber.
What is most telling is the fact that archaeologists accidentally stumbled across these bodies in a mere 90-yard sliver of the coast. “Certainly hundreds, if not thousands, of other victims are still interred along the beach and waterfront southeast and northwest of the excavations,” Sigurdsson observes, adding, “There is no other volcanic region on Earth that is poised for such a major disaster as the densely populated region around Vesuvius.” The volcano's slopes are crowded once again: 600,000 people inhabit the official eruption evacuation zone.
As he crisscrossed Vesuvius in his search for eruption deposits, Sigurdsson made his own archaeological discoveries. “The region around Vesuvius is peppered with gravel, sand, and pumice quarries for mining aggregate for the building industry in Naples,” he says. “They are actively trucking away the volcano.” Exposed in the quarry walls is the continuous layer of gray and white pumice that forms the eruption's distinctive signature. It rests on a stratum of brown, fertile clay that undulates in shallow ripples, remnants of the irrigation furrows in the vineyards that completely covered the mountain. “Sometimes you trace it along and you come across a farm, the remains of a so-called villa rustica, or sometimes just a wall or agricultural implements.”
Many of the best exposures are in quarries owned by the Camorra, the Neapolitan Mafia. During one of Sigurdsson's visits, suspicious workers pulled guns, fearing that he would call the archaeological authorities to save the sites. Sigurdsson backed out of the quarry with his hands up. “In one case, right in the middle of the quarry, there were some bronze candelabras and other bronze objects. They were going up on the truck when I came in, and I managed to grab a few. The rest went into the cement truck. We have no idea how many fine art objects there are now entombed in the concrete walls of Naples.”
Sigurdsson seems to make a habit of rescuing neglected resources. He found a long-forgotten article by E. T. Merrill in the 1918 volume of the American Journal of Archaeology which correctly proposed that pyroclastic flows destroyed Herculaneum and Pompeii. “It was never discussed,” he laments. “The archaeologists didn't read it, and the volcanologists didn't read it. You see this all the time in science.”
In his new book, Melting the Earth: The History of Ideas on Volcanic Eruptions (Oxford University Press), Sigurdsson describes how our bias for the new blinds researchers to the discoveries of the past. “Generally people are only really interested in the most recent works. They only cite the most recent references on a given topic, so the guy who made the original breakthrough is often forgotten.”
Archaeologists, too, sometimes have their own blinders, says Sigurdsson, noting that “[Classical] archaeologists have, until recently, been professionally trained as art historians. They are archaeologists, but their main motivation is to contribute to an understanding of art history.” In the case of Pompeii and Herculaneum, the eruption that covered the site was “kind of secondary, and certainly the material that enclosed it was just in the way.” This focus on the cultural import of the artifact, the message inherent in its form rather than the tale of the process that entombed it, meant that archaeologists often did not recognize or care to recognize external forces that changed the society. “That's the fundamental division or schism between archeology and volcanology, the approach to the study of sites of this type. That's changing of course.”
Sigurdsson, too, is enlarging his perspective, leaving the narrow confines of geology to study volcanism in a cultural context. “I've become more and more interested in volcanic hazards and in the symbols and the significance of volcanic activity in terms of history, art, culture, and literature. That's something that's been neglected, and it's a surprisingly rich vein to mine.” He is also the scientific director of the Metcalf Institute, a foundation created to improve marine and environmental journalism.  
Yet, despite his broader interests, Sigurdsson will never let his steel hammer rust. He is continuing to study Icelandic volcanoes as well as the debris left when a meteor the size of Manhattan slammed into the Gulf of Mexico and caused a global catastrophe that may have wiped out the dinosaurs. He is researching Krakatoa and Tambora volcanoes in Indonesia, and there is a village that was buried in the 1815 Tambora eruption that he is “itching” to unearth.
“The main thrust will always be the field work,” he says. “For me, that's the real joy, to be out there exploring in the field, turning over every rock, cracking them open to see what's inside, and just trying to understand the earth.”

Copyright © 2000 Tom Gidwitz