How the Largest Volcanic Eruption in Human History Changed the World

Our story begins on a very bad day about 74,000 years ago. The planet was starting to move out of one of its more recent ice ages, although in the tropics there was little change in climate between the Ice Age glacial episodes and the warmer interglacial episodes. A wide range of late Ice Age mammals inhabited the world, including woolly rhinoceroses and mammoths up in the cold regions of Eurasia, along with huge bison, giant deer, wild horses, and a variety of smaller mammals. Giant lions, sabertoothed cats, and huge bears fed on these large prey animals.

People lived in many parts of the Old World by then but had not yet reached Australia or the Americas. The bulk of the human population were archaic members of our species, Homo sapiens, which first appeared in southern Africa about 100,000 to 300,000 years ago. By 74,000 years ago, these people had spread out of Africa and may have occupied much of Asia, as well as parts of southeastern Europe. However, Europe was still dominated by another human species, the Neanderthals, who had adapted to life on the edge of the northern ice sheet. In contrast to archaic Homo sapiens, Neanderthals had a shorter, stockier, more muscular build and shorter limbs, a body type suitable for attacking large prey and adapted for reducing heat loss in the cold climate. In the far reaches of Asia, ancient humans had spread to many of the islands of modern-day Indonesia and Malaysia. On one now known as Flores, east of Java and Bali, they evolved into a dwarfed species, Homo floresiensis. Now nicknamed “hobbits,” these people stood only about 3 feet and 7 inches tall, shorter than any modern adult Pygmies (given their small brain size, some anthropologists question whether they are even in our genus, Homo). Flores is part of the island chain (including Sumatra, Java, and many smaller islands of the Malay Archipelago) that makes up of most of modern Indonesia. These islands are built completely of volcanoes, both active ones and ancient, dormant ones. Their climate is tropical and their jungle is dense. So much vegetation grows on the rich volcanic soil, in fact, that it’s often hard to recognize signs of volcanoes there.

About 1,200 miles northwest of Flores, in northern Sumatra, there are numerous volcanoes that have erupted over the past million years. On the very bad day in question, one particular volcano, now known as Mount Toba, had been active for a long time. It had bulged up gradually until it towered almost 3,000 feet above the jungle. All around this monstrous dome, cracks formed. Hot springs and fumaroles spewed out steam that smelled like rotten eggs because of all the sulfur in the mixture. Earthquakes both large and small had rocked the entire island of Sumatra for a year before Mount Toba started to blast out a few small eruptions of steam and ash that blanketed the surrounding jungle. These were likely terrifying to the local animals and people but soon forgotten once they had quieted down. After a few years of tropical rains and rapidly growing jungle, the blanket of ash had vanished. Yet recently the frequency of smaller eruptions venting steam and ash had begun to increase. Soon the sides of the volcano turned barren and rocky as red-hot ash and balls of scalding pumice burned away all the nearby jungle.

This was the state of things as that fatal day about 74,000 years ago dawned. Events really got rolling when the huge dome rumbled with a deep vibration that shook all of Sumatra. Jets of steam and ash shot one after another from the summit. Then came an explosion louder than any sound previously heard by humans in their entire evolution. For comparison, when the Krakatau (or Krakatoa) volcano, also in Indonesia, erupted in 1883, it created a sonic boom that could be heard 5,000 miles away and which traveled around the world seven times. That blast, 5,000 times as powerful as the Hiroshima nuclear bomb explosion, was the largest explosion heard in recent times. Yet the eruption of Mount Toba released the energy of a million tons of explosives, 40 times larger than the largest hydrogen bomb that humans have ever built, more than 1,000 times as powerful as Krakatau, and 3,000 times as powerful as the eruption of Mount Saint Helens in 1980. Thus, the sonic boom from Toba must have been deafening to animals and people for many miles around and must have bounced around the earth repeatedly, dwarfing any other sound produced on earth in the previous 28 million years.

After the explosion, a gigantic mushroom cloud of hot ash rose many thousands of meters into the stratosphere. Meanwhile, superheated ash and gases, as hot as 2,000°F, flowed down the sides of the mountain in gigantic, turbulent clouds traveling up to 200 miles per hour. They incinerated everything in the jungles for many miles. A dense blanket of ash and pumice covered not only Sumatra but also most of the nearby islands, causing death and devastation wherever it settled. The ashfall spread across southern Asia as well, leaving a thick deposit even in India, more than 1,800 miles away. The ash cover in India was on average about 6 inches thick; in the years following the eruption, it mingled with other layers and moved downslope, forming secondary ash deposits several meters thick (as happened in the Mount Saint Helens eruption in 1980).

Tropical rains turned the ash into mud with the texture of wet cement, which made rivers and pathways into impassable morasses and collapsed tree branches and sometimes even whole trees under its weight. Small huts, too, probably were crushed under thousands of kilograms of wet ash. Sea levels were lower at that time, but it is likely that a tsunami triggered by seismic activity associated with the eruption would have killed many people living along the coast. The people and animals of the jungle found their world in utter ruin, and most local survivors must have soon died of starvation, while others died from inhaling dry, dusty ash. Volcanic ash particles are microscopic shards of glass, and they cut up the insides of the lungs, which scar and then clog with fluid.

These were the effects on the life of the jungles within a few thousand miles of the erupting volcano. But areas beyond the densest ashfall were affected as well. Clouds spread around the world, leaving a blanket of ash on the ocean floor in many places thousands of miles from the eruption. The volcano spewed out about 11 billion short tons of sulfuric acid and 6.6 million short tons of sulfur dioxide, which combined with water in the atmosphere to make sulfuric acid. The sulfuric acid was devastating to life in many parts of the globe and can be detected even in the Greenland ice sheet.

The farthest-reaching impact of the eruption, however, was caused by the 720 cubic miles of dust-size particles of volcanic debris that were injected into the stratosphere, more than 6 miles above sea level. At that altitude, they were picked up by the jet stream, and soon a plume of ash began to circle the world. When something like this happens, the amount of sunlight that reaches our planet is reduced, resulting in abnormal cooling. When Krakatau erupted in 1883, the huge volume of ash that was shot into the stratosphere blocked sunlight, and global average temperatures dropped by about 2° to 4°F for more than a year. Weather patterns were erratic for years, and temperatures did not return to normal until 1888. The sky was dimmed, even darkened, for months after the eruption, and the large amount of particulate matter in the stratosphere changed its color, producing, for instance, spectacular orange-red sunsets, such as the one depicted in Edvard Munch’s The Scream (1893). As Munch wrote in his diary on January 22, 1892: “Suddenly the sky turned blood red. . . . I stood there shaking with fear and felt an endless scream passing through nature.” Rare atmospheric effects, including a literal blue moon, a Bishop’s ring (a faint brown halo around the sun), and volcanic purple light at twilight, were also seen around the world.

Sixty-eight years earlier, when Mount Tambora (also in Indonesia) had erupted in 1815, it injected so much dust into the stratosphere that the earth’s weather patterns changed. As the ash blocked sunlight, the resulting cooling led to crop failures, starvation of livestock, and widespread disease (including a typhus epidemic) and famine in human populations around the world. The following “Year without a Summer” (1816) saw cold, dark, rainy summer months in North America and Eurasia: even in June, it snowed in New York, New England, and many European cities. That month, Percy and Mary Shelley were staying at Lord Byron’s villa near Lake Geneva in Switzerland, and they told one another gothic horror stories to pass the long hours spent indoors. That wet, gloomy summer inspired Mary Shelley to write Frankenstein, or The Modern Prometheus.

The eruption of Toba about 74,000 years ago was 1,000 times as large as that of Tambora or Krakatau. It didn’t just trigger a summerless year or a short cold spell spanning several years: global temperatures dropped by 5° to 9°F, to a worldwide average of just 60°F after three years, and took a full decade to recover to pre-eruption levels. The tree line and the snow line fell to 10,000 feet lower than where they are today, making most high elevations uninhabitable. Ice cores from Greenland show the evidence of this dramatic cooling in trapped ash and ancient air bubbles.

What happened to people and animals during this terrible time? As we shall see in the rest of this book, many geneticists and archaeologists believe that the Toba catastrophe nearly wiped out the human race; afterward, they argue, only about 1,000 to 10,000 breeding pairs of people survived worldwide. Supporting this idea are both geologic evidence of Toba’s size and atmospheric effects and indications of a human genetic bottleneck that happened around the time of the eruption. A genetic bottleneck occurs when the number of individuals in a population drops so low that its genetic diversity is greatly reduced, and all descendants of that population carry the rare genes of the handful of survivors.

Several studies have found similarly timed bottlenecks in the genes of human lice and of our gut bacterium Helicobacter pylori, which causes ulcers; according to these organisms’ molecular clocks, which show how much time has passed since a genetic change took place, both bottlenecks date back to the time of Toba. The molecular clocks of a number of other animals, including tigers and pandas, indicate that they, too, passed through a bottleneck around that time. In short, Toba was the biggest eruption since modern humans appeared on earth, and it came very close to wiping out people, along with many other animals, altogether.

The Toba eruption was one of the greatest geological catastrophes ever to strike our planet. It was larger than any volcanic eruption in the previous 28 million years and hundreds to thousands of times larger than later eruptions such as Tambora, Krakatau, and Mount Saint Helens. Toba may even have been a disaster on the scale of the one 65 million years ago that wiped out the dinosaurs and many other creatures, and it may have been similar in its effects to other mass extinction events in our planet’s history.

However, the amazing story of the Toba eruption, and its aftereffects, is one that few people (and even few scientists) have heard. Not until the late 1990s did researchers even realize that the catastrophe had occurred; at that point, many scientists, working on many different kinds of problems in geology, genetics, and other fields, eventually came to recognize that they were all uncovering evidence of the same great disaster. The story of Toba’s discovery is one of surprise, serendipity, and continuing controversy.

Read more in When Humans Nearly Vanished, which is available from Smithsonian Books. Visit Smithsonian Books’ website to learn more about its publications and a full list of titles. 

Excerpt from When Humans Nearly Vanished: The Catastrophic Explosion of the Toba Volcano © 2018 by Donald R. Prothero

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