Fourteen thousand four hundred and ten feet above sea level, I have to pause for breath between each plodding step as I arrive at the crest of Mount Rainier, the highest point in the Cascade Range. I’ve climbed this huge volcano many times over the past thirty years—for fun, for exercise, for escape from the urban grind. On this occasion, however, the impetus is morbid curiosity.

Recent geological reports have suggested that Rainier poses a serious threat to thousands of people who live in the shadow of the mountain (as I do), even if it does not erupt. My hope is that this ascent will shed a little light on the matter. Two days of strenuous effort were required to reach the top. Now, a surly wind is screaming across the summit crater, flash-freezing the skin on my face and turning my mittened hands into blocks of ice.

Before my eyes is a swath of country that stretches from Canada to central Oregon, some 200 miles away. Scanning the craggy spine of the Cascades, one can’t help noticing that this corner of the planet is lousy with volcanoes: from this lofty vantage I count no fewer than nine of them. The most riveting—and most notorious—is Mount St. Helens, whose truncated cone squats immediately to the southwest. At the moment, its yawning crater is belching a plume of steam high into the troposphere, a gentle admonition that St. Helens—along with some two dozen other Cascade peaks—is very much an active volcano.

On Sunday, May 18, 1980, shortly before nine o’clock in the morning, I happened to look up from a salmon net I was mending on a dock near Tacoma, Washington, and saw what appeared to be a massive cumulonimbus cloud boiling toward the heavens, where a moment earlier there had been nothing but blue sky. Unbeknownst to me, avalanches had carried millions of tons of rock and ice off the top of Mount St. Helens, suddenly removing the geologic lid that had enabled the mountain to contain the molten rock and the superheated steam within. The resulting explosion obliterated the upper 1,300 feet of the volcano and sent it hurtling into the sky.

The blast devastated plant and animal life for 150 square miles. The cloud of ash that caught my eye soared to a height of more than 60,000 feet, turning day to night across much of Washington, and scattered 540 million tons of volcanic debris across the Western United States. Fifty-seven people were killed, a remarkably small number considering the violence of the eruption. The death toll was so low due to evacuation measures, and because the Mountain’s rugged environs were sparsely populated and largely undeveloped.

Fifteen years later, as I mull the scenery from atop Mount Rainier, the merest glance between my boots suggests that the consequences could be much more dire when it’s this peak’s turn to blow. Hard to the northwest sprawl Tacoma, Seattle, and their myriad suburbs. I can plainly make out the Space Needle, skyscrapers, and 747s landing at Sea-Tac Airport.

Given that Rainier has erupted numerous times in the past (most recently, just 150 years ago), the proximity of so much humanity is troubling. Geologists warn that there is no way of knowing when the mountain will blow again—it could go off in ten years or not for ten thousand—but blow it will, eventually.

Hunkered on the crest of Rainier, watching freighters and ferryboats ply the dazzling waters of Puget Sound, it’s easy to imagine that the volcano is ready to erupt at any moment. The rim of the summit crater is riddled with fumaroles actively venting hot gases from the bowels of the earth, filling my nostrils with their sulfurous stink. Even though the air temperature is well below freezing and most of the rest of the mountain wears a carapace of glacial ice hundreds of feet thick, the rocky ground beneath me is utterly bare of snow and disturbingly warm to the touch. Scientists have recorded surface temperatures on the crater rim as high as 176 degrees F. The heat I feel through my insulated trousers makes it impossible to forget that somewhere not too far below lies a reservoir of fidgety, red-hot magma, itching to elbow its way to daylight.

***

Sixty miles north of Rainier, in a cluttered, windowless room on the University of Washington campus, banks of humming machines keep close tabs on the mountain’s every seismological twitch, lest an eruption take the region by surprise. Antennas on the roof of the building gather signals from some thirty remote-sensing devices on or near various Cascade volcanoes: telephones and microwaves carry signals from 120 more. Twelve devices monitor Rainier. Tremors large and small show up as squiggly lines on an array of rotating drum graphs in the center of the room.

“Actually,” confesses Steve Malone—a cheerful, bearded geophysicist attired in shorts and sandals, who oversees the seismology lab— “the drums provide some useful data, but they are mostly for the benefit of news media. They give TV crews something to point their cameras at whenever there’s an earthquake. Here at the lab, we glance at the drums now and then, but mostly we rely on a fairly sophisticated computer system.” Should any remote sensor record a seismic event of a magnitude greater than 2.4, the computers are programmed to trigger a beeper that Malone wears on his belt. Additionally, if the event is stronger than 2.9, the system will automatically send out a flurry of faxes and email communiqués to scientists and emergency-management agencies throughout the region.

The big St. Helens blast in 1980 was preceded by a series of minor earthquakes prompted by movement of magma up the throat of the volcano; similar tremors would almost certainly give geologists plenty of notice before Mount Rainier next erupts. “With this system in place,” Malone concurs, “the available seismic data should give us warning of an impending eruption weeks or even months in advance.”

Truth be told, Malone doesn’t think Rainier is a very likely candidate for an explosive, St. Helens-like eruption, in any case. Extrapolating from past eruptions, he and most other volcanologists believe that when Rainier blows its lid, it is apt to do so in a much less histrionic fashion, producing relatively modest explosions or extrusions of lava rather than a cataclysmic detonation. Malone cautions, nevertheless, that it would be a grave mistake to conclude thereby that the mountain presents no great threat: “In fact, Rainier is perhaps far more dangerous than St. Helens. The frightening thing about Rainier is the hazard posed by catastrophic debris flows—a hazard most people aren’t even aware of.” Known to geologists as lahars (an Indonesian term), such flows are flash floods of semi-liquid mud, rock, and ice that surge down from the heights with terrifying speed and destructive power.

“Lahars have occurred throughout Rainier’s history,” warns Malone, “and they can happen more or less spontaneously, in the complete absence of an eruptive event, with practically no warning at all. Horribly, we remember what happened to Armero, and worry that something similar might happen around here.”

***

Armero was a prosperous farming community nestled in the Andes in Colombia, not far from Bogotá. On the evening of November 13, 1985, residents of the town felt the earth tremble and heard a series of rumbling explosions emanating from a 17,453-foot volcano called Nevado del Ruiz, thirty miles away. According to a local woman named Marina Franco de Huez, an ominous cloud rose from the mountain’s crater, raining ash down on Armero, “but we were told it wasn’t anything serious.”

Although the volcano was erupting, at first there seemed to be little reason for concern. Indeed, newspaper reports later described the event as a “relatively small eruption, a volcanic burp” that melted only about five percent of the ice and snow covering the uppermost reaches of the peak. The “burp” was sufficiently powerful, however, to collapse a steep buttress below the summit crater, initiating an avalanche of rock, snow, and ice—a classic lahar—that swept down the slopes of Nevado del Ruiz from an elevation of 15,000 feet.

Liquefying and gathering momentum as it rocketed down a river drainage toward the valley bottom, the lahar wiped out a natural dam, sending a colossal mud slide downvalley. As one resident of Armero remembered the ensuing cataclysm, “I heard a sound like a huge locomotive going at full steam, and then I felt water swirling around my neck.”

Within moments, the town was inundated with a slurry resembling wet concrete, burying the community beneath thirty feet of gray-brown muck. The next morning, where their homes had once stood, survivors gazed upon a stark, lunar-like plain covering 600 acres and littered with smashed cars, corpses, and uprooted trees. An estimated 23,000 people lost their lives, and more than 60,000 were left homeless. It was the worst natural disaster in Colombia’s history.

Kevin Scott, a senior geologist at the U.S. Geological Survey’s Cascades Volcano Observatory in Vancouver, Washington, warns that there are a number of disquieting parallels between Nevado del Ruiz and Mount Rainier. As he tells me this, he is standing in a field beside a new housing development in Orting, a rapidly growing town in the lowlands near Puget Sound. “The Mountain” (as Rainier is known locally), gleaming in the summer sun, looms to the southeast less than thirty miles away.

A solitary block of lava as big as a Volkswagen Beetle rests incongruously on a manicured sward behind a recently constructed house. “You know how that boulder got here?” Scott asks. “It was carried down from Rainier by a lahar. This development, like most of the rest of Orting, was built on twenty feet of debris deposited by the Electron Mudflow, a lahar that came down the Puyallup River Valley about 500 years ago.” The geologic record indicates that at least sixty major lahars have roared down from Rainier over the past 10,000 years; a few of them ran all the way to Puget Sound, more than fifty miles from the mountain.

Scott points out that Armero, like Orting, was built on debris from an old lahar: “Armero had already been destroyed at least once, in 1845, before the most recent disaster—by a mudflow that killed hundreds of people. Yet the town was rebuilt in the same place. We can be fairly certain that sooner or later another lahar is going to plow through Orting, too—we just don’t know when.

“Judging from the frequency of mudflows on Rainier in the past, we are reasonably confident that the recurrence interval for major lahars is between 500 and 1,000 years. That sounds like an awfully long time, long enough that we don’t really need to worry. Statistically, however, it’s been calculated that a house built on the floodplain of a lahar is many times more likely to be destroyed by a lahar than by fire. Almost nobody would consider owning a home without fire insurance and smoke alarms, yet people think nothing of living in the path of mudflows without safeguards. Most folks simply don’t take the risk of lahars seriously.”

***

Geologists take lahars very seriously, which is why Mount Rainier makes them so nervous. More than 100,000 people in these environs live in homes built on debris washed down by lahars. Two hundred thousand Puget Sound residents go to work each day at businesses lying in the path of documented mudflows. A report published in 1994 by the National Research Council warns, “This metropolitan area is the high-technology industrial center of the Pacific Northwest and one of the commercial aircraft manufacturing centers of the United States…. A major volcanic eruption or debris flow could kill thousands of residents and cripple the economy.”

Lahars are a hazard of virtually all volcanoes, but Rainier has some unique geologic traits that make it especially dangerous. Thanks to its great height and the sodden Northwest climate, Rainier wears a stupendously robust mantle of ice. Twenty-six named glaciers drape Rainier’s broad flanks, a reservoir of snow and ice approximately equal to that of all the other Cascade volcanoes combined. The melting of just a tiny fraction of this frozen water during a volcanic event could unleash lahars of biblical proportions.

“Compared to any other volcano in the Cascades,” Scott declares, “Rainier is in a class by itself in terms of risk to human life and property.”

“Rainier is covered with thirty-six square miles of perennial snow and ice,” muses Carolyn Driedger, a hydrologist with the Cascades Volcano Observatory who has carefully measured the size and thickness of the mountain’s glaciers. During the eruption of Mount St. Helens, she says, three-quarters of that volcano’s glacial ice melted over a very short time, creating gargantuan mudflows that ran all the way to the Columbia River, filling its channels with enough debris to disrupt international shipping for three months.

“And remember that St. Helens,” Driedger points out, “only had about four percent as much year-round ice and snow as Rainier has. That’s pretty sobering to think about.”

Rainier’s prodigious mantle of ice contributes to the potential danger in a less obvious, more insidious way, as well. The peak’s subterranean heat is continuously melting its glaciers from below, feeding water into a complex system of geothermal aquifers. Constantly circulating through the mountain, this abundance of hot liquid combines with sulfur-bearing gases to produce acids that are eroding Rainier from the inside out, undermining its structural integrity. “The entire edifice of the mountain is stewing in its own hot chemical juices,” explains Scott, “and as a consequence it’s becoming increasingly rotten and unstable.”

Geologists, who call this phenomenon hydrothermal alteration, have only recently begun to appreciate its impact on Mount Rainier. “We don’t have a very complete picture of what’s going on up there yet,” says Don Swanson, a geologist with the U.S. Geological Survey and a contributor to the 1994 National Research Council report. “But the hydrothermal alteration of all that rock is alarming to contemplate. I firmly believe that learning the extent of the alteration is among the most important things to find out about Rainier.”

***

Most of what we know thus far comes from the efforts of Tom Sisson and David Zimbelman, geologists who have spent many months on the mountain, painstakingly studying and mapping it. “It’s extremely hard work,” Zimbelman admits. Because altered rock is inherently unstable, he and Sisson had to climb some of Rainier’s more dangerous slopes to conduct their research.

The research has shed considerable light on the degree to which strong acids percolating through Rainier’s innards are transforming solid rock into soft, crumbly clay, a process that is readily apparent on the rim of the summit crater. The thin air up here reeks of rotten eggs: the telltale scent of hydrogen sulfide gas, which condenses and mixes with water to form sulfuric acid, the primary agent of alteration on Rainier. Around the steaming summit fumaroles, clumps of reddish brown mud cling to the spikes of climbers’ crampons. This spongy stuff is hydrothermally altered rock.

Zimbelman and Sisson have discovered bands of weak, highly altered rock that locally penetrate the mountain. It is the nature of clay to absorb water, and expand when it does so. As zones of newly altered rock on Rainier swell with moisture, dry out and re-expand, the clay acts as a sort of natural crowbar, prying apart the more solid rock around it, further weakening the edifice. As soon as part of Rainier grows sufficiently rickety, a catastrophic lahar is bound to result. “What would it take to trigger a significant mudflow?” asks Zimbelman. “Certainly an earthquake could do the job. But so could a much lesser event like a minor steam explosion. You have all this gravitationally unstable rock becoming weaker and weaker; eventually it’s going to reach a point where it won’t take much of a jolt to break off a big piece of mountain and send it tumbling toward Puget Sound. In fact, you could have a major sector collapse without any triggering mechanism at all. That’s what’s so scary: something huge could come down simply under its own weight, with no advance warning.”

***

When Rainier is viewed from southern Puget Sound, it looks like a large part of the summit cone is missing, as though removed by some Brobdingnagian ice-cream scoop. The cliffs that form the walls of the hollow are mottled with yellowish orange splotches: zones of rotten, clay-rich altered rock that hold clues to what happened here. The scooped-out area is called the Sunset Amphitheater, and its existence hints at what can happen when a really big piece of the mountain lets go. The hollow is where the Electron Mudflow started.

Around the corner from the Sunset Amphitheater, on the north face of the mountain, scars from an even bigger cataclysm are visible with the naked eye from downtown Seattle. They were left behind by the Osceola Mudflow, the largest known lahar ever spawned by Mount Rainier. It came thundering down about 5,000 years ago, when something—perhaps an earthquake or a buildup of steam within the volcano, perhaps nothing more than the tug of gravity on hydrothermally weakened rock—caused the upper 2,000 feet of the peak to cleave off and slide away. A dusting of airborne ash sprinkled over the region at the time of the mudflow indicates that it was accompanied by a modest volcanic eruption, but many geologists have come to believe that the landslide may have triggered the eruption, rather than vice versa.

The Osceola Mudflow began as an avalanche of mind-boggling size, carrying 60 times as much rubble as the lahar that wiped out Armero in 1985. Much of the flow consisted of clay saturated with geothermal fluids, an avalanche of semi-liquid muck that roared downslope at a speed in excess of 100 miles per hour.

As it reached the base of the mountain and churned down the valleys of the White, Green, and Puyallup rivers, the lahar slowed to between thirty and fifty miles per hour, but it nevertheless scoured the earth of everything in its path. Thanks to its high clay content, the Osceola Mudflow was what geologists call a cohesive lahar: a thick, gooey mass viscous enough to suspend house-size boulders and 200-foot trees within its flow. Indeed, it swallowed up entire forests and carried them down onto the plains below.

Cohesive lahars tend to travel farther and pack a greater punch than more dilute, clay-poor, noncohesive lahars, and the Osceola Mudflow was no exception. It ran all the way to Puget Sound, whose shores lay considerably east of where they do now. The lahar blanketed at least 200 square miles with a layer of concrete-like muck that averaged twenty-five feet deep, burying the sites of present-day Orting, Enumclaw, Auburn, Puyallup, Kent, Sumner and some of the Tacoma waterfront.

If a similar lahar came down Rainier tomorrow, it would take about one to two hours to reach the densely populated lowlands. According to Kevin Scott, residents of Orting would see “a wall of trees, rocks, and mud rolling down the canyon of the Carbon River at perhaps thirty miles per hour. The sound would be deafening and the earth would tremble. In Armero, survivors reported hearing a loud roar and feeling the ground shake when the lahar was still five kilometers away, but they didn’t know what was happening—they thought it was an earthquake—so they didn’t run to high ground.”

***

Hoping to avoid such deadly confusion, in February of last year the Orting Fire Department hand-delivered copies of an emergency evacuation plan to each of the town’s roughly 3,000 residents. The plan explains that if a debris flow is expected, sirens will sound throughout Orting, indicating that people should immediately leave town via evacuation routes described on an attached map. In conjunction with the plan, the local schools regularly practice lahar evacuation drills.

“The last time the schools held a drill,” says Orting Fire Chief Scott Fielding, “all the kids were out of the city and bused to high ground in fifteen minutes. I’m more concerned about our adult citizens. We’ve told them, ‘When you hear the sirens, get in your car and leave! Immediately!’ But adults are more likely to doubt the seriousness of the situation, to question it, to fool around. Realistically, I doubt we’d get more than fifty or sixty percent of the people to actually evacuate.”

Fielding acknowledges that for the evacuation plan to work at all, moreover, the town will have to have some advance warning that the mud is bearing down. “And at present,” he laments, “no early-detection system is in place. If someone up toward the mountain doesn’t happen to see the debris flow and phone the fire department, it’s going to be bad news for the people downstream. Personally, I live on a hill above town, so I sleep well at night. But I worry about my friends who live on the valley floor.”

Scientists and government officials have discussed establishing a network of electronic sensing devices in each of the threatened drainages to sound a warning when a lahar is on the way. Such a system, however, would probably be far from foolproof. “The technology for implementing something like that would be pretty straightforward,” says Steve Malone. “The problem will be maintaining it in the long term, and getting people to take it seriously when the alarm goes off after decades or even centuries without anything happening.”

***

What, then, can we who live in the shadow of the mountain do to protect our lives and property the next time a tsunami of mud comes rumbling down from the heights? Civil engineers have suggested constructing massive containment dams in each of the half-dozen river drainages that snake down from Rainier to Puget Sound. Such structures could be designed to catch most of the sediment released by a lahar.

Although containment dams would probably stop a lahar, says Patrick Pringle, a geologist with the Washington State Department of Natural Resources, “the cost of building and maintaining them would be substantial, and I don’t think the public is willing to commit those kinds of funds in this economic climate. It’s hard to galvanize people to do anything until after a disaster has already happened. Mudflows occur so infrequently that people would rather just take their chances. They’d rather spend their tax money on a new baseball stadium. Such dams would also pose environmental problems.”

In the absence of containment dams and a reliable early-warning program, an obvious way to reduce the risk would be to enact zoning laws that would prevent people from building homes or businesses in the path of documented mudflows. “But most of the land we’re talking about is prime real estate,” laments Don Swanson. “It’s probably unrealistic to think that very much of it will be placed off-limits.”

Given fiscal and political realities, Swanson, Scott, Pringle and other experts believe the best course of action for the time being is to learn as much as possible about the hazards posed by the mountain and aggressively share that knowledge with the public. Toward that end, the International Association of Volcanology and Chemistry of the Earth’s Interior has designated Rainier, along with fourteen other mountains worldwide, a “Decade Volcano:” an unusually dangerous volcano earmarked for intensive study.

We’re trying to come up with hard statistical data,” says Pringle, “actual numbers people can use to get a handle on the risk. We want to give people enough knowledge to make rational decisions concerning their options. The challenge is to get the public’s attention without alarming anybody unnecessarily.”

“It’s extremely hard to quantify these kinds of risks,” Swanson concedes. “How do you put large but infrequent hazards into any sort of meaningful statistical context? A catastrophic mudflow is not likely to happen on Rainier in our lifetime. But it will happen somewhere down the road, in one or more generations. And when it does, a lot of people will lose everything.”