On every clear day, Mount Rainier anchors the horizon like a steadfast sentinel. The mountain's mood may shift as sunlight and clouds play across its face, but its basic contours seem solid and unchanging. Viewed from a distance, its profile looks the same today as it did more than two centuries ago, when a member of Capt. George Vancouver's crew first sketched it on paper.

Get closer, though, and Rainier's restless soul is revealed.

When the air is cold, the summit rim seethes with steam from fumaroles that tap the mountain's scalding depths. On the lower flanks, water simmers in hot springs after percolating through heated rocks. Up to 100 times a year, small earthquakes rattle the edifice - further evidence of the turmoil within.

Mount Rainier is a living volcano.

It is, scientists say, the most dangerous volcano in the country.

As Mount Rainier National Park celebrates its centennial, researchers are wrapping up a decade of the most intense scientific scrutiny ever of the Northwest's signature peak.

The result is an unprecedented understanding of Rainier's volcanic history and temperament - the keys to predicting its future behavior and protecting the 2.5 million people who live in its shadow.

Scientists now know the volcano has erupted much more frequently than anyone had realized, and with unexpected fury. Several times in its half-million years of existence, Rainier has disgorged enough molten rock to bury an area the size of Tacoma and Seattle nearly 10 feet deep. Avalanches of red-hot rock and ash have roared down its steep flanks, transforming rivers of ice into torrents of water and mud.

Dozens of mudflows - including one of the largest in the world - hurtled off the mountain, uprooting old-growth forests and burying valleys under concrete-like sludge. Geologists have discovered entire sections of crumbly, rotten rock on Rainier that could easily give way, unleashing walls of mud.

All the new evidence leaves little doubt that Rainier will reawaken. The mountain that today epitomizes nature's glory for so many Northwesterners will one day stun the region with another demonstration of nature's awesome power, experts agree. The only question is when.

"Mount Rainier is behaving now the way it has for 500,000 years," said Tom Sisson, a federal geologist who has spent the past six years unraveling the volcano's eruptive past. "There's no indication it is dying out."

What has changed are the potential consequences of the mountain's actions.

When Mount St. Helens erupted in 1980, 57 people died in the sparsely settled area, and a pall of ash darkened the skies over four states. While not as explosive as its Cascades cousin, Rainier is vastly more threatening by virtue of its location at the edge of Puget Sound's most urbanized area.

The river valleys that radiate from the peak like the spokes of a wheel now harbor tens of thousands of people and one of the country's most vibrant economies. Scientists estimate that it could take less than an hour for a volcanic mudslide to reach - and destroy - the closest towns: Greenwater, Orting, Ashford and Elbe. Tacoma, Kent and Auburn also lie well within the mountain's reach, the new research shows.

Some geologists say the new data appear to temper the mountain's fearsomeness, suggesting that the most horrifying scenario - a killer mudflow without warning -Êis less likely than they feared just a few years ago. Others insist the dangers are not reduced.

A mountain with roots in an ancient epoch

Puget Sound's most entrenched natives might claim ancestry stretching back 10,000 years. That is when the first people ventured onto the ice-covered plains beneath the peak some called "Tacobet." The mountain was already ancient.

Rainier's time line is hard for the mind to wrap around. The mountain's roots extend into another age, where life spans are inconsequential and human memory cannot reach.

Sisson is one of the few who can traverse that epochal terrain, using a technique called geologic mapping as his time machine.

Tall and sinewy, he is as much mountain climber as scientist. Without knowledge of ice ax, crampons and carabiners, he could never reach his laboratory: the mountain's high, rocky places. He has worked on the summit, clambered into remote corners where few people ever venture and spent weeks at a stretch camped on snowfields and wind-swept saddles thousands of feet up Rainier's flanks.

Sisson reconstructs the volcano's past by tracing its lava layers. He notes their size and thickness and the places where they overlap. He collects samples and estimates their age through isotope-dating methods.

On the ridges around Paradise, he finds columns of petrified lava 40,000 years old. Jagged outcrops on Liberty Ridge, on the mountain's north side, date back 200,000 years. Along the glacier-scoured basin of the Mowich River, northwest of the summit, many of the rocks are remnants from the volcano's birth half a million years ago.

"It's like a three-dimensional jigsaw puzzle," said Sisson, who works for the U.S. Geological Survey.

The result of his work is the volcanic equivalent of a personality profile - a detailed breakdown of Rainier's peculiar quirks and behavior patterns.

"Each volcano acts in its own, unique fashion," Sisson said. "The more you understand those characteristics, the better you can anticipate how it will act in the future."

In the mid-1970s, geologists accurately predicted that Mount St. Helens would erupt before the end of the century, based on its frequent, regular outbursts.

Rainier, by contrast, is "calmer, more mature," Sisson said. By no means, though, is it headed for the volcano graveyard, as some experts believed before Sisson launched his mapping effort in 1993.

The pioneering work on Mount Rainier's geology was conducted in the 1960s and '70s, but those researchers kept largely to the mountain's outskirts. Lacking the technology to date lava layers, several concluded that Rainier was an old volcano whose fires were dwindling.

The lava flows and ash layers that Sisson and other geologists have studied high on the mountain in recent years tell a very different story. Rainier is an active, vigorous volcano that has erupted three to four times more frequently than previously suspected, Sisson said.

"One of the very first things we learned is that Mount Rainier is not on its last legs."

A history of eruptions, from burps to doozies

The volcano's most recent eruption - little more than a burp, really - was late in 1894. Before clouds obscured the peak, Tacoma and Seattle were abuzz with sightings of steam puffs and a plume of black smoke. Some experts pooh-poohed the observations. But a veteran climber, Maj. Edward Ingraham, led the first winter expedition to the mountain, using passenger pigeons to send back his observations of the sooty ash.

The last big eruptions occurred 1,100 and 2,200 years ago.

Some were doozies.

Rainier's size is testament to its voluminous output. Most of the major outpourings occurred in episodes, Sisson found. Over periods ranging from a few hundred to a few thousand years, the mountain sporadically let loose immense loads of lava that flowed down its flanks like thick, hot soup.

Sisson also discovered that as many as one in five of Mount Rainier's eruptions fall into the ferocious category called pyroclastic flows. These mixtures of glowing rock, gas and ash cascade down the mountain at up to 80 mph, obliterating everything in their paths.

New studies have added a third type of eruption. Jim Vallance, a geologist from McGill University in Montreal, analyzed layers of ash that earlier researchers had dismissed. Preserved in damp, alpine meadows, they show that the volcano has repeatedly belched clouds of black smoke. Some of the eruptions were small, Vallance found. Some were powerful explosions that flung rocks up to six miles.

"These things would have gotten everyone's attention," he said. "It would have been most unpleasant in places like Sunrise and Crystal Mountain."

In the last 10,000 years, geologists now estimate, Rainier has erupted at least two dozen times - an average of about once every 400 years. Though the eruptions haven't been regularly spaced over time, Sisson said, it is likely that Rainier will blow again within the next century.

What type of eruption will it be?

Playing the odds, he bets on a minor one.

"The most frequent eruptions are going to be pretty small and relatively inconsequential. The big, catastrophic eruptions are rare."

But not impossible. Which is what makes Rainier-watching so suspenseful.

As bad as they can be, Rainier's eruptions aren't the nastiest trick up the mountain's sleeve. Large, slow-moving lava flows wouldn't travel far enough to threaten people or communities. Ash clouds could be nuisances, but not killers. Even avalanches of hot rock aren't a direct threat beyond the boundaries of the park.

The scary thing about Mount Rainier, geologists say, is the mammoth mudflows it produces.

"The way to picture these things is to think of flowing concrete," said Kevin Scott of the U.S. Geological Survey's Cascades Volcano Observatory in Vancouver, Wash. "They're the biggest hazard to people because they move fast, they travel great distances, and they destroy everything in their paths."

About 5,600 years ago, a small eruption triggered a collapse on the northeast side of Rainier's summit. Glacier and snow mixed with clay and rock, creating a slurry called a lahar that barreled off the mountain at 130 mph. The leading edge was nearly as tall as the Space Needle when it plunged over the spot where the White River Campground is today. Spreading over 100 square miles, the flow left up to six stories of sludge in the White and Puyallup river valleys and reshaped the shoreline of Puget Sound. It was one of the world's biggest mudflows.

A smaller mudflow 500 to 600 years ago smothered the entire Puyallup Valley, from modern-day Orting to Puyallup.

Most Northwesterners have seen photographs of Mount St. Helens' north flank caving in as the volcano exploded. Most also have seen films of the North Fork of the Toutle River shortly afterward, a torrent of mud, rocks and logs sweeping up houses and slamming them into a bridge.

Rainier's lahars are like that, Scott said - just many, many times bigger. He and his fellow geologists search for traces of these ancient mudflows in the valleys around the mountain.

"You have to do a lot of thrashing through the brush, driving the back roads and walking the river channels," he said.

On the Nisqually River near Elbe, Scott used aerial photos to pinpoint a deep-cut bank where two mudflows sit, one atop the other. The site is a 20-minute hike from the nearest dirt road, through dense thickets.

Pulling on fingerless leather gloves, Scott grasped his ice ax and scrambled up the bank, using the sharp point to gouge out the soil. It is like a Christmas pudding, rocks suspended like plums in a muddy matrix.

Protruding from one layer is a geologist's jackpot - a tree swept up in the mudflow, which can be used to carbon-date the event.

"Sometimes you spend a whole day searching for a speck of charcoal," Scott said.

Uncovering evidence of ancient mudflows

These two flows occurred 400 years apart, Scott estimated, the first coming 2,600 years ago. Geologists have found the tracks of scores more all across the Puget Sound lowlands, where much of the region's population, industry and commerce lie. In the middle of Orting, and next to Emerald Downs Racetrack in Auburn, they have unearthed ancient old-growth forests, uprooted by the torrents of mud. From Kent to Packwood, earthen cores extracted from wells contain the characteristic debris layers laid down by the landslides.

Underneath the Puyallup Fairgrounds and the Port of Seattle, scientists have found tell-tale deposits.

Today, such landslides would be devastating, Scott said. Stripped of their old-growth forests, the lands around Rainier are "slick as snot." Mudslides could travel even faster and farther than they did in ancient times.

Many of the mudflows were triggered by eruptions. Mount Rainier is so tall and unstable - and is covered with so much ice and snow - that it wouldn't take a very big outburst to set off a mudflow, geologists say.

A few years ago, Scott and others came to an even more alarming conclusion. When they examined the deposits left by some of the big mudflows, they found no evidence that an eruption had occurred at the same time. If that is true, it could mean an earthquake could shake loose a big chunk of the mountain, or sections could simply collapse under their own weight.

And that would mean no warning and no time to evacuate the valleys downstream.

By contrast, even a small eruption would announce its coming with geologic rumblings. The number and frequency of earthquakes would increase. The mountain would swell as magma moved through it. The steam and gases escaping from the summit fumaroles would change.

Emergency planners would go on alert, and residents could be prepared to bolt.

Clearly, the absence of a warning is the worst possibility for Mount Rainier and the people who live in its shadow. The U.S. Geological Survey has emphasized it heavily in videos, pamphlets and publications warning of Rainier's volcanic hazards. A network of sensors was installed in the Puyallup and Carbon river valleys last year to sound the alarm in case of an unanticipated mudslide.

But some geologists now believe the worst case is not as likely as they once thought.

The reason for the re-evaluation is the work Vallance and his colleague Sue Donoghue have been doing in Rainier's high meadows.

Over the centuries, ash from volcanic explosions settled on these moist fields. Because the ground was damp, the ash didn't blow away, as it did elsewhere on the mountain. In places where streams cut through the meadows, the dark soot is revealed, stacked like layers in an earthy cake.

Early geologists paid little attention to these innocuous bands, but Vallance has found in them a mine of information.

10,000 years of history preserved in the meadows

"We're playing a little game of detective work here," he said, carving at a stream bank with his favorite scientific instrument, a small, hoe-like Japanese gardening tool. The five-foot bank is made up of about 30 layers in shades of brown, black and orange. Some are inches thick. Others are narrow slivers, barely distinguishable. Each represents a separate eruption.

"Your eyeballs get fine-tuned, and you can tell them apart," Vallance said.

About 10,000 years of history are preserved in the meadows, since the last ice age scoured the area. Vallance has found traces of nearly two dozen previously unknown eruptions during that window of time.

Most important, some of those volcanic episodes appear to coincide with mudflows that hadn't been previously linked with eruptions.

Vallance is looking for other connections between eruptions and mudflows and expects to find them.

"These mudflows are so much more likely to occur during eruptions than at other times," he said. "And if it's an eruption, you're almost sure to have warning signs before it touches off."

The latest research on the small earthquakes that jolt Mount Rainier several times a month also bolsters the view that the mountain may be less prone to unexpected mudslides than previously feared.

Seth Moran, who studies Alaskan volcanoes for the U.S. Geological Survey, used a network of nearly 100 seismometers to conduct a kind of CAT scan on Rainier. He analyzed earthquake vibrations and used the data to interpret what's going on underground.

His conclusion: Rainier's earthquakes are so small that they are not likely to trigger mudflows.

"I think that is something people can take comfort in," he said.

But not too much comfort.

Big chunks of the mountain could collapse at any time

While some geologists - including Sisson -Êbelieve the possibility of unanticipated mudflows might have been over-emphasized, all of the experts still agree they could happen.

"There's no reason why a part of the volcano can't fall off without an eruption," Sisson said. "Some mountains that aren't volcanos at all have lost big chunks."

In 1997, rain-soaked slopes gave way on Mount Adams, about 50 miles southeast of Rainier. Twice, Adams sloughed enough mud and rock to fill more than five Tacoma Domes. In 1963, a section of Little Tahoma, the 11,138-foot spire on Rainier's northeast side, broke off. The rockfall and debris flow tumbled nearly four miles, carrying stones the size of semi-trailers. A spontaneous mudflow swept down Kautz Creek on Rainier's southern flank 1947, obliterating the main park road beneath 20 feet of debris.

Scott, one of the deans of Rainier mudflow researchers, discounts the practical importance of the new research.

Even if mudflows coincide with volcanic activity, it doesn't necessarily mean there will be ample notice, he points out. The movement of magma through the volcano might be enough to dislodge a big piece long before any eruption occurs. While there would be some warning signals, the resulting mudflow could come so early and so unexpectedly that there would be little chance for evacuations.

"In my view, (the new studies) do not diminish the risk at Mount Rainier," he said.

The reason Rainier is so susceptible to massive mudflows is hidden in its rocks: In many places, they are as crumbly as cupcakes.

Some geologists liken the mountain to a termite-infested house. Others compare it to a pot roast in a pressure cooker, stewing in its own juices.

Geologist Dave Zimbelman puts it more simply: "It's basically a big pile of weak, crummy rock."

In scientific parlance, the phenomenon is called alteration: Water seeping through the mountain heats up and combines with volcanic gases to create a kind of acid bath that transforms hard rock into soft clay. At the same time, glaciers grind away at the mountain's stony surface.

"Look at this," Zimbelman said, grabbing a chunk of whitish-looking rock shot through with orange streaks. With his hands, he snapped the rock. It flaked apart like pastry.

He was working on the mountain's northeast side, where the White River pours off Emmons Glacier. To collect rock samples, he scrambled across a 100-foot-high bank of loose soil - a remnant of the world-record mudflow that occurred 5,600 years ago. Zimbelman was trying to figure out whether the section of mountain that fell away was weakened by crumbly rock.

He has been scouring the upper mountain for several years, looking for concentrations of unstable rock. He estimates that nearly one-tenth of the mountain's mass is rotten.

"It's not going to surprise geologists if the whole thing lets go," he said.

He has also singled out Rainier's most vulnerable spots - the places that are most likely to crumble next.

The mountain's west side is the worst: From Sunset Amphitheater to Puyallup Cleaver and down the Puyallup River drainage, the rock is very rickety. The east side is nearly as bad, leading into the White River drainage. The area above the Cowlitz River is relatively solid.

The patterns of weakened rock reflect the mountain's inner plumbing. Sheet-like fractures in the rock, oriented east and west, channel acidic water through the edifice.

The pattern also points up the areas most vulnerable to future mudflows: the Puyallup, White and Nisqually rivers.

"There are clearly higher-risk areas and lower-risk areas," Zimbelman said.

An independent researcher who sometimes works for the federal government, Zimbelman criticizes the Geological Survey for failing to point out these differences in the hazard maps it prepares for surrounding communities. The maps assume the risk of mudflows is roughly comparable in all of the valleys leading from Mount Rainier.

Better safe than sorry, is the agency's stance.

While it's true that some drainages are more hazardous than others, it's also true that all of the valleys have been hit by mudflows, said Scott, who prepared the maps.

And volcanic processes are rarely as neat as computer simulations might indicate. If a section high on the mountain collapses, the resulting debris flow could easily slop over into adjacent valleys, as has happened many times, Scott points out.

"You really can't tell anybody downstream they're going to be safe," he said.

For all its trappings of scientific certitude, the study of volcanos remains a murky endeavor.

The 1980 eruption of Mount St. Helens electrified the field like nothing before in the United States. It opened scientific eyes to the hazards posed by other Cascade volcanoes, particularly Rainier. And it shocked geologists with the limitations of their knowledge and understanding.

Even though they knew the volcano was going to blow, they were unprepared for its timing. The blast was three times more powerful than anyone anticipated. The mountain's lateral collapse was a surprise. In most of its aspects, the eruption was worse, by far, than any worst-case scenario.

Science has progressed exponentially since then, but is still humbled in the face of all that remains unknown and unfathomable. The primordial forces that power a volcano don't lend themselves to easy analysis. The ravages of time make it impossible to completely trace any volcano's fiery evolution.

Researchers realize that the eruptions and mudflows they have uncovered at Rainier are the bare minimum of what the mountain produced. When the volcano rumbles to life again, they are confident they will be able to track its progression. They are less sanguine about their ability to predict exactly how it will behave.

"We just don't have a very clear picture," Vallance said. "Our crystal ball is a hazy one."

(Published March 28, 1999)