| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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Feature Mammoth's Perilous Magma
Eleven years ago, Horseshoe Lake in the eastern Sierra Nevada was a splendid place to camp. Families staked tents under the shady boughs of lodgepole pine, mountain hemlock, and red fir. Swimmers splashed in the sparkling blue water and sunbathed on the pumice beach, soft as sand. Children tossed peanuts to squirrels. At night, smoke from campfires curled up to the stars. Today, Horseshoe Lake is a quiet place. Its trees are dead. Its Forest Service campground is closed. "Danger" signs warn children and dogs to stay away. The lake, still a sparkling blue, is considered hazardous for swimmers. No deer nibble here, no squirrels scamper, no birds chirp. The most common sound at Horseshoe Lake is an occasional wind whistling through spindly branches. When I visited Horseshoe Lake one August weekend, it seemed at first that a small forest fire had swept through. Or a beetle infestation. Or maybe a drought. But as I walked among the dead conifers, I noticed that their hunched profiles told a different story. Their weathered silhouettes all slumped downslope, branches pointing to the earth, as though accusing their assassin. Indeed, the earth at Horseshoe Lake is saturated with so much carbon dioxide (CO2) that a couple of whiffs in a hole there could make a person dizzy - and a few more could kill. In early 1990, U.S. Forest Service ranger Fred Richter almost died when he entered a ski hut at Horseshoe Lake to escape a snowstorm. He had rested in the same hut only two months earlier without any problem. But in March, as he climbed down into the snow-buried hut from a ladder on the ceiling, he became dizzy and would have lost consciousness if he hadnt managed to climb back out. "Thats how we know when the CO2 reached the surface," said Mike Sorey, a hydrologist with the U.S. Geological Survey (usgs). "The difference in the huts air between January and March." At the time of Richters visit, Horseshoe Lakes trees were still green. No one thought the lake had a problem, and Richters near-suffocation went unnoticed. It was only after the conifers started dying over the next few years - and drought and beetle infestations were ruled out - that the usgs started testing the ground for emissions. In 1994, a soil sample taken at Horseshoe Lake confirmed that lethal doses of CO2 were seeping from an underground source. Heavier than air, the odorless, colorless gas was concentrating near the ground, becoming trapped under snowbanks as well as in buildings like the ski hut. Even in the open forest in summer, the gas was concentrated enough to asphyxiate the conifers by interrupting their roots ability to take in oxygen and nutrients from the soil. Sorey and his co-workers determined that if a tree sank its roots deeper than two feet, it encountered lethal levels of CO2 - about 30 percent or higher. Nothing like this had happened at Horseshoe Lake before - at least for a couple of centuries, given that some of the dying trees were 250 years old. In the next few years, several trees crashed to the ground, and birds and rodents were found asphyxiated in tree wells. Gas samples taken from shallow depressions dug into the ground contained almost 90 percent CO2. After identifying the poisonous gas, it didnt take usgs scientists long to solve the mystery of its source: magma. During 1989, less than a year before Richters near-asphyxiation, Horseshoe Lake and nearby Mammoth Mountain were shaken for six months by swarms of small earthquakes - a sign of moving magma. The molten basalt, rich in CO2, rose into Mammoth Mountain to roughly one mile below the surface. "When the magma was injected, it probably opened cracks which allowed the CO2 to escape to the surface," according to Sorey. By 1995, the magmas gas had killed roughly 100 acres of trees at Horseshoe Lake as well as at five other sites on the mountain. Chemically the same at all sites, the gas was also discovered in effervescent cold springs and steaming fumaroles on the 11,053-foot volcano. Sorey believes that "the fact that the same kind of gas has been emitted all over Mammoth Mountain for a decade means there is a very large reservoir underneath the volcano." Although it first appeared that the 1989 magma was the sole source, Sorey and other usgs researchers now speculate that the gas must be from a larger, older, and deeper source. The gas reservoir - located roughly one mile below Mammoths surface - could be from a previous intrusion of magma whose CO2 was trapped for centuries and is now being replenished from new magma still degassing. Carbon dioxide emissions from magma have been observed on the flanks of other volcanoes such as Vulcano and Etna in Italy. What is strange at Mammoth is the prodigious quantity of gas without any accompanying eruption. The widespread tree kill is also unusual and has been seen in only a few places around the world. Although the emissions have decreased slightly from their peak in the early 1990s, Mammoth Mountain still fumes daily with 300 tons of CO2. Sorey anticipates that the gas will continue to seep, probably for many more years. Having monitored Mammoth Mountain for the last decade, he often peppers his statements with adverbs like "probably." He knows only too well that the dormant volcano is tucked into a sprawling volcanic complex known as Long Valley - where, geologically speaking, there are no short answers. As I discovered during my weekend there, Long Valley Caldera is aptly named. Calderas are craters formed by the collapse of volcanoes. This one is 20 miles long and ten miles wide. Its topographical contours are astounding, with spectacular 10,000-foot Sierran peaks along its western rim and a low, sagebrush-dotted plateau on its floor. It was created 760,000 years ago when a gigantic eruption blew ash as far east as Nebraska and caused the earth to collapse a mile deep. The eruption, which sent magma gushing over the Sierra Nevada and down the San Joaquin River, is believed to have been one of the largest ever in North America. More impressive yet, the caldera lies in the middle of an even greater volcanic region, known as the Long Valley/Mono-Inyo Volcanic Field. In his book Fire Mountains of the West, author Stephen Harris calls the region "Californias potentially most dangerous volcanic field." Bordered by the eastern slope of the Sierra, this volcanically restless area begins at Mono Lake and spreads 25 miles south past the town of Mammoth Lakes. Its eruptive landmarks include the Long Valley Caldera, framed by Glass Mountain on the northeast, and a 400,000-year-old volcanic chain called the Mono-Inyo Craters. Beginning as islands in the alkaline waters of Mono Lake, the Mono-Inyo chain ends at Mammoth Mountain, which has erupted frequently for 40,000 years and whose most recent eruption was 600 years ago. The Long Valley region exhibits all the classic symptoms of geologic unrest: pent-up magma, earthquakes, and gas. In addition to magma-rich Mammoth Mountain, the Inyo Craters - known for explosive blasts - have magma bodies beneath them. These craters have erupted regularly every 200 to 700 years for millennia, and their last eruption was 600 years ago. The Long Valley Caldera itself is swelling ominously as magma rises beneath its floor. The 100-square-mile-wide uplift was first noticed in 1980 when the caldera was rocked by four magnitude 6.0 earthquakes from May 25 to 27. Since then, Long Valleys "resurgent dome" - as it is scientifically termed - has risen almost three feet and continues to rise about an inch a year. For someone like me, who lived through the May 18, 1980 eruption of Mount St. Helens, there are so many signs of restless earth at Long Valley that its hard not to wonder when the next eruption is going to blast everything to smithereens. Gas emissions, ground swelling, and earthquakes all preceded Mount St. Helenss eruption. Back then, no one but a few wary geologists thought the mountain was going to blow. But Long Valley is not a small, young Cascade volcano with a predictable eruptive profile. It is an old, huge, and volcanically complicated region that has been active for four million years. In her 1902 work, The Land of Little Rain, Mary Austin described Long Valley as a "mysterious country" where "a hidden force works mischief, mole-like, under the crust of the earth." Even then, a century ago, it was obvious to Austin and her contemporaries that the ground was moving in elusive and monumental ways. She writes: Whatever agency is at work in that neighborhood, and it is popularly supposed to be the devil, it changes means and direction without time or season. It creeps up whole hillsides with insidious heat, unguessed at until one notes the pine woods dying at the top, and having scorched out a good block of timber returns to steam and spout in caked, forgotten crevices of years before. Austins passage sounds eerily prescient about Horseshoe Lakes dead trees. But she was describing another aptly named place in Long Valley: Hot Creek. After horseshoe lake i headed to Hot Creek. I descended north and east out of the shady, steep Sierra and found myself on an open, unpaved road winding gently through sagebrush, manzanita, and juniper. Great expanses of sky engulfed the car, and the desert heat made me rifle around for a water bottle. Puffy cumuli peppered the caldera floor with fleeting shadows. I parked on a pumice-sprinkled cliff overlooking a small gorge. Down below, steaming and bubbling around rocks and algal mats, flowed Hot Creek. A long-time favorite among hot-springs enthusiasts, Hot Creek gurgles and geysers its way straight through the caldera. The creek begins near Horseshoe Lake as Mammoth Creek, where it runs icy and clear. Then, as it flows northeast it merges with hot springs to form a sulfurous, gassy gorge full of milky-blue pools and baked boulders. Not far from its banks lies the Casa Diablo geothermal plant, which transforms Long Valleys heat into electricity. At Casa Diablo, acres of conifers have also died, due to acid poisoning and heat. The afternoon I visited Hot Creek, swimmers lounged in pools just beyond the "Danger Keep Out" signs. Since 1968, more than a dozen people and four dogs have drowned here or been scalded to death. Like Horseshoe Lake, Hot Creek lies inside the Inyo National Forest and is managed by the Forest Service. Both sites are posted but not policed. As with Horseshoe Lake - where a crosscountry skier died in 1998 possibly from CO2 asphyxiation - Hot Creek is both scenic and sinister, natural and unnatural. Mountain bikers cruise along fire roads near Horseshoe Lake, and bathers soak in Hot Creek - both lured to Long Valley not for volcanisms sake, as at Mount St. Helens or Hawaii, but for the recreation volcanism has created. I didnt envy the Forest Service its job. I bent down and stuck my hand in the creek. The currents swirled around it - sometimes burning hot, sometimes icy cold. The erratic temperatures seemed to sum up precisely the regions volcanism. Maybe Hot Creek will scald another swimmer to death tomorrow. Or maybe not. Maybe Long Valleys resurgent dome will explode with billions of gallons of magma next month. Or maybe not. Maybe a magnitude 7.5 earthquake will rip through the earth next year, and a heavy wave of carbon dioxide gas will surge out of the ground to strangle the 5,000 residents of Mammoth Lakes. Or maybe not. Cold, hot, cold, hot. I withdrew my hand and started back up the paved walkway to escape the nauseating odor of rotten-egg sulfur rising from scoured pits along the bank. As I topped the canyon, I passed a man on his way down with a T-shirt that read "faultline." I asked him if he thought there would be an eruption at Long Valley any time soon. "Nah," he said, rolling his eyes. "I wish. Then it wouldnt be so crowded." But talk to usgs seismologist David Hill and you might hear a different response. As scientist-in-charge at Long Valley, Hill shares the generally accepted view among geologists that Long Valley marks an area of crustal stretching - where the Earths crust is spreading. The stretching is occurring as the Sierra Nevada Range tilts up and westward about an inch a year, as it has been doing for millions of years. "As the crust stretches, it sucks up magma," according to Hill. In this process, eruptions are inevitable. Nonetheless, Hill chooses his words carefully when he talks about future eruptions at Long Valley. Having overseen the monitoring of the region since 1982, Hill can still remember when geologists were personae non gratae in the early 1980s due to a usgs hazard alert that angered residents of Mammoth Lakes, which depends heavily on tourists. It took county and city officials several years - and a few earthquakes - to understand the precarious position in which the Survey stood. Either it could do nothing and risk criticism, or issue an alert and risk criticism. Eventually, residents recognized that they were living on top of a swelling caldera. In 1983, after another earthquake swarm, an escape route was built out of Mammoth Lakes when officials realized that the town had only one road connecting it to Highway 395. But then the seismicity decreased, and the escape route was signposted as the "Mammoth Scenic Loop." There is nothing particularly scenic about the six-mile-long road, except perhaps the Jeffrey pines that shade it. But the euphemism used for the evacuation route attests to the power of tourism at Mammoth Lakes, where the winter population swells to 40,000 skiers, mostly from southern California. Snowy, sunny, and scenic, Mammoth Lakes is enjoying a tourism boom that is expected to intensify over the next decade. As one real-estate brochure claims, "Mammoth Real Estate is HOT." Vacant lots in Mammoth Lakes have doubled in price in the last year, according to Mike Vance of the Mammoth Lakes Visitors Bureau. The local airport and college are expanding. And Mammoth Mountain - a major ski resort - is being aggressively developed and marketed by Intrawest, a multinational corporation that plans to invest $700 million there over the next decade. Vacation homes are sprouting around Mammoth Mountain as fast as summer wildflowers. New lodges are being erected even as CO2 emissions have crept into some ski areas on the mountains slopes. Amidst the real-estate buzz, Hill and a team of usgs scientists keep a close watch on each of Long Valleys puffs, swells, and quivers. On Mammoth Mountain, Sorey and others constantly monitor the volcanos emissions with high-tech gear installed at some sites as well as with hundreds of one-time measurements throughout the tree-kill areas. Data from the decade-long monitoring show that Mammoths CO2 emissions peaked in 1991 and have leveled off since 1995. The caldera, too, is wired like a heart-surgery patient. Even the slightest geologic sneeze is caught on seismographs, and the resurgent dome is surveyed regularly for new deformation. The Survey has even drilled a 9,833-foot well into the domes center. Partially funded by state and other federal agencies, the well was completed in September 1998. Its core samples have helped scientists map the domes layers of rock, and the Survey also hopes to measure earthquakes, uplift, tension, pressure, and heat through special underground sensing devices in the future. Dying trees, shaky ground, scalding water, deadly gas. Mammoth real estate is indeed hot. And yet, as I discovered traveling around Long Valley, few residents consider themselves in any danger. After a sunny day spent at Horseshoe Lake and Hot Creek, I talked to a desk clerk at a lodge built in 1924 on the shores of Twin Lakes, hardly a mile from Horseshoe Lake. Having worked at a 1920s lodge that was destroyed by a volcano, I was curious to know whether the clerk was worried about a future eruption. "No," he replied confidently. "And do you ever feel earthquakes?" I wondered, having felt one myself at Mount St. Helens the summer before it erupted. I remember how the old lodges walls had shaken and its kitchen cabinets had rattled. He smiled and nodded. "All the time," he said. Christine Colasurdo is the author of Return to Spirit Lake, a natural history memoir of Mount St. Helens. She lives in San Francisco, where the usgs estimates the same statistical odds for an 8.0 earthquake along the San Andreas fault as a Long Valley eruption. |
Fall 2000
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