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Frequency of Volcanic Eruptions in the Mammoth Lakes Sierra

of years. How do we deal with volcanoes eruptions that incrementally built the eologists recognize and that have never erupted in recorded edifice from 100,000 to 50,000 years ago. Gash deposits from about 60 history but certainly could? Nearly all eruptions were extrusions of past eruptions in the area around For long-dormant but potentially viscous that solidified as knobby and Devils threatening volcanoes, geologists do labor- domes and thick lava flows. Only one Postpile, California. This raises intensive field mapping, sampling, and deposit represents an explosive eruption the unanswerable question, “When dating of deposits from all past eruptions that spread a cloud of and ash will it erupt again?” An alternative, to build detailed records of the magnitude, eastward across Long Valley. Surrounding answerable, and informative question style, and frequency of prehistoric activity. Mammoth Mountain, about 35 scattered is, “How often has it erupted?” Such records are useful indicators of vents, mostly basaltic and andesitic cinder In the Mammoth Lakes Sierra, what to anticipate when a again cones, bring the total number of eruptions geologists have mapped in great becomes restless. Studies of eruptive to about 60. detail all the lavas and ash deposits histories are now fairly detailed for several Thick viscous (, produced by those 60 eruptions. volcanoes in the , as is dacite, and ) are products of They have dated almost all of them the record for the Mammoth Mountain cooling and crystallizing in by laboratory methods, showing that of California. chambers and plumbing conduits within eruptions have been repetitive and the Earth’s crust, mostly at depths between persistent, though not quite regular, Mammoth Mountain Volcanology 2 and 15 miles. However, almost all over the last quarter-million years. magmatic activity is initiated by the rise of For few volcanoes in the world is Mammoth Mountain, site of a major hotter, more fluid basaltic magma that is the long-term eruptive frequency so regional ski area, rises 3,000 feet above produced by melting of the Earth’s mantle well calibrated as in the Mammoth the Town of Mammoth Lakes. Geologists at more than 20 miles deep. Batches of Lakes Sierra. have mapped the deposits of 25 separate molten sometimes rise right through 119° 118° 45’

Rising Magma and Volcanic Eruptions 395 Map Area Beneath active volcanic regions, I lass molten magma is held in chambers and n y Mountain o

conduits at many depths—from a few 37° 45’ C h a Owens River LO miles to more than 100. But its ascent to i N n AL the surface to produce lava flows or to LE CA effervesce its dissolved gas to produce LD E R explosions can seldom be predicted long A in advance. Not until telltale signs of Resurgent Dome Creek ascending magma (earthquakes, ground Sa Basaltic n Hot Jo a Periphery q uplift, and gas clouds) are recorded by u i n

R

i monitoring instruments, typically only v

e 203 r ot weeks before an outburst, can geologists Mammoth Creek Mammoth Lakes Flow consider an impending eruption likely. Mountain Mammoth Creek 37° 37’ 30” Mammoth A few volcanoes (Kīlauea, Hawaii; Devils osemite Postpile Airport Stromboli, ; Sakurajima, ) Crowley Lake erupt persistently for many years. Others Owens River Red 0 5 MILES tend to erupt every few years and still Cones 0 5 KILOMETERS others erupt every few decades. But many 395 volcanoes come back to life after lying dormant for centuries or even thousands Map of Mammoth Mountain and the vicinity. Banner: Devils Postpile basalt, eroded colonnade 3 miles west of Mammoth Mountain. Mammoth Mountain rises above the Town of Mammoth Lakes, California.

U.S. Department of the Interior Fact Sheet 2018–3059 U.S. Geological Survey September 2018 the crust and erupt as basaltic lava flows Mammoth Mountain and cinder cones. Devils Postpile is an Rhyolite example of basaltic lava that flowed ERPTION TPES comple out of a vent and ponded thickly on a Lava flow canyon floor. Other batches of rising Dacite basalt fail to erupt, and they stall within the crust, melting the surrounding crustal rocks with their greater heat. Cooling, Andesite crystallization, and mixing of mantle and crustal melts with different temperatures, water contents, and viscosities can lead Basalt to a wide range of erupting compositions. Progressively lower temperature magmas, 0 50,000 100,000 150,000 200,000 250,000 from basalt to andesite to dacite to ears before present rhyolite, are increasingly more viscous, Eruptive timeline showing the age and composition of 60 eruptions on and around contain more water, and erupt more Mammoth Mountain. explosively. The viscous dacite lavas of releasing enough carbon dioxide gas to 250,000 years, and the last eruption was Mammoth Mountain oozed out and piled kill the forest at 10 sites that hug the base 8,000 years ago. Time gaps between up above a crustal magma chamber that of Mammoth Mountain (see USGS Fact eruptions are variable, and we never lay near the middle of a scattered field Sheet 172–96, cited below). expect eruption frequency to be strictly of about 35 basaltic and andesitic vents. regular. We cannot predict exactly when Geologists infer that heating by the many How Frequently Has Mammoth the next eruption will take place, but the surrounding had focused the Mountain Erupted? USGS California Volcano Observatory crustal dacite chamber where the basaltic has a network of seismometers, ground injections were most concentrated. Looking at the eruptive timeline, deformation detectors, and gas sensors in The mass and heat of the basalts are there has been at least one eruption place that give us real-time alerts about fundamental to producing the dacites at every 10,000 years or so for the past unrest beneath the volcanic field. the center of the volcanic field.

Determining the Ages of Mammoth Eruptions Most of the 60 magma batches, which erupted lavas of many kinds, have been dated by argon radioisotopic methods in U.S. Geological Survey (USGS) laboratories. Basalts began erupting in the Mammoth Mountain area about 235,000 years ago and have been fairly persistent ever since. Between 140,000 and 100,000 years ago, cooling of basalts within the crust led to eruption of several batches of andesite. Continued cooling, magma mixing, and partial crystallization produced dacite melt that culminated in 29 dacite eruptions during a 50,000-year-long episode; 25 of these One of the twin basaltic cinder cones, called Red Cones, 3 miles southwest of Mammoth Mountain. eruptions built Mammoth Mountain in the center of the volcanic field. Dacite Wes Hildreth, Andrew Calvert, Judy Fierstein, and Mae Marcaida eruptions eventually ceased, but a few Edited by Katherine Jacques basaltic eruptions have continued during postglacial time—the past 16,000 years. Layout by Geraldine Dang The most recent eruption of all was the For Additional Information: basalt of Red Cones, a lava-flow apron https://volcanoes.usgs.gov/observatories/calvo/ and two cinder cones that erupted 8,000 Invisible CO gas killing trees at Mammoth Mountain, California (USGS Fact Sheet 172–96); California’s years ago, just 3 miles southwest of the 2 summit of Mammoth Mountain. Since restless giant—the Long Valley (USGS Fact Sheet 2014–3056); The California Volcano Observatory—Monitoring the State’s restless volcanoes (USGS Fact Sheet 2014–3120); U.S. Geological 1990, batches of basalt that failed to erupt, Survey’s alert-notification system for volcanic activity (USGS Fact Sheet 2006–3139). but instead stalled in the crust, have been

ISSN 2327-6916 (print) ISSN 2327-6932 (online) https://doi.org/10.3133/fs20183059