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Earth Chgs More than 150 earthquakes strike near Mammoth Lakes (California) Long Valley Caldera
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  1. #1

    More than 150 earthquakes strike near Mammoth Lakes (California) Long Valley Caldera

    More than 150 earthquakes strike near Mammoth Lakes (California) – Long Valley Caldera imminent eruption

    There is currently an earthquake swarm hitting Mammoth Lakes in California.

    The series, which included a M3.9 quake on Sunday, may be tied to the movement of magma in the nearby Long Valley caldera.

    The historically restless Mammoth Lakes area has experienced more than 150 tiny and small earthquakes over the past week, including a magnitude 3.0 temblor that hit at 7:13 p.m. on Monday, says the USGS.

    The series — which included a 3. 9 quake on Sunday — may be tied to the movement of magma in the nearby Long Valley caldera, said Susan Hough, a USGS seismologist.

    According to All News Pipelines and USGS, in 1915, Lassen Peak erupted and wrecked a huge portion of the state. Over the last 100 days, the much larger Long Valley Caldera has begun acting-up. And what it’s doing has Geologists at the US Geological Survey “concerned.” The Caldera – the mouth of the Volcano – is . . . . moving.

    According to scientific instruments monitored by the United States Geological Survey (USGS) the area in vicinity of the Long Valley caldera is deforming and moving rapidly compared to previous records.

    The seismic data is showing on a recent timespan that the amount of movement is causing STATISTICALLY SIGNIFICANT STRAIN in the rock in the area. This is not conspiracy-theorist conjecture or amateur geology antics, this is from the USGS itself.

  2. #2
    Join Date
    Apr 2009
    Central Iowa
    And Long Valley is overdue for an eruption.
    People are quick to confuse and despise confidence as arrogance but that is common amongst those who have never accomplished anything in their lives and who have always played it safe not willing to risk failure.

  3. #3
    Join Date
    Apr 2009
    Central Iowa

    Long Valley Caldera

    From Wikipedia, the free encyclopedia
    Long Valley Caldera
    Long Valley caldera NE rim.jpg
    View from northeast rim of caldera
    Location Mono County, California
    United States
    Floor elevation 6,500–8,500 feet (2,000–2,600 m)
    Long-axis direction EW
    Long-axis length 20 miles (32 km)
    Width 11 miles (18 km)
    Depth up to 3,000 feet (900 m)
    Type Caldera
    Age 760,000 yrs
    Coordinates 37°43′00″N 118°53′03″WCoordinates: 37°43′00″N 118°53′03″W[1]

    Long Valley Caldera is a depression in eastern California that is adjacent to Mammoth Mountain. The valley is one of the Earth's largest calderas, measuring about 20 mi (32 km) long (east-west) and 11 mi (18 km) wide (north-south), and up to 3,000 ft (910 m) deep.

    Long Valley was formed 760,000 years ago when a colossal volcanic eruption released very hot ash that later cooled to form the Bishop tuff that is common to the area. The eruption emptied the magma chamber under the area to the point of collapse. The second phase of the eruption released pyroclastic flows that burned and buried thousands of square miles. Ash from this eruption blanketed much of the western part of what is now the United States.

    See also: Volcanic crater
    The caldera is a giant bowl-shaped depression, approximately 20 miles (32 km) wide, surrounded by mountains, but open to the southeast. The elevation of the bottom of the bowl ranges from 6,500 to 8,500 ft (2,000 to 2,600 m), being higher in the west.[2]

    Map of the Long Valley Caldera

    Near the center of the bowl, there is a resurgent dome formed by magmatic uplift. The southeastern slope from the caldera down towards Bishop, California is filled with the Bishop Tuff, solidified ash that was ejected during the stupendous eruption that created the caldera. The Bishop tuff is thousands of feet thick and is cut by the Owens River Gorge, formed during the Pleistocene when the caldera filled with water and overtopped its rim.

    The rim of the caldera is formed from pre-existing rock, rising about 3,000 ft (910 m) above the caldera floor.[2] However, the eastern rim is less high, only about 500 ft (150 m).[2]
    Mammoth Mountain is a lava dome complex in the southwestern corner of the caldera, consisting of about 12 rhyodacite and dacite overlapping domes.[2][3] These domes formed in a long series of eruptions from 110,000 to 57,000 years ago, building a volcano that reaches 11,059 feet (3,371 m) in elevation.[4]

    The Mono–Inyo Craters are a 25 mi (40 km)-long volcanic chain situated along a narrow, north–south-trending fissure system extending along the western rim of the caldera from Mammoth Mountain to the north shore of Mono Lake.[5] The Mono-Inyo Craters erupted from 40,000 to 600 years ago, from a magma source separate from the Long Valley Caldera.[6]

    The caldera has an extensive hydrothermal system. Casa Diablo Hot Springs at the base of the resurgent dome hosts a geothermal power plant. Hot Creek cuts into part of the resurgent dome and passes through hot springs. The warm water of Hot Creek supports many trout, and is used at the Hot Creek Fish Hatchery.[7] The creek was closed to swimming in 2006 after geothermal activity in the area increased, and was still closed as of 2016.[7][8] There are a number of other hot springs in the area, some of which are open to bathers.

    The tectonic causes of the Long Valley volcanism are still largely unexplained and are therefore a matter of much ongoing research. Long Valley is not above a hotspot as is Yellowstone or Hawaii, nor is it the result of subduction such as that which produces the volcanism of the Cascades.

    Layers of the Bishop tuff, laid down in phases of the major eruption 760,000 years ago.

    The known volcanic history of the Long Valley Caldera area started several million years ago when magma began to collect several miles below the surface. Volcanic activity became concentrated in the vicinity of the present site of Long Valley Caldera 3.1 to 2.5 million years ago with eruptions of rhyodacite followed by high-silica rhyolite from 2.1 to 0.8 million years ago. After some time a cluster of mostly rhyolitic volcanoes formed in the area. All told, about 1,500 sq mi (3,900 km2) were covered by lava.

    All but one of these volcanoes, 1–2 million year old Glass Mountain (made of obsidian),[9]:264 were destroyed by the major (VEI 7) eruption of the area 760,000 years ago, which released 600 km3 (140 cu mi) of material from vents just inside the margin of the caldera (the 1980 Mount St. Helens eruption was a VEI 5 eruption releasing 1.2 km3 or 0.29 cu mi). About half of this material was ejected in a series of pyroclastic flows of a very hot, 1,500 °F (820 °C), mixture of noxious gas, pumice, and ash that covered the surrounding area hundreds of feet (meters) deep. One lobe of this material moved south into Owens Valley, past where Big Pine, California now lies. Another lobe moved west over the crest of the Sierra Nevada and into the drainage of the San Joaquin River. The rest of the pyroclastic material along with 300 km3 (72 cu mi) of other matter, was blown as far as 25 mi (40 km) into the air where winds distributed it as far away as eastern Nebraska and Kansas. However, much of the material ejected straight into the air fell back to earth to fill the 2–3 km (1.2–1.9 mi) deep caldera two-thirds to its rim.

    Cross-section through Long Valley
    Subsequent eruptions from the Long Valley magma chamber were confined within the caldera with extrusions of relatively hot (crystal-free) rhyolite 700,000 to 600,000 years ago as the caldera floor was upwarped to form the resurgent dome followed by extrusions of cooler, crystal-rich moat rhyolite at 200,000-year intervals (500,000, 300,000, and 100,000 years ago) in clockwise succession around the resurgent dome.[2] The declining volcanic activity and increasingly crystalline lava extruded over the last 650,000 years, as well as other trends, suggest that the magma reservoir under the caldera has now largely crystallized and is unlikely to produce large-scale eruptions in the future.[10]

    The Long Valley volcano is unusual in that it has produced eruptions of both basaltic and silicic lava in the same geological place.[11]

    Water from the Owens River filled the caldera to a depth of 300 m (1,000 ft) as of 600,000 years ago. At that time, the lake surface was at an elevation near 7,500 feet (2,300 m).[12] The lake drained sometime in the last 100,000 years after it overtopped the southern rim of the caldera, eroded the sill, and created the Owens River Gorge. A human-made dam in the gorge has created Lake Crowley, a partial restoration of the original lake. Since the great eruption, many hot springs developed in the area and the resurgent dome has uplifted.

    During the last ice age, glaciers filled the canyons leading to Long Valley, but the valley floor was clear of ice. Excellent examples of terminal moraines can be seen at Long Valley: these moraines are the debris left from glacial sculpting. Laurel Creek, Convict Creek, and McGee Creek all have prominent moraines.

    Recent activity[edit]
    In May 1980, a strong earthquake swarm that included four Richter magnitude 6 earthquakes struck the southern margin of Long Valley Caldera associated with a 10 in (25 cm), dome-shaped uplift of the caldera floor.[13][14] These events marked the onset of the latest period of caldera unrest that continues to this day.[13] This ongoing unrest includes recurring earthquake swarms and continued dome-shaped uplift of the central section of the caldera (the resurgent dome) accompanied by changes in thermal springs and gas emissions.[13] After the quake another road was created as an escape route. Its name at first was proposed as the "Mammoth Escape Route" but was changed to the Mammoth Scenic Loop after Mammoth area businesses and land owners complained.

    In 1982, the United States Geological Survey under the Volcano Hazards Program began an intensive effort to monitor and study geologic unrest in Long Valley Caldera.[13] The goal of this effort is to provide residents and civil authorities in the area reliable information on the nature of the potential hazards posed by this unrest and timely warning of an impending volcanic eruption, should it develop.[13] Most, perhaps all, volcanic eruptions are preceded and accompanied by geophysical and geochemical changes in the volcanic system.[13] Common precursory indicators of volcanic activity include increased seismicity, ground deformation, and variations in the nature and rate of gas emissions.[13]
    Hydrothermal system[edit]
    People are quick to confuse and despise confidence as arrogance but that is common amongst those who have never accomplished anything in their lives and who have always played it safe not willing to risk failure.

  4. #4
    Join Date
    Apr 2009
    Central Iowa
    Btw there's a thread here from about two or three weeks ago about the increased activity in the area.

    The 16 x 32 km (20 x 10 mi) Long Valley caldera east of the central Sierra Nevada Range formed as a result of the voluminous Bishop Tuff eruption (considered a "supereruption") about 760,000 years ago. Resurgent doming in the central part of the caldera occurred shortly afterwards. During early resurgent doming the caldera was filled with a large lake that left lake-shore traces (strandlines) on the caldera walls and the resurgent dome peninsula; the lake eventually drained through the Owens River Gorge.

    Along the caldera's ring fault, Mammoth Knolls is the youngest eruption about 100,000 years ago. In the topographic basin, Cone 2652 in West Moat is about 33,000 years old and dacite change in NW Moat is 40,000-27,000 years old. The mafic chain along the west rim is 16,000 to 17,000 years old. The caldera remains thermally active, with many hot springs and fumaroles, and has had significant deformation, seismicity, and other unrest in recent years. A robust geothermal system inside the caldera fuels the Casa Diablo power plant, which generates enough power for 40,000 homes.

    The late-Pleistocene to Holocene Mono-Inyo Craters, which cut the northwest topographic rim of the caldera, along with Mammoth Mountain, on the southwest topographic rim, are west of the structural caldera and are chemically and tectonically distinct from the Long Valley magmatic system. The most recent activity in the area was about 300 years ago in Mono Lake. Both Long Valley Caldera and Mammoth Mountain have experienced episodes of heightened unrest over the last few decades (earthquakes, ground uplift, and/or volcanic gas emissions). As a result, the USGS manages a dense array of field sensors providing the real-time data needed to track unrest and assess hazards.

    Location: California, Mono County
    Latitude: 37.7° N
    Longitude: 118.87° W
    Elevation: 2,600 (m) 8,530 (f)
    Volcano type: caldera
    Composition: basalt to rhyolite
    Most recent eruption: 16,000-17,000 years ago
    Nearby towns: Mammoth Lakes
    Threat Potential: Very High *
    People are quick to confuse and despise confidence as arrogance but that is common amongst those who have never accomplished anything in their lives and who have always played it safe not willing to risk failure.

  5. #5
    Join Date
    Apr 2009
    Central Iowa
    Future Eruptions in California's Long Valley Area - What's Likely?

    Long Valley Caldera and the Mono-Inyo Craters chain form a large volcanic complex in eastern California that has had persistent earthquake activity and ground uplift in recent decades. Volcanoes have been active in the area for millions of years, and future eruptions are certain to occur. When the next eruption in the area does occur, it will most likely be small and from a site in the Mono-Inyo chain.

    Inyo Craters in Long Valley Caldera

    The three Inyo Craters, part of the Mono-Inyo Craters volcanic chain, stretch northward across the floor of Long Valley Caldera, a large volcanic depression in eastern California. During the past 1,000 years there have been at least 12 volcanic eruptions along the chain, including those that formed the Inyo Craters and South Deadman Creek Dome (seen here just beyond the farthest Crater).

    After four strong (magnitude 6) earthquakes rocked the Long Valley area of eastern California in May 1980, U.S. Geological Survey (USGS) scientists also detected evidence of renewed volcanic unrest in the region. They discovered that the central part of Long Valley Caldera, a broad depression formed in a cataclysmic volcanic eruption 760,000 years ago, was slowly rising. Because such ground deformation and earthquakes are common precursors of volcanic eruptions, the USGS has continued to closely monitor the unrest in this region.
    It is natural to wonder when and where the next volcanic eruption might occur in the Long Valley area. Geologic processes generally proceed at a slow pace, and when viewed on the scale of a human lifetime, volcanic eruptions and destructive earthquakes happen rarely. Nevertheless, the long history of volcanic activity in the Long Valley area indicates that future eruptions will occur.

    Geologists studying the Long Valley Caldera have found that following its creation in the violent eruption 760,000 years ago, clusters of smaller volcanic eruptions have occurred in the caldera at roughly 200,000-year intervals. About 100,000 years ago, the most recent of these eruptions formed the Mammoth Knolls, low hills just north of the Town of Mammoth Lakes.

    Mammoth Mountain, a young volcano on the rim of Long Valley Caldera, was built by numerous eruptions between 220,000 and 50,000 years ago. Volcanoes in the Mono-Inyo Craters volcanic chain, which extends from just south of Mammoth Mountain to the north shore of Mono Lake, have erupted often over the past 40,000 years. During the last 5,000 years, an eruption has broken out somewhere along this chain every 250 to 700 years. The Inyo Craters and nearby lava domes were formed by a series of small to moderate eruptions 550 to 600 years ago, and the most recent eruptions along the volcanic chain took place about 250 years ago at Paoha Island in Mono Lake.

    The pattern of volcanic activity over the past 5,000 years suggests that the next eruption in the Long Valley area will most likely happen somewhere along the Mono-Inyo volcanic chain. However, the probability of such an eruption occurring in any given year is less than 1%. This is comparable to the annual chance of a magnitude 8 earthquake (like the Great 1906 San Francisco Earthquake) along the San Andreas Fault in coastal California or of an eruption from one of the more active Cascade Range volcanoes in the Pacific Northwest, such as Mount Rainier.

    As long as increased volcanic unrest (including earthquake swarms, ground deformation, and CO2 gas emissions) continues in the Long Valley area, the chances of an eruption occurring in the near future will remain somewhat increased. However, evidence from large volcanic areas and calderas worldwide shows that unrest, such as the current activity in eastern California, can persist for decades or even centuries without leading to an eruption. Nevertheless, recent eruptions at Rabaul Caldera in Papua New Guinea (1994) and the Izu volcanic complex in Japan (1989) following short periods of unrest emphasize the need to closely monitor restless calderas.

    When an eruption does break out in the Long Valley area, its impact will depend on the location, size, and type of eruption, as well as the wind direction. Also, an eruption during the winter months could melt heavy snow packs, generating mudflows and locally destructive flooding.

    Most likely, the next eruption will be small and similar to previous eruptions along the Mono-Inyo volcanic chain during the past 5,000 years. Such eruptions typically begin with a series of steam-blast explosions as rising molten rock (magma) encounters and vaporizes underground water near the Earth's surface. These blasts can throw large blocks of rock and smaller fragments hundreds of feet into the air, leaving deep, circular pits like the Inyo Craters.

    If magma reaches the surface, gases trapped within it can escape explosively, hurling volcanic ash (tiny fragments of the solidifying magma) as high as 6 miles or more. Airborne volcanic ash can be carried hundreds of miles downwind, and the amount and size of falling ash decrease with distance from the eruption site. Thin accumulations of ash pose little threat to life or property, especially in areas where the roofs of most buildings are constructed to withstand heavy snow loads. However, even a light dusting of fine volcanic ash can close roads and seriously disrupt communications and utilities for weeks or months after an eruption.

    The eruptions that led to the creation of the 600-year-old South Deadman Creek Dome covered the area of what is now the Town of Mammoth Lakes with a layer of volcanic ash about 1 inch thick. During these eruptions, the wind first blew toward the northeast (tan) and later toward the southwest (pink), spreading volcanic ash in the pattern shown on the map. These eruptions also produced fiery flows of hot ash (pyroclastic flows). Depending on the wind direction and the location of an eruption site, future eruptions in the Long Valley area could spread volcanic ash over the communities of Mammoth Lakes, June Lake, or Lee Vining (see also eruption chart). Pyroclastic flows incicated by yellow and orange area.

    Explosive volcanic eruptions may also produce fiery flows of hot ash (pyroclastic flows) that can sweep over the ground at speeds greater than 100 miles an hour, devastating everything in their paths. In the past 5,000 years, eruptions from several sites along the Mono-Inyo chain have produced narrow, tongue-like pyroclastic flows that extended more than 5 miles. Fortunately, the main population centers in the Long Valley area are far enough from probable eruption sites that they are unlikely to be directly impacted by future pyroclastic flows.

    Less violent eruptions have also taken place in the Long Valley area. These eruptions typically began with mild explosions that formed relatively small volcanic cones less than 1,000 feet in diameter and then produced hot, fluid lava flows that extended a few miles. Eruptions of this type about 5,000 years ago created the Red Cones, just south of Mammoth Mountain. Flows of fluid lava were also erupted from sites near the base of Mammoth Mountain between 400,000 and 60,000 years ago. Such flows are highly destructive to property, but seldom endanger people because lava flows rarely move faster than a brisk walk.

    Although the chance of a volcanic eruption in any given year is small, future eruptions will occur in the Long Valley area. Because volcanic unrest can escalate to an eruption in a few weeks or less, USGS scientists are closely monitoring activity in this region. To be able to provide the public with reliable and timely warnings before an eruption, the USGS has joined local and State authorities in developing procedures for responding to changing levels of volcanic unrest in the Long Valley area. The ongoing work of the USGS Volcano Hazards Program in this and other volcanic regions of the United States helps to better protect people's lives and property from volcano hazards.
    People are quick to confuse and despise confidence as arrogance but that is common amongst those who have never accomplished anything in their lives and who have always played it safe not willing to risk failure.

  6. #6
    Join Date
    Apr 2009
    Central Iowa

    Long-term outlook for volcanic activity in Long Valley caldera

    The area of eastern California that includes the Long Valley Caldera and the Mono-Inyo Craters volcanic chain has a long history of geologic activity that includes both earthquakes and volcanic eruptions. This activity is likely to continue long into the future. When measured in the time-scale of a human lifetime, volcanic eruptions or destructive earthquakes are infrequent events. How does this ongoing geologic activity affect those who live in or visit this area of spectacular eastern Sierra scenery?

    The best guide to the future behavior of a volcano or volcanic system is its past behavior. Geological studies of Long Valley Caldera and the Mono-Inyo Craters volcanic chain indicate that:

    Future eruptions are more likely to occur somewhere along the Mono-Inyo Craters volcanic chain than from the resurgent dome or south moat area within the caldera.
    In the absence of unrest (earthquake swarms, ground deformation, gas emissions, and fumarole activity), the odds of an eruption occurring in any given year along the chain are one in a few hundred (comparable to the odds for a great [magnitude 8] earthquake along the San Andreas fault in coastal California).

    Unrest can temporarily increase the odds of an eruption, depending on the nature, intensity, and location of the unrest.

    Future eruptions are likely to be explosive in style but small to moderate in size. For the Mono Chain, any eruption would probably include both. For the Long Valley Caldera and Mammoth area, the most likely eruption is a lava fountaining eruption that builds a scoria cone and feeds lava flows.

    The odds that a small eruption somewhere along the chain will have a significant impact on any specified place along the chain are roughly one in a thousand in a given year.

    Larger eruptions are possible but less common (and thus less likely) than smaller ones (true for most volcanoes).

    Massive eruptions of the size that accompanied formation of Long Valley Caldera 760,000 years ago are extremely rare (none have occurred during the period of written human history). Scientists see no evidence that an eruption of such catastrophic proportions might be brewing beneath Long Valley caldera.
    Additional Information About the Outlook at Long Valley Caldera

    Looking at the history of volcanic eruptions at any volcano helps scientists understand what might happen in the future at any volcano. Find out more about the recent eruptions in the Long Valley region.

    All but three of the 20 or so eruptions over the past 5,000 years at Long Valley Caldera have been explosive in nature. Those three were of the effusive, Hawaiian type (the Red Cones eruptions south of Mammoth Mountain about 5,000 year ago, the Negit Island eruption about 2,000 years ago, and the Paoha Island eruption just 300 years ago). All have been small to moderate in scale. So what type of eruptions can we expect at Long Valley Caldera?
    People are quick to confuse and despise confidence as arrogance but that is common amongst those who have never accomplished anything in their lives and who have always played it safe not willing to risk failure.

  7. #7
    Join Date
    Apr 2009
    Central Iowa

    Map of the Long Valley area, California.
    (Click image to view full size.)
    Map of the Long Valley area, California.

    Persistent earthquake and volcanic activity over the past 4 million years has formed the spectacular eastern Sierra landscape in the vicinity of Long Valley caldera and the Mono Basin. Beginning about 3 million years ago (3 Ma), the Sierra Nevada and White Mountains fault systems became active with repeated episodes of fault movement (earthquakes) gradually producing the impressive relief of the eastern Sierra Nevada and White Mountain escarpments that bound the northern Owens Valley-Mono Basin region. Two distinct, but interrelated, magmatic systems have dominated the volcanic evolution of the Long Valley - Mono Basin region over this time interval. The compositions of lava produced by both systems have evolved similarly, becoming more silica-rich with time from early, predominantly basaltic eruptions to later, predominantly rhyolitic eruptions.

    The older magmatic system is centered on Long Valley Caldera and covers a 4,000 km 3 (1,544 mi3) area straddling the eastern Sierra Nevada escarpment at the northern end of Owens Valley. This system produced widespread eruptions of basalt and andesite between 3.8 and 2.8 Ma over much of the Long Valley-Mono Basin region. The Sierra Nevada fault system has offset some of these early lava flows as much as 1,000 m (3,280 ft), leaving one side high on the Sierra crest and the other in the valley below. Volcanic activity became concentrated in the vicinity of the present site of Long Valley caldera 3.1 to 2.5 Ma with eruptions of rhyodacite followed by high-silica rhyolite from 2.1 to 0.8 Ma. Lava from the latter eruptions form Glass Mountain on the northeast rim of the present caldera.

    Simplified geologic map of Long Valley area.

    (Click image to view full size.)
    Simplified geologic map fo Long Valley area with inset map showing ash distribution across western U.S.

    Long Valley Caldera

    The Glass Mountain eruptions, which were fed by a large, chemically evolving magma chamber in the shallow crust, culminated in the cataclysmic eruption of 600 km3 (144 mi3) of high-silica rhyolite 760,000 years ago. This massive eruption resulted in the widespread deposition of the Bishop Tuff and the simultaneous 2 to 3 km (1.2 to 1.7 mi) subsidence of the magma chamber roof to form the present 17 by 32 km (10 by 20 mi), oval depression of Long Valley Caldera. Subsequent eruptions from the Long Valley magma chamber were confined within the caldera with extrusions of relatively hot (crystal-free) rhyolite 750,000 to 640,000 years ago as the caldera floor was upwarped to form the resurgent dome followed by extrusions of cooler, crystal-rich moat rhyolite at 200,000-year intervals (500,000, 300,000, and 100,000 years ago) in clockwise succession around the resurgent dome. Repeated eruption of dacite and rhyodacite from vents on the southwest rim of the caldera 220,000 to 50,000 years ago formed Mammoth Mountain, a dome complex.

    Mono-Inyo Craters Volcanic Chain

    The younger system, the Mono-Inyo Craters volcanic chain, is localized along a narrow, north-trending fissure system that extends from north of Mammoth Mountain through the western moat of Long Valley caldera to the north shore of Mono Lake. This system began by erupting basalt and andesite first in the west moat of Long Valley caldera 400,000 to 60,000 years ago and then in the Mono Basin 40,000 to 13,000 years ago. Dacite and rhyodacite also were erupted in the Mono Basin 100,000 to 6,000 years ago. The Mono Craters were formed by multiple eruptions of high-silica rhyolite 40,000 to 600 years ago, and the Inyo Craters were formed by eruptions of low- silica rhyolite 5,000 to 500 years ago.

    During the past 3,000 years the Mono-Inyo Craters have erupted at intervals of 700 to 250 years, the most recent eruptions being from Panum Crater and the Inyo Craters in 1350 C.E., and Paoha Island about 300 years ago. Evidence from both seismic soundings of the crust and studies of the fabric and composition of the lava indicate that these eruptions probably originated from small, discrete magma bodies rather than from a single, large magma chamber of the sort that produced the caldera-forming eruption 760,000 years ago.

    This geologically recent volcanic activity, together with unrest in Long Valley Caldera that began in 1980 and the frequently felt earthquakes in the region, are reminders the processes that have sculpted the eastern Sierra landscape over the past 4 million years continue today.
    People are quick to confuse and despise confidence as arrogance but that is common amongst those who have never accomplished anything in their lives and who have always played it safe not willing to risk failure.

  8. #8
    Join Date
    Apr 2009
    Central Iowa
    Link for recent activity, I'm not sure how one would go about embedding all of the information here so... just go to the USGS link.
    People are quick to confuse and despise confidence as arrogance but that is common amongst those who have never accomplished anything in their lives and who have always played it safe not willing to risk failure.

  9. #9
    Join Date
    Apr 2009
    Central Iowa
    From February of this year

    Long Valley Caldera-Mammoth Mountain Unrest: The Knowns and The Unknowns

    By David P. Hill
    Posted in Perspective and tagged February 2017
    February 2017 Issue Table of Contents

    David P. Hill worked as a staff seismologist at the USGS Hawaiian Volcano Observatory (1964–1966) and as Scientist-in-Charge of the USGS Long Valley Observatory (1982–2009). His responsibilities in the latter role included coordinating monitoring of the unrest in Long Valley Caldera and Mammoth Mountain and explaining the hazard implications to civil authorities and the public. In 2002, David received the Mineralogical Society of America's Distinguished Service Medal for his work with the public and civil authorities at Mammoth Lakes. He continues to pursue research on Long Valley/Mammoth Mountain unrest as Scientist Emeritus with the USGS.

    This perspective is based largely on my study of the Long Valley Caldera (California, USA) over the past 40 years. Here, I’ll examine the “knowns” and the “known unknowns” of the complex tectonic–magmatic system of the Long Valley Caldera volcanic complex. I will also offer a few brief thoughts on the “unknown unknowns” of this system.


    Figure 1. Seismicity and structural map of the Long Valley Caldera, Mammoth Mountain, and surrounding area (California, USA). Symbols are as follows: HCF – Hilton Creek Fault; HSF – Hartley Springs Fault; Hwy – Highway; ML – Mammoth Lakes; SMFZ – South Moat Fault Zone; WCF – Wheeler Crest Fault; heavy barbed lines are Sierra Nevada range-front faults; colored circles are high-resolution epicenters for magnitude 2–6 earthquakes from 1982–2014; solid black circles are epicenters of the May 1980, magnitude 6 earthquakes; opposing half-arrows indicate the sense of strike-slip motion across faults or fault zones; large open arrows indicate the relative sense of displacement of the Basin and Range to the east with respect to the Sierra Nevada block; heavy dashed red line indicates approximate location of the dike feeding the Inyo Domes vents with opposing arrows indicating the sense of extension across the dike; the orange circle with opposing arrows indicates the best-fit location of the compact inflation source driving tumescence of the resurgent dome. Inset: Simplified kinematics of the SMSZ as a “leaky transform fault”. B&R – Basin and Range; LVC – Long Valley Caldera; SN – Sierra Nevada. Stuart Wilkinson produced the shaded relief and seismicity background.

    The Long Valley Caldera, located along the eastern escarpment of the Sierra Nevada mountain range, formed 760 ky ago with the eruption of ~600 km3 of rhyolite we now call the Bishop Tuff. The caldera sits between two range-front normal faults: the Hilton Creek Fault to the south and the Hartley Springs Fault to the north (Fig. 1). The 1,300–650-year-old silicic vents forming the Inyo Domes extend into the west moat of the caldera. Mammoth Mountain is a 100–50 ka dacitic, cumulo-volcano surrounded by mafic volcanic vents as young as 8 ka that lies on the southwest rim of Long Valley Caldera (Hildreth 2004). Mammoth Mountain at 11,000 feet and the town of Mammoth Lakes at its base serve as a year-round resort and one of the largest ski areas in the USA.

    No notable volcanic activity was documented in the caldera from the time of the early settlers in the mid-1800s through to early 1980. The onset of current caldera unrest occurred in May 1980, just a week after the May 18 eruption of Mount St. Helens (Washington, USA), with four magnitude 6 (M 6) earthquakes. Three were located beneath the Sierra Nevada just south of the caldera and the third was located beneath the southern margin of the caldera. A leveling survey later that summer revealed that the resurgent dome in the center of the caldera had bowed upward by 25 cm since the late 1970s, implying magmatic, rather than tectonic, processes were at work (Savage and Clark 1982).

    Attempts by US Geological Survey (USGS) geologists to explain the implications of the ongoing unrest were initially greeted with outrage and denial in the resort community of Mammoth Lakes—a population largely unaware of the long history of volcanism in the area. Antagonism toward Earth scientists gradually waned through the 1980s and early 1990s as the caldera’s unrest continued to produce many locally felt earthquakes. The community began to accept the message presented by scientists through frequent public lectures and geological field trips open to the public. Outreach has included USGS support for civil authorities from Mammoth Lakes and Mono County to attend the 10th anniversary meeting on the eruption of Mount St. Helens.

    Seismic unrest of the Long Valley Caldera has continued with recurring earthquake swarms in the south moat seismic zone (SMSZ), accompanied by elevated seismicity in the Sierra Nevada block to the south. Inflation of the resurgent dome has continued at rates as high as 20 cm/y (1980–1982 and 1997–1999), with a relatively stable interval from 2000–2010 (Hill 2006). Uplift resumed in 2011 at a rate of ~2 cm/y, and continues to this day. The center of the resurgent dome currently stands ~80 cm higher than before the onset of inflation in 1979–1980 (Montgomery-Brown et al. 2015).

    Mammoth Mountain has a magmatic system that is distinct from that of the Long Valley Caldera. But it, too, joined in the regional unrest with a nine-month earthquake swarm in 1989–1990.

    Mid-way through this sequence, long-period “volcanic” earthquakes began occurring at mid-crustal depths (10–20 km). By early 1990, diffuse emissions of magmatic CO2 began killing trees in several areas around the mountain, and elevated levels of 3He/4He were detected from a fumarole on the upper flank of the mountain. The CO2 emissions have since resulted in four fatalities when skiers fell into CO2-rich snow pits. Swarms of lower-crustal, brittle-failure earthquakes, centered at depths of 20–30 km, occurred from June 2006 to September 2009, each followed by a seismicity increase in the upper 10 km of the crust beneath Mammoth Mountain. There was a doubling in uptake of magmatic CO2 in the 2009–2012 tree-rings in a large tree near the CO2 tree-kill area (Lewicki et al. 2014). This post-1989 seismicity illuminates the crustal roots of the magmatic system and the path of CO2-rich magmatic fluids from the base of the crust (depth ~30 km). This seismicity stands in contrast to the Long Valley Caldera, which has produced no earthquakes deeper than 10 km. Both findings limit our ability to access the state of the magmatic system beneath the caldera.

    Included with the knowns are the surface geology and structure of the upper 5 km of the crust.

    Geophysical studies of the area have sometimes produced conflicting results. Just how the results of these studies relate to one another and to the actual physical properties of the crust beneath the caldera are the known unknowns. Without a clear image of the deep structure beneath the caldera, a critical known unknown is the process inflating the resurgent dome. Two views prevail. One holds that inflation is *dominated by activation of hydrothermal fluids, while the other involves renewed intrusion of magma and associated volatiles into the upper crust. Careful mapping and analysis of the eruptive history of the Long Valley Caldera led Hildreth (2004) to suggest that current inflation is driven by hydrous volatiles from secondary boiling of the final stages of a moribund, 760 ka Bishop magma chamber. Others suggest that inflation is driven by advection of a melt-fraction into the upper crust. The simplest model providing a good fit to the deformation data is a volume increase in a compact magma body centered at a depth of 7 km beneath the center of the resurgent dome (Montgomery-Brown et al. 2015).

    The difference between these two views carries important implications for hazard assessment. If the volume of low seismic wave-speeds 10–15 km beneath the caldera inferred by Weiland et al. (1995) and Seccia et al. (2011) holds up to further testing, does it correspond to a zone of secondary boiling in a moribund Bishop magma chamber or to recent (last ~10,000 years) emplacement of a melt fraction at mid-crustal depths resulting from, say, basalt underplating and collapse of a lower-crustal crystal mush?

    Geothermal fluids upwelling beneath the Inyo Domes flow eastward down the hydrologic gradient within the postcaldera fill (2–3 km deep). Magmatic CO2 carried by this thermal water is apparently derived from a basaltic reservoir somewhere beneath the Inyo Domes (Brown et al. 2013). The elevated 3He/4He ratios in thermal springs in the east moat have been attributed to fluids ascending from upper-mantle sources along an extension of the Hilton Creek Fault into the caldera (Suemnicht et al. 2015). At issue here is lack of evidence for postcaldera (760 ka) displacement along the Hilton Creek Fault into the caldera (Hildreth 2004; Hill and Montgomery-Brown 2015). This leaves a question of whether the magmatic CO2 and the elevated 3He/4He in the thermal water in the east moat might be, in part, derived from a recent melt intrusion beneath the resurgent dome.

    The temporal correlation between the onset of seismicity in the Sierra Nevada south of the caldera and caldera unrest suggests a tectonic–magmatic interaction. A related issue is the possibility of a local tectonic earthquake triggering the onset of eruptive activity in a magmatic system that has reached a tipping point in its evolution. Indeed, Hildreth (2004) points to the possibility that a major earthquake on the Hilton Creek Fault may have triggered the onset of the 760 ka *caldera-forming eruption of Bishop Tuff. In mapping eruptive deposits of the Bishop Tuff, he found that onset of the eruption began near the point where the Hilton Creek Fault intersects the southeastern margin of the ring fracture system. Modern examples of proximal triggering include the summit eruption of Kilauea volcano (Hawaii, USA), which began half an hour after the 29 November 1975, M 7.5 Kalapana earthquake, and the M 5 earthquake that triggered the onset of the catastrophic 18 May 1980 eruption of Mount St. Helens. In many cases, seismic waves (dynamic stresses) from large regional earthquakes and major (M > 7.5) earthquakes at global distances have triggered increased seismicity at volcanic and geothermal sites around the globe, including Long Valley Caldera and Mammoth Mountain. In a few cases, this remote dynamic triggering may have accelerated onset of eruptive activity in magmatic systems already in a near-critical state (Hill and Prejean 2015)

    These examples underscore the importance of considering tectonic–magmatic interactions in parallel with processes within an internally evolving magmatic system when forecasting eruptions or making volcanic hazard assessments. A recent report on earthquake hazards in the Long Valley–Mono Lake region (Chen et al. 2014) represents one step in this direction. The above known unknowns illustrate the limits of current understanding of the state of magmatic systems for both the caldera and Mammoth Mountain and their proximity to criticality or tipping points. Moreover, these known unknowns present a challenge in communicating to the local residents and authorities the significance of the ongoing unrest, potential volcanic hazards, and reliable eruption forecasts.

    Unknown unknowns further complicate the challenge of making socially useful eruption forecasts. Even the most successful models developed for Earth’s structure and active processes are simplified versions of reality. The gap between models and reality is potentially a rich source of unknown unknowns. Similarly, unrecognized regional strain perturbations in an evolving tectonic–magmatic environment can compromise long-to-intermediate-term eruption forecasts based either on models or on probabilistic analysis of the recurrence history of past eruptions. Earthquake-triggered eruptions represent an extreme example of the challenge in making short-term eruption forecasts. On a more positive note, a number of successful short-term (days to weeks) eruption forecasts have been based on on-site experience and pattern recognition during accelerating unrest episodes leading to eruptions (White and McClausland 2016).
    People are quick to confuse and despise confidence as arrogance but that is common amongst those who have never accomplished anything in their lives and who have always played it safe not willing to risk failure.

  10. #10
    Join Date
    May 2004
    Potato Country
    When I first noticed activity in the Truckee, Ca area, the end of June; I wondered what was causing it. I was told it was the LVC....looks like activity has been increasing for some time. With the eclipse in August; I think we will see earthquakes or volcanic activity increase....I'm keeping an eye on August 21st.....a week before and a week after....


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