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Active Tectonic and Magmatic Active Tectonic and Magmatic Processes Beneath Long Processes Beneath Long Valley Caldera, Eastern Valley Caldera, Eastern California: An OverviewCalifornia: An Overview
By: David P. Hill, Roy A. Bailey, and Alan S. Ryall
Presented by: Kristin Hepper
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Long Valley Caldera at a Glance…
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Long Valley CalderaLong Valley Caldera
150 years of earthquake activity
M 5-6 level
The last 10,000-20,000 years
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Most Recent EruptionsMost Recent Eruptions
Most recent = 500-600 years ago along Inyo/Mono craters chain
These are the most recent caldera forming eruptions within the boundaries of the US– Except for the Cascades
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Inyo Craters
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The Long Valley RegionThe Long Valley Region
Is the boundary between the Sierra Nevada and Basin and Range Provinces
Contemporary encroachment of Basin and Range into the Sierra Nevada
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Recent HistoryRecent History October 1978- M 5.7
earthquake
1 ½ years later- swarmlike sequence of M6 earthquakes within 48hrs of each other
This was the onset of the inflation of the resurgent dome
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Next 3 years-– Moderate earthquake swarms and rapid fire
sequences– Continued inflation
This raised concerns– Large earthquake– Renewed volcanic activity in the area
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History of Volcanic ActivityHistory of Volcanic Activity
Volcanism began ~3.2mya
Began with displacement of the Sierran frontal fault scarp
There were basaltic eruptions extending over a 4,000km2 area
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Volcanic Activity cont…Volcanic Activity cont…
The area existed in a potentially explosive state for 400,000 years
0.73mya a 600km3 of magma was ejected from the magma chamber– Now called the Bishop Tuff
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The Bishop TuffThe Bishop Tuff
Covered 1500km2
Thickness=1500m within the 17 by 32 km Long Valley Caldera
The caldera formed by collapse
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The Bishop Tuff
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Youngest EruptionsYoungest Eruptions
Inyo/Mono craters chain
Erupted roughly every 500 years for the last 2-3000 years
N trending dike 8-12km long, 200 m from the surface
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Future eruptions will be from the younger and more frequently active Mono/Inyo crater chain
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The magma chamberThe magma chamber Seismic measurements
Roof=6-7km beneath resurgent dome
Volume of present magma chamber is between 500 and 1000km3
Resurgent dome
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Closer to recent businessCloser to recent business
Began in fall of 1978
Inflation of resurgent dome
Cumulative uplift of resurgent dome=50cm
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The cause of the earthquakesThe cause of the earthquakes
NE-SW extension
Between mid 1979 and ~1984, resurgent dome increased its volume by 0.1-0.2km3
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Geothermal…Geothermal…Intermediate between the giant Yellowstone
and the smaller Valles calderas
Shallow convective hydrothermal system
There are active thermal springs and fumaroles at Casa Diablo and Hot creek– Fed by 100-170°C (212-338 °F) water
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The Geothermal System-Mammoth geothermal power facility (produces 40Mw of electrical energy)
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Comparison with other Comparison with other CalderasCalderas
200 calderas around the world have shown unrest
During a “typical” year, 14 calderas show some signs of unrest– Ground deformation in 3– Eruptions from 6-7
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Other calderas out there…Other calderas out there…
Long Valley is one of the 4 major calderas in the world
Includes: Rabaul in New Guinea, the Phlegraean Fields in Italy, Long Valley in California, and Yellowstone in Whyoming
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Keep in mindKeep in mindThe Inyo/Mono craters erupted 500-600
years agoErupted ~2km3 of material
– Each vented ejected ~0.2km3 of material Produced ash falls and pyroclastic flows that
extended 6-9km from the vents
Mono Lake
Sierra Nevada
Active Fumaroles
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Earlier eruptions- ejected ~0.2km3 of magma (Long Valley)
Comparable to Mt. St. Helens which erupted on May 18, 1980
Caldera forming eruption in Long Valley
Was 700,000 years ago
Produced 600km3 of magma
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Mt. St. HelensMt. St. Helens
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Mt. St. Helens in all its glory…Mt. St. Helens in all its glory…
May 18, 1980
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ConclusionsConclusionsLong lived silicic magma chamber beneath
Long Valley
A smaller magma chamber beneath Inyo/Mono Craters
Eruptions may be preceded by multiple episodes of unrest
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This can be separated by decades of quiet
Most likely, the Inyo/Mono Crater chain will produce the next large eruption
Inyo/Mono last erupted 500-600 years ago…and it erupts every 500 years…is it due again?
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An eruption from either system though will be preceded by accelerated levels of unrest
There should be ample warning
A large earthquake in the area however, could set the whole system off
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View from on top Mammoth Mountain
Horseshoe Lake
Area where trees were killed recently by high levels of carbon dioxide gas in the soil