Active Tectonic and Magmatic
Processes Beneath Long
Valley Caldera, Eastern
California: An Overview
By: David P. Hill, Roy A. Bailey, and
Alan S. Ryall
Presented by: Kristin Hepper
Long Valley Caldera at a Glance…
Long Valley Caldera
150 years of earthquake activity
M 5-6 level
The last 10,000-20,000 years
Most 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
Inyo Craters
The 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
Recent History
October 1978- M 5.7
1 ½ years laterswarmlike sequence of
M6 earthquakes within
48hrs of each other
This was the onset of
the inflation of the
resurgent dome
Next 3 years– Moderate earthquake swarms and rapid fire
– Continued inflation
This raised concerns
– Large earthquake
– Renewed volcanic activity in the area
History 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
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
The Bishop Tuff
Covered 1500km2
Thickness=1500m within the 17 by 32 km
Long Valley Caldera
The caldera formed by collapse
The Bishop Tuff
Youngest 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
eruptions will
be from the
younger and
crater chain
The magma chamber
Resurgent dome
Seismic measurements
Roof=6-7km beneath
resurgent dome
Volume of present
magma chamber is
between 500 and
Closer to recent business
Began in fall of 1978
Inflation of resurgent dome
Cumulative uplift of resurgent dome=50cm
The cause of the earthquakes
NE-SW extension
Between mid 1979 and ~1984, resurgent
dome increased its volume by 0.1-0.2km3
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
The Geothermal System-Mammoth geothermal
power facility (produces 40Mw of electrical energy)
Comparison with other
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
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
Keep in mind
The Inyo/Mono craters erupted 500-600
years ago
 Erupted ~2km3 of material
– Each vented ejected ~0.2km3 of material
 Produced ash falls and pyroclastic flows that
extended 6-9km from the vents
Sierra Nevada
Mono Lake
Active Fumaroles
Earlier eruptionsejected ~0.2km3 of
magma (Long
Comparable to Mt.
St. Helens which
erupted on May 18,
Caldera forming
eruption in Long
Was 700,000 years
Produced 600km3 of
Mt. St. Helens
Mt. St. Helens in all its glory…
May 18, 1980
Long lived silicic magma chamber beneath
Long Valley
A smaller magma chamber beneath
Inyo/Mono Craters
Eruptions may be preceded by multiple
episodes of unrest
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?
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
View from on top Mammoth Mountain
Horseshoe Lake
Area where trees were killed recently by
high levels of carbon dioxide gas in the soil
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