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
earthquake

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
sequences
– 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
Future
eruptions will
be from the
younger and
more
frequently
active
Mono/Inyo
crater chain
The magma chamber
Resurgent dome

Seismic measurements

Roof=6-7km beneath
resurgent dome

Volume of present
magma chamber is
between 500 and
1000km3
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
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
The Geothermal System-Mammoth geothermal
power facility (produces 40Mw of electrical energy)
Comparison with other
Calderas

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
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
Mt. St. Helens
Mt. St. Helens in all its glory…
May 18, 1980
Conclusions

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