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

Long Valley Caldera at a Glance…

Long Valley CalderaLong Valley Caldera

150 years of earthquake activity

M 5-6 level

The last 10,000-20,000 years

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

Inyo Craters

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

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

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 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

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

The Bishop TuffThe Bishop Tuff

Covered 1500km2

Thickness=1500m within the 17 by 32 km Long Valley Caldera

The caldera formed by collapse

The Bishop Tuff

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

Future eruptions will be from the younger and more frequently active Mono/Inyo crater chain

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

Closer to recent businessCloser to recent business

Began in fall of 1978

Inflation of resurgent dome

Cumulative uplift of resurgent dome=50cm

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

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

The Geothermal System-Mammoth geothermal power facility (produces 40Mw of electrical energy)

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

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

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

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

Mt. St. HelensMt. St. Helens

Mt. St. Helens in all its glory…Mt. St. Helens in all its glory…

May 18, 1980

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

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