By: David P. Hill, Roy A. Bailey, and Alan S. Ryall 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 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- This raised concerns 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 Seismic measurements 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 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 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. 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