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Roger Weller, geology instructor

Long Valley Caldera
by Jonathan McCracken
Physical Geology
Fall 2007

                                               Building to Blow The Long Valley Caldera

Long Valley Caldera from Mammoth Mountain. Photo Courtesy of


A Brief History

          Approximately 760,00 years ago, a volcanic eruption took place in the northern portions of present day California which was 2000 times greater than the more recent eruption of Mount Saint Helens in Washington state. This eruption caused a caldera 10 miles wide and 20 miles in length. It is estimated that the eruption left a lava flow that covered 1,500 square miles, and was the cause of the placement of 150 cubic miles of volcanic material. The ash from the volcano was thrown as far East as Nebraska and Kansas, all the way to the Pacific Ocean in the west, into Idaho and Wyoming in the north, and even into the northern parts of Mexico.  

          In the modern day world however, the volcano in question has not had any volcanic activity in over two hundred years. The last eruption that occurred in this area formed a new depression called Mono Lake. This area is now known as the Long Valley Caldera. The Long Valley Caldera in northern California known for its skiing, fishing and breath taking views. The caldera is not as well known for its potential to erupt.

          In 1980, volcano watch was at an all time high after the Mount Saint Helens eruption, which took the lives of many bystanders caught in the path of the violent explosion. With such a disaster fresh in the minds of the American people, the United States Geological Society (USGS) began searching for other volcanoes in the continental United Sates that might also present a danger to America. In their search they located an ancient caldera named the Long Valley Caldera. The reason the geologists were drawn to this location was because of a four seismic geological events that took place in may 1980. Four magnitude six earthquakes shook the valley of the Caldera, three of them happening in the same day. These seismic events caused geologists to become worried about the possibility of another event like Mount Saint Helens, so intense monitoring of the caldera began.

Ways the caldera is monitored.

          The Long Valley Caldera is being constantly monitored by the USGS at the Long Valley observatory using the most advanced technology available.  

         Remote Sensors

          The USGS is making use of the Global Positioning System (GPS) by measuring any movement of the caldera by placing numerous GPS receivers that are regularly recording their own locations and sending them to the Long Valley observatory. The data is then read to see if the ground has had any significant movement over any given period of time.


          Constantly receiving and interpreting the data received, the USGS is finding the location, depth, and magnitude of the numerous earthquakes that take place in and around the Long Valley Caldera. This helps predict the movement of the underground magma bed that sits below the caldera.

         Ground deformation

          By studying the heights of the valley and surrounding area, the USGS may perhaps predict if pressure is building below the surface of the earth. If the ground is rising, it is usually a sign of a rising magma flow, or at least the expansion of a magma chamber.

         Low lying atmospheric gasses

          By monitoring the atmospheric gasses in the area the USGS may see what the underground pressure is doing to the gasses. Large leaks of Carbon Dioxide have been recorded around Horseshoe Lake. The gas leak is a fact that increases the chance of another eruption in the area.


          All lakes, wells, fumaroles, springs and precipitation in the caldera are continually monitored and recorded in order to be read by the USGS. The data is then compared to other known volcanic patterns, that the USGS might be able to make a safe and timely prediction on the possibility of another eruption.

What Monitoring has Shown.

          Ten years after the caldera had started its initial monitoring, trees on Mammoth Mountain began unexpectedly dieing. Upon researching the matter the USGS found that large amounts of carbon dioxide gas had began pouring into the local atmosphere around Horseshoe lake. The gasses being released are in excess of 300 tons of Carbon Dioxide gas per day. Despite the large amount of gas, the USGS has declared that the area is still inhabitable. This out pouring of gas is similar to two other volcanoes Kilauea Volcano on the big island of Hawaii, and Mount Etna in Sicily. Both of these volcanoes have a constant lava flow and Mt. Etna is the largest volcano in Europe.

          This implies the possible strength at which the Long Valley Caldera may erupt. Having similar traits to these monster volcanoes without being active yet could mean a very large eruption is around the corner for the inhabitants in and around the California area

          Another disturbing fact about the caldera is the observation of a very large resurgent caldera measuring 100 square miles within the original caldera. Since 1980 the resurgent caldera has risen almost two feet. As time has passed the resurgent caldera has been picking up speed, rising faster each year.

          The seismic activity in the area of the Long Valley Caldera has geologist worried just as much as any of the other geologic activity. The largest earthquake in recent years happened southeast of the caldera registering a 5.4 magnitude. The earthquake has renewed the interest in the caldera, with officials looking to new evacuation routes and methods

          Even thought the resurgent caldera is in the Long Valley caldera, geologists at the USGS believe that if an eruption were to take place it would take place south of Mono Lake in the Inyo Craters. Mono Lake is north of the original caldera. The Inyo Craters are the result of the most recent volcanic activity in the area that created a line of calderas that seem to make a line heading toward Mono Lake. 

What are they doing?

          The local authorities in the long valley area have devised a color code system called the Avian system that is now used widely by the USGS in the monitoring of other volcanoes in the united states. With four colors green (The lowest chance for eruption), yellow, orange, and red (Red indicates the volcano has already begun erupting)

          Apart from that there is little more the authorities can do, the only other option they have is to permanently evacuate the entire area. This option is not even considered, the area is sparsely populated, but does sit close to Yosemite National Park. With such a large tourist attraction for the state, it is highly unlikely there will ever be a move to take such a large-scale evacuation seriously. Since the Caldera has the same chance of erupting as the San Andreas fault has a chance of producing an eight magnitude earthquake.

          It seems the only action that can be taken is to sit back, and be ready to run from an eruption whose size we do not know, and whose damage will be great. The Caldera rises and reminds humanity that they are not invincible, and one geological event can change the lives of hundreds, and end the lives of even more. 

Work Cited


Thank you to the United States Geological Society for all of the pictures (Unless otherwise Noted) United States Geological Society publication Vault Super Volcanoes Discovery Channel Long Valley Caldera Monitoring website