Introducing Volcano Mammoth Mountain

[BrickFielder]

Introduction
My intention is to produce a series of articles (depending on interest) which introduces ideas about volcanic and earthquake activity. I want to go slightly off the beaten track to explore oddities, volcano hazards, analysis methods, Wonders and Mankind’s impacts. Keep in mind I am not expert (corrections gratefully received), but hopefully these will at least give a flavour of some different places in the world and provide a few minutes escape from people’s troubles.

Introducing Mammoth Mountain

The Setting

Mammoth mountain is a volcano on the rim of the long valley caldera in California. The caldera is about 20 miles across which formed as part of a VEI7 eruption some 760,000 years ago. It is thought that the magma reservoir under the caldera has now largely crystallized and is unlikely to produce large scale eruptions again. The caldera has many hot springs and fumaroles with deformation, seismicity, and other unrest in recent years. Separately Mammoth Mountain has had seismic activity, gas emission, and tree kill, which is thought to be related to a dike intrusion beneath Mammoth Mountain in 1989. Mammoth Mountain and Mammoth Lakes town have an alpine landscape that has become one of California’s busiest recreational playgrounds. The town has spread around the eastern base and onto lower slopes of Mammoth Mountain. The volcano is covered with chair lifts, gondolas, ski runs, and mountain bike paths. Both winter and summer tourism bring nearly 3 million visitors a year to the area.  Visitor Lodges and restaurants are increasing being built up the slopes of the Mountain and the closest Inns being less than 3km from the summit. Forests around Mammoth Lakes are home to black bears, marmots, mountain lions, deer, elk, bighorn sheep, coyotes, falcons, osprey and eagles. Summers are mostly dry and winters snowy with temperatures typically ranging from -12C to 25C during the year.
The Tectonic Environment
The long valley caldera lies in a depression between the White Mountains to the east and the Sierra Nevada to the west. The most recent volcanic activity in the area occurred along the Mono Inyo dike chain with a chain of domes and craters stretching more than 30km. The area is one of the most seismically active areas in California with major earthquake swarms often preceded by uplift of the resurgent dome. Nearly a meter of uplift of the dome has been observed since the 1970s.Most earthquakes are tectonic in nature and associated with major faults like the Hilton Creek Fault and the Hartley spring fault. A complex fault zone in the south moat of the caldera has been the focus of most activity. It is thought that this complex fault with rotational slipping is due regional tectonic process. A localized area of tectonic extension might be providing conduits for magma and magmatically derived fluids to move to the surface. This localized stretching and twisting is thought to allow magma movement and uplift rather than any new major influx of magma.
Mammoth Mountain earthquake swarms are driven by different processes than those in the Caldera with upward moving magmatic processes the most likely source. 

The Volcano

Mammoth mountain is a lava dome complex which reaches just over 11,000 feet in height and is on the southwest rim of the Long Valley Caldera. Major building of the mountain was from 100,000 to 50,000 years ago from Eruptions from over 25 different vents and domes spreading across 2km. Fumaroles exist on the north side of the mountain and to the south above horseshoe lake which emit Carbon Dioxide and Helium. Tomographic imaging suggests that there is a 3km deep reservoir of CO2 which is capped by a low permeable seal which is refreshed from a deep magmatic source. This reservoir is estimated to be able to release 500 tons day for the next 1000 years.

Recent Activity

In May 1989 a small earthquake swarm began with seismicity at a depth of 7–9 km in a NNE trending keel like structure beneath the center of Mammoth Mountain.  The swarm continued to November with seismicity moving upward and concentrating between 6 km to 1 km depth. This is thought to be related to a magma injection into a dike below the mountain.

During the early 1990's a 100 acres of trees began dying off at several places on Mammoth Mountain Studies conducted by USGS and U.S. Forest Service scientists show that the trees are being killed by large amounts of carbon dioxide (CO2) gas seeping up through the soil from magma deep beneath Mammoth Mountain.

In February 1999, a seismic swarm occurred along a north west striking, steeply southwest dipping fault beneath the southeast flank of Mammoth Mountain.

In April 2006, three members of the Mammoth Mountain Ski Area ski patrol died while on duty. All three died from suffocation by carbon dioxide when they fell into a fumarole on the slopes of the mountain while attempting to fence it off.

More than 2400 earthquake events have occurred between 2009 and 2012 beneath Mammoth Mountain.

Discussion Ideas

Mammoth mountain is the picture book ski resort and by all rules the volcanic risks our low. There are unusual aspects like the large volumes of carbon dioxide and the local regional the tectonics which make me nervous. What do you think?