I've been following this resent activity at mammoth for about a week now, this is really the sleeping giant of the American volcanic systems, this puppy blew out 150 cubic miles of magma around the same time as Yellowstone was ripping up eastern Idaho, ash fell as far east as Kansas and Nebraska and the ground subsidence after the magma chamber emptied was over one mile! leaving the visible caldera we see today along the Long valley system.

The history of this system is impressive by any standards, it has had continued activity over the last several hundred thousand years slowly shifting northward to the Mono lakes region around 35,000 years ago, the most resent activity was only some 200 years back in Mono craters!

During the last big eruption, it covered areas around the surrounding Sierras with as much as 3000ft of ash that held temperatures of 1300 degrees F for months after the the event! and traces of this same ash were some 3 ft deep in the Kansas area! Bellow is the link to the USGS real time map for the Mammoth lakes region, the resent swarm activity during this week was as high as 4.6

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USGS Real Time Map
Wikipedia Page : http://en.wikipedia.org/wiki/Long_Valley_Caldera  

The known volcanic history of the Long Valley Caldera area started several million years ago when magma began to collect several miles below the surface. Volcanic activity became concentrated in the vicinity of the present site of Long Valley Caldera 3.1 to 2.5 million years ago with eruptions of rhyodacite followed by high-silica rhyolite from 2.1 to 0.8 million years ago. After some time a cluster of mostly rhyolitic volcanoes formed in the area. All told, about 1,500 square miles (4,000 square kilometers) were covered by lava.

All but one of these volcanoes, 1-2 million year old Glass Mountain (made of obsidian), was completely destroyed by the major eruption of the area 760,000 years ago, which released 600 cubic kilometres of material from vents just inside the margin of the caldera (the 1980 Mount St. Helens eruption was 1.2 km³). About half of this material was ejected in a series of pyroclastic flows of a very hot (1,500 degree Fahrenheit or 800 degree Celsius) mixture of noxious gas, pumice, and ash that covered the surrounding area hundreds of feet deep. One lobe of this material moved south into Owens Valley, past where Big Pine, California now lies. Another lobe moved west over the crest of the Sierra Nevada and into the drainage of the San Joaquin River. The rest of the pyroclastic material along with 300 km³ of other matter, was blown as far as 25 miles (40 km) into the air where winds distributed it as far away as eastern Nebraska and Kansas. However, much of the material ejected straight into the air fell back to earth to fill the 2 to 3 km deep caldera two-thirds to its rim.

The Long Valley Caldera at a Glimpse : Recent History

The Caldera. Long Valley Caldera a 15- by 30-km oval-shaped depression located 20 km south of Mono Lake along the east side of the Sierra Nevada in east-central California. This area of eastern California has produced numerous volcanic eruptions over the past 3 million years, including the massive caldera-forming eruption 760,000 years ago. The most recent eruption occurred just 250 years ago in Mono Lake at the north end of Mono-Inyo Craters volcanic chain.

Volcanic Unrest. In May of 1980, a strong earthquake swarm that included four magnitude 6 earthquakes struck the southern margin of Long Valley Caldera associated with a 25-cm, dome-shaped uplift of the caldera floor. These events marked the onset of the latest period of caldera unrest that continues to this day. This ongoing unrest includes recurring earthquake swarms and continued dome-shaped uplift of the central section of the caldera (the resurgent dome) accompanied by changes in thermal springs and gas emissions.

USGS Monitoring. In 1982, the U.S. Geological Survey under the Volcano Hazards Program began an intensive effort to monitor and study geologic unrest in Long Valley caldera. The goal of this effort is to provide residents and civil authorities in the area reliable information on the nature of the potential hazards posed by this unrest and timely warning of an impending volcanic eruption, should it develop. Most, perhaps all, volcanic eruptions are preceded and accompanied by geophysical and geochemical changes in the volcanic system. Common precursory indicators of volcanic activity include increased seismicity, ground deformation, and variations in the nature and rate of gas emissions.