Foray into Geology : The Long Valley Caldera

*Disclaimer : I am not a geologist; I don’t have a geology degree, I never majored or minored in geology.  I've taken exactly one college geology class, and I've read three books on geology - four, if you want to count the one I read twice.  (More on those gems in another post, no pun intended.)  But I'm learning.  And since California is such a geologically rich place, it's the perfect learning ground.  Therefore, what you read here is just that : a glimpse into a learning process, as I study and read and glean information from my travels to sites of geologic significance, then share my findings with you.  Please correct me if anything is amiss!

During the summer of 2015, my husband and I spent a week in the Mammoth Lakes area, on the breathtaking eastern side of the Sierra Nevada.  The cabin where we stayed was perched on the edge of the Long Valley Caldera, below Mammoth Mountain, in the Lakes Basin region of Mammoth Lakes.  (I had learned, from my one college geology class, about the Long Valley eruption and the volcanic potential of the region… and may have down-played these facts to my husband during our trip.) 
 
The entire week was one fantastic self-guided geology field trip, and will become a series of posts, the first of which being a look at the Long Valley Caldera.
On the floor of the Long Valley Caldera, looking toward the northeast rim
Nearly 10 miles wide and 20 miles long, the Long Valley Caldera was formed approximately 760,000 years ago by a massive volcanic eruption.  Geologists estimate that volcanic activity in the Long Valley region began between three and four million years ago with the eruption of basalts and andesites. 

The chemical composition of Long Valley lavas changed over time, from low silica or mafic lavas, which are less viscous, effusive lavas that form basalts and andesites, to lavas with a higher silica content, or felsic lavas.  Felsic lavas produce dacite and ryolite, and have a higher viscosity which makes them explosive rather than effusive. 
Looking toward the southeast rim of the caldera, with a hot spring in the foreground
Between 2.1 and 0.8 million years ago, the Long Valley Caldera produced an eruption of rhyolite lava, which formed Glass Mountain at the northeast edge of today's caldera and indicated that silica-rich magma with explosive potential was building up in a magma chamber below.
The floor of the caldera contains a number of springs and creeks filled with water that has been heated by
underground magma, attesting to the area's present day volcanic activity.
By 760,000 years ago, enough ryolitic magma had built up in a magma chamber a mere four miles beneath the surface to produce a colossal eruption.  So great was the pressure that was released, so high its velocity and so large the volume of ejected material, that tephra and pyroclastic flow deposits buried an estimated 580 square miles of California and Nevada.  75 miles from the volcano, ash covered the ground to a depth of four feet.  125 miles from the volcano, the ash blanket was 16 inches deep.  Ash from the Long Valley eruption reached as far as Nebraska and Kansas, recognizable by geologists even today. 

Bishop tuff is the name given to deposits of ash and pumice produced by the Long Valley eruption, and numerous examples can be found in the area.
Photo of a photo display in the Mammoth Lakes Welcome Center, describing Bishop tuff.

So great was the amount of magma which erupted from the Long Valley volcano that the roof of the magma chamber collapsed, causing the ground to subside and create a caldera one mile in depth.  Some of the material from the eruption fell back into the caldera, and more material has continued to fill it in the many years since.  But from the center of the present-day Long Valley caldera, the rim of the massive volcano is still evident, rising up from the caldera floor in every direction. 
Map of the Long Valley Caldera, outlined in purple.  Orange shows the resurgent dome; yellow denotes areas of geologic interest.  Red circles mark sites of volcanic vents and fumaroles of the last 2 million years.
Image courtesy of the Mammoth Lakes Welcome Center. 
After the caldera formed, volcanic activity continued as pressure from rising magma caused the floor of the caldera to bulge, creating a resurgent dome which is about 1,600 feet higher than the surrounding caldera floor.  Approximately 100,000 years ago, Mammoth Mountain, along the southwest edge of the Long Valley caldera, began producing dacite flows (higher viscocity, felsic lavas) at the estimated rate of one every 5,000 years, for the next 50,000 years. 
Mammoth Mountain in the background; a portion of the resurgent dome can be seen at the right, covered in dark vegetation.  Hot Creek flows through the foreground.
Geologists don't expect another massive eruption in the immediate future, but the region is still volcanically active, and any number of smaller eruptions are possible, if not likely, near the Long Valley caldera and Mammoth Mountain.
Enjoying beautiful hot springs and stunning views of the eastern Sierra.  If you visit, please respect this remarkable place.
 

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