Sunday, October 2, 2016

California's Cascade Volcanoes: Lassen Volcanic National Park

At the southern-most end of the Cascade Range lies Lassen Volcano National Park, seemingly a world away from the rest of California.  This beautiful park was the last stop on our geology field studies trip to Northern California, and this post is the last in my short series about California's Cascade volcanoes.

Within Lassen Volcanic National Park are examples of the four main types of volcanoes - stratovolcano, plug dome, shield volcano and cinder cone - as well as other volcanic features including hot springs, a basalt plateau, and a very large caldera.  Of course, the most prominent and recognizable of these is Lassen Peak itself, perhaps the world's largest plug dome volcano.
Lassen Peak, from the Devastated Area, northeast of the peak.
Lassen Peak formed between 26,000 and 28,000 years ago, and is composed largely of dacite and rhyolite.  Dacite and rhyolite magma has a high silica content, making it quite viscous and as a result, highly explosive.  
 
Only a century ago, Lassen Peak sprang to life.  The volcano erupted in a series of phreatic eruptions, or steam eruptions, between May 1914 and May 1915, as the volcano began "clearing its throat," so to speak.  In early May of 1915, new dacite lava appeared for the first time.  On May 19, 1915, a steam explosion broke up the dacite dome, resulting in a volcanic mudflow.  Volcanic mudflows, or lahars, are caused as hot volcanic material comes into contact with ice and snow on the volcano, melting the ice and snow to create a hot slurry of pyroclastic material that flows rapidly down the volcano.  Lahars are one of the most destructive aspects of a volcanic eruption, as they may reach depths of 30 feet and spread many miles downslope, often following creek beds.  
 
On May 22, 1915, just a few days after the volcanic mudflow swept 20 miles down Lost Creek and Hat Creek on the flanks of Mount Lassen, the volcano erupted violently, sending a mushroom-shaped cloud of ash over five miles into the air.  As ash fell, a high-speed volcanic avalanche, or pyroclastic flow, was generated, sending a wave of ash, pumice, rock fragments and gases boiling down the flanks of Mount Lassen.  The result of this pyroclastic flow was the total destruction of the forest in a three-square-mile area, still known as the Devastated Area.  After the main eruption, steam eruptions continued through 1917, followed by a smaller eruption in 1921.
View west from the Bumpass Hell trail, looking across the caldera of Mount Tehama to Brokeoff Mountain (left),
Mount Diller (center), and Piolot Pinnacle (right).
Lassen Peak, the focal point of Lassen Volcanic National Park, was not always the prominent peak in this area.  In fact, it would have been dwarfed by its predecessor, the 12,000-foot stratovolcano, Mount Tehama.  Mount Tehama came into being between 700,000 and 800,000 years ago, producing 50 to 80 cubic miles of tuff when it erupted.  But even mighty volcanoes are not eternal when considered in the vast span of geologic time; Mount Tehama went extinct around 400,000 years ago, and geologists believe that rather than ending in a catastrophic eruption, the volcano quietly eroded away.  Glaciers covering this area under hundreds of feet of slowly flowing, grinding ice took their toll on the old volcano.
Glacial erratic (boulder in foreground) with Brokeoff Mountain (left) and Mount Diller (right) in the
background.  Brokeoff Mountain and Mount Diller are part of the edge of the caldera of former Mount Tehama.
The dark speck in the sky is a California Tortoise Shell butterfly, one of perhaps thousands that are carried over
 the mountain on wind currents.
Glaciation is really a topic for another time, but evidence such as glacial erratics, large boulders carried along by the flow of ice until they are dropped, and glacial striations and grooves in the area around Bumpass Hell are clear signs that ice once covered this area.
First view of Bumpass Hell from the hiking trail.  You'll smell it before you see it!
Geologists once thought that Bumpass Hell was the main vent of Mount Tehama, but have since discovered this to be untrue, as Bumpass Hell lies outside the area of the caldera; the main vent actually lies within the Sulphur Works region of the park, which we didn't have the chance to visit on this trip.  Instead, Bumpass Hell is the eroded vent of a different dormant volcano, Bumpass Mountain.
Bumpass Hell, Lassen Volcanic National Park
The trail to Bumpass Hell is a beautiful 1.5 mile hike through stands of mountain hemlock (Tsuga mertensiana) and meadows of lupine (Lupinus sp.).  Rounding the last bend, the trail opens up onto a scene one would perhaps associate more closely with Yellowstone National Park than California: a sulfurous valley of steaming fumaroles and bubbling mudpots.  The geothermal activity in this area is evidence that magma chambers are alive and well, a mere three miles below the surface.
Fumaroles (steam vents) at Bumpass Hell
Three of the four major geothermal features can be found here: hot springs, mudpots, and fumaroles; only geysers are absent, due to a lack of sufficient pressure build-up belowground.  Hot springs are what they sound like: areas where super-heated ground water reaches the surface, with an extremely low (acidic) pH around 1.  Mudpots are an intermediate stage between hot springs and steam vents, forming when less water present.  The hot water turns the surrounding rock into clay, creating a pot of boiling mud.  Fumaroles occur in the presence of even less water, or water farther belowground. What little water that is present is vaporized to steam before it escapes through a steam vent.
Mudpots at Bumpass Hell
Lassen Volcanic National Park, tucked away in a sparsely populated corner of California, is a truly remarkable place.  For a good overview of everything volcanic, as well as some stunningly beautiful scenery, it is well worth a visit - especially if you don't have time for a visit to Yellowstone National Park!



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