Friday, September 30, 2016

California's Cascade Volcanoes: Mount Shasta

Mount Shasta from the east; looking west, from Lava Beds National Monument (September, 2016)
Mount Shasta, a large stratovolcano in Northern California, is the second-tallest volcano in the Cascade Range.  (Only Washington's Mount Rainier is taller.)  At 14,162 feet in elevation, 25 miles across at its base, and with a volume of 108 cubic miles, Mount Shasta reigns over its surroundings with a quiet presence and ominous beauty.
Mount Shasta from Lava Beds National Monument (September, 2016).  Note that a portion of Sargent's Ridge is visible,
 just barely standing out as a small, dark ridge on the left (south) flank of the volcano.
Last week, I had the privilege of viewing Mount Shasta from the east, seeing a side of the mountain I'd not yet seen.  In past summers, I've spent time in the town of McCloud, soaking in views of snow-covered Mount Shasta from it's southern base.  But the first time I laid eyes on the mountain, while traveling with my parents, I was just two years old.  And it must have made an impression, because my family still recounts the story that upon returning home, I formed "Mount Sashtas" with my mashed potatoes.
Mount Shasta, from the south.  (June, 2010)
Mount Shasta has a 600,000 year history of eruptions, and is not the first volcano to occupy this location at the southern end of the Cascades.  Geologists have discovered that other volcanos have previously stood where Shasta now stands.  A region of low hills, called hummocks, partially fills a fault graben (a valley that sunk between mountain ranges along fault lines) north of Mount Shasta.  Geologists were puzzled by this geological formation until the 1980 eruption of Mount St. Helens, when an earthquake triggered the collapse of part of the mountain and resulted in a debris avalanche, characterized by its hummocky topography.
Mount Shasta's debris avalanche, characterized by hummocky topography.  Looking north (September, 2016)
From this evidence, geologists have concluded that the original Mount Shasta was destroyed in a massive debris avalanche about 400,000 years ago, the volcano's remains now forming the hummocks that span 5 to 8 miles in width and reach 28 miles down the valley.  The Mount Shasta debris avalanche is the largest recorded debris avalanche.
View from the south.  Photo taken from the lumber mill in McCloud. (June, 2009)
Mount Shasta is considered a compound stratovolcano (also called a composite cone), made up of four overlapping cones.  These four cones were formed during separate cone-building periods; during each period, cones produced eruptions of andesite lava, ash flows, and mud flows. 
View from the south.  Photo taken at the lumber mill in McCloud (June 2010)
The oldest remaining cone is Sargent's ridge (see second photo in this post), which erupted 250,000 years ago.  Misery Hill, a former summit near the top of the volcano, erupted between 100,000 and 130,000 years ago.  Most notable are Shastina, a parasitic cone and second summit which erupted 9,800 years ago, and Hotlum Cone, the very highest cone and current summit, which erupted 9,000 years ago.
Quiet backroads of McCloud, at the southern base of a massive stratovolcano.
According to the United States Geological Survey (USGS), in the past 10,000 years, Mount Shasta has erupted once about every 300 to 600 years; in the past 750 years, it has averaged one eruption every 250 years.  The last eruption, which probably occurred in 1786, was described, though vaguely, by the explorer La Perouse.  A future (possibly near-future) eruption of Mount Shasta is likely.  But take heart: many brilliant geologists have their eyes (and monitoring equipment) trained on the volcano, and an eruption would likely be preceded by a series of earthquakes and steam explosions, giving locals fair warning and allowing excited geologists to get into place to watch.
Shasta and Shastina (at left)
(P.S. If you're not a trained geologist, I would never advise trying to watch an eruption; I would suggest you heed their warnings and evacuate!)

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