Lassen (KQED Perspective aired September 2003)
By Michael Ellis
Mt. Harkness Lassen Volcanic National Park
Below the surface of the earth anywhere from 15 to 60 miles deep is a very large reservoir of molten rock called magma. When this molten rock gets to the surface we then call it lava. There are four different kinds of volcanoes throughout the world that funnel this magma upward. There is a place in California from where you can see all four types. This is very rare and yet another superlative for our great state.
Seventy miles to the north from this spot is a very large volcano. This is the classic Mt. Fuji type called a composite or strato-volcano. These occur where an ocean seafloor plunges underneath a continental land mass and the seafloor is melted into a huge pool of magma which then rises. These volcanoes are famous for cataclysmic eruptions because they contain large quantities of gases and high concentrations of explosive silica. This one last erupted in the 1790’s.
In the foreground about five miles away is the classic cinder cone or tephra volcano. These are probably the most common type. They are often not the main vent but parasitic cones. Their explosions are usually violent and they are composed of lava thrown into the air and cooled into ash, obsidian and/or cinders.
To the west is the world’s largest volcanic plug dome. This one rises over 10000′ above sea level and was formed by pasty lava so thick that it could essentially not flow and plugged the neck of volcano.
And finally this mystery spot itself is a mountain composed of nothing but basalt. This very runny lava usually is found in spreading ocean sea floor bottoms or in so-called hot spots like the kind that form Hawaii and the Galapagos. This lava tends to flow smoothly and quietly and forms volcanoes that have a very rounded top hence the name shield volcanoes.
So where is this location? This is Michael Ellis with a Perspective.