Wednesday, September 4, 2013

A geological wonderland: Lassen Volcanic National Park in northeastern California

California's Lassen Volcanic National Park is an easily accessible “Ring of Fire” hot spot rich in volcanic episodes. Visitors enjoy to explore this dramatic landscape. Volcanologists are dreaming of solving the intriguing puzzles that lie and evolve underneath its surface. Tim I. Purdy introduces Lassen Volcanic as follows [1]:

Lassen Volcanic National Park is a geological wonderland—home to four different types of volcanoes. While Lassen Peak is the most prominent feature in the park, it just happens to be the world's largest plug dome volcano. The peak gained notoriety with its eruptions of 1914-15.

The four different volcano types—to which Purdy refers—have to be the same types illustrated in the Kohm Yah-mah-nee Visitor Center and mentioned in the 20-minute park film The Story Behind the Landscape shown there and at the Loomis Museum auditorium: plug dome (like Lassen Peak), cinder cone, shield volcano and composite volcano (or strata dome or stratovolcano). These are the four main kinds into which geologists group volcanoes [2]. But there are other terms as well as type variations including cryptodome, mud volcano, supervolcano, submarine volcano and subglacial volcano.

A plug dome, generally named lava dome or volcanic dome, forms and evolves by the flow of highly viscous lava, like dacite, through its vent or veins. Typically the lava does not flow very far—otherwise it would build a gently sloping shield volcano. The viscous mass piles up, fills the volcano's crater, is cooling and solidifying around the rim or at the cone top, and grows a plugging dome by expansion from within the volcanic vent—hence its name. According to the park brochure, this is how Lassen Peak originated, since it started to take shape about 27,000 years ago as a volcanic vent on the northern flank of ancestral Brokeoff Volcano. The latter was a big composite volcano, which built up 400,000 to 600,000 years ago through countless eruptions until it broke down due to hydrothermal activity and weathering. 

Keywords: geology, volcanology, volcano classification, lava flow.

References and more to explore
[1] Tim I. Purdy: Lassen Volcanic. Lahontan Images, Susanville, California, 2009: Introduction.
[2] USGS: Principal Types of Volcanoes [].

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