What happened to the top?
San Francisco Peaks are the remnants of the only stratovolcano in the San Francisco volcanic field. For decades, volcanologists suggested that the mountain now called San Francisco Peaks had simply worn away over time, eroded bit by bit to form its current bowl-shaped top. Then, in 1980, the catastrophic explosion of Mount St. Helens forced us to rethink our ideas about volcanoes.
You see, stratovolcanoes are known for their powerful explosive eruptions, but they usually force their way upwards, producing a gaping crater at the top. Mount St. Helens on the other hand, blasted sideways, leaving a bowl-shaped amphitheater where a nearly symmetrical mountain top once stood.
Take a look at the two photographs below. On the left side is Mount St. Helens before the 1980 eruption; on the right is a photograph taken after the eruption.
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Mount St. Helens before the 1980 eruption.
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Mount St. Helens after the 1980 eruption
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Compare the photographs above with the two below. On the left side you'll see post-1980 eruption Mount St. Helens again; on the right are the San Francisco Peaks with a line showing the pre-1980 eruption outline of Mount St. Helens.
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Mount St. Helens after the 1980 eruption
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San Francisco Peaks with Mount St. Helens outline superimposed.
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Notice any resemblance? Well, so did several volcanologists! Many volcanologists now feel that the scooped-out shape of the San Francisco Peaks may be the result of a catastrophic sideways blast like that of Mount St. Helens.
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