Origins
The still largely unexplained tectonic causes of the volcanism that has produced the Silverthrone Caldera are a matter of ongoing research. Silverthrone is not above a hotspot as are Nazko or Hawaii. However, it may be a product of the Cascadia subduction zone because andesite, basaltic andesite, dacite and rhyolite can be found at the volcano and elsewhere along the subduction zone. At issue are the current plate configuration and rate of subduction but Silverthrone's chemistry indicates that Silverthrone is subduction related.
The Cascadia subduction zone is a long convergent plate boundary that separates the Juan de Fuca, Explorer, Gorda and North American Plates. Here, the oceanic crust of the Pacific Ocean sinks beneath North America at a rate of 40 millimetres (1.6 in) per year. Hot magma upwelling above the descending oceanic plate creates volcanoes, each of which erupts for a few million years. It is estimated that the subduction zone has existed for at least 37 million years; in that time it has created a line of volcanoes, called the Cascade Volcanic Arc, that stretches over 1,000 kilometres (620 mi) along the subduction zone from Northern California to Vancouver Island. Several volcanoes in the arc are potentially active. All of the known historic eruptions in the arc have been in the United States. Two of the most recent were Lassen Peak in 1914 to 1921 and the major eruption of Mount St. Helens in 1980. It is also the site of Canada's most recent major eruption, about 2,350 years ago at the Mount Meager volcanic complex.
Read more about this topic: Silverthrone Caldera, Geology
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