http://volcanoes.usgs.gov/volcanoes/mount_shasta/ every 600 years or so Mount Shasta Mt. Shasta volcanoes.usgs.gov Shasta Shasta Shasta suncups, USGS/Cascades VolcanoObservatory Mount Shasta Mount Shasta Mount Shasta Mount Shasta at least 70 times every 600 years or so at least 70 times California Volcano Observatory-Mount Shasta Mount Shasta