The volcanoes of northeastern California—Lassen Peak, Mount Shasta, and Medicine Lake
volcano—along with the rest of the Cascade Mountains of the Pacific Northwest, are the
result of subduction of the Juan de Fuca plate beneath the North American plate (Figure
6.23). Mount St. Helens, which erupted explosively on May 18, 1980, is the most famous
and currently the most active of the Cascades volcanoes.
Figure 6.23: The Cascade Mountains of the Pacific Northwest are formed by the subduction
of the Juan de Fuca plate beneath the North American plate. The Juan de Fuca plate forms
near the shoreline at the Juan de Fuca ridge. ( 5 )
Sometimes the magma does not rise all the way through the continental crust beneath a
volcanic arc. This usually happens if the magma is rich in silica. These viscous magmas
form large areas of intrusive igneous rock, called batholiths, which may someday be uplifted
to form a mountain range. The Sierra Nevada batholith cooled beneath a volcanic arc