last 10 million years and might be buoyed by a mass of hot rock in the
upper mantle.
In the Pacific Northwest of the United States and British Columbia, the
Juan de Fuca plate dove into the Cascadia subduction zone located beneath
the continent. As the 50-mile-thick crustal plate subducted into the mantle,
Earth’s interior heat melted parts of the descending plate and the adjacent
lithospheric plate, forming pockets of magma. The magma rose toward the
surface, forming the volcanoes of the Cascade Range (Fig. 185), which
erupted in one great profusion after another.
India and the rocks that comprise the Himalayas broke away from
Gondwana early in the Cretaceous, sped across the ancestral Indian Ocean,
and slammed into southern Asia about 45 million years ago. As the Indian
and Asian plates collided, the oceanic lithosphere between them thrust
under Tibet, destroying 6,000 miles of subducting plate.The increased buoy-
Figure 184The San
Andreas Fault in Choice
Valley, San Luis Obispo
County, California.
(Photo by R. E.Wallace,
courtesy USGS)TERTIARY MAMMALS