Chapter 8 Plate Tectonics

200 CHAPTER 8: PLATE TECTONICS
sphere resists being drawn into Earth in the process of subduction.
This lighter material forms intensely folded and
faulted mountains near the subduction zones. Felsic rocks
drawn into Earth often melt and come back to the surface in
volcanic eruptions. Due to the water in these continental
rocks changing to steam, volcanic eruptions of felsic rocks are
often violent and explosive. Both the highest mountains and
the deepest parts of the oceans occur near zones of subduction
and both of them are related to the process of subduction.
Plate Tectonics
You may recall that one of the most important objections to
Wegener’s idea of continental drift was that there was no
mechanism to explain how continents could move through the
ocean basins. Seafloor spreading provided an explanation for
the motion of Earth’s lithosphere. When the discoveries
of seafloor spreading were added to Wegener’s ideas about continental
drift, geologists recognized a larger theory of plate
tectonics. (The word tectonics means large-scale motions of
Earth’s crust.) Tectonic forces are responsible for uplift and
mountain building. (See Figure 8-5.) In 1965, Canadian geologist
J.Tuzo Wilson proposed that the whole Earth is covered by
about a dozen rigid sections called lithospheric plates. The
plates include the crust as well as the rigid upper mantle.
Figure 8-5 The San Rafael
Swell in Utah is the result
of tectonic forces associated
with the movement of
Earth’s plates pushing up
these formerly flat-lying rock
layers. Note the large truck
for scale.