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Arc
Trench
Ridge
Loss of
Warm
low-density
Cold
materials

Pressure
400 km
transformations
of minerals
to denser forms

670 km

?
?
?
FIGURE 18.20 Slab-pull. The dense, leading edge of a subducting plate pulls the rest of the plate along. Plate density increases due to
cooling, loss of low-density material, and pressure transformation of minerals to denser forms.

SUMMARY

Earth has a layered interior that formed as Earth’s materials under- The shapes of the continents suggested to some that the conti-
went differentiation, the separation of materials while in the molten nents were together at one time and have shifted positions to their
state. The center part, or core, is predominantly iron with a solid present locations. This idea, first developed in the early 1900s, came to
inner part and a liquid outer part. The core makes up about 15 per- be known as continental drift and was generally dismissed by most sci-
cent of Earth’s total volume and about one-third of its total mass. The entists. Evidence from the ocean floor that was gathered in the 1950s
mantle is the middle part of Earth’s interior that accounts for about and 1960s revived interest in the idea that continents could move.
two-thirds of Earth’s total mass and about 80 percent of its total vol- The evidence for seafloor spreading came from related observations
ume. The mantle is predominantly composed of the ferromagnesian concerning oceanic ridge systems, sediment and fossil dating of
silicate olivine, which undergoes structural changes at two depths materials outward from the ridge, and magnetic patterns of seafloor
from the increasing heat and pressure. The outer layer, or crust, of rocks. Confirmation of seafloor spreading led to the plate tectonic
Earth is separated from the mantle by the Mohorovicic discontinuity. theory. According to plate tectonics, new basaltic crust is added at
The crust of the continents is composed mostly of less dense, granite- diverging boundaries of plates, and old crust is subducted at converging
type rock. The crust of the ocean basins is composed mostly of the boundaries. Mountain building, volcanoes, and earthquakes are seen
denser basaltic rocks. as related geologic features that are caused by plate movements. The
Another way to consider Earth’s interior structure is to consider force behind the movement of plates is uncertain, but it may involve
the weak layer in the upper mantle, the asthenosphere that extends convection in the deep mantle.
around the entire Earth. The rigid, solid, and brittle layer above the
asthenosphere is called the lithosphere. The lithosphere includes the
entire crust, the Moho, and the upper part of the mantle.

18-17 CHAPTER 18 Plate Tectonics 471