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Eurasian Plate
North
American
Juan De Fuca Plate Plate
Caribbean
Arabian San Andreas Plate
Plate Fault
Philippine Plate
Mid
Pacific Plate
Rise Andes
Indian-Australian Atlantic
Plate Nazca Plate South
Pacific American
African Mountains Plate Ridge
Plate
East

Antarctic Plate
Antarctic Plate

FIGURE 18.13 The major plates of the lithosphere that move on the asthenosphere. Source: After W. Hamilton, U.S. Geological Survey.

Sea level
Magma in fissures
Coastal
Ocean
mountains
Ocean and
volcanoes

Oceanic crust Magma Lithosphere
100 Km Trench Continental
lithosphere
Asthenosphere
Oceanic Magma
lithosphere
FIGURE 18.14 A diverging boundary at a mid-oceanic ridge. Shallow Deep-seated
Hot asthenosphere wells upward beneath the ridge crest. Magma earthquakes earthquakes
forms and squirts into fissures. Solid material that does not melt
remains as mantle in the lower part of the lithosphere. As the FIGURE 18.15 Ocean-continent plate convergence. This type
lithosphere moves away from the spreading axis, it cools, becomes of plate boundary accounts for shallow and deep-seated earthquakes,
denser, and sinks to a lower level. an oceanic trench, volcanic activity, and mountains along the coast.

As an example of ocean-continent plate convergence, consider volcanic mountains on the continent (the Andes Mountains).
the plate containing the South American continent (the South The trench is formed from the down-bending associated with
American Plate) and its convergent boundary with an oceanic subduction and the volcanic mountains from subducted and
plate (the Nazca Plate) along its western edge. Continent- oceanic melted crust that rise up through the overlying plate to the sur-
plate convergence produces a characteristic set of geologic face. The earthquakes are associated with the movement of the
features as the oceanic plate of denser basaltic material is sub- subducted crust under the overlying crust.
ducted beneath the less dense granite-type continental plate Ocean-ocean plate convergence produces another set of
( Figure 18.15). The subduction zone is marked by an oceanic characteristics and related geologic features (Figure 18.16). The
trench (the Peru-Chile Trench), deep-seated earthquakes, and northern boundary of the oceanic Pacific Plate, for example,

18-11 CHAPTER 18 Plate Tectonics 465