for instance, has opened up in the last
quarter of a billion years at an average
rate of the growth of a human finger-
nail. This rearrangement is enough to
cause major deformations of the crust
in the zones of seafloor spread and
plate collisions.
Plate boundaries are where “the
action is.” They are the sites of large
volcanoes, earthquakes, and intense
crustal deformation. The boundaries
are of various types: (1) Divergent
boundaries are associated with sea-
floor spreading and are the locations
of numerous shallow earthquakes
because of the outpouring of lava to
make the basaltic rocks of the sea-
floors. Divergent boundaries have
constructed the midocean ridges. If
the divergence happens under a con-
tinent the surface expression is vol-
canoes along and in continental rift
valleys such as the East African Rift
Valley.(2)Ifthereisdivergencein
one area there must be convergence
in another. Some converging boundaries bring together oceanic (dense) and
continental (less dense) crust. The oceanic plate subducts (“slides”) under the
continental plate and the crustal rock melts as it reaches deeper than 100 km.
The subduction is thought to pull the entire plate along and keeps the divergent
boundaries active. Long, linear mountain ranges and oceanic trenches are found
inregionsof subduction. The subduction zones have a mixture of shallow and
deep earthquakes caused by rearrangement of material around the margins of the
plate. These materials can become the intrusive and extrusive forms ofvulcanism.
Perhaps the grandest example of subduction is the Andes of South America.
Studded with many huge volcanoes, this 200-km-wide range extends the entire
7,000 km length of western South America over the collision zone of the Pacific
and South American plates. They are paralleled by the Peru-Chile Trench offshore
in the Pacific Ocean. (3) Another sort of convergence brings together oceanic
plates underneath the oceans. The crustal expression is a deep trench with a paral-
leling island arc. A prominent example of this is the Mariana Trench and the258 Plate Tectonics
Many notable landforms result from plate
tectonics. This view across Lake Geneva from
southern Switzerland shows the titanic barrier of
the French Alps, which top out at well over 3,000 m
above sea level and were formed by the African
plate colliding with the Eurasian plate. (Mihai-
Bogdan Lazar/Dreamstime.com)