Chapter 8 Plate Tectonics

212 CHAPTER 8: PLATE TECTONICS
rocks from the South American side of the subduction zone
are drawn down.
You may recall from Chapter 5 that felsic minerals generally
melt at a lower temperature than do mafic minerals.
Therefore, as they absorb heat from their surroundings and
from friction caused by plate movements, felsic rocks are the
first to melt. The change to magma makes the rock fluid. Due
to felsic magma’s low density, it rises toward the surface. For
this reason, subduction zones are regions of volcanic activity.
Unlike the mafic volcanoes of Hawaii, which generate longlasting
streams of lava, felsic volcanoes tend to be more violent.
Felsic magma is likely to contain water, which expands
explosively when it reaches the surface. The 1980 eruption of
Mount St. Helens in Washington is a good example of the
eruption of less mafic lava.
The edges of converging plates may both be continental
crust. Because continental crust resists subduction, this type
of collision can produce a great mass of jumbled rock that
builds the world’s highest mountains. The Himalaya Mountains
of Asia are the result of a collision between the northmoving
Indian-Australian Plate and the giant plate that
contains most of Europe and Asia. Measurements conducted
in the Himalaya Mountains have shown uplift of several centimeters
per year.
Volcanoes are not restricted to land areas. Volcanoes are
common where the oceanic portion of one plate dives under
another ocean floor segment. Partial melting of the descending
plate may result in a curved line of volcanic islands
known an island arc. The Aleutian Islands, which extend to
the westward from Alaska, and the islands of Japan are island
arcs created at subduction zones.
TRANSFORM BOUNDARIES Some plates do not converge or diverge.
A transform boundary occurs when two plates slip
past each other without creating new lithosphere or destroying
old lithosphere. If you could straddle a transform fault for
a long enough time, you would see one foot heading in one
direction parallel to the fault and your other foot going in the
opposite direction. The San Andreas Fault in California is