Geology Reference
In-Depth Information
As all the magma from beneath the continental crustal plate breaks through
the surface, a chain of volcanoes is created along the surface, parallel to the edge
of the subducting oceanic plate. This feature is called a
continental volcanic arc.
The Cascade Mountain range along the West Coast of the United States is an ex-
ample of a volcanic arc created by subduction of an oceanic plate under a contin-
ental plate.
The friction of the plates against one another also causes earthquakes to oc-
cur near the surface where the plates meet; gradually they occur more deeply as
the oceanic plate is subducted beneath the continental plate.
Diving into the abyss
When two plates of oceanic crust move toward each other, they create an
oceanic con-
vergent boundary.
The force of their movement causes one of the plates to be driven be-
neath the other, down into the mantle rocks below. This interaction of oceanic plates is
another type of subduction zone, and the resulting features are in some ways similar to
those I describe for a continental-oceanic convergence.
The subduction of one oceanic plate beneath another oceanic plate creates friction,
melting the lithosphere and creating volcanoes. But in this case, the volcanoes erupt be-
neath the ocean rather than on a continent. Eventually, the volcanoes along the seafloor
may build up high enough to be visible above the surface of the sea, creating a
volcanic
island arc.
The islands of Japan are an example of a volcanic island arc.
A unique feature occurs along the subduction boundary of two oceanic
plates, due to their similar density. As the subducting plate moves downward into
the mantle, it pulls the overlying plate downward slightly, creating a deep
trench.
(Trenches also form along the edge of continental-oceanic plate subduction but
are usually not as deep.) This feature and other oceanic-oceanic plate subduction
zone characteristics are illustrated in Figure 9-5.
Reaching for the sky
