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Ring of Fire
The Volcanic Cascade Range makes up part of the Ring of Fire... which is
where some 80% of the world's earthquakes and volcanoes occur.
Large areas of the Pacific Plate are being subducted, which
cause bucking, cracking, and other deformations that shape the mountains of
the regions that make up the ring.
Oceanic-Continental Convergence
 If by magic we could pull a plug and drain the Pacific Ocean, we would
see a most amazing sight -- a number of long narrow, curving trenches
thousands of kilometers long and 8 to 10 km deep cutting into the ocean
floor. Trenches are the deepest parts of the ocean floor and are created by
subduction.
Off the coast of South America along the Peru-Chile trench, the oceanic
Nazca Plate is pushing into and being subducted under the continental part
of the South American Plate. In turn, the overriding South American Plate is
being lifted up, creating the towering Andes mountains, the backbone of the
continent. Strong, destructive earthquakes and the rapid uplift of mountain
ranges are common in this region. Even though the Nazca Plate as a whole is
sinking smoothly and continuously into the trench, the deepest part of the
subducting plate breaks into smaller pieces that become locked in place for
long periods of time before suddenly moving to generate large earthquakes.
Such earthquakes are often accompanied by uplift of the land by as much as a
few meters.
Oceanic-continental convergence also sustains many of the Earth's active
volcanoes, such as those in the Andes and the Cascade Range in the Pacific
Northwest. The eruptive activity is clearly associated with subduction, but
scientists vigorously debate the possible sources of magma: Is magma
generated by the partial melting of the subducted oceanic slab, or the
overlying continental lithosphere, or both?
Oceanic-oceanic convergence
 As with oceanic-continental convergence, when two oceanic plates
converge, one is usually subducted under the other, and in the process a
trench is formed. The Marianas Trench (paralleling the Mariana Islands), for
example, marks where the fast-moving Pacific Plate converges against the
slower moving Philippine Plate. The Challenger Deep, at the southern end of
the Marianas Trench, plunges deeper into the Earth's interior (nearly 11,000
m) than Mount Everest, the world's tallest mountain, rises above sea level
(about 8,854 m).
Subduction processes in oceanic-oceanic plate convergence also result in
the formation of volcanoes. Over millions of years, the erupted lava and
volcanic debris pile up on the ocean floor until a submarine volcano rises
above sea level to form an island volcano. Such volcanoes are typically
strung out in chains called island arcs. As the name implies,
volcanic island arcs, which closely parallel the trenches, are generally
curved. The trenches are the key to understanding how island arcs such as
the Marianas and the Aleutian Islands have formed and why they experience
numerous strong earthquakes. Magmas that form island arcs are produced by
the partial melting of the descending plate and/or the overlying oceanic
lithosphere. The descending plate also provides a source of stress as the
two plates interact, leading to frequent moderate to strong earthquakes.
Continental-continental convergence
 The
Himalayan mountain range dramatically demonstrates one of the most visible
and spectacular consequences of plate tectonics. When two continents meet
head-on, neither is subducted because the continental rocks are relatively
light and, like two colliding icebergs, resist downward motion. Instead, the
crust tends to buckle and be pushed upward or sideways. The collision of
India into Asia 50 million years ago caused the Eurasian Plate to crumple up
and override the Indian Plate. After the collision, the slow continuous
convergence of the two plates over millions of years pushed up the Himalayas
and the Tibetan Plateau to their present heights. Most of this growth
occurred during the past 10 million years. The Himalayas, towering as high
as 8,854 m above sea level, form the highest continental mountains in the
world. Moreover, the neighboring Tibetan Plateau, at an average elevation of
about 4,600 m, is higher than all the peaks in the Alps except for Mont
Blanc and Monte Rosa, and is well above the summits of most mountains in the
United States.
Above: The collision between the Indian and Eurasian plates has pushed
up the Himalayas and the Tibetan Plateau. Below: Cartoon cross sections
showing the meeting of these two plates before and after their collision.
The reference points (small squares) show the amount of uplift of an
imaginary point in the Earth's crust during this mountain-building process.
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