Convergent Boundaries
The size of the Earth has not changed significantly during the past 600 million years, and
very likely not since shortly after its
formation 4.6 billion years ago. The Earth's unchanging size implies that the crust must
be destroyed at about the same rate as it
is being created, as Harry Hess surmised. Such destruction (recycling) of crust takes
place along convergent boundaries where
plates are moving toward each other, and sometimes one plate sinks (is subducted) under
another. The location where sinking
of a plate occurs is called a subduction zone.
The type of convergence -- called by some a very slow "collision" -- that takes
place between plates depends on the kind of
lithosphere involved. Convergence can occur between an oceanic and a largely continental
plate, or between two largely
oceanic plates, or between two largely continental plates.
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.
On 9 June 1994, a magnitude-8.3 earthquake struck about 320 km northeast of La Paz,
Bolivia, at a depth of 636 km. This
earthquake, within the subduction zone between the Nazca Plate and the South American
Plate, was one of deepest and largest
subduction earthquakes recorded in South America. Fortunately, even though this powerful
earthquake was felt as far away as
Minnesota and Toronto, Canada, it caused no major damage because of its great depth.
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.