(source: USGS: http://pubs.usgs.gov/publications/text/understanding.html)
Scientists now have a fairly good understanding of how the plates move
and how such movements relate to earthquake activity. Most movement occurs
along narrow zones between plates where the results of plate-tectonic forces
are most evident. There are three well understood types of plate boundaries:
Divergent boundaries -- where new crust is generated as the plates pull
away from each other.
Convergent boundaries -- where crust is destroyed as one plate dives under
Transform boundaries -- where crust is neither produced nor destroyed as
the plates slide horizontally past each other.
Divergent boundaries occur along spreading centers where
plates are moving apart and new crust is created by magma pushing up from
the mantle. Picture two giant conveyor belts, facing each other but slowly
moving in opposite directions as they transport newly formed oceanic crust
away from the ridge crest.
Perhaps the best known of the divergent boundaries
is the Mid-Atlantic Ridge. This submerged mountain range, which extends
from the Arctic Ocean to beyond the southern tip of Africa, is but one
segment of the global mid-ocean ridge system that encircles the Earth.
The rate of spreading along the Mid-Atlantic Ridge averages about 2.5 centimeters
per year (cm/yr), or 25 km in a million years. This rate may seem slow
by human standards, but because this process has been going on for millions
of years, it has resulted in plate movement of thousands of kilometers.
Seafloor spreading over the past 100 to 200 million years has caused the
Atlantic Ocean to grow from a tiny inlet of water between the continents
of Europe, Africa, and the Americas into the vast ocean that exists today.
The volcanic country of Iceland, which straddles
the Mid-Atlantic Ridge, offers scientists a natural laboratory for studying
on land the processes also occurring along the submerged parts of a spreading
ridge. Iceland is splitting along the spreading center between the North
American and Eurasian Plates, as North America moves westward relative
Map showing the Mid-Atlantic Ridge splitting Iceland and separating
the North American and Eurasian Plates. The map also shows Reykjavik, the
capital of Iceland, the Thingvellir area, and the locations of some of
Iceland's active volcanoes (red triangles), including Krafla.
The consequences of plate movement are easy to see around Krafla Volcano,
in the northeastern part of Iceland. Here, existing ground cracks have
widened and new ones appear every few months. From 1975 to 1984, numerous
episodes of rifting (surface cracking) took place along the Krafla
fissure zone. Some of these rifting events were accompanied by volcanic
activity; the ground would gradually rise 1-2 m before abruptly dropping,
signalling an impending eruption. Between 1975 and 1984, the displacements
caused by rifting totalled about 7 m.
In East Africa, spreading processes have already
torn Saudi Arabia away from the rest of the African continent, forming
the Red Sea. The actively splitting African Plate and the Arabian Plate
meet in what geologists call a triple junction, where the Red Sea
meets the Gulf of Aden. A new spreading center may be developing under
Africa along the East African Rift Zone. When the continental crust stretches
beyond its limits, tension cracks begin to appear on the Earth's surface.
Magma rises and squeezes through the widening cracks, sometimes to erupt
and form volcanoes. The rising magma, whether or not it erupts, puts more
pressure on the crust to produce additional fractures and, ultimately,
the rift zone.
East Africa may be the site of the Earth's next
major ocean. Plate interactions in the region provide scientists an opportunity
to study first hand how the Atlantic may have begun to form about 200 million
years ago. Geologists believe that, if spreading continues, the three plates
that meet at the edge of the present-day African continent will separate
completely, allowing the Indian Ocean to flood the area and making the
easternmost corner of Africa (the Horn of Africa) a large island.
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
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?
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
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
The zone between two plates sliding horizontally past one another is called
a transform-fault boundary, or simply a transform boundary. The
concept of transform faults originated with Canadian geophysicist J. Tuzo
Wilson, who proposed that these large faults or fracture zones connect
two spreading centers (divergent plate boundaries) or, less commonly, trenches
(convergent plate boundaries). Most transform faults are found on the ocean
floor. They commonly offset the active spreading ridges, producing zig-zag
plate margins, and are generally defined by shallow earthquakes. However,
a few occur on land, for example the San Andreas fault zone in California.
This transform fault connects the East Pacific Rise, a divergent boundary
to the south, with the South Gorda -- Juan de Fuca -- Explorer Ridge, another
divergent boundary to the north.
The Blanco, Mendocino, Murray, and Molokai fracture zones are some
of the many fracture zones (transform faults) that scar the ocean floor
and offset ridges (see text). The San Andreas is one of the few transform
faults exposed on land.
The San Andreas fault zone, which is about 1,300 km long and in places
tens of kilometers wide, slices through two thirds of the length of California.
Along it, the Pacific Plate has been grinding horizontally past the North
American Plate for 10 million years, at an average rate of about 5 cm/yr.
Land on the west side of the fault zone (on the Pacific Plate) is moving
in a northwesterly direction relative to the land on the east side of the
fault zone (on the North American Plate).
Oceanic fracture zones are ocean-floor valleys that
horizontally offset spreading ridges; some of these zones are hundreds
to thousands of kilometers long and as much as 8 km deep. Examples of these
large scars include the Clarion, Molokai, and Pioneer fracture zones in
the Northeast Pacific off the coast of California and Mexico. These zones
are presently inactive, but the offsets of the patterns of magnetic striping
provide evidence of their previous transform-fault activity.
See also http://www.wiley.com/college/strahler/sc/strach11.html