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Understanding plate motions
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
plain.
There are four 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 slipes under another.
- Transform boundaries -- where crust is neither produced nor destroyed as the plates slide horizontally past each other.
- Plate boundary zones -- broad belts in which boundaries are not well defined and the effects of plate interaction are
unclear
Divergent boundaries (two examples)
Divergent boundaries appear 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.)
1.) Perhaps the best known of the divergent boundaries is the Mid-Atlantic Ridge. This submerged mountain range, which
strechtes from the Arctic Ocean to beyond the southern tip of Africa, is one part 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 map is showing the Mid-Atlantic Ridge splitting Iceland
the North American and Eurasian Plates.The map also shows
and separating Reykjavik, the capital of Iceland, the Thingvellir
area and the locations of some of Iceland\'s active volcanoes
(red triangles), including Krafla.
2.) 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, if or not it erupts,
puts more pressure on the crust to produce additional fractures and, finally, 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 believes that, if spreading
continues, the three plates that meet at the edge of the present-day African continent will fall apart completely, allowing the
Indian Ocean to flood the area and making the eastern corner of Africa (which is also known as \" the Horn of Africa\") a large
island.
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 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.
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.
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.
Left: The collision between the Indian and Eurasian plates
has pushed up the Himalayas and the Tibetan Plateau.
Right: 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.
Transform boundaries
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 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.
Plate-boundary zones
Not all plate boundaries are as simple as the main types listed above. In some regions, the boundaries are not well
defined because the plate-movement deformation occurring there extends over a broad belt (called a plate-boundary zone).
One of these zones marks the Mediterranean-Alpine region between the Eurasian and African Plates, in which several
smaller fragments of plates (microplates) have been recognized. Because the plate-boundary zones involve at least two large
plates and one or more microplates caught up between them, they tend to have complicated geological structures and
earthquake patterns.
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