(understanding this reason of an earthquake)

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


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





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 Mid- Atlantic Ridge

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: showing the plate boundaries,

and Historically Active Volcanoes

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



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


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 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).

San Andreas Fault

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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