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The Earth's tectonic plates courtesy of the U.S. Geological Survey

Collission or convergent boundaries. Where plates collide. Plate edges may be either oceanic crust or continental crust. So when plates collide, we have only three possibilities: oceanic-oceanic, oceanic-continental, or continental-continental collisions. If oceanic crust collide with continental crust, the denser oceanic crust is subducted under the less dense continental crust (as at the Ring of Fire). If continental crust collide with continental crust they push each other up in a mountain range (like the Himalayas).

When two oceanic tectonic plates converge, one is subducted or pulled under, the other. This convergent activity allows for magma from the earth's core to erupt at the point of contact. The cooled erupted lava and debris build up over time. When the pile of debris and lava build up high enough, land is formed above sea level. This land is called an island. Often, these island volcanoes stay active, continuously growing.

The Ring of Fire, also known as the Circum-Pacific Belt, is a string of islands with active volcanoes in the Pacific Ocean. This geographical region is the most active volcanic and seismic zone in the world, and corresponds with oceanic-continental convergence of multiple tectonic plates.

The short answer is that earthquakes are caused by faulting, a sudden lateral or vertical movement of rock along a rupture (break) surface.     Here's the longer answer: The surface of the Earth is in continuous slow motion. This is plate tectonics--the motion of immense rigid plates at the surface of the Earth in response to flow of rock within the Earth. The plates cover the entire surface of the globe. Since they are all moving they rub against each other in some places (like the San Andreas Fault in California), sink beneath each other in others (like the Peru-Chile Trench along the western border of South America), or spread apart from each other (like the Mid-Atlantic Ridge). At such places the motion isn't smooth--the plates are stuck together at the edges but the rest of each plate is continuing to move, so the rocks along the edges are distorted (what we call "strain"). As the motion continues, the strain builds up to the point where the rock cannot withstand any more bending. With a lurch, the rock breaks and the two sides move. An earthquake is the shaking that radiates out from the breaking rock.

Tsunamis race across the sea at up to 500 miles (805 kilometers) an hour

A tsunami is a series of ocean waves that sends surges of water, sometimes reaching heights of over 100 feet (30.5 meters), onto land. These walls of water can cause widespread destruction when they crash ashore.These awe-inspiring waves are typically caused by large, undersea earthquakes at tectonic plate boundaries. When the ocean floor at a plate boundary rises or falls suddenly it displaces the water above it and launches the rolling waves that will become a tsunami.

PARIS - THE magnitude 7.0 earthquake that struck Haiti in January, killing a quarter of a million people, also unleashed a string of tsunamis on the country's western coast, scientists reported on Sunday. Several waves measuring up to 75cm were caused by a combination of earth movement and coastal landslides. They included banks of sediment on the sea bed, accumulated at river deltas, which were displaced by the shock and unleashed the waves as they moved, the study says. Waves were reported west, north and south of the epicentre, which was within a few kilometres of the surface on the Enriquillo-Plaintain Garden fault, on the boundaries of two microplates in the Caribbean. Such 'slide-generated' tsunami are rare, but their risk is underestimated, according to the paper, published online in the journal Nature Geoscience. 'Even modest, predominantly strike-slip earthquakes can cause potentially catastrophic slide-generated tsunami,' it says. A strike-slip earthquake occurs when one side of the fault moves along horizontally and in parallel to the other side of the fault, rather than down or up. Vertical displacement, especially of the seabed, is best known for creating tsunamis. The study was led by a team led by Matthew Hornbach of the University of Texas at Austin. The January 12 quake was in tectonic terms a surprisingly complex affair, according to new research. Two-thirds of the movement was strike-slip, and a third was a thrust, or upward, movement. The quake inflicted huge damage to the capital, Port-au-Prince, injuring 300,000 people and leaving 1.5 million homeless. -- AFP

An earthquake is what happens when two blocks of the earth suddenly slip past one another. The surface where they slip is called the fault or fault plane. The location below the earth’s surface where the earthquake starts is called the hypocenter, and the location directly above it on the surface of the earth is called

the epicente

How are earthquakes recorded? Earthquakes are recorded by instruments called seismographs. The recording they make is called a seismogram. (figure 4) The seismograph has a base that sets firmly in the ground, and a heavy weight that hangs free. When an earthquake causes the ground to shake, the base of the seismograph shakes too, but the hanging weight does not. Instead the spring or string that it is hanging from absorbs all the movement. The difference in position between the shaking part of the seismograph and the motionless part is what is recorded.

Causes of earthquakes If seen broadly we can say that earthquakes are caused due to two major reasons. The first reason is the eruption of volcanoes, which are sudden, and as is known volcanoes are seat of inner disturbance and can effect the plates which is the second cause of earthquakes. Earthquakes are caused due to disturbance in the movement of plates, which again can be caused due to various reasons like under crust waves or cracks in the plates. Plate Tectonic Theory The outer layer of the earth is divided into many sections known as plates, which are floating on the molten magma beneath the earth’s crust. Now the movement of these plates is determined by the convection current in the molten magma. The heat makes these plates rise and vice versa. Therefore after intervals there are plates that get submerged in the molten magma and there plates that rise upwards and at times even new crust is formed from the molten magma which in turn forms a new plate until it connects itself with the already existing ones. At times these plates and can be pushed up to form mountains and hills and the movement is so slow that it is really hard to comprehend that there is any movement at all. The movement and the results come out to be visible suddenly. Now these plates are the bases on which the continents stand and when these plates move the continents also move. Most of the earthquakes occur on the edges of the plates where a plate is under one or across. This movement disrupts the balance and position of all plates, which leads to tremors, which are called earthquakes.