Introduction

The Earth is composed of several plates that move in various directions on the lithosphere (Figure 1). When these plates collide with one another it is called convergence. Convergent boundaries are regions that have maximum shortening and have the highest shear motion on the surface, which produces large earthquakes. When there is convergence of two plates a process called subduction occurs. There are four types of convergence between plates: oceanic-oceanic, oceanic-continental, oceanic-continental along island arcs, and continental-continental.

1. Subduction process

Oceanic-Oceanic
When two oceanic plates collide (Figure 2), the younger of the two plates will ride over the edge of the older plate. The younger plate is less dense than the old, cold one.
The older, heavier plate bends and plunges steeply through the athenosphere. As it is descending into the earth it heats up due to the friction and the oceanic plate starts to melt forming magma. This magma is now less dense then the surrounding rock and it pushes its way up through the crust and forms a chain of volcanoes. These generally volcanoes form in a semi-circular pattern called an island arc. An example of an island arc is the Aleutian islands of Alaska.
Subductions of plates cause oceanic trenches. When two oceanic plates collide they form a trench in the area where one plate is being subducted. The Marianas Trench is an example of an oceanic trench. It plunges deeper into the Earth's interior (nearly 11,000 m) than Mount Everest. Volcanic island arcs closely parallel trenches. 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.

Oceanic-Continental
When an oceanic plate converges with a continental plate (Figure 3), the oceanic plates is pushed beneath the continental plate. Oceanic plates are denser than continental plates, so the continental plate rides above. As the oceanic plate moves down into the Asthenosphere it heats up and begins to melt. This magma begins to push its way up to the surface and form volcanoes called a Volcanic Arc.
The continental crust that rides over the subduction area experiences deformation. The subducted oceanic crust compresses and pushes up on the continental crust. This process folds the rock producing mountains. An example of an oceanic-continental convergence and subduction is the Appalachian Mountains along the eastern North America.

Oceanic-Continental along Island Arcs
When two oceanic plates collide, they form a chain of islands called island arcs. These islands continue to move with the plate they lie on. Over time these oceanic plates they lie on will come in contact with either another oceanic plate or a continental plate. When they become in contact with a continental plate and subduction starts to occur they do not go down with the oceanic plate they lie on. They are less dense that the oceanic plates so they attach to the continent they collided with. This is very common along the western coast of North America. Most of the terrain is foreign, meaning it was not initially there. It was once part of an island arc that is now cemented to the North America plate. This process is called continental accretion.

Continental-Continental
In continental-continental collisions (Figure 4), neither plate subducts because the continental rocks are relatively light and resist downward motion. Instead, the crust tends to buckle and be pushed upward or sideways. The convergent zone becomes a site of intense mountain building (thrust or reverse faults, folds, metamorphism, etc.). Currently, this can be seen in the Himalayas where India is crashing into Asia.

Subduction Zones
Subduction zones are associated with the deepest earthquakes on the planet. Nearly 80% of the total seismic energy released is in subduction zones. Earthquakes are generally restricted to the shallow, brittle parts of the crust, generally at depths of less than 20 km. However, in subduction zones, earthquakes occur at depths as great as 700 km. These earthquakes define inclined zones of seismicity known as Wadati-Benioff zones, which outline the descending lithosphere.

2. Examples of present subduction
An interesting subduction zone is along the Casades in the northwestern United States and Canada (Figure 5). Here the Juan de Fuca plate is being subducted under the North American plate. The Juan de Fuca plate is relatiely young only ten million years old and has a low density so it is concluded that this should be a very active area in terms of earthquakes. However, the opposite is occuring. There are no recent earthquakes.
The Juan de Fuca plate is moving in a northwestern direction into the North American plate. The subduction is at a steep angle and is forming a very large accretionary wedge due to this steep angle. However, the Casade Mountains serve as a backstop to the wedge. The backstop is composed of accreted terranes, which are basalts and volcanic rock.

The subducted plate is divided into three areas of movement occuring: locked, transition, and slipping (Figure 6). The area that is considered to be locked is below the accretionary wedge. This area is determined to be locked due to the low temperatures. The temperatures range between 100-150 degrees Celsius. This area is experiencing the most strain. The transition area is located below the backstop, this is between the locked and slipping areas. So it experiences both extremes. Further down the subduction is where slipping is occuring. This is the area of intense heat, which might explain why it slips easier than the area further up subduction. This intense heat begins to melt the plate and that is why you see a volcanic arc located above. An earthquake would occur when the tension in the locked area when it has surpassed its limit.
Cores have been taken from the Casadian basin and a regular turbiditic cycle was found every 700 years. It is believed that these cycles are caused by earthquakes. There have also been effects on land in this region that indicate earthquakes have in fact occurred in this subduction zone and from cycles it looks like we are in store for another earthquake in this region.

Another area of interest that has increased the effort to understand subduction and possible zones of hazard is the recent Sumatra Earthquake/Tsunami. The earthquake was recorded to be a magnitude 9.3. The devastating megathrust earthquake of December 26th, 2004 occurred on the interface of the India and Burma plates and was caused by the release of stresses that develop as the India plate subducts beneath the overriding Burma plate. The India plate begins its descent into the mantle at the Sunda trench which lies to the west of the earthquake's epicenter.
One indicator of a large earthquake is the rupture area. The Sumatra Earthquake’s rupture area was 1200 km by 100 km, which is about the the same area of the Gulf side of Florida. When an area experiences an earthquake along a subduction zone the stresses that had been built up are released. However, when there is movement along an area of a subduction zone the rest of the zone must react. So aftershocks occur along the subduction zone to try to equilibrate the zone again. Aftershocks tend to propogate away from the epicenter.

3. An example of past subduction

There is a schematic cross-section through the island of Groix (Figure 7) that separates the island into two units. The first unit is located on the western half of the island and it is an anticline. The main rocks of this area are composed of green shales and micaschistes with traces of graphite, indicating a weak metamorphism. The second unit is located on the eastern half of the island, and this unit is superior to the previous. The presence of blue shales and micaschistes with garnets indicates a more elevated metamorphism. One wonders why the unit the with lower métamorphism is located in the anticline, and the unit the more métamorphism in the syncline.
Two hypotheses exist:
- The first is that the floor that was being subducted broke off and a piece went back up.
- The second is more of a tectonic phenomenon: the plate went back up on a rail with obduction. Even though it is not possible to opt for one or the other of these hypotheses on this day, we decide to describe the second more precisely. At the time of subduction the oceanic crust went under the Armorican microplate, a prism of accrétion formed itself in the front of this continent, by the accumulation of sediments coming from the erosion (Figure 8).
The subduction continues under this prism for a major part of the oceanic crust, but a part is detached and goes back up over it by the phenomenon of obduction (Figure 9).
While the continental sedimentation continues, the oceanic crust obductée is dismantled. The new prism is a mixture of continental materials and oceanic materials (Figure 10).
As the subduction continues, the oceanic floor disappears completely under the continent, and the two plates collide. The Aquitania microplate passes under the Armorica microplate, burying the prism of accrétion and undergoes intense temperature and pressure (Figure 11).
The continental crust is thickened by the overlaps and begins the formation of a mountain chain. This chain of mountain immediately starts to erode. The isostatic compensation drives to the vertical ascent of the mixture, bringing closer it of the surface (Figure 12). The rocks present in these outcrops have been formed 400 millions years ago.

Conclusions

The subduction is a complex phenomenon present in the different ages of the Earth. Different types of subduction exist. It is possible to describe from the present observations of the past subductions.