Subduction is the sinking of the edge of one plate under the edge of another.
The term subduction has its etymological origin in the Latin word subductio , which refers to "to lift." The notion of subduction is used in the field of geology to refer to what happens when the edge of a plate slides beneath the edge of another .
Tectonic plates
Before moving forward, it is important to know what plates are in geology. This is the name given to the parts of the lithosphere (the outer crust of the Earth) that are semi-rigid and float on the Earth's mantle (the layer located between the central core and the crust). The collision zones of these plates produce tectonic , seismic and volcanic activity .
Sinking phenomenon
Returning to the idea of subduction, it is the sinking of the edge of a plate, which is situated beneath the edge of another. This phenomenon develops in the so-called subduction zones , most of which are distributed throughout the Pacific Ocean . Subduction is the result of the action of two tectonic forces .
It is important to note that the displacement and contacts of the plates cause the accumulation of elastic potential energy . Seismic activity is concentrated on the margins where they converge, with earthquakes generated by subduction being frequent in Haiti , Chile and Japan , among other nations.
In addition to earthquakes , subduction produces magma through the partial melting of parts of the Earth's mantle. This magma can rise and lead to large volcanic eruptions .
Forces
Regarding the phenomenon of subduction, we can point out different related concepts, such as the forces that promote it. In this case, we have two: the thrust and the gravitational forces . The first is the one that most often causes subduction, and it occurs as a result of the activity of the oceanic ridges . The thrust of the younger stones in these ridges generates a force that causes the oldest ones to collide against the crust of the continent, and the latter responds with an opposite force.
Regarding gravitational forces, on the other hand, which are those exerted by gravity on the plates, the difference between these due to the density of each tectonic plate is one of the main reasons for subduction. An opposing force to gravity is buoyancy , and the value of each is inversely proportional to that of the other: the greater one, the smaller the other.
In general terms, we can say that the density of the oceanic crust is usually higher than that of the continental crust, which is explained by its chemical composition: the former is composed of basic and ultrabasic rocks , such as dunites, basalts and gabbros, and weighs much more than the latter, whose composition includes acidic and intermediate rocks such as rhyolites, granites and andesites. The oceanic crust is made up of rocks with a great richness in chromium, magnesium, iron and other heavy elements; the continental crust, on the other hand, is made up of rocks with abundant potassium, aluminium and sodium.
An example of high subduction angles occurs between the Pacific and the Philippines.
Subduction angles
Another essential concept is the angle of subduction , which depends largely on the characteristics of the sinking crust and affects the surface . In short, we can say that the angle of subduction varies according to the region and can range from 10° to 90° .
Those with the highest values are called high angles and can be observed, for example, in the subduction between the Pacific and Philippine plates. The others are grouped under the name of low angles and are caused by the protuberances of the sinking plate; we can notice this phenomenon in the subduction between the Nazca and South American plates.