Tectonic activity is limited to sections of the Earth's surface long and relatively narrow: it is an example of the so-called Ring of Fire around the Pacific. For this reason, a rough identification of plate boundaries are he obtained according to the distribution of the epicenters of earthquakes and volcanic activity.
The seismic sections mark the boundaries of the plates. We can distinguish four types of seismic zones.
· The first type is represented by the axis of the mid-ocean ridges, where earthquakes are shallow (less than 70 km) and accompanied by volcanic activity and high heat flow. Along the axes of the ridges began the expansion of the ocean floor and the plates move away from each other.
· The second type of seismic zone is characterized by shallow earthquakes, volcanic activity but is absent. Examples are the San Andreas Fault in California and the Anatolian fault in northern Turkey. Along both of these fractures have been documented considerable horizontal displacements.
· The third type of seismic zone is closely connected with the ocean trenches and island arcs systems such as those that border the Pacific Ocean. In these areas may experience earthquakes surface (up to 70 km), intermediate (70 to 300 km) or deep (300 to 700 km). The depth of earthquakes increases with distance from the pit, following a hypothetical inclined plane, said plane of Benioff, highly active from seismic point of view.
· The fourth type of seismic zone is represented by the band, across Asia, stretching from Burma to the Mediterranean. It is a vast continental zone with diffuse seismicity where earthquakes, generally superficial, are associated with high mountain ranges clearly originate from the effects of compression.
Seismic zones define a mosaic of plates in motion relative to one another, separated by three types of margins:
Divergent margins, along which two plates move away from each other, leaving room for the ascent of magma from the mantle to form new oceanic crust, so I'm also known as constructive. The increase in margins, are delimited by ridges. The actual direction of the relative motion is not necessarily perpendicular to this line, although it often is.
Margins converging, along which two plates move toward each other, causing the subduction of one plate beneath the other, in the case in which at least one or both margins are constituted by oceanic lithosphere (in the first case is the oceanic plate to dive), or causing the collision of the two plates, in the case in which the facing edges are both constituted by continental lithosphere. The margins in consumption, also known as destructive because it absorbs lithosphere (only one of the two plates sinks), are marked by oceanic trenches or mountains recently. Even in this case, the direction of the relative motion is not necessarily perpendicular to this line, and in general it is not.
Margins transform, along which two plates slide horizontally relative to one another, without the formation or destruction of lithosphere, for this are said also conservative. Since the relative motion along the fault is exactly parallel to the direction of the fault itself, it follows that transform faults are the only lines that give us the direction of the relative motion between the plates.
Typically, a plaque is limited by a combination of margins belonging to these three categories. Any movement along one of the edges of a plate has the consequence readjustments along other margins. Where there are three plaques identify the so-called triple junctions. The margins of the continents may be located distant from the plate margins, or coincide with the plate boundaries. In the first case you may have rifted continental margins or turn in the second case you have active continental margins.
The rifted margins are characterized by absence of seismic and volcanic phenomena. Are typical of the continents that are located at the edges of the ocean basins expanding as the Atlantic Ocean. These margins mark the boundary between the continent and ocean of the same plate. Respect to the direction of motion of the plate, they are "in the rear", that is on back of the continent. For this reason they are tectonically inactive. The main phenomenon that occurs in these margins is the sedimentation of materials of continental origin, which accumulate undisturbed.
The continental margins correspond to abrupt turn discards or deviations in the initial split, at which will be implemented then the continental separation. These margins are typical of the oceans expanding and characterize the edge of the continent in the rear with respect to the direction of movement.
The active continental margins are subject mainly to efforts compression and are affected by endogenous phenomena. These margins are situated on the edge of the continents in the front position with respect to the direction of movement, and coincide with the boundary of the plate. It is tectonically active margins that characterize oceans in decline, such as the Pacific Ocean.