How do subduction zones initiate?

Question

I have often wondered how subduction zones form. How does the transition from a quiet ocean floor to an active vulcanic arc (ocean-ocean subduction) occur? Or the transition from a passive continental margin to Andean type subduction zone (ocean-continent subduction). In either case one plate needs to divide into two plates and some kind of decoupling is required. How does this happen? What is the driving mechanism? And can this transitional proces be observed at present?

Kenni, Denmark

Answer 1

Subduction initiates when a plaque supporting an ocean collides with a plane supporting a les important charge. then the paque supporting the ocean dives in the mantel

There is a good explanation on my link

Answer 2

I think the processes that start a subduction zone are the same ones that keep it going. There are large convection cells that appear to carry the crust along. And at spreading centers the upwelling that forms new crust creates a bit of a topographic high (a.k.a. mid-ocean ridge), so I'm sure gravity plays a role in plate motion, too, especially for oceanic plates.

As to what would cause decoupling of a passive margin to form a subduction zone, the simplest answer would be that the forces acting on the oceanic crustal segment become strong enough relative to the forces acting on the rest of the plate to break the segment. I'm sure some deformation occurs as well as breakage. And, since Mother Nature's heterogeneity tends to lead to imprecision, this breakage may not always occur at an exact boundary between oceanic and continental crust. Consequently, there is quite a bit of oceanic crust exposed on the western coast of the United States. I've seen it in California as far inland as the Sierra Nevadas and in Alaska, and assume it is exposed elsewhere.

It’s been suggested that the Puerto Rican Trench may be currently extending itself to form a subduction zone than could eventually extend along the entire eastern edge of North America and South America. The current trench seems to have started at a transform margin, where plate segments are moving past each other. A bit of transpression as the segments on either size of the transform are pressed toward each other (perhaps by motion of other plates), a bit of up-and-down, and we have ourselves a subduction zone.

Another sort of dividing of a plate is happening in Baja California, in the Gulf of Cortez (a.k.a. the Gulf of California). A spreading center is docking with the North American plate, and the combination of spreading and transform motion has led to the opening of the Gulf as well as the formation of the San Andreas Fault.

Africa is being pulled apart along the East Africa Rift. The rifting there seems to have been preceded by the eruption of flood basalts and is a result of the formation of a new spreading center. Such new spreading centers don’t always succeed in tearing continents apart; the Mississippi Vallley appears to be a failed rift system.

There seems to be enough variability in the composition of the crust and in the dynamics of the convection to allow just about any sort of plate change we can imagine.

Answer 3

Lime stoneses-CaCO3, usually removed from water by a biological process (e.g. corals and .... Convergent Boundaries: Subduction Zones: high pressure/low temperature metamorphism ... Here partial melting is initiated at about 700 degrees Centigrade .

Answer 4

conventional currents are the main source of the plate momnets in the earth crust. The plates are moving on the astenospher which was in the molten state where the conventional currents will occur.