To the left, I state a conventional definition of the derivative of the function in question. When x is negative as it is on the second line of the derivation, the numerator (|x + dx| - |x|) is -dx because dx is always a positive number. Thus the derivative is -1. On the third line when x is positive, this simplifies to +1.
Plate subduction occurs when two tectonic plates converge and at least one of them is an oceanic plate. Oceanic plates are formed at divergent plate boundaries from dense basaltic magma. This magma originates from the mantle beneath and fills the gap between the spreading plates.
Key points. 1  Subduction is a geological process in which the edge of a lithospheric plate slides underneath the edge of an adjacent plate. 2  Subduction is one of the two major processes of plate tectonics, the other being seafloor spreading.ey points. 1  Subduction is a geological process in which the edge of a lithospheric plate slides underneath the edge of an adjacent plate. 2  Subduction is one of the two major processes of plate tectonics, the other being seafloor spreading.