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.
There are 3 types of plate boundaries-convergent, divergent, and transform. Convergent boundaries occur when 2 plates collide with each other and push each other. Neither plate will give way, so the crust on the plates rise up and form mountains and mountain ranges. Divergent boundaries form rift valleys, volcanoes, and deep-ocean trenches, and transform boundaries cause earthquakes. Plates slide, collide and shift.