[1] Earth's outer rigid shell, known mechanically as the lithosphere, is broken into approximately a dozen major tectonic plates that perpetually drift across the semi-fluid asthenosphere. The kinetic motion of these immense crustal slabs is primarily powered by deep mantle convection currents. Where two tectonic plates collide at a convergent boundary, the difference in physical density between the interacting plates dictates the geological outcome.
[2] Because oceanic crust is composed of dense basaltic rock whereas continental crust consists of comparatively buoyant granitic rock, the denser oceanic plate inevitably undergoes subduction. During subduction, gravitational slab pull forces the oceanic plate to descend beneath the continental margin at a steep angle into the high-temperature mantle. As the descending oceanic slab reaches depths exceeding one hundred kilometers, extreme thermal heat and immense lithostatic pressure cause trapped hydrated minerals to release water vapor into the overlying mantle wedge.
[3] This addition of volatile water significantly lowers the melting point of the surrounding mantle rock—a geochemical phenomenon known as flux melting. Consequently, partially melted magma becomes buoyant relative to the surrounding solid rock and ascends vertically through fractures in the crust. Over millions of years, repetitive volcanic eruptions accumulate layers of solidified igneous rock, ultimately constructing volcanic island arcs and coastal mountain ranges, such as the Cascade Range in North America.
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