According to Alexander I. Shemenda of Montpellier University, subduction is the process whereby one tectonic plate moves, or slides, below another tectonic plate, then sinks into the Earth's mantle, or interior. For example, an oceanic crust might collide with a continental crust then dive below its lithosphere, on account of its greater density. The subduction zone is the name of the point of contact. Subduction movement rates are approximately equivalent to the rate of fingernail growth.
Professor Philippe Agard, et al., writing in Earth and Planetary Science Letters asserts that obduction processes remain largely unexplained by the geological and scientific community. In brief, obduction is where oceanic crust or mantle rocks gradually creep over continental crust. In the process, which may take millions of years, mountains may be formed. The Appalachian Mountains reveal many examples of historical obduction processes, according to Timothy Kusky, Ph.D.
The primary mountain-forming force on the Earth is orogenesis, which occurs when continental crusts collide. The Himalayas, Asia's vast mountain range, was formed this way, when the Indian landmass collided with Asia, 40 to 50 million years ago. The forces at work were no doubt incredible, considering that over a hundred of the peaks formed by the collision are taller than the tallest mountain of The Americas, Aconcagua. Ten of these are over 26,000 feet, according to Kusky.
The Earth's outer shell is broken into about a dozen pieces, almost like the cracked shell of an egg. However, the pieces of the Earth's crust are enormous, rigid and moving, relative to each other, all the time. Below continents, the lithosphere has a thickness of approximately 80 miles. Below oceans, the lithosphere is about 50 miles thick. The movement of plates occurs because of the partially molten zone within the mantle, the layer between the crust and the outer core.