Series:
How the Earth was Made (2009)
Episode: The Deepest Place on Earth
Channel: History Channel (yes, really)
Using the Marianas Trench (5-7 miles deep) as a central focus, the program follows the history of discovery of plate tectonics. It explains better than anything else I've ever read or watched why plates move (ridge-push and slab-pull), why sea floor subducts (it cools as it moves from the ridge, gradually growing denser with age and finally diving into the mantle), how magnetic sea floor striping occurs, why ridges of volcanic activity form a couple hundred miles behind subduction zones, and finally, why trenches form at subduction zones.
This program might be hard to find. Though it's a series and I've programmed my TiVo to find it, I've only picked up this one episode so far.
--Percy