[editor-surveyor]

"sometimes they slide beneath each other (creating trenches, like the Marianas in the Pacific)"

Current tectonic theory holds that the trenches are created by plates receding from each other, not by colliding. The best known place where one plate is being forced under the other is the northwest coast of the U.S. from Gualala to Juneau. This is the engine that fires the Cascades (as in Mount St. Helens)

[Cincinatus]

Current tectonic theory holds that the trenches are created by plates receding from each other, not by colliding.

Uh, actually, no. Trenches are found where one plate is thrust under another. It's called a subduction zone. The western Pacific rim is the best example -- the volcanoes of Japan are caused by the Pacific plate being subducted beneath the Asian plate.

The plates recede from each other at either mid-ocean ridges (the most common case; example: the mid-Atlantic ridge) or a mid-continental rift (as in the east African rift valley).