As one tectonic plate moves under its adjoining plate, what happens?
The process of one tectonic plate sliding under another is called subduction. Generally subduction zones are areas with lots of earthquake activity, especially deep earthquakes.
Because the plates are coming together into the same space, there is a lot of compression happening at the subduction zone. The plate that is driven down into the earth is melted and its material is incorporated into the mantle and recycled.
In some cases the interaction of the plates results in the uplifting of the upper plate in a mountain-building process known as orogeny.
You can find a number of excellent animations that will help you understand these processes at this link.
When 2 plates collide, a part of their sides gets ruined. The result of this distructions depends on the type of crusts that are at the side of the plates that collide. So:
-if an oceanic plate collides with a continental one, the oceanic, beeing thiner and denser, will be forced to go under the continental one, which is heavier and thicker. This phenomenon is called "subduction".
-when 2 oceanic plates collide, it's also possible that one of them may go under the other one.
-when 2 continental plates collide, mountain areas are created because the sides that meet will wrinkle, compress and will be pushed to the outside. The area where the 2 plates collide is called a "convergent zone".
In the convergent zones, when the 2 plates collide, the underneath plate will be pushed into the hot rock ocean. With time, as it goes further into the lava river, a part of it melts down gradually and will form a pressure. When this pressure becomes too intense it'll force the hot rock to go up through the crust until it reaches surface, forming the volcanous. :)