The main features of plate tectonics are:

• The Earth's surface is covered by a series of crustal plates.
• The ocean floors are continually, moving, spreading from the center, sinking at the edges, and being regenerated.
• Convection currents beneath the plates move the crustal plates in different directions.
• The source of heat driving the convection currents is radioactivity deep in the Earths mantle.

The Mid-Oceanic Ridges

The mid-oceanic ridges rise 3000 meters from the ocean floor and are more than 2000 kilometers wide surpassing the Himalayas in size. The mapping of the seafloor also revealed that these huge underwater mountain ranges have a deep trench which bisects the length of the ridges and in places is more than 2000 meters deep. 

Deep Sea Trenches

The deepest waters are found in oceanic trenches, which plunge as deep as 35,000 feet below the ocean surface. These trenches are usually long and narrow, and run parallel to and near the oceans margins. They are often associated with and parallel to large continental mountain ranges. There is also an observed parallel association of trenches and island arcs. Like the mid-oceanic ridges, the trenches are seismically active, but unlike the ridges they have low levels of heat flow. 

Island Arcs

Chains of islands are found throughout the oceans and especially in the western Pacific margins; the Aleutians, Kuriles, Japan, Ryukus, Philippines, Marianas, Indonesia, Solomons, New Hebrides, and the Tongas, are some examples.. These "Island arcs" are usually situated along deep sea trenches and are situated on the continental side of the trench.