Trenches are long, narrow depressions that make up the deepest part of the ocean floor.
As ocean ridges spread, the sides of the plates furthest from the ridge often encounter subduction zones, in which the oceanic plate is forced under the continental plate it encounters. This forms an ocean trench that runs along the length of the plate being subducted. The deepest trench to be measured so far, known as the Mariana Trench, is almost 11,000 m (36,000 ft) deep.
Subduction zones also have increased potential for destructive earthquakes.Click on the headings in the table below to see information about a massive subduction zone earthquake that occurred in 2004.
| Destructive Subduction Zone Earthquake in 2004 | |
| Location: | Near Sumatra in Indonesia, over 18 miles below sea level. |
| Magnitude: | 9.1 on the Richter Scale |
| Effect: | A tsunami (giant wave) spread out from the epicenter toward surrounding land areas. In addition, the power of the quake caused the Earth to wobble on its axis by a few centimeters. |
| Destruction: | The tsunami killed over 230,000 people, making it one of the worst natural disasters in human history. |
