What Is Required for an Ocean Trench to Form?

Plate Tectonics

• The most basic requirement for the formation of trenches is the process of plate tectonics. While erosion and weathering are the primary agents for geological destruction, plate tectonics is responsible for the construction of almost all the planet's major formations, including trenches. The interaction between crustal plates creates three major types of boundaries. Divergent zones occur where plates are moving apart and transform boundaries occur where plates are moving laterally past one another. However, trenches are created along convergent zones, where plates are colliding.

Convergent Boundaries

• There are three types of convergent boundaries, with each defined by the types of plates involved in the collision. When oceanic plates collide, the process will result in the formation of a trench. When an oceanic plate collides with a continental plate, the process will also result in the formation of a trench. However, the third type of convergent boundary, one involving two continental plates, will not result in the formation of a trench. The important difference is that the first two boundaries involve the creation of a subduction zone, which is a critical ingredient for trench formation.

Subduction Zones

• When two plates collide, the denser plate will be forced under the more buoyant plate. This process is known as subduction. In the case of two oceanic plates, the older plate will have had more time to cool. As an oceanic plate cools, its density increases. This causes the older plate to sink below the younger plate. Oceanic plates are always denser than continental plates. Therefore, when they collide, the oceanic plate is always subducted under the continental plate. It is this subduction that creates the planet's trenches. As one plate is bent and thrust under the other, a vertical v-shaped deformation is created. This geological feature, called a trench, can have steep sides and stretch for hundreds of miles.

Tectonic Examples

• The Mariana Trench was formed by the collision of two oceanic plates. The Pacific plate is subducting under the Philippine plate, creating a 1,554-mile long trench. A segment of this trench, known as Challenger Deep, reaches the deepest known point on Earth. At a depth of more than 36,000 feet, the trench is far deeper than Mount Everest is tall. The Peru-Chile Trench, also known as the Atacama Trench, was formed by the collision of an oceanic and continental plate. Here, the Nazca plate is subducting under the South American plate. At 3,650 miles, this trench is the longest oceanic trench in the world. It has a maximum depth of more than 26,000 feet.