The tallest mountains in the world would still be beneath one mile of water in the Mariana trench.
Deep ocean trenches are formed through interactions between tectonic plates, also called lithospheric plates. As these plates slide past one another, the heavier of the two plates slide beneath the lighter plate, creating a convergence point. This point creates an ocean trench, which is long, deep but very narrow. These trenches are surficial proof that lithospheric plates are returning to the mantle of the Earth.
Lithospheric plates make up the fractured surface of the earth. These plates come in two types: oceanic and continental. The oceanic plates are typically composed of basaltic type rocks. Continental plates are composed of granite based rocks. Trenches are formed when heavier oceanic plates are forced down into the mantle, since basaltic rocks are heavier than granite rocks and create the steep V shape of an deed oceanic trench.
According to plate tectonic theory, new crust is being produced on the plates at a rate of a few centimeters a year. With new crust being created, you might expect that the size of the Earth would grow over time. However, this has not been the case. One theory to explain why is that the subduction zone present at deep ocean trenches is pushed under another at roughly the same rate as new crust is being created.
Island Volcanic Arcs
Deep ocean trenches typically run parallel to island volcanic arcs. Islands such as the Marianas and the Aleutian are located very closely to deep oceanic trenches. It is theorized, these volcanic islands are the result of magma partially melting the plates at the subduction zone and creating new islands. Around the Pacific, volcanic island arcs are found along the oceanic plates, creating what is known as the “Ring of Fire.”
Another natural phenomenon associated with deep oceanic trenches are earthquakes. The earthquakes occur as the two plates are sliding past one another. The two plates move very slowly and pressure builds between the plates as they try to pass one another. When the pressure becomes too great, the plates move and the energy released creates earthquakes.
The Challenger Deep
The deepest oceanic trench is the Challenger Deep located in the Mariana trench. Challenger Deep is 10,911 meters or 35,798 feet below sea level. This point was first reached by Jacques Piccard and Don Walsh with the Bathyscape Trieste in 1960. The trip to the bottom of the world took more than five hours and the observers were only able to stay for 20 minutes. Fish and shrimp were observed at this location. Many unmanned robotic vehicles have traveled to Challenger Deep but Piccard and Walsh are the only explorers to reach the depth in a manned vehicle.