Exploring Deep-ocean Trenches

The Deepest Regions on Earth

Ocean trench
The Deep Discoverer ocean vessel exploring the Mariana Trench. It studied geological features similar to rocks and canyons found in the Alps and canyons in California. This was done during the 2016 Deepwater Exploration of the Marianas. NOAA Office of Ocean Exploration and Research.

There are places deep beneath the waves of our planet's oceans that are as far away from us on the surface as the upper reaches of our atmosphere. These regions are the deep ocean trenches. These are long, narrow depressions on the seafloor, extending so deep that their life forms and conditions were once considered "unknown". These dark, once-mysterious canyons plunge down as far as 11,000 meters (36,000 feet) into our planet's crust. That's so deep that if Mount Everest were placed at the bottom of the deepest trench, its rocky peak would be 1.6 kilometers beneath the waves of the Pacific Ocean.

Why Do Ocean Trenches Exist?

Trenches are some pretty amazing topography. The seafloor itself contains volcanoes and mountains that tower higher than any of the continental peaks. And the deep ocean trenches dwarf any of the continental canyons. How do those trenches form? The short answer comes from Earth science and the study of tectonic plate motions, which applies to earthquakes as well as to volcanic activity

Earth scientists know that deep layers of rock ride atop Earth's molten mantle layer. As they float along, these "plates" jostle each other. In many places around the planet, one plate dives under another. The boundary where they meet is where deep ocean trenches exist. For example, the Mariana Trench, which lies beneath the Pacific Ocean near the Mariana island chain and not far from the coast of Japan, is the product of what's called "subduction." Beneath the trench, the Eurasian plate is sliding over a smaller one called the Philippine Plate, which is sinking into the mantle and melting. That sinking and melting formed the Mariana Trench.

Finding Trenches

Ocean trenches are found around the world and make up the deepest part of any ocean. They include the Philippine Trench, Tonga Trench, the South Sandwich Trench, the Eurasian Basin and Malloy Deep, the Diamantina Trench, the Puerto Rican Trench, and the Mariana. Most (but not all) are directly related to subduction actions or plates moving apart, which take millions of years to occur. For example, the Diamantina Trench formed when Antarctica and Australia pulled apart many millions of years ago. That action cracked Earth's surface and the resulting fracture zone became the trench. Most of the deepest trenches are found in the Pacific Ocean, which overlies the so-called "Ring of Fire". That region gets the name due to tectonic activity that also spurs the formation of volcanic eruptions deep beneath the water.

The lowest part of the Mariana Trench is called the Challenger Deep and it makes up the southernmost part of the trench. It has been mapped by submersible craft as well as surface ships using sonar (a method that bounces sound pulses from the sea bottom and measures the length of time it takes for the signal to return). Not all trenches are as deep as the Mariana. Time seems to erase their existence. That's because, as they age, trenches are filled with sea-bottom sediments (sand, rock, mud, and dead creatures that float down from higher in the ocean). Older sections of the sea floor have deeper trenches, which happens because heavier rock tends to sink over time.

Exploring the Deeps

The fact that these deep-ocean trenches existed at all remained a secret until well into the 20th century. That's because there were no vessels that could explore those regions. Visiting them requires specialized submersible craft. These deep ocean canyons are extremely inhospitable to human life. Although people did send diving bells into the ocean prior to middle of the last century, none went as deep as a trench. The pressure of the water at those depths would instantly kill a person, so no one dared venture into the deeps of the Mariana Trench until a safe vessel was designed and tested.

That changed in 1960 when two men descended in a bathyscaphe called the Trieste. In 2012 (52 years later) filmmaker and underwater explorer James Cameron (of Titanic film fame) ventured down in his Deepsea Challenger craft on the first solo trip to the bottom of the Mariana Trench. Most other deep-sea explorer vessels, such as Alvin (operated by the Woods Hole Oceanographic Institution in Massachusetts), do not dive nearly quite so far, but still can go down around 3,600 meters (around 12,000 feet).

The Weird Life in the Deep Ocean Trenches

Surprisingly, despite the high water pressure and cold temperatures that exist at the bottoms of trenches, life does flourish in those extreme environments. Tiny one-celled organisms live in the trenches, as well as certain types of fish, crustaceans, jellyfish, tube worms, and sea cucumbers. In addition, the bottoms of many trenches are filled with volcanic vents, called "black smokers". These continually vent lava and chemicals into the deep sea. Far from being inhospitable, however, these vents supply much-needed nutrients for types of life called "extremophiles", which can survive in the alien conditions. 

Future Exploration of Deep Sea Trenches

Exploring the deep sea is expensive and difficult, although the scientific and economic rewards can be very substantial. Human exploration (like Cameron's deep dive) is dangerous. Future exploration may well rely (at least partially) on robotic probes, just as planetary scientists reply on them for the exploration of distant planets. There are many reasons to keep studying the ocean depths; they remain the least-probed of Earth's environments. Continued studies will help scientists understand the actions of plate tectonics, and also reveal new life forms making themselves at home in some of the most inhospitable environments on the planet.