Exploring Deep Ocean Trenches:

Ocean trenches are long, narrow depressions on the seafloor, hidden deep beneath Earth's oceans. These dark, once-mysterious canyons can plunge as deep 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.

WHAT CAUSES OCEAN TRENCHES?

Some of the most amazing topography exists beneath the waves of Earth's oceans.

There are 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 volcanic activity. 

Earth scientists have discovered that deep layers of rock ride atop Earth's molten mantle layer, and as they float along, they 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 have formed the Mariana Trench.

FINDING TRENCHES

Ocean trenches exist around the world and are routinely the deepest part of the 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. Interestingly, 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 Diamantina Trench. Most of the deepest trenches are found in the Pacific Ocean, which is also known as the "Ring of Fire" 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. As they age, trenches can get 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

Most trenches weren't really known until the late 20th century. Exploring them requires specialized submersible craft, which didn't exist until the second half of the 1900s.

These deep ocean canyons are extremely inhospitable to human life. The pressure of the water at those depths would instantly kill a human, so no one dared venture into the deeps of the Mariana Trench for years. That is, until 1960, when two men descended in a bathyscaphe called the Trieste. It wasn't until 2012 (52 years later) that another human being ventured into the trench. This time, it was filmmaker and underwater explorer James Cameron (of Titanic film fame) who took 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).

DOES LIFE EXIST 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.

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.