Deep Ocean Trench: Definition & Examples

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  • 0:00 Definition of an Ocean Trench
  • 1:22 Formation of an Ocean Trench
  • 2:36 Location of Ocean Trenches
  • 3:12 Life in Ocean Trenches
  • 4:02 Lesson Summary
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Lesson Transcript
Instructor: Terry Dunn

Terry has a master's degree in environmental communications and has taught in a variety of settings.

Ocean trenches are pretty impressive geological features - so it's too bad we can't easily see them. But here you will learn what they are, how they are formed, and what makes them so impressive.

Definition of an Ocean Trench

Have you ever tucked one piece of paper under another? Maybe you were trying to hide a personal note from a teacher in elementary school, or possibly you were just organizing papers in an essay. Either way, it was probably an easy maneuver. Much simpler than trying to tuck one whole continent under another, at any rate. And yet, in theory, these two moves are very similar - continents can tuck under each other just like one piece of paper can tuck under another, in a process called subduction.

In subduction, one tectonic plate (a large portion of the earth's crust) slides under another. The plate that is slipping under the other plate bends and forms an ocean trench. The subduction zones where ocean trenches form become long, deep valleys. . . very deep valleys.

The deepest valley on Earth is an ocean trench called the Challenger Deep, which is part of the Mariana Trench in the Pacific near Guam. The deepest point of the Challenger Deep is 36,070 feet below the ocean surface. That makes it over 13,000 feet deeper than Everest is tall. If it was possible to drive the length of the Mariana Trench, you should pack some snacks because it would take you a while. It would be like driving from Los Angeles, California to Kansas City, Missouri. Driving the width though, would be an easier task. It averages only 43 miles wide.

Formation of an Ocean Trench

If you could fast-forward through the process of ocean trench formation, I doubt you would feel comfortable staying close by. The bottom tectonic plate kind of melts where it presses against the top plate, often forming a chain of volcanoes and setting off some of the world's most dramatic earthquakes. The heavier tectonic plate always tucks under the lighter tectonic plate, and in terms of plate weights, the continental plates are lighter than oceanic plates. 

Often when you see an arc of volcanic islands, like the Aleutian Islands, they are there because a subduction zone is near. The earthquakes that are set off near subduction zones can be quite dangerous to people. Because they happen on the seafloor, they can spawn tsunamis like the disastrous tsunami in the Indian Ocean in 2004 and the one that hit Japan in 2011. 

Another reason for discomfort near ocean trenches is the tremendous pressure of being underwater that deep. The bottom of the trench can have 1,000 times the pressure you would feel at sea level - not to mention there are underwater vents at subduction zones that spew out gasses, like liquid sulfur and carbon dioxide, that can reach temperatures of 217°. And despite those hot gas temperatures, elsewhere the water is near-freezing.

Location of Ocean Trenches

The Mariana Trench is the biggie, but there are others. Along the West coast of South America, there is the Peru-Chile trench, and even in the Atlantic, which doesn't have as many trenches, there is the Puerto Rico Trench, which is an impressive 28,232 feet deep and 175 miles long.

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