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Encyclopedic Entry ocean trench submarine valley

For the complete encyclopedic entry with media resources, visit: http://education.nationalgeographic.com/encyclopedia/ocean-trench/

Ocean trenches are the deepest parts of the ocean. They are also called submarine valleys. An ocean trench is a long, deep depression in the ocean floor, similar to deep chasms on the Earth’s dry land. Some trenches are near continental shelves. Others are found near chains of volcanic islands, often called volcanic arcs. Some volcanic arcs include the Aleutians, and the island nations of Japan and the Philippines.

Trenches are formed as a result of plate tectonics, or the movement of the Earth’s crust. Tectonic plates slip underneath each other in a process known as subduction.

Subduction When the leading edge of a heavy plate meets the edge of a lighter plate, the heavier plate bends downward. This place where the heavier plate melts (subducts) beneath the lighter one is called the subduction zone. In the ocean, subduction zones can create huge, deep trenches. Ocean trenches can be formed by subduction between continental crust and oceanic crust. Continental crust is always lighter. The long series of Peru-Chile Trenches off the west coast of South America is formed by the oceanic crust of the Nazca plate subducting beneath the continental crust of the South American plate. Ocean trenches can also be formed when two plates carrying oceanic crust meet. These are more rare. The Mariana Trench, in the South Pacific Ocean, is formed as the massive Pacific plate subducts beneath the Philippine plate. In a subduction zone, some of the molten material—the former seafloor—can rise through volcanoes located near the trench. The volcanoes often build mountain ranges or chains of volcanic islands that lie parallel to the trench.

Exploring Trenches The deepest place on Earth is called the Challenger Deep. It is found in the Mariana Trench in the Pacific Ocean, near the island of Guam. The Challenger Deep is 10,994 meters (36,070 feet) below the ocean’s surface. For comparison, Mount Everest, the world’s tallest mountain, is 8,850 meters (29,035 feet) above sea level. Mount Everest could fit inside the Mariana Trench with more than 2 kilometers (1 mile) to spare.

In June 2009, scientists sent an unmanned deep-sea robot, the Nereus, to explore the Mariana Trench and Challenger Deep. The vehicle had to be designed to withstand extreme pressure of 15,000 pounds per square inch—more than 1,000 times the pressure felt at sea level. Scientists want to learn more about the subduction process that created the trench.

The deepest part of the Atlantic Ocean is the Puerto Rico Trench. It is just over 8,600 meters (28,232 feet) deep and is about 280 kilometers (175 miles) long. The Puerto Rico Trench was formed by the North Atlantic plate 1 of 3 sliding beneath the Caribbean plate.

Animals that live in ocean trenches have to survive a habitat of extreme pressure. Most organisms collected from the Challenger Deep have been microscopic. The organisms, or foraminifera, are similar to algae or slime-molds. Scientists believe the foraminifera they found at the bottom of the Challenger Deep are similar to Earth’s earliest life forms.

VOCABULARY

Term Part of Speech Definition

algae plural noun (singular: alga) diverse group of aquatic organisms, the largest of which are seaweeds.

Challenger noun deepest measured point in the ocean (part of the Mariana Trench), about 11,000 Deep meters (36,198 feet), located in the South Pacific Ocean.

chasm noun a deep opening in the earth's surface.

continent noun one of the seven main land masses on Earth.

continental noun thick layer of Earth that sits beneath continents. crust

continental noun part of a continent that extends underwater to the deep-ocean floor. shelf

crust noun rocky outermost layer of Earth or other planet.

depression noun indentation or dip in the landscape.

foraminifera plural noun (singular: foraminifer.) Type of microscopic organism (protist) that forms a shell and lives in marine or salty conditions.

habitat noun environment where an organism lives throughout the year or for shorter periods of time.

Mariana noun deepest place on Earth, located in the South Pacific Ocean at 11,000 meters (36,198 Trench feet) at its deepest.

microscopic adjective very small.

molten adjective solid material turned to liquid by heat.

Nereus noun deep-sea diving remote operated vehicle (ROV).

ocean noun large body of salt water that covers most of the Earth.

oceanic noun thin layer of the Earth that sits beneath ocean basins. crust

ocean noun a long, deep depression in the ocean floor. trench

plate noun movement and interaction of the Earth's plates. tectonics

pressure noun force pressed on an object by another object or condition, such as gravity.

Puerto Rico noun deepest place in the Atlantic Ocean, 8,400 meters (27,560 feet) deep. Trench

robot noun machine that can be programmed to perform automatic, mechanical tasks.

2 of 3 slime-mold noun simple organism similar to fungus that uses spores to reproduce.

subduction noun process of one tectonic plate melting or going beneath another.

subduction noun area where one tectonic plate slides under another. zone

tectonic noun large, moveable segment of the Earth's crust. plate

unmanned adjective lacking the physical presence of a person.

volcanic arc noun chain of volcanoes formed at a subduction zone.

volcanic noun land formed by a volcano rising from the ocean floor. island

volcano noun an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.

For Further Exploration

Articles & Profiles National Geographic News: Life is Found Thriving at Ocean’s Deepest Point National Geographic: DEEPSEA CHALLENGE—The Mariana Trench NOAA: Ocean Explorer—The Puerto Rico Trench

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