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R E S O U R C E L I B R A R Y E N C Y C L O P E D I C E N T RY

Ocean are long, narrow depressions on the seafloor. These chasms are the deepest parts of the ocean—and some of the deepest natural spots on .

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For the complete encyclopedic entry with media resources, visit: http://www.nationalgeographic.org/encyclopedia/ocean-trench/ Ocean trenches are long, narrow depressions on the seafloor. These chasms are the deepest parts of the ocean—and some of the deepest natural spots on Earth. Ocean trenches are found in every ocean basin on the planet, although the deepest ocean trenches ring the Pacific as part of the so-called “” that also includes active volcanoes and zones.

Ocean trenches are a result of tectonic activity, which describes the movement of the Earth’s . In particular, ocean trenches are a feature of convergent plate boundaries, where two or more tectonic plates meet. At many convergent plate boundaries, dense lithosphere melts or slides beneath less-dense lithosphere in a process called , creating a trench.

Ocean trenches occupy the deepest layer of the ocean, the hadalpelagic zone. The intense pressure, lack of sunlight, and frigid of the hadalpelagic zone make ocean trenches some of the most unique habitats on Earth.

How Ocean Trenches Form

Subduction Zones When the leading edge of a dense tectonic plate meets the leading edge of a less-dense plate, the denser plate bends downward. This place where the denser plate subducts is called a subduction zone.

Oceanic subduction zones almost always feature a small hill preceding the ocean trench itself. This hill, called the outer trench , marks the region where the subducting plate begins to buckle and fall beneath the more buoyant plate.

Some ocean trenches are formed by subduction between a plate carrying continental and a plate carrying . is always much more buoyant than oceanic crust, and oceanic crust will always subduct.

Ocean trenches formed by this continental-oceanic boundary are asymmetrical. On a trench’s outer slope (the oceanic side), the slope is gentle as the plate gradually bends into the trench. On the inner slope (continental side), the trench walls are much more steep. The types of rocks found in these ocean trenches are also asymmetrical. The oceanic side is dominated by thick sedimentary rocks, while the continental side generally has a more igneous and metamorphic composition.

Some of the most familiar ocean trenches are the result of this type of convergent plate boundary. The Peru-Chile Trench off the west coast of South America is formed by the oceanic crust of the subducting beneath the continental crust of the . The Ryukyu Trench, stretching out from southern , is formed as the oceanic crust of the Philippine plate subducts beneath the continental crust of the .

More rarely, ocean trenches can be formed when two plates carrying oceanic crust meet. The , in the South , is formed as the mighty subducts beneath the smaller, less-dense 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 volcanic arcs— ranges that lie parallel to the trench. The is formed where the Pacific plate subducts beneath the in the Arctic region between the U.S. state of and the Russian region of Siberia. The Aleutian form a that swings out from the Alaskan Peninsula and just north of the Aleutian Trench.

Not all ocean trenches are in the Pacific, of course. The is a tectonically complex in part formed by the Lesser Antilles subduction zone. Here, the oceanic crust of the enormous North American plate (carrying the western Atlantic Ocean) is being subducted beneath the oceanic crust of the smaller .

Accretionary Wedges Accretionary wedges form at the bottom of ocean trenches created at some convergent plate boundaries. The rocks of an are so deformed and fragmented they are known as melange—French for “mixture.”

Accretionary wedges form as from the dense, subducting tectonic plate are scraped off onto the less-dense plate. Sediments often found in accretionary wedges include from the deep oceanic lithosphere, sedimentary rocks from the seafloor, and even traces of continental crust drawn into the wedge. The most common type of continental crust found in accretionary wedges is volcanic material from islands on the overriding plate.

Accretionary wedges are roughly shaped like a triangle with one angle pointing downward toward the trench. Because sediments are mostly scraped off from the subducting plate as it falls into the , the youngest sediments are at the bottom of this triangle and the oldest are at the more flattened area above. This is the opposite of most rock formations, where geologists must dig deep to find older rocks.

Active accretionary wedges, such as those located near the mouths of rivers or glaciers, can actually fill the ocean trench on which they form. (Rivers and glaciers transport and deposit tons of into the ocean.) This accreted material can not only fill trenches, but rise above level to create islands that “hide” the ocean trenches beneath. The Caribbean island of , for example, sits atop the ocean trench created as the South American plate subducts beneath the Caribbean plate.

Life in the Trenches

Ocean trenches are some of the most hostile habitats on Earth. Pressure is more than 1,000 times that on the surface, and the is just above freezing. Perhaps most importantly, no sunlight penetrates the deepest ocean trenches, making photosynthesis impossible.

Organisms that live in ocean trenches have evolved with unusual adaptations to thrive in these cold, dark canyons. Their behavior is a test of the so-called “visual interaction hypothesis,” which states that the greater an organism’s visibility, the more energy it must expend to catch prey or repel predators. In general, in dark ocean trenches is isolated and slow-moving.

Pressure Pressure at the bottom of the , the deepest spot on Earth, is about 12,400 tons per square meter (8 tons per square inch). Large ocean animals, such as sharks and whales, cannot live at this crushing depth.

Many organisms that thrive in these high-pressure environments lack gas-filled organs, such as lungs. These organisms, many related to sea stars or jellies, are made mostly of water and gelatinous material that cannot be crushed as easily as lungs or bones. Many of these creatures navigate the depths well enough to even make a vertical migration of more than 1,000 meters (3,281 feet) from the bottom of the trench—every day.

Even the fish in deep trenches are gelatinous. Several species of bulb-headed snailfish, for example, dwell at the bottom of the Mariana Trench. The bodies of these fishes have been compared to tissue paper.

Dark and Deep Shallower ocean trenches have less pressure, but may still fall outside the photic or sunlight zone, where light penetrates the water.

Many fish species have adapted to life in these dark ocean trenches. Some use bioluminescence, meaning they produce their own “living light” in order to attract prey, find a mate, or repel a predator. Anglerfish, for instance, use a bioluminescent growth on the top of their heads (called an esca) to lure prey. The anglerfish then snaps up the little fish with its huge, toothy jaws.

Food Webs Without photosynthesis, marine communities rely primarily on two unusual sources for nutrients.

The first is “.” Marine snow is the continual fall of organic material from higher in the . Marine snow is mostly detritus, including excrement and the remains of dead organisms such as seaweed or fish. This nutrient-rich marine snow feeds such animals as sea cucumbers and vampire squid.

Another source of nutrients for ocean-trench food webs comes not from photosynthesis, but from chemosynthesis. Chemosynthesis is the process in which producers in the ocean trench, such as bacteria, convert chemical compounds into organic nutrients. The chemical compounds used in chemosynthesis are methane or carbon dioxide ejected from hydrothermal vents and cold seeps, which spew these toxic, hot gases and fluids into the frigid ocean water. One common animal that relies on chemosynthetic bacteria for food is the giant tube worm.

Exploring Trenches

Ocean trenches remain one of the most elusive and little-known marine habitats. Until the 1950s, many oceanographers thought that these trenches were unchanging environments nearly devoid of life. Even today, most research on ocean trenches has relied on seafloor samples and photographic expeditions.

That is slowly changing as explorers delve into the deep—literally. The Challenger Deep, at the bottom of the Mariana Trench, lies deep in the Pacific Ocean near the island of . Only three people have visited the Challenger Deep, the deepest ocean trench in the world: a joint French-American crew ( and Don Walsh) in 1960 and National Geographic Explorer-in-Residence in 2012. (Two other unmanned expeditions have also explored the Challenger Deep.)

Engineering to explore ocean trenches is presents a huge set of unique challenges. Submersibles must be incredibly strong and resilient to contend with strong ocean currents, no visibility, and intense pressure of the Mariana Trench. Engineering a to safely transport people, as well as delicate equipment, is even more challenging. The sub that took Piccard and Walsh to the Challenger Deep, the remarkable , was an unusual vessel called a .

The , Cameron’s submersible, successfully addressed engineering challenges in innovative ways. To combat deep-sea currents, the sub was designed to spin slowly as it descended. Lights on the sub were not incandescent or fluorescent bulbs, but arrays of tiny LEDs that illuminated an area of about 30 meters (100 feet). To adapt to the pressure of the deep, the sub was shaped like a sphere—the walls of a square or cylinder- shaped vessel would need to be at least three times thicker to avoid being crushed. The sub’s fuel was augmented by to prevent the oil from compressing. Perhaps most startlingly, the Deepsea Challenger itself was designed to compress. Cameron and his team created glass-based syntactic foam that allowed the vehicle to compress under the ocean’s pressure—the Deepsea Challenger came back to the surface 7.6 centimeters (3 inches) smaller than when it descended. Vocabulary

Part of Term Definition Speech accrete verb to build up or grow together. mass of sediments scraped off from oceanic crust during subduction accretionary noun and piled up at the edge of the overriding plate. Also called an wedge accretionary . that has had a recorded eruption since the last glacial active volcano noun period, about 10,000 years ago. a modification of an organism or its parts that makes it more fit for adaptation noun existence. An adaptation is passed from generation to generation. Arctic noun region at Earth's extreme north, encompassed by the Arctic Circle. asymmetric adjective not identical on both sides. augment verb to enlarge or add to. plural (singular: bacterium) single-celled organisms found in every bacteria noun ecosystem on Earth. noun type of dark volcanic rock. basin noun a dip or depression in the surface of the land or ocean floor. vehicle used to explore the deep ocean. Developed after the bathyscaphe noun bathysphere. light emitted by living things through chemical reactions in their bioluminescencenoun bodies. adjective, bone structure composing the skeleton of vertebrate animals. noun buckle verb to bend, , or fall apart quickly. buoyant adjective capable of floating. canyon noun deep, narrow valley with steep sides. Part of Term Definition Speech greenhouse gas produced by animals during respiration and used carbon dioxide noun by plants during photosynthesis. Carbon dioxide is also the byproduct of burning fossil fuels. chasm noun a deep opening in the earth's surface. process by which some microbes turn carbon dioxide and water into chemosynthesis noun carbohydrates using energy obtained from inorganic chemical reactions. coast noun edge of land along the sea or other large body of water. marine environment where hydrogen sulfide and methane seep up noun from beneath the seafloor and mix with the ocean water. substance having at least two chemical elements held together with compound noun chemical bonds. compress verb to press together in a smaller space. contend verb to sincerely assert. continental noun thick layer of Earth that sits beneath continents. crust convergent area where two or more tectonic plates bump into each other. Also noun plate boundary called a collision zone. current noun steady, predictable flow of fluid within a larger body of that fluid. cylinder noun tube or long, circular object. deform verb to put out of shape or distort. delicate adjective fragile or easily damaged. delve verb to research or investigate thoroughly. dense adjective having parts or molecules that are packed closely together. depression noun indentation or dip in the landscape. descend verb to go from a higher to a lower place. detritus noun non-living organic material, often decomposing. devoid adjective lacking or not having something. dominate verb to overpower or control. the sudden shaking of Earth's crust caused by the release of energy earthquake noun along lines or from volcanic activity. eject verb to get rid of or throw out. elusive adjective difficult to capture. the art and science of building, maintaining, moving, and engineering noun demolishing structures. Part of Term Definition Speech enormous adjective very large. equipment noun tools and materials to perform a task or function. esca noun long, thin, fleshy growth from the head of an anglerfish. to develop new characteristics based on adaptation and natural evolve verb selection. excrement noun waste material discharged from the body. expedition noun journey with a specific purpose, such as exploration. pre-eminent explorers and scientists collaborating with the National Explorer-in- Geographic Society to make groundbreaking discoveries that noun Residence generate critical scientific information, conservation-related initiatives and compelling stories. type of electric light in which an electrical gas discharge is fluorescent noun maintained in a tube with a thin layer of phosphor on its inside surface. food web noun all related food chains in an ecosystem. Also called a food cycle. fragment noun piece or part. frigid adjective very cold. fuel noun material that provides power or energy. state of matter with no fixed shape that will fill any container gas noun uniformly. Gas molecules are in constant, random motion. gelatinous adjective resembling or behaving like a jelly, gel, or gelatin. geologist noun person who studies the physical formations of the Earth. glacier noun mass of ice that moves slowly over land. environment where an organism throughout the year or for habitat noun shorter periods of time. hadalpelagic deepest zone of the open ocean, starting at around 6,000 meters noun zone (20,000 feet). land that rises above its surroundings and has a rounded summit, hill noun usually less than 300 meters (1,000 feet). hostile adjective confrontational or unfriendly. hydrothermal noun opening on the seafloor that emits hot, mineral-rich solutions. vent noun rock formed by the cooling of or . illuminate verb to shine light on. Part of Term Definition Speech a type of electric light in which light is produced by a filament incandescent adjective heated by electric current. inner slope noun landward or continental side of an ocean trench. innovative adjective new, advanced, or original. (light emitting diode) device (semiconductor) that emits light when LED noun an electric current passes through it. lithosphere noun outer, solid portion of the Earth. Also called the geosphere. lung noun organ in an animal that is necessary for breathing. lure noun object used to attract an animal or other organism. mantle noun middle layer of the Earth, made of mostly solid rock. marine adjective having to do with the ocean. continuous fall of organic and inorganic particles (including the marine snow noun remains of marine organisms, fecal matter, shells, and sand) from the upper layers of the water column to the seafloor. mate noun one of a breeding pair of animals. disordered mixture of rocks of different shapes, sizes, ages, and melange noun origins. that has transformed its chemical qualities from igneous or noun rock sedimentary. methane noun chemical compound that is the basic ingredient of natural gas. movement of a group of people or animals from one place to migration noun another. molten adjective solid material turned to liquid by heat. mountain range noun series or chain of that are close together. place where a river empties its water. Usually rivers enter another mouth noun body of water at their mouths. navigate verb to plan and direct the course of a journey. nutrient noun substance an organism needs for energy, growth, and life. oceanic crust noun thin layer of the Earth that sits beneath ocean basins. oceanographer noun person who studies the ocean. ocean trench noun a long, deep depression in the ocean floor. ocean trench noun a long, deep depression in the ocean floor. fossil fuel formed from the remains of marine plants and animals. oil noun Also known as petroleum or crude oil. organ noun group of tissues that perform a specialized task. Part of Term Definition Speech organic adjective composed of living or once-living material. outer slope noun oceanic side of an ocean trench. outer trench hill on the seafloor near an ocean ridge, where the oceanic noun swell lithosphere begins to subduct beneath the overriding plate. parallel adjective equal distance apart, and never meeting. penetrate verb to push through. peninsula noun piece of land jutting into a body of water. process by which plants turn water, sunlight, and carbon dioxide photosynthesis noun into water, oxygen, and simple sugars. predator noun animal that hunts other animals for food. force pressed on an object by another object or condition, such as pressure noun gravity. prey noun animal that is hunted and eaten by other animals. organism on the food chain that can produce its own energy and producer noun nutrients. Also called an autotroph. rely verb to depend on. remarkable adjective unusual and dramatic. repel verb to resist or push back. resilient adjective able to recover. horseshoe-shaped string of volcanoes and earthquake sites around Ring of Fire noun edges of the Pacific Ocean. river noun large stream of flowing fresh water. rock noun natural substance composed of solid mineral matter. seafloor noun surface layer of the bottom of the ocean. base level for measuring elevations. is determined by sea level noun measurements taken over a 19-year cycle. marine algae. Seaweed can be composed of brown, green, or red seaweed noun algae, as well as "blue-green algae," which is actually bacteria. sediment noun solid material transported and deposited by water, ice, and wind. formed from fragments of other rocks or the remains of plants noun rock or animals. region of land stretching across from the Ural Mountains to Siberia noun the Pacific Ocean. spew verb to eject or discharge violently. sphere noun round object. Part of Term Definition Speech startling adjective surprising or astonishing. steep adjective extreme incline or decline. process of one tectonic plate melting, sliding, or falling beneath subduction noun another. submersible noun small used for research and exploration. The upper zone of the ocean. This zone goes down to about 200 sunlight zone noun meters (660 feet). Also called the photic, euphotic, or epipelagic zone. material consisting of tiny hollow "microballoons" made from syntactic foam noun material such as glass or carbon. movement of tectonic plates resulting in geologic activity such as tectonic activity noun volcanic eruptions and . massive of solid rock made up of Earth's lithosphere (crust and tectonic plate noun ). Also called lithospheric plate. degree of hotness or coldness measured by a thermometer with a temperature noun numerical scale. thrive verb to develop and be successful. tissue paper noun very thin, translucent paper often used for wrapping delicate items. toxic adjective poisonous. transport verb to move material from one place to another. unique adjective one of a kind. unmanned adjective lacking the physical presence of a person. vertical noun up-down direction, or at a right angle to Earth and the horizon. the ability to see or be seen with the unaided eye. Also called visual visibility noun range. visual theory of behavior pattern stating that the greater an organism's interaction noun visibility, the more energy it must expend to catch prey or repel hypothesis predators. volcanic arc noun chain of volcanoes formed at a subduction zone. water column noun area reaching from the sediment of a body of water to its surface. Articles & Profiles National Geographic News: Life is Found Thriving at Ocean’s Deepest Point AGU Geojourneys blog: Geology Word of the Week: A is for Accretionary Wedge NOAA: How Does Pressure Impact Animals in the Ocean?

Interactives BBC: Ocean trench: Take a dive 11,000m down

Maps National Geographic: Puerto Rico Trench

Worksheets & Handouts University of Texas at Dallas: Ocean Trenches

Video National Geographic: Deepsea Challenge

Websites Woods Hole Oceanographic Institution: HADES—Hadal Ecosystem Studies

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