E N C Y C L O P E D I C E N T RY
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.
G R A D E S
5 - 12+
S U B J E C T S
Earth Science, Geology, Geography, Physical Geography
C O N T E N T S
11 Images, 1 Video, 2 Links 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 “Ring of Fire” that also includes active volcanoes and earthquake zones.
Ocean trenches are a result of tectonic activity, which describes the movement of the Earth’s lithosphere. 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 subduction, creating a trench.
Ocean trenches occupy the deepest layer of the ocean, the hadalpelagic zone. The intense pressure, lack of sunlight, and frigid temperatures 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 swell, 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 crust and a plate carrying oceanic crust. Continental crust 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 Nazca plate subducting beneath the continental crust of the South American plate. The Ryukyu Trench, stretching out from southern Japan, is formed as the oceanic crust of the Philippine plate subducts beneath the continental crust of the Eurasian plate.
More rarely, ocean trenches can be formed when two plates carrying oceanic crust meet. The Mariana Trench, in the South Pacific Ocean, is formed as the mighty Pacific plate 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—island mountain ranges that lie parallel to the trench. The Aleutian Trench is formed where the Pacific plate subducts beneath the North American plate in the Arctic region between the U.S. state of Alaska and the Russian region of Siberia. The Aleutian Islands form a volcanic arc 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 Puerto Rico Trench is a tectonically complex depression 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 Caribbean plate.
Accretionary Wedges
Accretionary wedges form at the bottom of ocean trenches created at some convergent plate boundaries. The rocks of an accretionary wedge are so deformed and fragmented they are known as melange—French for “mixture.”
Accretionary wedges form as sediments from the dense, subducting tectonic plate are scraped off onto the less-dense plate. Sediments often found in accretionary wedges include basalts 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 mantle, 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 sediment into the ocean.) This accreted material can not only fill trenches, but rise above sea level to create islands that “hide” the ocean trenches beneath. The Caribbean island of Barbados, 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 water temperature 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, life in dark ocean trenches is isolated and slow-moving.
Pressure
Pressure at the bottom of the Challenger Deep, 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.” Marine snow is the continual fall of organic material from higher in the water column. 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 Guam. Only three people have visited the Challenger Deep, the deepest ocean trench in the world: a joint French-American crew (Jacques Piccard and Don Walsh) in 1960 and National Geographic Explorer-in-Residence James Cameron in 2012. (Two other unmanned expeditions have also explored the Challenger Deep.)
Engineering submersibles 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 submersible 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 Trieste, was an unusual vessel called a bathyscaphe.
The Deepsea Challenger, 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 cylindershaped vessel would need to be at least three times thicker to avoid being crushed. The sub’s fuel was augmented by seawater 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
accrete
Definition
to build up or grow together.
Speech
verb
mass of sediments scraped off from oceanic crust during subduction and piled up at the edge of the overriding plate. Also called an accretionary prism.
accretionary wedge
noun
volcano that has had a recorded eruption since the last glacial period, about 10,000 years ago.
active volcano noun
a modification of an organism or its parts that makes it more fit for existence. An adaptation is passed from generation to generation. region at Earth's extreme north, encompassed by the Arctic Circle.
adaptation
noun noun
Arctic asymmetric augment
adjective not identical on both sides.
verb
to enlarge or add to.
plural noun noun noun
(singular: bacterium) single-celled organisms found in every ecosystem on Earth.
bacteria basalt basin
type of dark volcanic rock. a dip or depression in the surface of the land or ocean floor. vehicle used to explore the deep ocean. Developed after the bathysphere.
bathyscaphe
noun
light emitted by living things through chemical reactions in their bodies.
bioluminescencenoun
adjective,
bone
structure composing the skeleton of vertebrate animals.
noun verb
buckle
to bend, fold, or fall apart quickly.
buoyant canyon
adjective capable of floating. noun deep, narrow valley with steep sides.
Part of Speech
- Term
- Definition
greenhouse gas produced by animals during respiration and used by plants during photosynthesis. Carbon dioxide is also the byproduct of burning fossil fuels.
carbon dioxide noun chasm
noun
a deep opening in the earth's surface. process by which some microbes turn carbon dioxide and water into carbohydrates using energy obtained from inorganic chemical reactions.
chemosynthesis noun coast
noun noun
edge of land along the sea or other large body of water. marine environment where hydrogen sulfide and methane seep up from beneath the seafloor and mix with the ocean water. substance having at least two chemical elements held together with chemical bonds.
cold seep compound
noun
compress contend continental crust
verb verb
to press together in a smaller space. to sincerely assert.
noun noun
thick layer of Earth that sits beneath continents.
convergent plate boundary current
area where two or more tectonic plates bump into each other. Also called a collision zone.
noun noun verb
steady, predictable flow of fluid within a larger body of that fluid. tube or long, circular object.
cylinder deform
to put out of shape or distort.
delicate delve
adjective fragile or easily damaged.
verb
to research or investigate thoroughly.
dense
adjective having parts or molecules that are packed closely together.
depression descend detritus devoid
noun verb noun
indentation or dip in the landscape. to go from a higher to a lower place. non-living organic material, often decomposing. adjective lacking or not having something.
dominate
verb noun verb
to overpower or control. the sudden shaking of Earth's crust caused by the release of energy along fault lines or from volcanic activity. to get rid of or throw out.
earthquake eject elusive
adjective difficult to capture. the art and science of building, maintaining, moving, and demolishing structures.
engineering
noun
Part of
- Term
- Definition
Speech
enormous equipment esca
adjective very large.
noun noun
tools and materials to perform a task or function. long, thin, fleshy growth from the head of an anglerfish. to develop new characteristics based on adaptation and natural selection.
evolve
verb
excrement expedition
noun noun
waste material discharged from the body. journey with a specific purpose, such as exploration. pre-eminent explorers and scientists collaborating with the National Geographic Society to make groundbreaking discoveries that generate critical scientific information, conservation-related initiatives and compelling stories.
Explorer-inResidence
noun
type of electric light in which an electrical gas discharge is maintained in a tube with a thin layer of phosphor on its inside surface.
fluorescent
noun
food web fragment frigid
noun noun
all related food chains in an ecosystem. Also called a food cycle. piece or part. adjective very cold.
fuel
noun
material that provides power or energy. state of matter with no fixed shape that will fill any container uniformly. Gas molecules are in constant, random motion.
gas
noun
gelatinous geologist glacier
adjective resembling or behaving like a jelly, gel, or gelatin.
noun noun
person who studies the physical formations of the Earth. mass of ice that moves slowly over land. environment where an organism lives throughout the year or for shorter periods of time.
habitat
noun noun noun
hadalpelagic zone
deepest zone of the open ocean, starting at around 6,000 meters (20,000 feet). land that rises above its surroundings and has a rounded summit, usually less than 300 meters (1,000 feet).
hill hostile
adjective confrontational or unfriendly.
hydrothermal vent
noun
opening on the seafloor that emits hot, mineral-rich solutions.
igneous rock illuminate
noun verb
rock formed by the cooling of magma or lava. to shine light on.
Part of Speech
- Term
- Definition
a type of electric light in which light is produced by a filament heated by electric current.
incandescent
adjective
inner slope innovative
noun
landward or continental side of an ocean trench. adjective new, advanced, or original.
(light emitting diode) device (semiconductor) that emits light when
LED
noun
an electric current passes through it.
lithosphere lung
noun noun noun noun
outer, solid portion of the Earth. Also called the geosphere. organ in an animal that is necessary for breathing. object used to attract an animal or other organism. middle layer of the Earth, made of mostly solid rock.
lure mantle 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. one of a breeding pair of animals.
mate
noun noun
disordered mixture of rocks of different shapes, sizes, ages, and origins.
melange metamorphic rock
rock that has transformed its chemical qualities from igneous or sedimentary.
noun noun noun
methane
chemical compound that is the basic ingredient of natural gas. movement of a group of people or animals from one place to another.
migration molten
adjective solid material turned to liquid by heat.
mountain range noun
series or chain of mountains that are close together. place where a river empties its water. Usually rivers enter another body of water at their mouths.
mouth
noun
navigate
verb noun noun
to plan and direct the course of a journey.
nutrient
substance an organism needs for energy, growth, and life. thin layer of the Earth that sits beneath ocean basins. person who studies the ocean.
oceanic crust oceanographer noun ocean trench ocean trench
noun noun
a long, deep depression in the ocean floor. a long, deep depression in the ocean floor. fossil fuel formed from the remains of marine plants and animals. Also known as petroleum or crude oil.
oil
noun noun
organ
group of tissues that perform a specialized task.
Part of Speech
Term
organic
Definition
adjective composed of living or once-living material.
outer slope outer trench swell
noun
oceanic side of an ocean trench. hill on the seafloor near an ocean ridge, where the oceanic lithosphere begins to subduct beneath the overriding plate.
noun
parallel
adjective equal distance apart, and never meeting.
penetrate peninsula
verb
to push through.
noun
piece of land jutting into a body of water. process by which plants turn water, sunlight, and carbon dioxide into water, oxygen, and simple sugars. animal that hunts other animals for food. force pressed on an object by another object or condition, such as gravity.
photosynthesis noun predator pressure prey
noun noun noun noun verb
animal that is hunted and eaten by other animals. organism on the food chain that can produce its own energy and nutrients. Also called an autotroph.
producer rely
to depend on.
remarkable repel
adjective unusual and dramatic. verb to resist or push back. adjective able to recover.
resilient
horseshoe-shaped string of volcanoes and earthquake sites around
Ring of Fire
noun
edges of the Pacific Ocean.
river
noun noun noun
large stream of flowing fresh water.
rock
natural substance composed of solid mineral matter. surface layer of the bottom of the ocean. base level for measuring elevations. Sea level is determined by measurements taken over a 19-year cycle. marine algae. Seaweed can be composed of brown, green, or red algae, as well as "blue-green algae," which is actually bacteria. solid material transported and deposited by water, ice, and wind. rock formed from fragments of other rocks or the remains of plants or animals.
seafloor sea level
seaweed
noun noun noun noun
sediment sedimentary rock
region of land stretching across Russia from the Ural Mountains to the Pacific Ocean.
Siberia
noun
spew
verb
to eject or discharge violently.
sphere
noun
round object.
Part of
Term
startling
Definition
Speech
adjective surprising or astonishing. adjective extreme incline or decline.
steep
process of one tectonic plate melting, sliding, or falling beneath another.
subduction submersible
noun noun
small submarine used for research and exploration. The upper zone of the ocean. This zone goes down to about 200 meters (660 feet). Also called the photic, euphotic, or epipelagic zone.
sunlight zone noun
material consisting of tiny hollow "microballoons" made from material such as glass or carbon.
