Ocean trenches
By-Mr_dad_digvijay
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.
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| Seafloor 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.
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| Pacific Ring of Fire Most ocean trenches circle the Pacific in the "Ring of Fire," which 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.
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| Mariana Trench Ocean trenches are a result of the process of subduction, where dense oceanic crust slides beneath less-dense crust. The red arrow indicates the subduction zone forming the massive Mariana Trench. |
More rarely, ocean trenches can be formed when two plates carrying oceanic crust meet. The Marina trench, in the South Pacific Ocean, is formed as the mighty Pacific plate subducts beneath the smaller, less-dense Philippine plate.
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| Mariana Trench The Mariana Trench—including the Challenger Deep, the deepest spot on Earth—is formed where the mighty Pacific plate is subducting beneath the less-dense oceanic crust of the Philippine plate. Be sure to visit our ocean trench map for more information on the Mariana Trench. |
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.
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Ocean trenches can contribute to the formation of volcanic arcs, island mountain chains that run parallel to the trench. The Aleutian Islands, swinging out from the Alaska Peninsula, run parallel to the Aleutian Trench, where the Pacific plate is subducting beneath the North American plate.
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.
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| Caribbean Sea Not all ocean trenches ring the Pacific, of course. The Puerto Rico Trench, the deepest spot in the Atlantic Ocean, is created where the oceanic crust of the North American plate (carrying the western Atlantic Ocean) is being subducted beneath the oceanic crust of the smaller 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.
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| Shrimp Krill There are few food sources in the cold, isolated, and high-pressure habitats of deep ocean trenches. Marine snow, the steady fall of organic material from higher in the water column, is a vital source of nutrients. Marine snow may include decaying parts of dead organisms or fecal material. |
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.
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| Deep Sea Ecosystem: Titled and Labeled Many organisms are well-adapted to survive in the deep ocean trenches. In order to "see" in the dark and find food and mates, many organisms use bioluminescence. In order to survive in the intense pressure, many organisms have soft, gelatinous bodies. |
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
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| Chemosynthetic Community Lacking sunlight, photosynthesis is impossible in deep ocean trenches. Instead, producers in this unusual food web convert chemical compounds, such as methane, into nutrients, a process called chemosynthesis. The orange bacterial mat above is part of a chemosynthetic community (though not in an ocean trench). |
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.)
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| The Bathyscaphe Trieste Ocean trenches are among the most remote places on the planet. Only three people have ever descended to the Challenger Deep. Two of them (Jaques Piccard and Don Walsh) did so in this vessel (the bathyscaphe Triestse) in 1960 |
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 cylinder-shaped 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.
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