How does subduction create trenches

He is known for his research on the ecology and evolution of fauna in deep-ocean hydrothermal, seamount, canyon and deep trench systems. He has conducted more than 60 scientific expeditions in the Arctic, Atlantic, Pacific, and Indian Oceans. Sunita L. Her research explores how the larvae of seafloor invertebrates such as anemones and sea stars disperse to isolated, island-like habitats, how larvae settle and colonize new sites, and how their communities change over time.

Kirstin also has ongoing projects in the Arctic and on coral reefs in Palau. Her work frequently takes her underwater using remotely operated vehicles and SCUBA and carries her to the far corners of the world. What are ocean trenches? How are trenches formed? What is it like in a trench? How does life survive there? What do we know about trenches?

Why are ocean trenches important? What can ocean trenches tell us about earthquakes? What can ocean trenches tell us about human health? What can ocean trenches tell us about Earth's climate? What's next for trench exploration and discovery? July 22, Life at Rock Bottom This digital photo essay brings you the forms, figures, and facts of life more than a mile and half deep.

Ocean Robots: Challenger Deep. Nereus in the Challenger Deep. Related Topics Hydrothermal Vents. Mid-ocean Ridges. Natural Oil Seeps. The addition of water to the already hot mantle rocks lowers their melting temperature resulting in partial melting of ultramafic mantle rocks to yield mafic magma. Melting aided by the addition of water or other fluid is called flux melting.

It is somewhat more complicated than this, but metamorphic dewatering of suducting crust and flux melting of the mantle wedge appears to account for most of the magma at subduction zones.

Magma formed above a subducting plate slowly rise into the overriding crust and finally to the surface forming a volcanic arc , a chain of active volcanoes which parallels the deep ocean trench. Beneath the active volcanic arc lie intrusive igneous rocks formed from magma that didn't make it all the way to the surface before crystallizing. The volcanic arcs may be volcanic island arcs e. A descending plate is usually referred to as a "slab. Subduction, in the form of gravitational "slab pull," is thought to be the largest force driving plate tectonics.

At a certain depth, the high pressure turns the basalt in the slab to a denser rock, eclogite that is, a feldspar - pyroxene mixture becomes garnet -pyroxene. This makes the slab even more eager to descend. It's a mistake to picture subduction as a sumo match, a battle of plates in which the top plate forces the lower one down.

In many cases it's more like jiu-jitsu: the lower plate is actively sinking as the bend along its front edge works backward slab rollback , so that the upper plate is actually sucked over the lower plate.

This explains why there are often zones of stretching, or crustal extension, in the upper plate at subduction zones. Where the subducting slab bends downward, a deep-sea trench forms. The deepest of these is the Mariana Trench, at over 36, feet below sea level.

Trenches capture a lot of sediment from nearby land masses, much of which is carried down along with the slab. In about half the world's trenches, some of that sediment is instead scraped off. It remains on top as a wedge of material, known as an accretionary wedge or prism, like snow in front of a plow. Slowly, the trench is pushed offshore as the upper plate grows. Once subduction begins, the materials on top of the slab—sediments, water, and delicate minerals—are carried down with it. The water, thick with dissolved minerals, rises into the upper plate.

There, this chemically active fluid enters an energetic cycle of volcanism and tectonic activity. This process forms arc volcanism and is sometimes known as the subduction factory.

Accretionary wedge s form at the bottom of ocean trenches created at some convergent plate boundaries. Accretionary wedges form as sediment s from the dense, subducting tectonic plate are scraped off onto the less-dense plate. Sediments often found in accretionary wedges include basalt s 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 geologist s must dig deep to find older rocks. Active accretionary wedges, such as those located near the mouth s of river s or glacier s, can actually fill the ocean trench on which they form. Rivers and glaciers transport and deposit tons of sediment into the ocean. The Caribbean island of Barbados, for example, sits atop the ocean trench created as the South American plate subducts beneath the Caribbean plate.

Ocean trenches are some of the most hostile habitats on Earth. Pressure is more than 1, times that on the surface, and the water temperature is just above freezing. Perhaps most importantly, no sunlight penetrate s the deepest ocean trenches, making photosynthesis impossible. Organisms that live in ocean trenches have evolve d with unusual adaptation s to thrive in these cold, dark canyon s.

In general, life in dark ocean trenches is isolated and slow-moving. Pressure at the bottom of the Challenger Deep, the deepest spot on Earth, is about 12, 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 organ s, such as lung s. 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, meters 3, 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. 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. 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. Without photosynthesis, marine communities rely primarily on two unusual sources for nutrient s. 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 producer s in the ocean trench, such as bacteria , convert chemical compound s into organic nutrients.

The chemical compounds used in chemosynthesis are methane or carbon dioxide eject ed from hydrothermal vent s and cold seep s, 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. Ocean trenches remain one of the most elusive and little-known marine habitats.