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Here’s How Oceanic Trenches Are Formed

Oceanic trenches are some of the most mysterious places on earth. They go so deep that it’s difficult to explore them properly. The explorations that scientists have managed to conduct suggest that there are life forms and sea creatures in the trenches that we don’t even have names for. Oceanic trenches occur in oceans throughout the world, and you may wonder how ocean trenches formed and why they exist

Ocean trenches form on tectonic plate boundaries where the dense plate subducts below the less dense plate due to plate movement. These destructive plate boundaries result in a steep V-shaped valley or trench on the ocean floor. Ocean trenches occur in all the world’s ocean basins.

Ocean trenches are deep narrow rifts or valleys in the ocean floor. They are the deepest places in the ocean and are mostly unexplored. If you took Mount Everest and put it into some ocean trenches, it would completely disappear into the sea.

Many people have scaled the many high mountain tops globally, but only a handful have ever gone down into the deepest sea trenches. To understand how ocean trenches are formed, you need to understand the structure of the earth and tectonic plate movement.

This article will explain these concepts and provide you with explanations on how the trenches form. We’ll also look at the structure and features of the trenches and what life forms exist in the ocean trenches.  But first, let’s get an understanding of the structure of the Earth, so that we can better understand the oceanic trenches…

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The Structure Of The Earth

There are various models and terminology to describe the structure of the earth and the different zones. For the purposes of this article, we will use the simplest models and terms to make it easy to understand. There are four basic parts to the structure of the earth:

The crust is the outer layer of the earth and is the least dense, varying in thickness at different places on the earth. The width is between 3.7 miles -56 miles (6 – 90 km), with an average width is between 24 miles – 25 miles (40 km). The thickest places of the earth’s crust are at mountain ranges such as the Himalayan Mountains, and the thinnest areas are on the ocean floor.

Sometimes people confuse themselves by thinking the crust only extends over the continents, but it covers the surface of all the earth – continents and oceans included. The different parts of the crust are sometimes referred to as oceanic crust and continental crust. The crust is the thinnest part of the structure of the earth. 

The mantle lies under the crust. It is denser than the crust and is approximately 1802 miles (2900 km) thick. The mantle consists of molten rock that flows like a thick viscous liquid. This molten rock is known as magma. The mantle contains up to three times more water than all the oceans in the world combined. The temperature of the magma is between 932°F – 1652°F (500°C – 900°C) at the upper border where the mantle meets the crust.

The lower boundary where the mantle meets the core has temperatures of 7230°F  (4000°C). There are convection currents in the magma because of the differences in the heat. The hot material moves upwards because it is less dense and cooler magma moves downwards as its density is higher.

As the magma meets the earth’s crust, it begins to cool, and eventually, the temperature drops to a point where that portion of the magma sinks through the mantle towards the core. In this way, there are currents formed in the mantle. Scientists believe that these convection currents cause the instability of the earth’s crust. 

The outer core is approximately 1242,7 miles (2000 km) thick and consists of very dense material. The outer core is molten and has higher temperatures than the mantle. 

The inner core is at the center of the earth. It is approximately 746 miles thick (1200 km), and scientists believe it is an extremely dense solid metal ball.

Seismic Waves

Many people are confused as to how scientists have developed this model of the earth’s structure. We cannot drill down below the earth’s crust, so how do we know the structure of the planet. Most of the information scientists use comes from seismic waves. Seismic waves are the waves generated when there is an earthquake. The waves move through the earth and behave in different manners according to the difference in temperature and density.

Seismological studies give information that scientists can interpret to identify boundaries of different parts of the earth’s structure and the composition of the various sections. Gravitational and magnetic field analysis also give information that scientists use to develop models. If you are interested in reading further on this topic, follow this link; Evidence for Internal Earth Structure and Composition

Tectonic Plates

Subduction and plate tectonics diagram

The earth’s crust is not one solid mass. It is divided into different regions known as tectonic plates. They are also sometimes called lithosphere plates. The majority of tectonic plates carry continents and oceans, but there are some, such as the Nazca plate, that carry only the ocean.  There are seven major tectonic plates:

  • African plate
  • Antarctic plate
  • Eurasian plate
  • Indo-Australian plate
  • North American plate
  • South American plate
  • Pacific plate

There are eight minor plates:

  • Nazca plate
  • Scotia plate
  • Caribbean plate
  • Cocos plate
  • Arabian plate
  • Philippine plate
  • Juan de Fuca plate
  • New Hebrides plate

There are also micro-tectonic plates. For more information on tectonic plates, click here.

Tectonic Plate Boundaries

Tectonic plate boundaries are the regions where two plates meet. Earthquakes and volcanoes occur commonly in these areas due to tectonic plate movement. Tectonic plates ‘float’ on the liquid mantle, and convection currents in the mantle cause the movement of the plates. 

Different Types Of Tectonic Plate Boundaries

The tectonic plates may move towards each other, and this is known as a convergent plate boundary. Convergent plate boundaries are also called destructive boundaries. At these boundaries, land is destroyed, which is why they are known as destructive boundaries. One plate moves under the other plate at destructive boundaries.

If the plates move away from each other, then it is a divergent plate boundary. Divergent boundaries are also known as constructive boundaries. At constructive boundaries, land is created. Constructive boundaries form underwater volcanoes that may become so large that they emerge from the water to form a volcanic island, for example, Ascension Island in the Atlantic Ocean.   

Sometimes the plates slide horizontally past each other, and this is a passive plate boundary or a conservative boundary. There is no creation or destruction of land at conservative boundaries, but there are often earthquakes as the plates move. The most famous conservative plate boundary is the San Andreas Fault which causes earthquakes in California around Los Angeles. 

How Do Oceanic Trenches Form

Oceanic plates consist of denser rocks than continental plates. When an oceanic plate and a continental plate converge, the dense oceanic plate slides under the continental plate – the oceanic plate is said to subduct beneath the continental plate. The result is a very deep trench that runs along the subduction zone and is known as an oceanic trench

The portion of the oceanic plate that is pushed under the continental plate heats up and becomes molten forming magma. The magma has nowhere to go and so is forced up through gaps and cracks in the continental plate. The magma that is forced upwards forms mountain ridges or volcanic islands that run parallel to the ocean trench.

The continental plate is thrust upwards by the oceanic plate. This process is known as folding and gives rise to fold mountains. These may be underwater or project above the surface of the water. 

Sometimes the oceanic trench may form along the subduction zone of two oceanic plates. The plate that subducts will be the least dense plate. The density of the plates is usually determined by the age of the plate. An older plate is cooler and denser and so will usually subduct beneath a younger plate.

The older plate forms magma that rises through the less dense oceanic plate and forms volcanoes known as island arcs. An example of this is the Mariana Islands in the North Pacific Ocean. This island arc is associated with the Mariana Trench that is located 124 miles east of the islands.  

The Structure Of Ocean Trenches

Ocean trenches are deep V-shaped trenches in the hadal zone of the earth. (The hadal zone is named after Hades and is the deepest part of the ocean). The landward side is steeper than the seaward side. There are two kinds of trenches: 

  • those associated with island arcs – formed from two oceanic plates converging
  • those associated with continents – formed from oceanic plates and continental plates converging

The trenches associated with island arcs are deeper than those associated with continents. Sometimes trenches become filled with sediment and can fill entirely if there are high rates of sediment in the water. This is the case in ocean trenches off the coast of Oregon.

Ocean trenches can be found off the coast of Oregon

At times there may be so much sediment deposited that the trench is completely filled, and the depositions form a landmass. Barbados, an island formed when the South American plate subducted beneath the Caribbean plate, is basically an island sitting on top of a deep-sea trench.  

The ocean trenches formed from oceanic plates subducting below continental plates are asymmetrical. The landward side is steep, but the oceanic side may have a gentler slope created by the bending of the oceanic plate as it subducts. Rock composition also differs on the two sides. The oceanic side has thick sedimentary rocks, and the landward side has igneous and metamorphic rocks. An example of this type of trench is the Peru-Chile trench. 

Features Of Ocean Trenches 

Scientists have found that the force of gravity is lower in ocean trenches, but magnetic fields show no abnormalities. Ocean trenches are extremely deep – 24 000 to 36 000 feet (7300 m – 11 000 m) deep. The pressure at the bottom of ocean trenches is eight tons per square inch.

This is more than a thousand times the pressure that you would experience at the sea surface. Temperatures are very low – almost constantly just above freezing. Light cannot penetrate to the depths of the trenches, and so no photosynthesis is possible. 

Where Are Oceanic Trenches Situated?

Ocean trenches are found throughout the world’s oceans, but the majority of them occur in the Pacific Ocean. Ocean trenches are found in every ocean basin in the world. Oceanographers have identified forty-six different trenches that form part of the hadal zone in the world. 

What Is The Ring Of Fire?

The Ring of Fire is also known as the Circum-Pacific Belt. It is an area where seven tectonic plates meet in various places. The plates are:

  • the Pacific plate
  • the Nazca plate
  • the North American plate
  • the Philippine plate
  • the Juan de Fuca plate
  • the Cocos plate
  • the Indo-Australian plate

As you can imagine, this area is very unstable due to its vulnerability in shifting tectonic plates. Ninety percent of the earth’s earthquakes originate in this area. There are more than four hundred and fifty volcanoes ( seventy-five percent of all the world’s volcanoes). The number of volcanoes in the area is where the name ‘Ring of Fire’ came from. There are several ocean trenches associated with the Ring of Fire.

The deepest ocean trenches in the world are situated in the Ring of Fire that lies on the edges of the Pacific Ocean. The Ring of Fire is 24 900 miles (40 000 km) long and includes the Andes Mountains, the United States west coast, and the Aleutian Islands. From there, it passes along Asia through Japan. Indonesia, the Philippines, and New Zealand. 

What Is The Deepest Ocean Trench?

The deepest oceanic trench is the Mariana Trench at 36 201 feet. The trench is 1585 miles long and 42.8 miles wide and is formed when the Pacific Plate subducted beneath the Philippine Sea Plate.  The reason that the Mariana Trench is so deep is that the crust at the edge of the Pacific plate is one of the oldest portions of oceanic crust in the world. This makes it very dense.

When the Pacific plate subducted, its density pulled it deeply under the Philippine plate.  The Mariana Trench has been declared a protected zone by the United States. Any expeditions or scientific studies in the area must first apply for permits. 

The first two people to go down into the Mariana trench were Jacques Piccard and Navy Lieutenant Don Walsh. This was done in 1960 in a navy submersible called the Trieste. It took them five hours to reach the bottom, and they could only stay for twenty minutes at the bottom of the trench. Their descent stirred up so much silt that they could not take photographs.

The deepest area of the Mariana Trench is known as the Challenger Deep. It is named after the first expedition, the Challenger expedition, which attempted to determine its depth in 1875. A weighted rope was hung over the ship’s side and measured 4475 fathoms or 26850 feet (8184 m). Since then, sonar systems and seismometers have been used to measure and map the trench. 

The following trenches all have depths of more than 33 000 feet (10 000 m):

  • Kuril Kamatcha trench
  • Kermadec trench
  • Philippine trench
  • Tonga trench

Is There Life In Oceanic Trenches?

Many scientists and even philosophers like Socrates felt that life could not exist at these depths. During the Victorian era, Edward Forbes dredged the Aegean Sea. He asserted from his findings that life could not exist at depths of more than 1804 feet (550 m). This notion was challenged in1864 by naturalists Michael and George Ossian Sars. They found a sea lily ( which is an animal) in a Norwegian fjord at a depth of 10 000 feet (3000 m). 

The British decided to explore deep-sea life. Between 1872 and 1876, the British HMS Challenger sailed over seventy-nine thousand miles on this endeavor. They found a fantastic variety of life and categorized four thousand seven hundred new marine species.    

Piccard claimed to see a large flatfish that was about twelve inches long. Scientists have been doubtful about this claim and postulate that what Piccard saw was actually a sea cucumber. A new species of snailfish was detected at 26722 feet (8145 m) was found in December 2014.

This was a record-breaking find as no other living fish has been found deeper than this. Recently there have been many discoveries, and the conclusion is the deep-sea trenches are teaming with life. Gigantism seems to affect life in the hadal zone. All species seem much larger than their counterparts that exist at shallower ocean depths. 

Adaptations Of Organisms In Oceanic Trenches

Amphipods can be found deep in the ocean trenches

Oceanic trenches have no light, and it is utterly dark in these depths. Some animals have responded to the challenge by developing huge eyes to capture the faintest trace of light. An example of this is the Stout Blacksmelt fish. The vision of other animals has completely vanished, and they rely on vibrations and touch to move around and catch prey. The tripod fish is an excellent example of this.

Other animals have learned to make their own light, known as bioluminescence, as seen in the lanternfish. The lack of light does not allow for plant growth, and so food is limited. Animals in deep-sea trenches have adapted to eat scraps of dead organisms and detritus that filter down through the water. These scraps of organisms are known as marine snow. Sometimes dead whales are heavy enough to sink into the ocean trenches, which provides a feast for the animals that occupy the trench depths. 

The second challenge that animals face is the extreme cold -1°C to 4°C and the immense pressure at the lower reaches of the trenches. Normal body fat would solidify in these conditions. The animals have adapted by storing unsaturated fats, which remain liquid even in extreme temperatures.

The cells in deep-sea animals have been adapted to include a tiny organic molecule known as piezolytes. Piezolytes bind to the water and give the proteins space to expand and change shape as they function in the body. Proteins in normal cells would not be able to operate at such extreme temperatures and pressures. 

What Kind Of Animals Live At The Bottom Of Ocean Trenches?

Fish seem to live at a maximum-depths of 26 000 to 27 000 feet. Beyond that, there are other life forms, such as crustaceans known as amphipods. The amphipods grow to giant sizes in the deep cold water. Most normal amphipods are approximately an inch to two inches long, but they grow to more than a foot long in the trenches. As they look like giant shrimp, it gives an impression of an alien world down in the depths. 

Single-cell organisms, similar to amoeba, known as foraminifera, have been found in giant sizes in ocean trenches. Huge sea cucumbers were also discovered inhabiting deep areas. Most of the animals found at great ocean depths do not have standard bone structure or air-filled spaces – the enormous pressure would shatter them if they did. Jellyfish and soft-bodied animals cope much better with the increased pressure. 

Dumbo octopuses have been found at 32 152 feet (9800 m) in the Mariana Trench. They do not look like the octopuses found closer to the surface. They have very deep-set eyes, with tiny tentacles and two ear-type appendages. Ping-Pong Tree Sponges look like transparent ping pong balls attached to a stalk. The ping pong ball sponges trap any creature that comes near and slowly consume it. These unusual creatures are found at the deepest levels in the trenches.

Frilled sharks have six or seven gills ( most normal sharks have five gills) and more closely resemble eels than sharks. They live at depths of 16 404 feet (5000 m). The scariest feature of frilled sharks is their considerable number of teeth – three hundred teeth – not something anyone wants to face.  

Many bacteria inhabit the bottom of trenches, feeding off-gases. The bacteria provide food for organisms in ocean trenches by changing the gases from chemical compounds into organic nutrients. The life forms that live in the trenches feed off the organic nutrients as well as marine snow. Scientists believe that studying these bacteria could significantly advance medical knowledge. 

Pollution In Ocean Trenches

Mt. Everest is said to be littered with rubbish that hikers have thrown down and left. Sadly people have managed to spread their waste from the highest heights to the depths of the sea. Scientists have found evidence of waste material at the very bottom of trenches. There is plastic waste material, lead, mercury, and nuclear waste knowns as carbon-14.

The ubiquitous and indestructible plastic bag has even been spotted in the Mariana Trench – the deepest point of the ocean. It would be impossible to clean these ocean depths, and who knows how long these pollutants will exist in the depths. Our best hope is to begin changing people’s habits to create a cleaner environment from the height of Mt. Everest to the depths of the Mariana Trench.  


For many years more was known about outer space than the deepest ocean trenches. The difficulties encountered in exploring these trenches made it challenging to find out anything about them. The darkness, extreme cold, and crushing pressure seemed to preclude life, but findings from recent years show a plethora of life down in the deep-sea trenches. The steep V-shaped valleys are formed at destructive tectonic plate boundaries and are closely associated with volcanoes and earthquakes.

There is still a great deal that can be learned about the ocean trenches that could benefit man in innumerable ways. Bacteria that live in the ocean’s depths could revolutionize human medical knowledge and give us valuable insight into climate changes and changes in the earth’s structure. 

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