Like on land, we know that there is life in the sea. And if there's life, there are also ecosystems, habitats, and so on. Here on land, we have mountains as high as 8,000 meters above the ground. But in the ocean, there are inverted mountains that are hollow inside and can go farther into the earth than the mountains can go farther in the sky. These are what we call "Deep Sea Trenches".
Trenches are long, narrow depressions forming the deepest parts of the ocean. Their openings are found on the ocean floor, and these trenches can reach a depth of 6,000 meters to more than 11,000 meters or from 24,000 feet to about 36,000 feet. The deepest known depression is the Marianas Trench which lies east of the Mariana Islands in the North Pacific Ocean. Its depth is around 10,915 meters or some 35,800 feet. The Philippine trench ranks as the third deepest among the major ocean trenches with a depth of 10,033 meters or about some 32,907 feet.
Trenches mostly lie parallel or adjacent to island arcs or mountain ranges of the continental margins. Inside, they are steep and V-shaped. The slopes' steepness range between 4-16 meters, although slopes as steep as 45 meters have been measured in the Tonga Trench of the equitorial South Pacific. Sediments mostly occupy the insides of trenches.
Trenches may look good, but did you know that trenches are associated with earthquakes? At the Pacific Ocean, earthquakes occur close to the trenches, at depths within the Earth of 55 Km or less. Farther away from the trenches, earthquakes occur at greater depths starting from 500 Km or more. The trench margins are destructive because this is the region where the oceanic rocks are carried down after colliding with a continental plate. See Seafloor Spreading Hypothesis to learn more about this.
Of the Earth's 20 major trenches, 17 are found in the Pacific. The only trenches in the Atlantic are the Puerto Rico Trench found north of the Caribbean islands, and the South Sandwich Trench found at the east of the Drake Passage located between South America and Antartica. The single major Indian Ocean trench is the Java Trench found in the south of Indonesia. Now, how are these inverted mountains formed? There are two theories that attempts to explain that.
Plate Tectonics
The theory of the Plate Tectonics tells us that the lithosphere is divided into a small number of plates that float on, and travel independently over the Earth's mantle. Much of the Earth's seismic activity and volcanism occurs at the boundaries of these plates. The trenches form and are found in these boundaries. The plates may either be oceanic or continental. This theory provided an explanation on why from one massive piece of land (Pangea), everything broke down and the lands separated. The plates are said to be moving away from each other and also colliding with other plates. Because of these movements, bounderies experience earthquakes.
The Plate Tectonics theory states that the oceanic plate is carried down when it meets a continental plate, and that some plates moving away from each other caused rock formation, which is otherwise known as mantle convection. Changes in the mantle convection initiated the formation of the Atlantic and Indian Oceans by splitting the two major blocks, Laurasia and Gondwanaland some 160 to 180 million years ago. Similarly, past continental collisions are recorded by largely eroded chains, such as the Appalachian Mountains of eastern North America and the Caledonian-Hercynian Mountains of Europe and Africa, which were formed when these continents collided on succesive occasions. The rate of mantle convection depends basically on the square root of heat production within the mantle. This means that convection rates must have been at least twice as fast about 3 billion years ago, when the radiogenic heat being produced was about five times greater than today.
Seafloor Spreading Hypothesis
The Seafloor Spreading Hypothesis is a theory that speculates that the oceanic crust forms along the submarine mountain zones, known as the mid-ocean ridge system, and spreads out latterally away from them. This theory was proposed by the American geophysicist Harry H. Hess in 1960. Hess postulated that molten material from the Earth's mantle continuously wells up along the crests of the mid-ocean ridges that wind up for 60,000 km through all the world's ocean. To have an easier understanding of this, refer to the picture and to the next paragraph.
Imagine two plates under the ocean moving away from each other. The gap they form allows magma or molten material to rise up to the surface. But when it gets to the level of the ocean floor, it cools since the temperature is very cold. These cooled magma form ridges on the ocean floor. The ridges decrease in height when they are farther from the openings.
So where do trenches go into the picture? As one end of the oceanic plate is moving away from another oceanic plate, the other end is colliding with another plate, either to another oceanic plate or a continental plate. When the plate collides to another oceanic plate, mountain-like figures are made but when the plate collides with a continental plate, the oceanic plate is carried down or subducted since the oceanic plate is denser than the continental plate. Why is it denser? Imagine the continental plate as a plain. The only variable that will bring pressure to it is the pressure from the atmosphere making it less dense. Now, imagine the ocean floor, having to deal with the pressure from the atmosphere and the pressure of the water. That is what makes the oceanic plate denser than the continental plate. The oceanic plate is subducted in the zone called the subduction zone. Trenches form and last long because the rate of the plates moving away or towards each other is just 1-2 inches per year.
Since the Seafloor Spreading Hypothesis was introduced, it has been more accepted than the Plate Tectonics Theory as being the closer explanation to the question of why the continents move and along with it, why ridges and trenches are formed. There are distinctive similarites between the two theories because they have almost the same concept that they want to bring out. The only problem with Plate Tectonics was that the nature of Plate Tectonics in Earth's history being relatively faster than today are still uncertain, and that the models of the continental rocks are highly speculative. So, as to be more closer to the more highly accepted explanation, stick to the Seafloor Spreading Hypothesis because the Plate Tectonics theory still has some problems of its own.