Oceanic trenches

Ocean trenches exist as ocean rifts or mid-ocean ridges exist. These regions are interconnected.
An oceanic trench or submarine trench is a very deep underwater depression as low as 11,000 m below the level of the sea. These are areas where the oceanic lithosphere is recycled back down into the terrestrial upper mantle.
Ocean trenches are located mainly along the seismic lines.
The great depths oceans are usually found on the outskirts of continents, along the continental shelves and along the island arcs and not in the middle of the ocean. This is due to the fact that the oceanic plate, supported by the lithosphere plunges, due to its own weight, usually by subduction under the continental plate as a "moving walkway" that disappears beneath the continental crust. It is here that we will find the deepest abyssal trenches.
This great moving walkway of the ocean plates continuously creates new surfaces along the oceanic ridges, in the center of the oceans. Then it runs down along of the trenches, beneath the continents, and thus makes them disappear at the same rate the old surfaces. In other words, the recycling of the lithosphere creates many new oceanic crust in the ridges than it makes them disappear in large subduction trenches. These layers of subduction of tens of kilometers thick sink beneath the continental crust, and thus cause frequent earthquakes in the oceanic trenches.
Our Earth is alive "active" is a thermal machine that produces heat, lots of heat that needs to evacuate. It can not be discharged through conductivity (heat propagation by molecular agitation step by step) because the inner layers of the Earth are far too thick.

It therefore removes essentially by convection (movement of matter) in mid-ocean ridges form ejection of hot gas and hot lava.
The continents are a set of rigid tectonic plates that interact with each other (movements affecting geological structures) or moving away from each other by creating rifts as the middle of the oceans, or approaching from each other, descending into the submarine trenches or sliding over each other as along the San Andreas Fault, located in California, where there is neither creation nor destruction of surface area.
In summary at any time there is equality between creation and destruction of surface area. This results in a permanent renewal of the ocean floor, plunging continuously inside the Earth. This explains why the average age of oceanic plates (the ocean floor) is ≈100 million years, much younger than continental plates (continents) that they can reach 2 to 3 billion years. It is the mobility of the ocean which causes the mobility of the continents.

NB: Alfred Lothar Wegener (1880-1930) German astronomer and climatologist, proposed the hypothesis of the mobility of continents. He published his theory of continental drift in 1912. His theory was based on many arguments geodesical, geophysical, geological and paleontological. In 1915 Wegener emits the hypothesis the existence in the late Paleozoic or primary Era (−541 to −252.2 millions years), a single previous supercontinent, he calls Pangea "all land" which would then be broken.

While the abyssal plains are averaging 5,000 feet deep, the ocean trenches, they can reach 11,000 feet.
However if they are highly mobile (≈ 15 cm per year), they are also relatively narrow, a few dozen kilometers away.
The Mariana Trench in the Pacific, along the Mariana Islands archipelago, is the underwater trench with the deepest 11 033 meters.
The Tonga Trench, reaches 10 882 m.
The Kuril Trench, near Japan, reached a depth of 10,542 m.
The Philippine Trench, east of the Philippines reached a depth of 10,540 m.
The Admiralty Trench, in the Indian Ocean, reached a depth of 9500 m.
The Puerto Rico Trench, Atlantic Ocean, reached a depth of 9218 m.
The topography of the Earth, because of the presence of water, is less well known than other celestial bodies like the Moon or Mars.

The planet is currently mapped by acoustic surveys from ships, but less than 10% of the surface area of the ocean is covered by measures of ocean depth finders. Know if the satellites measure the height of the sea surface, the seamounts are less easily measured. However they can be mapped from their signatures on the ocean surface.
These satellite maps do not replace the traditional data, but offer a global vision.
Geophysical methods can now calculate an overall map of the topography of the seabed from satellite altimeter measurements of ocean surface.

Image: The trenches of the Pacific Ocean, Atlantic Ocean and Indian Ocean.

Planets are assumed to consist of successive layers of increasing density. The materials are in order of their density, the iron center and the iron sulfide, silicates, water, nitrogen, carbon dioxide, ammonia, methane, helium, hydrogen . There is yet no two identical planets in their structures, each with its own characteristics. The native iron first abundant condensate Solid grains of chemical and mineralogical birth fused in the nebulae, the following is called: the sequence of condensation. The first compounds that condense at 1300 ° C, are rich in titanium oxides, aluminum and calcium. To 1050 ° C condenses massive metallic iron and then to 950 ° C, the first case the silicate magnesium silicate and iron. To 800 ° C, to form silicates looser structures, feldspars and iron sulfide. At even lower temperatures condenses silicate containing water at 0 ° C and the water condenses into ice. is a constituent of the Earth's core. Silicon, magnesium silicate and iron are essential components of Earth's mantle. Feldspar, condensate gives the basalt, forms the floor of Earth's oceans. The internal structure of the Earth is divided into several successive envelopes, the Earth's crust, mantle and core. This representation is very simplified since the envelopes can themselves be decomposed. To identify these layers, seismologists use seismic waves, when the speed of a seismic wave suddenly changes, there is change of environment, so layer. This method allowed, for example, determine the state of matter at great depths (deep mantle, core). These layers are bounded by discontinuities such as the Moho between the crust and mantle, that of Gutenberg between the mantle and core. The Earth formed by accretion of meteorites and the different layers are implemented in accordance with the density of its constituents. Theory of plate tectonics is now recognized since the late 1960s and widely needed in the scientific world. In the 19th century it was hard to believe that entire continents could drift.

We know now that the solid mantle is driven by huge convection currents that flow over millions of years.
The image we have now is a complex and active planet whose crust is composed of oceanic and continental plates of different mineral compositions, constantly moving under the combined action of convection currents and internal gravity land. Continental blocks formed by collision of continental plates and tear, according to a cycle of 400 million years. Oceanic plates are heavier, are participating in this ballet constant for several billion years and often end up diving in the Earth's interior by subduction, thus helping to recycle the Earth's crust whose thickness varies between a few kilometers and 65 km. The seed (inner core) is a solid ball of 1220 km radius, located at the center of the Earth. Seismologists suspect the existence of an almond in it. It is surrounded by the liquid core composed of an alloy of molten iron. The seed increases, by crystallization of liquid iron core, which cools slowly.

Image: The structure of the Earth: the continental crust on the surface has a thickness of 30 to 65 km, upper mantle with a thickness of 670 km, the lower mantle with a thickness of 2180 km, the outer core with a thickness of 2270 km, the inner core of a thickness of 1220 km.

The satellites are among the tools of observation of the deep, are studied since the 1990s, the ocean floor from space. From April 1994 to March 1995, the European satellite ERS-1, with its radar altimeter, conducted a comprehensive mapping of the seafloor topography. It has provided an overview of the global ocean with a resolution of few kilometers. It includes the already known structures such as ridges, large pits, but we also discover millions of small submarine volcanoes unknown. These altimetry also inform us about other aspects of the deep: plate tectonics (study by volcanic alignments and direction of faults), the underwater volcanoes, the structure of the lithosphere,...

NB: The topography can be measured and then represented on a map the shapes and details visible from Earth. Bathymetry is the science of measuring ocean depths to determine the topography of the seafloor.

Image: Bathymetry and topography of the land and oceans to -12,000 + 9000 meters (satellite data). Ifremer source.