5.4 How deep are ocean trenches such as the Mariana Trench and the Tonga Trench?



Between the continental margin and the oceanic ridge lies the deep ocean basin. The size of this region, almost 30% of earth’s surface, is roughly comparable to the percentage of land above sea level. This region includes:
1) extremely deep linear depressions in the ocean floor called deep ocean trenches;
2) remarkably flat areas known as abyssal plains;
3) tall volcanic peaks called seamounts and guyots; and
4) large sediment filled and flood basalt provinces called oceanic plateaus.




Deep ocean trenches

Deep ocean trenches are long, relatively narrow creases in the seafloor that form the deepest parts of the ocean. Most trenches are located along the margins of the Pacific Ocean, where many exceed 30,000 ft in depth. A portion of one trench, the Challenger Deep In the Mariana Trench, has been measured at 36,163 ft below sea level making it the deepest known part of the world ocean. The Tonga Trench is over 35,000 feet deep. Only two trenches are located in the Atlantic Ocean, the Puerto Rico trench adjacent to the Lesser Antilles Arc, and the South Sandwich Trench, both in excess of 25,000 feet deep.

Although deep ocean trenches represent only a small portion of the area of the ocean floor, they are nevertheless significant geologic features. Trenches are sites of plate convergence where lithospheric plates subduct and plunge back into the mantle. In addition to earthquakes being created as one plate “scrapes” against another, volcanic activity is also associated with these regions. The trenches are often paralleled by an arc-shaped row of active volcanoes called a volcanic island arc. Furthermore, continental volcanic arcs, such as those making up portions of the Andes and Cascades, are located parallel to trenches that lie adjacent to continental margins. The large number of trenches and associated volcanic activity along the margins of the Pacific Ocean explains why the region is known as the Ring of Fire.

Abyssal Plains

Abyssal plains are deep, incredibly flat features. In fact, these regions are likely the most level places on Earth. The abyssal plain found off the coast of Argentina, for example, has less than 10 feet of relief over a distance exceeding 800 miles. The monotonous topography of abyssal plains is occasionally interrupted by the protruding summit of a partially buried volcanic peak.

Using seismic profilers, instruments that generate signals designed to penetrate far below the ocean floor, researchers have determined that abyssal plains owe their relatively featureless topography to thick accumulations of sediment that have buried an otherwise rugged ocean floor. The nature of the sediment indicates that these plains consist primarily of fine sediments transported far out to sea by turbidity currents, deposits that have precipitated out of seawater, and shells and skeletons of microscopic marine organisms.

Abyssal plains are found in all oceans. However, the Atlantic Ocean has the most extensive abyssal plain because it has few trenches that act as traps for sediment carried down the continental slope.

Seamounts and Guyots

Dotting the ocean floor are submarine volcanoes called seamounts, which may rise hundreds of feet above the surrounding topography. It is estimated that more than a million of these features exist. Some grow large enough to become oceanic Islands but these are rare. Most do not have a sufficiently long eruption history to build a structure above sea level. Although these conical peaks are found on the floors of all the oceans, the greatest number have been identified in the Pacific. Furthermore, seamounts often form linear chains, or in some cases a more continuous volcanic ridge, not to be confused with mid-ocean ridges.

Some, like the Hawaiian Island Emperor seamount chain in the Pacific, which stretches from the Hawaiian Islands to the Aleutian Trench, form over a volcanic hotspot in association with a mantle plume. Others are born near oceanic ridges. If the volcano grows large enough before being carried from the magma source by plate movement, the structure may emerge as an island. Examples in the Atlantic include the Azores, Ascencion, Tristan da Cunha, and Saint Helena.

During the time they exist as islands, some of these volcanic structures are lowered to near sea level by the forces of weathering and erosion. In addition, islands will gradually sink and disappear below the water surface as the moving plate slowly carries them away from the elevated oceanic ridge or hot spot where they originated. Submerged, flat-topped seamounts which formed in this manner are called guyots or tablemounts.

The term guyot is named after Princeton University’s first geology professor, Arnold Henri Guyot.

Oceanic Plateaus

Mantle plumes have also generated several large oceanic plateaus, which resemble the flood basalt provinces found on the continents. Examples of these extensive volcanic structures include the Ontong Java plateau and the Caribbean plateau, which formed from vast outpourings of fluid basaltic lavas onto the ocean floor. Hence, oceanic plateaus are composed mostly of basalts and ultramafic rocks that in some cases exceed 15 miles in thickness.



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