The Mariana Trench or Marianas Trench[1] is the deepest part of the world's oceans. It is located in the western Pacific Ocean, to the east of the Mariana Islands. The trench is about 2,550 kilometres (1,580 mi) long but has a mean width of only 69 kilometres (43 mi). It reaches a maximum-known depth of about 10.91 kilometres (6.78 mi) at the Challenger Deep, a small slot-shaped valley in its floor, at its southern end,[2] although some unrepeated measurements place the deepest portion at 11.03 kilometres (6.85 mi).[3] If Mount Everest, the highest mountain on Earth at 8,848 metres (29,029 ft), was set in the deepest part of the Mariana Trench, there would be 2,060 metres (6,760 ft) of water left above it.[4]
The Mariana Trench is part of the Izu-Bonin-Mariana Arc geological boundary system that forms the boundary between two tectonic plates. In this system, the western edge of one plate, the Pacific Plate, is subducted beneath the smaller Mariana Plate that lies to the west. Because the Pacific plate is the largest of all the tectonic plates on Earth, crustal material at its western edge has had a long time since formation (up to 170 million years) to compact and become very dense; hence its great height-difference relative to the higher-riding Mariana Plate, at the point where the Pacific Plate crust is subducted. This deep area is the Mariana Trench proper. The movement of these plates is also indirectly responsible for the formation of the Mariana Islands (which are caused by volcanism as a result of subduction of water trapped in minerals).
At the bottom of the trench, where the plates meet, the water column above exerts a pressure of 1,086 bars (15,750 psi), over one thousand times the standard atmospheric pressure at sea level.
Because the Earth is not a perfect sphere the trench is not the part of the seafloor closest to the center of the Earth – parts of the Arctic Ocean seabed are at least 13 kilometres (8.1 mi) closer to the Earth's center than the Challenger Deep seafloor.
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The Mariana Islands were claimed by Spain in 1668. Spain established a colony there, and gave the islands the official title of Las Marianas in honor of Spanish Queen Mariana of Austria, widow of Philip IV of Spain. Thus the name of the trench derived from this. The islands are part of the island arc that is formed on the over-riding plate, the Mariana Plate, on the western side of the trench. The Pacific plate is being subducted under the islands, forming the trench.
The trench was first sounded during the Challenger expedition (December 1872 – May 1876), which recorded a depth of 4,475 fathoms, 8,184 m (26,850 feet). In 1877 a map was published called Tiefenkarte des Grossen Oceans by Petermann, which showed a Challenger Tief at the location of that sounding. In 1899 USS Nero, a converted collier, recorded a depth of 5269 fathoms (9,636 m, 31,614 ft).[5] Challenger II surveyed the trench using echo sounding, a much more precise and vastly easier way to measure depth than the sounding equipment and drag lines used in the original expedition. During this survey, the deepest part of the trench was recorded when the Challenger II measured a depth of 5,960 fathoms (10,900 m, 35,760 ft) at , known as the Challenger Deep.[4]
In 1957, the Soviet vessel Vityaz reported a depth of 11,034 m (36,201 ft), dubbed the Mariana Hollow.[3]
In 1962, the surface ship M.V. Spencer F. Baird recorded a maximum depth of 10,915 m (35,840 ft), using precision depth gauges.
In 1984, the Japanese survey vessel Takuyō (拓洋), collected data from the Mariana Trench using a narrow, multi-beam echo sounder; it reported a maximum depth of 10,924 m, also reported as 10,920 metres ± 10 metres.[6]
In 2003, a spot was found along the Mariana Trench, the depth of which is around the same depth as the Challenger Deep, possibly even deeper. It was discovered while scientists from the Hawaii Institute of Geophysics and Planetology were completing a survey around Guam; they used a sonar mapping system towed behind the research ship to conduct the survey. This new spot was named the HMRG (Hawaii Mapping Research Group) Deep, after the group of scientists who discovered it.[7]
On 1 June 2009 sonar mapping of the Challenger Deep by the Simrad EM120 sonar multibeam bathymetry system for deep water (300 - 11,000 m) mapping aboard the RV Kilo Moana (mothership of the Nereus vehicle), has indicated a spot with a depth of 10,971 m (35,994 ft). The sonar system uses phase and amplitude bottom detection, with an accuracy of better than 0.2% of water depth across the entire swath (implying the depth figure is accurate to less than ± 11 metres).[8][9]
The Swiss-designed, Italian-built, United States Navy bathyscaphe Trieste reached the bottom at 1:06 p.m. on 23 January 1960, with U.S. Navy Lieutenant Don Walsh and Jacques Piccard on board.[4] Iron shot was used for ballast, with gasoline for buoyancy.[4] The onboard systems indicated a depth of 11,521 m (37,799 ft), but this was later revised to 10,916 m (35,814 ft).[10] At the bottom, Walsh and Piccard were surprised to discover sole or flounder about 30 cm (1 ft) long,[10] as well as a shrimp.[1] According to Piccard, "The bottom appeared light and clear, a waste of firm diatomaceous ooze".[10]
Only three descents have ever been achieved. The first was the manned descent by Trieste in 1960. This was followed by the unmanned ROVs Kaikō in 1996 and Nereus in 2009. These three expeditions directly measured very similar depths of 10,902 to 10,916 m.
Like other oceanic trenches, the Mariana Trench has been proposed as a site for nuclear waste disposal,[11][12] in the hope that tectonic plate subduction occurring at the site might eventually push the nuclear waste deep into the Earth's mantle. However, ocean dumping of nuclear waste is prohibited by international law.[13][11][12] Furthermore, plate subduction zones are associated with very large megathrust earthquakes of which the effects on any specific site are unpredictable and possibly adverse to the safety of long-term disposal.[12]
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