Introduction
At the bottom of the Mariana Trench, pressure exceeds 1,100 atmospheres — equivalent to the weight of 50 jumbo jets stacked on a single square metre. Yet even here, life persists. The deep ocean trenches, where one tectonic plate dives beneath another, are not just the deepest places on Earth — they are the source of its most destructive earthquakes and tsunamis.
On 23 January 1960, Swiss oceanographer Jacques Piccard and US Navy Lieutenant Don Walsh climbed into the bathyscaphe Trieste and descended into the Challenger Deep — the deepest known point in the Mariana Trench, in the western Pacific Ocean. Five hours of descent. At 10,916 metres (later surveys have measured slightly varying depths, with recent estimates converging around 10,935 m (35,878 ft)), they touched the seafloor and reported the most astonishing thing: a flatfish — a sole, they thought — resting on the sediment. Life at the extreme bottom of the ocean. The observation, later disputed by scientists who suggested the animal might have been a sea cucumber, nonetheless established that even the deepest ocean is inhabited.
The Mariana Trench represents one end of a geological process that also produces the most destructive earthquakes and tsunamis on Earth: subduction. Where two tectonic plates converge and one descends into the mantle, the collision creates an asymmetric topography — a deep ocean trench on the ocean side and a volcanic arc on the overriding plate. The trench and its associated subduction zone are not passive scars in the seafloor; they are active machinery. The descending slab carries oceanic sediment and water into the hot mantle, which returns to the surface as arc volcanism. Great megathrust earthquakes occur when the two plates lock and then suddenly release — the 2004 Indian Ocean earthquake (magnitude 9.1), the 2011 Tōhoku earthquake in Japan (magnitude 9.0), and the 1960 Valdivia earthquake in Chile (magnitude 9.5, the largest ever recorded) all occurred on subduction zone faults adjacent to ocean trenches.
The deepest ocean — conventionally defined as water below 6,000 metres (19,686 ft), called the hadal zone — represents only about 0.15% of the total ocean area, yet it has emerged as a scientifically fascinating environment with distinct biology, chemistry, and geophysics. The trenches and the hadal zone are among the least explored places on Earth.
Key Terms
The region where one tectonic plate descends beneath another into the mantle. Subduction zones are the sites of ocean trenches, volcanic arcs, and the world's largest earthquakes (megathrust earthquakes). Oceanic plates subduct preferentially because their mafic rock (density ~3.0 g/cm³) is denser than the felsic continental crust (~2.7 g/cm³) they encounter.
The largest type of earthquake, occurring on the thrust fault interface between a subducting plate and the overriding plate. The two plates lock together due to friction; strain accumulates over decades to centuries; when the lock breaks, the sudden elastic rebound generates seismic waves and, often, tsunamis. Megathrust earthquakes can reach magnitude 9.0–9.5.
Ocean depth below 6,000 m (19,686 ft), corresponding to the deepest ocean trenches. Named after Hades (the underworld). Characterised by extreme hydrostatic pressure (the weight of the overlying water column, 600–1,100 atm), near-freezing temperatures (1–4°C (34–39°F)), complete darkness, and low but not zero food availability. About 0.15% of total ocean area.
A wedge of sediment and crustal rock scraped off the top of the subducting plate and accreted to the leading edge of the overriding plate. Grows over millions of years as subduction continues. Can build up significant topographic features on the landward wall of ocean trenches.
A chain of volcanoes formed on the overriding plate above a subduction zone, roughly parallel to the trench. Oceanic arcs form where oceanic crust overrides oceanic crust (e.g. the Aleutian Islands, the Mariana Islands). Continental arcs form where oceanic crust subducts beneath a continent (e.g. the Andes, the Cascades of the US Pacific Northwest).