Ocean trenches cover just half a percent of the sea, yet hold its deepest point
Oceanic trenches are long, V-shaped gashes in the seafloor where one tectonic plate dives beneath another. More than 50 major trenches exist, covering only about 0.5 percent of the oceans. The deepest spot ever measured, the Challenger Deep in the Mariana Trench, lies 10,994 metres below sea level.
A typical trench is 50 to 100 kilometres across and sits 3 to 4 kilometres below the surrounding ocean floor, but it can run for thousands of kilometres. Together they stretch roughly 50,000 kilometres, mostly ringing the Pacific, with others in the eastern Indian Ocean and a few elsewhere. They mark convergent plate boundaries, where plates close in on each other at anything from a few millimetres to more than ten centimetres a year, and where oceanic crust bends down to be recycled into the mantle. A chain of volcanoes usually runs parallel, about 200 kilometres away.
Not every seafloor furrow qualifies. Troughs have flat bottoms and steep walls, whereas trenches are V-shaped, though a partly filled trench, such as the Makran Trough, may take the trough name. The Cascadia subduction zone is a trench buried entirely under sediment. When buoyant continental crust reaches a trench, subduction stalls and a collision zone forms instead, as in the Himalayas.
Trenches host strange life. Fluids squeezed out of sediments on the sinking plate return to the surface as mud volcanoes and cold seeps, which feed communities built on chemical-eating microbes. Researchers worry that plastic waste collecting in trenches could threaten them.
Mapping them took a century. The Challenger expedition of 1872 to 1876 made 492 deep soundings and found the Challenger Deep at its station No. 225. In the 1920s and 1930s Felix Vening Meinesz measured gravity from submarines and found belts of low gravity near island arcs. The Second World War and postwar echo sounders revealed the linear shape of the deeps, and in 1960 the bathyscaphe Trieste reached the bottom of the Challenger Deep. Seafloor spreading and plate tectonics in the 1960s then made the trench a key idea in geology.
Source: Oceanic trench
