Introduction
The ocean floor is the largest and least explored terrain on Earth — less than 25% has been mapped at the resolution we routinely apply to the surface of Mars. Its average depth of 3.7 kilometres (2.3 mi) means the ocean holds a volume so vast that if you melted all of Antarctica, sea level would rise only about 60 metres (197 ft) — still leaving most of the deep seafloor untouched.
The ocean is not simply a uniform basin filled with water. It is a topographically complex world — mountain ranges thousands of kilometres long, plains larger than any continent, trenches so deep that Mount Everest could disappear within them with a kilometre to spare. Yet until the mid-twentieth century, almost none of this was known. The ocean floor was assumed to be a featureless, sediment-draped abyss, largely because the only tools available for sounding its depth were weighted lines dropped by hand from ships — a process so slow and imprecise that a comprehensive global survey was essentially impossible.
The technology that changed everything was sonar (Sound Navigation and Ranging), developed during World War I to detect submarines. By emitting pulses of sound from the ship and measuring the time for the echo to return from the seafloor, ships could measure depth continuously as they sailed. The first systematic sonar surveys, carried out by the US Navy and civilian research ships from the 1940s onward, began to reveal the true character of the ocean bottom. Marie Tharp, a geologist working at Columbia University's Lamont-Doherty Geological Observatory, spent years compiling thousands of sonar profiles into the first scientific maps of the Atlantic seafloor. Her maps revealed the Mid-Atlantic Ridge — a continuous underwater mountain range running the full length of the ocean basin — and convinced her colleague Bruce Heezen that the ocean floor was geologically active. The combination of Tharp and Heezen's work, and the broader Navy mapping programme, provided much of the data that would eventually confirm seafloor spreading (the continuous creation of new ocean floor at mid-ocean ridges, which pushes existing seafloor outward) and plate tectonics (the theory that Earth's surface is divided into large moving plates).
Today, multibeam sonar and satellite-derived gravity measurements have mapped the ocean floor at resolutions of 1–2 kilometres (0.6–1.2 mi). Remarkably, less than 25% of the ocean floor has been mapped at the resolution of 100 metres (328 ft) or better — the standard we apply routinely to land surfaces and to the surfaces of Mars and the Moon. The deep ocean remains the least explored terrain on Earth.
Key Terms
The gently sloping, shallow (0–200 m (0–656 ft)) submerged extension of the continent, averaging about 75 km (47 mi) wide but ranging from nearly absent (off active margins like the US west coast) to over 1,000 km (621 mi) wide (off Australia and the Arctic). Geologically it is continental crust, not oceanic. It is where most marine fisheries and hydrocarbon resources are found.
The steeper seaward face of the continental shelf, descending from the shelf break (at roughly 200 m (656 ft) depth) to the continental rise or ocean floor at 2,000–5,000 m (6,562–16,405 ft). Gradients of 3–6°, cut by submarine canyons. The boundary between continental and oceanic crust is typically somewhere beneath the slope.
The vast, nearly flat ocean floor at depths of 3,000–6,000 m (9,843–19,686 ft), accounting for about 40% of Earth's total surface. Formed by the gradual accumulation of fine-grained sediment that buries the rough basaltic topography of old oceanic crust. The flattest terrain on Earth.
A continuous underwater volcanic mountain system formed where tectonic plates diverge and new oceanic crust is created by seafloor spreading. The Mid-Ocean Ridge system is 65,000 km (40,391 mi) long — the longest mountain range on Earth — and rises 2,000–3,000 m (6,562–9,843 ft) above the surrounding seafloor.
A long, narrow, steep-sided depression in the ocean floor formed where one tectonic plate subducts beneath another. The deepest places on Earth. The Mariana Trench in the western Pacific reaches 11,034 m (36,203 ft) below sea level at Challenger Deep.