Introduction
Every year, mid-ocean ridges produce roughly 3 cubic kilometres (0.72 cu mi) of new volcanic rock — more than all of Earth's land volcanoes combined. This hidden mountain system, stretching 65,000 kilometres (40,391 mi) beneath every ocean, is the most volcanically active feature on the planet and the engine that drives plate tectonics.
In 1977, marine geologists aboard the research submersible Alvin descended to the Galapagos Rift, a section of the mid-ocean ridge system in the eastern Pacific. They were testing a hypothesis: if seafloor spreading is driven by hot magma rising from the mantle, the seafloor at the ridge crest should be warm, and seawater percolating through cracks in the basalt should absorb that heat and emerge somewhere. What they found exceeded all expectations. At a depth of 2,500 metres (8,202 ft), where the temperature of normal bottom water is just 2°C (36°F), they encountered shimmering columns of superheated water pouring from cracks in the seafloor at temperatures above 350°C (662°F). Around these hydrothermal vents, improbably, lived dense colonies of tubeworms, giant clams, shrimp, crabs, and fish — entirely independent of sunlight for energy. Instead, bacteria oxidised hydrogen sulphide emerging from the vents, producing the chemical energy that fuelled the entire food web. The discovery of hydrothermal vent ecosystems overturned the assumption that all life on Earth depends ultimately on photosynthesis, and opened new hypotheses about where life might exist elsewhere in the solar system.
The hydrothermal vents are a consequence of a larger geological system: the mid-ocean ridge. Running through every ocean basin on Earth, the mid-ocean ridge system is a continuous volcanic mountain chain approximately 65,000 kilometres (40,391 mi) long — roughly the circumference of Earth, and ten times the length of the Andes. It is the most volcanically active feature on the planet's surface. Every year, mid-ocean ridges produce approximately 3 km³ (0.72 cu mi) of new basaltic oceanic crust — vastly more volcanic material than all of the world's subaerial volcanoes combined. This continuous creation of new seafloor is the engine of plate tectonics: new crust forms, pushes older crust away from the ridge axis, and ultimately drives the movement of the tectonic plates.
The ridge system was first identified as a continuous global structure by Marie Tharp and Bruce Heezen in the 1950s, using thousands of sonar profiles collected by ships of Columbia University's Lamont-Doherty Geological Observatory. Tharp's meticulous hand-drafting of the seafloor bathymetry, frustrated at several points by the refusal of colleagues to take her data seriously, ultimately produced the first maps to show the global mid-ocean ridge system in its full extent — a scientific contribution that helped launch the plate tectonics revolution.
Key Terms
The process by which new oceanic crust is continuously created at mid-ocean ridges as magma rises from the mantle, solidifies into basalt and gabbro, and is pushed laterally away from the ridge axis. The rate varies from ~2 cm/yr (Mid-Atlantic Ridge) to ~15 cm/yr (East Pacific Rise). First proposed by Harry Hess in 1960.
The central graben (down-dropped block between two faults) that runs along the crest of a mid-ocean ridge, where new oceanic crust is being pulled apart. Most prominent at slow-spreading ridges like the Mid-Atlantic Ridge, where it can be 30–50 km (19–31 mi) wide and 1–2 km (0.6–1.2 mi) deep. At fast-spreading ridges it is shallower and less pronounced.
A type of hydrothermal vent that emits superheated, mineral-rich water at temperatures up to 400°C (752°F). The dark colour results from precipitation of fine-grained metal sulphide minerals (iron, copper, zinc, lead) as the hot vent fluid mixes with cold seawater. Build chimney structures up to 60 m (197 ft) tall over decades.
The process by which microorganisms use the chemical energy stored in reduced compounds (especially hydrogen sulphide, H₂S) to synthesise organic molecules, without sunlight. Chemosynthetic bacteria at hydrothermal vents and cold seeps are the primary producers of deep-sea vent ecosystems, supporting food chains independent of photosynthesis.
A type of fault that offsets segments of a mid-ocean ridge laterally. Transform faults connect ridge segments and absorb the differential spreading rates between them. They are a third type of plate boundary (beside divergent and convergent) and are sites of shallow earthquakes. The Romanche Transform in the equatorial Atlantic is one of the largest.