Divergent plate boundary (seafloor spreading at 10–200 mm/yr)
typical scale
65,000 km continuous global ridge system (~80,000 km total); crest depth ~2,600 m
characteristic lithology
Tholeiitic mid-ocean ridge basalt (MORB) pillows over sheeted dikes and gabbro
examples
Mid-Atlantic Ridge (Iceland), East Pacific Rise, Gakkel Ridge, Central Indian Ridge
Core Principles & Analysis
Mapped across the global ocean basins in the 1950s by Marie Tharp and Bruce Heezen, the mid-ocean ridge system forms a 65,000-kilometer continuous submarine mountain chain — the longest mountain range on Earth.
Along the ridge axis, decompression melting of upwelling asthenospheric mantle generates ~2.7 km² of new seafloor (~19 km³ of basaltic and gabbroic oceanic crust) each year, recording geomagnetic reversals as symmetric magnetic stripes.
Slow-spreading ridges such as the Mid-Atlantic Ridge (~25 mm/yr) feature deep axial rift valleys 10–20 km wide, whereas fast-spreading ridges like the East Pacific Rise (80–145 mm/yr) form smooth axial topographic highs.