Birth of Ocean Floor
At the crest of every mid-ocean ridge, the lithosphere is being pulled apart and hot mantle rock rises to fill the gap. As this peridotite undergoes decompression melting, basaltic magma erupts onto the seafloor, solidifying into pillow lavas and sheeted dikes. This new oceanic crust — about 7 km thick — is then carried laterally by plate motion, cooling and subsiding as it ages. The process has been continuous for at least 200 million years.
Magnetic Memory
When basalt cools through its Curie temperature (~580°C), iron-bearing minerals lock in the ambient geomagnetic field direction. Because Earth's field reverses polarity every few hundred thousand years on average, the seafloor becomes a striped magnetic tape. Fred Vine and Drummond Matthews recognized in 1963 that these symmetric stripes flanking the ridge are the 'smoking gun' for seafloor spreading — and the definitive proof of continental drift.
Ridge Morphology
Spreading rate controls ridge shape. Fast ridges (>6 cm/yr full rate) like the East Pacific Rise are broad domes with axial highs, sustained magma lenses, and frequent eruptions. Slow ridges (<4 cm/yr) like the Mid-Atlantic Ridge develop deep axial valleys bounded by normal faults, with intermittent magma supply and exposure of mantle rock (serpentinized peridotite) at the surface. Ultra-slow ridges can have almost no volcanism at all.
The Global System
The mid-ocean ridge winds through every ocean basin for 65,000 km — the longest geological feature on Earth. It is segmented by transform faults every 50–500 km, offsetting the ridge axis laterally. At ridge-transform intersections, complex three-dimensional geometry creates deep fracture zones that persist as scars on the seafloor for hundreds of millions of years, recording the spreading history of entire ocean basins.