Mid-Atlantic Ridge

The undersea mountain chain running down the Atlantic, where the North American and Eurasian/African plates pull apart and new ocean crust forms.

Seafloor spreading: The process by which new oceanic crust forms at ridges and spreads away on both sides as plates diverge. The ridge is the surface expression of spreading; the two cannot be explained apart.
Alfred Wegener: The German meteorologist who proposed continental drift in 1912 without a plausible driving mechanism. The ridge gave drift a mechanism: continents ride apart as crust forms between them.
Iceland: A volcanic island straddling the ridge, the only place where the Mid-Atlantic Ridge stands above sea level. A hotspot adds magma on top of spreading, raising the ridge above the waves.
East Pacific Rise: The fast-spreading Pacific ridge, spreading about 10 times faster than the Atlantic's mid-rate. Fast spreading builds a broad swell without a rift valley, unlike the Atlantic ridge.
Mohole project: The 1950s-60s American attempt to drill through the crust into the mantle, abandoned amid cost overruns. Drilling the full oceanic crust at ridges remains unfinished; its deepest holes fell far short.
Divergent boundary: A plate boundary where two plates move apart, thinning the crust and allowing magma to rise. The ridge is the longest and best-studied example of this boundary type.
Harry Hess: The American geologist who proposed in 1962 that seafloor spreading drives continental motion. He had surveyed the ridge during the war and called it 'an oceanic rift'.
Thingvellir rift: The rift valley in Iceland where the Eurasian and North American plates separate visibly on land. A place to walk between the plates; the site of the world's first parliament too.
Subduction zone: A convergent boundary where oceanic plate descends into the mantle, recycling crust. Ridges create crust; subduction zones destroy it. The Atlantic has few of the latter.
Deep biosphere: Microbial life living in rock pores and fractures kilometers beneath the seafloor. Ridge flanks circulate vast volumes of seawater, feeding microbes far from sunlight.

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