Seafloor Spreading
Seafloor spreading is the process by which new oceanic crust forms at mid-ocean ridges as magma rises and older crust moves away on both sides.
Definition
Seafloor spreading is how new ocean floor is made: melted rock rises up through a long crack in the middle of the ocean, cools into new rock, and pushes the older rock out to both sides. It occurs at mid-ocean ridges, where tectonic plates diverge; hot mantle rises to fill the gap, partially melts, and erupts as basalt to form new oceanic crust, which then moves away from the ridge on both sides like a conveyor belt. Evidence includes the symmetric pattern of magnetic stripes on either side of ridges, the increase in crust age and sediment thickness away from ridges, and high heat flow at the ridge crest. Proposed by Harry Hess in 1962 and confirmed by Vine and Matthews in 1963 through magnetic anomaly stripes, seafloor spreading produces oceanic crust by decompression melting of upwelling asthenosphere, yielding about $6$ to $7\ \text{km}$ of basalt and gabbro. Full spreading rates range from under $2\ \text{cm/yr}$ (slow, with a deep axial rift valley, as at the Mid-Atlantic Ridge) to over $15\ \text{cm/yr}$ (fast, with a smooth axial high, as at the East Pacific Rise). New crust records the geomagnetic polarity at formation, so stripe widths give spreading rate when matched to the reversal timescale, and ridge elevation follows from thermal contraction, with depth increasing roughly with the square root of crustal age.
Example
The Atlantic Ocean gets about $2.5$ centimeters wider every year, about the length of your thumbnail, because of spreading along the Mid-Atlantic Ridge. That ridge runs the length of the Atlantic and surfaces in Iceland, where you can stand on the boundary between the North American and Eurasian Plates, while the East Pacific Rise spreads faster, at over $10\ \text{cm/yr}$. About $3\ \text{km}^2$ of new seafloor forms worldwide each year, balanced by subduction elsewhere, and the global ridge system is over $65{,}000\ \text{km}$ long. Hydrothermal vents at ridges circulate seawater through hot crust, precipitating metal sulfides and supporting chemosynthetic ecosystems.
Key Insight
The youngest ocean floor is always in the middle of the ocean and the oldest is near the continents, the opposite of what most people guess. Seafloor spreading was the missing mechanism for continental drift: continents move because the ocean floor between them grows, not because they push through it. Spreading rate also influences global sea level. Fast spreading builds broad, elevated ridges that displace seawater onto continents, which is one reason Cretaceous seas flooded the interior of North America.