Schematic view of an oceanic spreading zone.

An oceanic spreading zone or mid-oceanic ridge is a sub-marine tectonic plate boundary of two diverging plates. Such ridges form a network through the Earth's oceans. The largest oceanic spreading zone is the Mid-Atlantic Ridge spreading from the Arctic Ocean, where it continues the Gakkel Ridge, through the whole Atlantic to the junction with the Atlantic-Antarctic Ridge in the Southern Ocean.

The faster the plates diverge, the larger is the region in the solid mantle that contains partially melted rock. Magma ascends into the lower crust where it rests in magma chambers. Sometimes it ascends to the surface where it forms igneous rock. Sea water intrudes into the fissures along the plate boundary, gets heated up by the magma and material from the crust's rock is solved in the up to 400 to 450 C hot water. The water then ascends and comes back up to the sea ground through black or white smokers. The color of the smoke depends on the material that is solved in the water. By cooling the water down the solved minerals precipitate and settle on the ground forming mineral deposits of copper, iron and mangan and other metals.

Major earthquakes are rare along oceanic spreading zones, one example was the earthquake on March 6, 2010, on the East Pacific Rise with a magnitude of 6.3. These earthquakes tend to be not very dangerous, although they are usually shallow, because oceanic ridges are rarely high enough to form inhabitable land and therefore these earthquakes usually occur far away from any inhabited place.

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