The Mid-Ocean Ridges:Page II

Numerous earthquakes occur along the Mid-Ocean Ridges. These are rarely of magnitudes greater than 7 on th riechter scale. Also the mid-ocean ridges are the site of 90% of the worlds volcanism. Rarely the ocean ridge emerges above the sea as in Iceland and the volcanism is prodigious with an eruption on average every 5 years.The volcanic country of Iceland straddles the Mid-Atlantic Ridge;This offers scientists an ideal site for studying on land the processes also occurring along the submerged parts of a spreading ridge. Iceland is splitting along the spreading center between the North American and Eurasian Plates, as North America moves westward relative to Eurasia, this is shown below.

The most distinct structures on the sea floor are the Mid-Ocean ridges. The ridges form a long continuous chain standing upto 3km above the abyssal plains of the sea floor. In places valleys parallel to the ridge axis follow a characteristic staggered path, being offset by a series of transform faults. In the mid-Atlantic ridge there is a major central valley at the ridge crest, typically about 10km wide, known as the Central Ridge, this ridge reprsents the spreading centre of the constructive plate boundary.
Seafloor_spreading

The tectonic plates, made of crust and upper mantle that form the Earth's outer shell, move apart at the mid-ocean ridges. Where they separate, white-hot soft mantle rises from great depths in the Earth to fill the gap between. As the mantle rises beneath the axis of the mid-ocean ridge, it melts to yield abundant basalt magma, which seeps up through the overlying rock to gather in a magma chamber a kilometre or so below the ocean floor. From the magma chamber, lava periodically erupts through fissures and pipes to form pillow lava flows. All of this magmatic activity creates new ocean crust, about three square kilometres of new crust, six kilometers, every year. This volume of seafloor volcanism far exceeds that of any type of volcano above sea-level.

The moving plates stretch the young hot crust and crack it open, allowing cold seawater to percolate down close to the magma chamber. There it is heated where it fluxes the mantle peridotite and becomes a hot sulphide and metal-rich fluid, this rises to reach the sea floor at temperatures of up to 400 degrees C. Where this hot hydrothermal fluid emerges, it mixes with cold seawater and precipitates iron, copper, and zinc sulphides, partly as seafloor sulphide deposits and partly as a cloud of black smoke. Here it is possible to see ore deposits in the course of formation.