Geoscience Reference
In-Depth Information
of crystalline rock which is partially melting. Along this line, the hot, soft asthenosphere
rises to meet a thin ocean crust with no hard lithospheric mantle in between. Because this
mantle material is hot it is less dense and so makes the ridge rise. About 4% of the mantle
rock melts to form the basalt magma which percolates up through pores and fissures into a
magma chamber a kilometre or so beneath the ridge. Seismic profiles reveal magma cham-
bers several kilometres wide under parts of the Pacific ridge, though they are harder to see
under the Atlantic ridge. Material in the magma chamber is slowly cooling, so some crys-
tallizes out and accumulates at the bottom of the chamber to form a coarse-textured rock
called gabbro. The remaining melt periodically erupts from fissures along the ridge. It is
quite fluid and doesn't contain much gas or steam, so the eruptions are fairly gentle. But
the lava is rapidly quenched by the sea water and tends to form into a series of pillow-like
structures.
11. The principal components of a mid-ocean ridge.
Black smokers
Even where there is not an active eruption, the rocks close to the ridge are still very hot. Sea
water gets drawn into cracks and pores in the dry basalt, where it is heated and dissolves
minerals such as sulphides. The hot water then rises out of vents, precipitating sulphide to
form tall, hollow chimneys. Bacteria that can withstand the hot water contribute to the pro-
cess by reducing soluble sulphates to sulphides. As the sulphides come out of solution in
the cooling water, they form a cloud of black particles, so these vents are often known as
black smokers. Water can gush out of them at high velocity and at temperatures in excess
of 350 degrees Celsius, making them hazardous but fascinating to explore in deep-diving
submersibles. The mineral chimneys can grow at a rate of several centimetres per day until
