Seafloor spreading

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Spreading at a mid-ocean ridge
Diagram of oceanic ridge
World Distribution of Mid-Oceanic Ridges: the big picture.

Seafloor Spreading happens when magma at the bottom of the earth comes out of a crack in a mid-ocean ridge at the bottom of an ocean. When the magma comes out of the mid-ocean ridge, the magma starts to cool down because of the cold water. As it cools down, it flows down the mid-ocean ridge. It keeps going down until it is finally under the crust of the earth once again. There it starts to boil so it gets hot again. After this, The whole process is repeated. So now this means that the newer crust is always at the top of the mid-ocean ridge because new magma is always flowing out of the ground. The older crust is the farthest from the peak of the mid-ocean ridge.

Age of oceanic crust: youngest (red) is along spreading centres.

Mid-ocean ridge[change | change source]

A mid-ocean ridge is an underwater mountain system. This consists of mountain chains, with a rift valley running along its spine, formed by plate tectonics. A mid-ocean ridge marks the boundary between two tectonic plates which are moving apart. A mid ocean ridge is made by a divergent boundary.

The mid-ocean ridges of the world are connected and form a single global mid-oceanic ridge system that is part of every ocean. The mid-oceanic ridge system is the longest mountain range in the world. The continuous mountain range is 65,000 km (40,400 mi) long. It is several times longer than the Andes, the longest continental mountain range. The total length of the oceanic ridge system is 80,000 km (49,700 mi) long.[1]

Description[change | change source]

Mid-ocean ridges are geologically active, with new magma constantly emerging onto the ocean floor and into the crust at and near rifts along the ridge axes. The crystallized magma forms new crust of basalt and gabbro.

The rocks making up the crust below the sea floor are youngest at the axis of the ridge and age with increasing distance from that axis. New magma of basalt composition emerges at and near the axis because of decompression melting in the underlying Earth's mantle.[2]

The oceanic crust is made up of rocks much younger than the Earth itself: oceanic crust in the ocean basins is everywhere less than 200 million years old. The crust is in a constant state of 'renewal' at the ocean ridges. Moving away from the mid-ocean ridge, ocean depth progressively increases; the greatest depths are in ocean trenches. As the oceanic crust moves away from the ridge axis, the peridotite in the underlying mantle cools and becomes more rigid. The crust and the relatively rigid peridotite below it make up the oceanic lithosphere.

Slow spreading ridges like the Mid-Atlantic Ridge have large, wide rift valleys, sometimes as big as 10-20 km wide and very rugged terrain at the ridge crest. By contrast, fast spreading ridges like the East Pacific Rise are narrow, sharp incisions surrounded by generally flat topography that slopes away from the ridge over many hundreds of miles.

References[change | change source]

  1. Cambridge Encyclopedia 2005 - Oceanic ridges
  2. Marjorie Wilson. (1993). Igneous petrogenesis. London: Chapman & Hall. ISBN 9780412533105 .