The global mid-ocean ridge system

Australia: The Land Where Time Began

The global mid-ocean ridge system

This system of mountain ranges is mostly 2.5 km below the sea surface, and up to 3 km above the adjacent ocean floor, the world’s longest chain of mountain ranges that are 75,000 km long and reaching up to 1,000 km wide. They extend from the Arctic Ocean, almost directly beneath the North Pole, a range running down the axis of both the North and South Atlantic Ocean to connect with a range that completely encircles the Antarctic continent. From this range encircling Antarctica branches run north in the Pacific and Indian Oceans. The ridge system is a continuous chain of volcanoes that exude the black basaltic lava of the seafloor along the length of the ridge system along the axis of a wide axial rift. The lava is brought to the surface by huge convection cells in the mantle. As the magma approaches the surface it accumulates in a large magma chamber a few kilometres beneath the seafloor, from where it forces it way up through the crust, that is fractured and weakened crust, repeatedly into the deep oceans in the form of sheets and pipes. These deep submarine eruptions are in the form of oozing larva, rather than violent eruptions, that result in the formation of pillow lava, small rounded domes. The lava at more than 1,000^o C meets ocean water that is close to freezing, glassy rims of the ‘pillows’ indicating that the lava is chilled almost immediately.

A new crest of the range is continually being formed as the older sides are being moved apart. The author³ suggests the range is so high because of the great heat at its centre, and as it moves further from the central vents it slowly cools and sinks to become part of the moving plates. It is also continually accumulating sediment from the upper levels of the ocean and the eroded material from the continents that helps push it down.

It has been estimated that about 3.5 km² of ocean crust is produced each year, and the spreading rates from the spreading centres in the mid-ocean ridges is a few cm/yr, though the rates can vary. It has been found that the ocean floor increases in age away from the ridge crests.

Mid-ocean islands

At a few places in the oceans the high points on the mid-ocean ridges break through the sea surface as islands. These include St Helena in the South Atlantic, where Napoleon was exiled, and a much larger island is Iceland in the North Atlantic, that lies atop a mantle plume that has remained there since the Atlantic first formed. The site of the first Icelandic ‘parliament’ was in the Thingvellir Valley that is the central axial rift of the mid-Atlantic spreading centre.

The island archipelago that is the Azores in another portion of the mid-ocean ridge that has been raised above the surface by a long-lived hotspot beneath the ridge, the 9 main islands are volcanic and are all isolated and rugged. On the island of Faial there is a lighthouse that is now just inland from the shore and has layers of fresh volcanic ash around it. In the 1940s a new volcanic dome rose above the surface of the sea, continuing to erupt intermittently for months. In the same year the island of Faial was extended and now has sharp cliffs towering 400 m above the sea.

There are 2 other islands, Flores and Corvo that rose from the sea at the same time as Faial but on the other side of the ridge that are now 120 miles west of Faial and still moving imperceptivity away. The other islands are on the same side of the ridge as Faial. It was these islands that were visited by Christopher Columbus. The Azores are on a triple junction of tectonic plates where 2 arms of the mid-Atlantic ridge intercept the Azores Fracture Zone, a very ancient fracture line running east across the Atlantic and into the Mediterranean Sea through the Straits of Gibraltar, where it becomes a gigantic suture that has healed at the point of final closure of the Tethys Ocean. The author³ suggests there are fragments of the seafloor and spreading centre, as well as possibly some islands that broke the surface of the Tethys Ocean that are preserved as ophiolites.

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