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Australia: The Land Where Time Began |
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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
1,000o 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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Oceanography |
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |