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Australia: The Land Where Time Began |
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Recycling and mountain uplift Thousands of kilometres of mid-ocean ridge
continued producing new ocean crust as
India moved
towards Eurasia, steadily increasing the size of the Indian Ocean. As in
all other major oceans, the process in the Indian Ocean has been
continuing for more the 100 My. If ocean crust continued to be produced
with no way of removing crust at the same rate the surface of the Earth
would need to continually increase to accommodate it, but it is now
known that oldest crust, the furthest from the spreading centre, is
being continually taken back down in to the Earth at subduction zones at
the bottom of deep trenches near the edges of continents. The heavy
oceanic crust dives beneath the lighter continental crust, after which
it is melted and merges with the mantle rock. At the surface of the
descending plate melting occurs due to extremely large levels of
friction in the upper levels of the subduction zone. Rocks carried down
on the plate surface and wet sediments, as well as in the zone of
friction just above, begin melting at depths of 50-100 km, where by this
depth the temperatures have risen to 1200-1500o C. Molten
rocks accumulate in chambers (localised pockets) and begin rising as
Plutons (intrusions) into the lower crust where they form granites as
they cool and crystallise, the main rock type of continents. Where some
of the molten rock breaks through the surface the reduced pressure
allows the magma to erupt with explosive force on land and beneath the
sea. Over time these can form archipelagos (island arcs) that are strung
out behind the subduction zone. The highest number of earthquakes and volcanoes in
the world occur around the edges of the Pacific Ocean.
The Ring of Fire, as it is
surrounded by subduction zones, trenches and island arcs. Some of the
greatest earthquakes and eruptions ever recorded have occurred on the
eastern margin of the Indian Ocean where the oceanic plate plunges into
the subduction zone under the Indonesian arc. According to the author3
the same would have occurred on the northern margin of Tethys, a line of
trenches being strung out in an east-west direction around ¼ of the
circumference of the globe, close to the continent, while in other
places separated from the continent by an island arc and a marginal sea.
There would have been a subtropical climate along its the full length
and there would have been islands surrounded by coral reefs, though it
would also have had what the author3 calls a
Girdle of Fire, the Tethys
Ocean version of the Ring of Fire
around the Pacific Ocean, the whole region being subjected to powerful
earthquakes, violent volcanic eruptions and very large tsunamis. As a result of this geological violence the land
above the subducting plate would have been pushed up, as frictional heat
above the subduction zone caused the formation of deep-seated granite
melts that are less dense than the mantle rock that surrounds them
causing them to rise up through the crust until they are above the land
surface. As the granite plutons are emplaced they are accompanied by
volcanic eruptions producing copious volumes of granite, the rock of the
continents that is lighter than the basalt lavas that form the oceanic
plates. Some of the most common rock types composing granite are
andesite, rhyolite, dacite and others that are rich in quartz.
The pressure required to trigger a volcanic eruption of granite
is much high than that required for basalt and as a result the eruptions
of granitic lava are much more violent than those of basaltic lava.
Granitic eruptions occur far less frequently than basaltic eruptions but
that are far more explosive, spewing out not only lava but also volcanic
dust and ash, but also blocks and bombs. The author3 suggests
that similar mountains would have
been thrust up along the northern margin of Tethys. The material that is eroded and weathered from the
rising mountains as they are gradually denuded is eventually carried by
the torrential rivers to the ocean where most is plastered against the
continental margin. The small amount that reaches the true ocean far
from shore is eventually returned to the continent as the plates
inexorably move towards the subduction zones. Huge slices of sediment,
as well as parts of the ocean crust are scraped off and added to the
overriding continent, being thrust up away from the plunging slab. This
scraped off material is badly deformed, being crumpled and folded and
broken up by faults, and extreme pressures and temperatures partly
convert them to metamorphic rocks. The rocks of the margin of the
continent also fold and buckle upwards as a result of the huge and
relentless pressure of the subducting oceanic plate. Exotic terranes,
giant rafts of oceanic and continental material, foreign to the local
area, are also fused to the growing belt of mountains. Exotic terranes
are often of unknown origin as their source land mass may have been
previously destroyed. The growing mountains preserve at least some clues
to their history. Eventually the ocean closes and there is a
continent to continent collision, this happened when the northern part
of the Tethys Ocean between Eurasia and the approaching Indian Plate was
squeezed out of existence once they had come into contact. The northern
Tethys Ocean Trench was overridden as the rocks of the Eurasian
continent, as with all continents, was too light and buoyant to be
subducted. The rocks of both continents folded and buckled more than
they had done in their previous history, with whole fragments of the
continental margin being thrust up over the growing mountains. The
superimposed slices of rock are called nappes (from the French word for
‘sheet’) The extreme complexity of the Himalayas has meant that there is
still a lot to understand, exactly what happened after the collision,
such as the great height of the mountains, the elevation of the vast
Tibetan Plateau, the over-thickening of their roots that extend far into
the mantle. It has been found that the Indian Plate has penetrated
nearly 2000 km into Asia following the first contact. The Asian crust
was bent and almost flowed as it was pushed aside forming a great
tectonic arc from the Tibetan Plateau to the Three Gorges region of
western Yunnan. Continued slippage and plate movement still occurs
at the line of fusion of the plates, the Shan Shear Zone.
Stow, Dorrik, 2010, Vanished Ocean; How Tethys Reshaped the World, Oxford University Press.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |