![]() |
||||||||||||||
Australia: The Land Where Time Began |
||||||||||||||
Supercontinents
Blewett describes supercontinents as amalgamations of nearly all the
continental blocks of the Earth. Geologists in India coined the name
Gondwana in the
late 1800s to describe the concept of a geological entity. This was
first recognised from the floral
Glossopteris
assemblage dating from the
Palaeozoic that was
found only in the continents of the Southern Hemisphere. These
continental landmasses were South America, Africa, Madagascar, India and
Australia, with Antarctica
being added later, and it was believed at the time that this widespread
distribution could only be explained by the presence of ‘land bridges’
that are no longer present that connected all the continents. It has
since become apparent the blocks of continental lithosphere have come
together and then fragmented into separate entities that were
subsequently dispersed by continental drift a number of times in the
Proterozoic and the
Palaeozoic since their earliest formation. The period of maximum packing
of the continental blocks to form a supercontinent defines a time span,
as the amalgamation of a supercontinent has been found to commonly
overlap with the dispersal of the constituent continental blocks.
Fundamental to supercontinent formation and breakup is the interplay of
continental blocks and the dynamics of the mantle, as it is believed
likely that continents move from the locations of upwelling mantle and
towards mantle cells that are downwelling. There are a number of
mechanisms that could power dispersion. Warming of the mantle beneath
the supercontinent could lead to uplift and lateral stresses, because
the upper mantle cannot be cooled by subduction in the interior of the
large landmass. According to an alternative proposal thermal plumes that
are linked to reorganisation of mantle flow can drive dispersal of the
constituent continental blocks. Blewett suggests these 2 mechanisms may
be linked.
The global climate is influenced by the formation and breakup of
supercontinents, as wind patterns and ocean circulation are affected by
the location of continents. Separation of the continents can also affect
eustatic sea level by the volume of the ocean basins being decreased by
the rapid spreading of the seafloor, as occurred in the
Eocene. This
condition results from young
oceanic crust
being relatively buoyant as well as elevated when compared with old
oceanic crust.
Mid-ocean ridges are examples. Feedbacks also exist between global
tectonics, climate
and sea level. Major glacial accumulations (‘icehouse’ episodes, as have
occurred in the end
Precambrian,
end-Ordovician and end-Carboniferous)
have been linked to times when the continents were amalgamated, and
therefore global sea levels were lowered. As occurred in the
Silurian and mid-Cretaceous,
supercontinental dispersal correlates with greenhouse regimes and
continents that were flooded.
An intense monsoonal system was induced in the
Triassic by the
meridional distribution of continents in
Pangaea, with the seasonal
aridity being recorded in red beds on many continents. Glaciation at the
poles can be driven by the opening of circumpolar circulation, such as
resulted from the rifting in the
Cainozoic of Australia
from Antarctica, and the opening of Drake Passage, which led to the ice
sheets that cover Antarctica. In the Northern Hemisphere, glaciation in
the last 1-2 Myr is probably linked to the closure of equatorial
circulation between the
Pacific Ocean and the Caribbean Sea.
|
|
|||||||||||||
|
||||||||||||||
Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |