Irene Preston's Historical Writing
GEOLOGY
In the late Carboniferous and early Permian periods a Super Continent
called Gondwanaland lay far south, with a South Pole located near the
centre. Rocks range from the Carboniferous to the Jurassic, 350 to 150
million years ago. There were 5 land masses known as, India, Africa,
South America, Australia and Antarctica. 265 million years ago glaciers
moved over the landscape with periods of ice melting, sea-rising and
then freezing again, and were responsible for the deposition of layers
of coal, sand and shale called cyclothems. Assorted rocks, boulders and
clays were also deposited called tillites. Coal was deposited in these
ancient ice-age swamps. Fossil remains of glossopteris trees growing at
the time show growth rings, reflecting the effects of changing seasons.
Club mosses, such as lycopsid are not found in these colder regions but
belong to an unchanging tropical environment.
During the early Jurassic period Gondwanaland was isolated by climate
and not by a physical severance. It was actually connected to the
northern continent called Pangea.
At the centre of the earth is a solid metallic core. About 1200km from
the centre metals become liquid and temperatures reach those of the
sun's surface, 5800° cel. Pressures exceed a million atmospheres and the
molten mass churns around a solid inner core of iron propelled by the
rotation of the earth's magnetic field. Convection is a way of
transferring heat from the interior of Earth to its surface. The heat is
carried by the movement of the hot mantle rising, cooling; and the cold
mantle sinking again. Convection cells of the mantle move at rates of
1-10 cent per year taking with it the plates beneath continents and
oceans. 350 million years ago the continent of Gondwanaland was isolated
but it later merged with the continent of the Baltic and Lauratia. This
vast continent, called Pangea, covering most of the earth's surface,
began to break up about 208 million years ago during the Triassic
period. Two great pulses of the earth during the end of the Triassic
period, 213 and 225 million years ago, caused the extinction of 20% of
marine life.
Great land changes brought a transition from seed-fern domination in the
lowland habitats of Gondwanaland, for most of the Triassic, to a conifer
and other gymnosperm flora. This transition reflected a climatic change
on a world scale to more arid conditions. The Jurassic and younger
faunas were vastly different from those previous. Around a 100 million
years ago a shallow inland sea entombed the Cretaceous creatures and
their bones were replaced by a natural silica gel which hardened into
opal.
India's breakaway was finished by the early Eocene period, 150 million
years ago, but Australia's was just beginning. Its separation from
Antarctica started about 70 million years earlier during the early
Cretaceous, along what is now its south coast. A rift valley had
developed in the late Mesozoic, when Gondwanaland began to break up.
Masses of volcanic debris from igneous centres poured into this ever
widening lowland and the open sea gradually encroached from the west.
The British Isles were part of the northern continent of Laurasia, also
known as Caledonia, which crossed the equator. All the continents joined
during the Permian period as Pangea at 10° north; at the same time as
the Rheic ocean, in the area of Devon and Cornwall, closed.
At the same time the southern forests contained few species and show
seasonal growth rings together with other features of high latitudes.
Very poor thin layers of limestone and virtual absence of corals also
suggests high latitudes.
North America separated 180 million years ago and Greenland separated
from Canada 85 million years ago, but the direction of spreading changed
to separate Greenland from Europe 50 million years ago. In the southern
hemisphere, 110 million years ago, the South Atlantic began to form and
South America pivoted away from Africa. The final link between the South
and Central Atlantic was not established until 90 million years ago. The
Indian Ocean started to form 160 million years ago when India and
Antarctica began to separate from Africa.As India moved north it
collided with the island arcs and Asia. Australia and New Zealand
remained attached to Antarctica until 88 million years ago when New
Zealand started to separate, then 45 million years ago Australia also
separated opening up the Gulf of California 10 million years ago. All
these new ocean developments have occurred in areas without subduction
zones, except the Java trench in the East Indian Ocean. This means that
all these new oceans must have expanded at the expense of the Pacific
Ocean where most of the subduction zones lie. Thus the Pacific Ocean
must have been drastically reduced during the last 200 million years,
despite this ocean being the site of the fastest sea-floor spreading
during most of this great period of time. The Siberian side of the
Arctic Basin is still opening.
MODERN-DAY PLATE MOVEMENTS
Apart from the constructive and destructive plates there is a
conservative plate. This type of plate just slides past another in jerky
movements due to friction giving rise to earthquakes. At present there
is only one such plate and it is in the area of California called the
San Andreas Fault. The North American plate comes into contact with the
spreading ridge of the East Pacific Rise. The Californian Coast and
Peninsular are moving towards Alaska at an average of 5cm per year.
FUTURE TRENDS
The Great Rift Valley in East Africa is believed to be the area of a
failed ridge. The Red Sea is at present rifting as a result of tension
and has allowed a section of continental crust to subside providing a
weak point for magma to reach the surface. This may be the beginning of
a new ridge system and eventually Africa will split from Arabia and
collide with Southern Europe. The Mediterranean will develop into
another mountain system. The growth of the rift is likely to have
extended down the East African Rift system causing the area to break
away and become an island similar to Madagascar. Europe, Asia and North
America will approach each other across the North Polar Regions forming
yet another mountain chain. Australia will have developed a much more
extensive chain on its northern borders as it moves towards the island
arcs of South East Asia.