Geoscience Reference
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50% of the cliffs. If you stand in the city of Hobart in Tasmania similar sills
can be seen in road cuttings or in the rocky cliffs of Mt Wellington behind.
Volcanic rocks of a similar age, but different in composition, in Southern South
America may even be part of the same extensive Gondwana-wide volcanic province.
The lateral extent of the province has raised intriguing questions as to why and
where this magma was generated and how it related to the ultimate disintegration
of Gondwana. There is little doubt that the Karoo Province, the name given to
the volcanic rocks in southern Africa, came from an abnormally hot part of the
Earth
s mantle that was located exactly where Africa and Antarctica ultimately
separated. Geologists refer to this as a
'
or mantle plume where partial
melting of rising hot mantle forms the volcanic magma. Some geologists suggest
that all the Gondwana magma came from a couple of those hotspots located in this
South Atlantic region. Present day representation of these hot spots may be the
volcanic islands of Marion and Bouvet. If this is the case, magma has
'
hot spot
'
flowed for
thousands of kilometres through the dykes and sills that we see exposed today.
Other geologists prefer a local source for these magmas in the mantle beneath
the exposed sills and dykes. We are also unsure as to whether the volcanic activity
caused the breakup of Gondwana or whether it was simply a secondary response
to a continent that was already rifting apart due to other reasons, and by chance
broke where the mantle was hotter than usual to produce a signi
cant event that
heralded the breakup of Gondwana leading to the continents as we know them
today. The volcanism also had a major effect on the Earth system at that time
affecting atmospheric composition, climate,
flora and fauna.
Gondwana breakup
The initial rifting stage started either synchronous with or just after the
outburst of volcanic activity ultimately leading to a seaway forming between
West (South America and Africa) and East Gondwana (Antarctica, Australia,
India and New Zealand) and to sea
floor spreading in the Somali, Mozambique
and possibly Weddell Sea basins. The second stage occurred in Early Cretaceous
times (circa 130million years ago) when this two-plate system was replaced by three
with South America separating from an African-Indian plate, and the African-
Indian plate from Antarctica. In late Cretaceous times (90
100 million years ago),
New Zealand and Australia started to separate from Antarctica until
-
finally at
approximately 32 million years ago, the breakup of that once large continent
was complete when the tip of South America separated from the Antarctica
Peninsula, opening up the Drake Passage and allowing the formation of the
circum polar current resulting in the cooling of Antarctica and the formation
of the ice sheets.
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