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Figure 2.23
Geophysicist reading a gravimeter at Haag Nunataks, an isolated exposure of
Precambrian rock. (Credit: BAS)
Passage in the Scotia Sea region. The South Georgia block has moved from a
position much closer to southern South America to its present position; the South
Orkney block separated from the northern tip of the peninsula to form Powell Basin,
and the South Shetland Islands have separated from the peninsula by sea
oor
spreading in Brans
eld Strait. Although the driving mechanisms may not always be
the same, the history of rotated microplates has been repeated in the South Atlantic
region and raises intriguing questions about breakup processes. The opening of
Drake Passage has also led to the formation of the South Sandwich Islands, an
isolated chain of 11 volcanic islands in the South Atlantic sector of the Southern
Ocean, and one of the Earth
s classic and best preserved arc trench subduction
systems formed entirely of volcanic rocks on oceanic crust devoid of any continental
in
'
uence. The volcanic arc is tectonically simple being located on the small oceanic
Sandwich Plate, which is overriding the southernmost part of the South American
Plate at the South Sandwich Trench. The 11 islands are small, typically 2
-
12 km
across and entirely volcanic in origin. They typically rise 500
-
1000m above sea level
and are subaerial summits of edi
ces that rise some 3 km above the surrounding
sea
oor. The arc is currently volcanically active with intense fumarolic activity on
several of the islands and there have been at least six historic eruptions. Summit
calderas are present on several of the islands with small persistent lava lakes
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