South Georgia and its associated islands & rocks are visible parts of a submerged ridge of mountains that runs in an arc from the tip of South America to the tip of the Antarctic Peninsula which also comes to the surface at the South Orkney Islands and the volcanic South Sandwich Islands. It is called the Scotia Arc or ridge after the ship of the Scottish National Antarctic Expedition of 1902-4.
The island’s origins lie in the separation of the present-day southern hemisphere continents by continental drift about 150 million years ago. As the Atlantic Ocean opened, the Pacific Plate dived under the South American Plate ( a process called subduction), with the boundary running down the coast of Chile. The continental land bridge that joined South America and Antarctica did not start to break apart until about 30 million years ago, since when the Scotia Sea has formed and continues to grow. During the formation of the Scotia Sea several small blocks of continental crust became distributed around its margins and South Georgia is one of these micro continental blocks, having been displaced from Tierra del Fuego. While the Atlantic Ocean is still spreading slowly westward at the expense of the Pacific Ocean, the South Atlantic oceanic crust is subducting beneath the South Sandwich Islands leading to the growth of the Scotia Sea and creating the volcanic islands in the last 3 million years.
South Georgia is not of direct volcanic origin, unlike most isolated oceanic islands. The majority of South Georgia’s rocks are derived from the sediments washed down from volcanoes as they eroded. The rock formations in the south western part of the island, south of the Cooper Bay Dislocation Zone, are from more than 200 million years old (pre Jurassic rocks on Cooper Island) to 150 million years old (igneous Jurassic rocks & metamorphic sandstones forming the Salvesen Range of mountains ). Between 110 - 130 million years ago volcanic activity occurred in the very south of the island leading to lava flows near Larsen Harbour and deposits of ash at Annenkov Island.
The bulk of the island from Bird Island in the west to Gold Harbour in the south east is made up of a sequence of sandstones & shales, up to 8 km thick, which were laid down in an Early Cretaceous sedimentary basin around 130 million years ago. Huge quantities of sand & mud were carried across the ocean floor suspended in turbidity currents from opposite sides of the basin and as these currents slowed the sediment particles were deposited, the larger particles first & the silt and mud later. The Cumberland Bay formation came from the volcanic arc side of the basin while the Sandebugten formation came from the Gondwana side. The result of this style of deposition leads to the layered appearance of the rocks forming the Allardyce Range which forms the main spine of the island, which reaches 2915m at the summit of Mount Paget.
Following this prolonged period of sedimentation a change in the movement of the tectonic plates resulted in the closure of the sedimentary basin and the sedimentary rocks were squeezed , folded & uplifted. This took place about 80 million years ago when South Georgia was still attached to South America and evidence of the folding can be seen in many places around the coast. Because of the tectonic plate movement, seismic activity is common at south Georgia with small tremors recorded daily by a seismometer at King Edward Point.
Glaciers first formed on South Georgia about 5 million years ago and around 20,000 years ago the island was smothered in a huge icecap that extended well beyond the present coast and left only the tops of the mountains protruding. Since then the ice covering has gradually shrunk to the present state of over 160 glaciers filling valleys or forming icecaps, but there have been several periods of advance and retreat in between. For example, although there has been general glacial retreat since the 1880’s, meteorological records show South Georgia weather cooled between 1920’s to 1940’s and this is linked to a temporary advance of the glaciers between 1925-35.
Currently half of the island’s area of 3,755 sq.km. is covered with ice and permanent snow and the permanent snow-line generally sits at 450-600 masl on the north coast 300 masl on the colder south side. The land that is not permanently covered is subjected to regular freeze-thaw cycles throughout the year which leads to shattering, weathering and sorting of the rocks, Some of which are covered with a brownish stain or glaze known as "desert varnish"
On the scree slopes below the peaks only a few mosses, lichens & hardy grass tufts can survive on the rocks and in the mineral poor soil. Enrichment of lower slopes by excreta & corpses of animal & bird life, leads to the formation of a richer soil which supports a total of 25 native species of flowering plants and around 60 alien species, inadvertently introduced by whalers, sealers & tourists.
TASKS
1) Within a 50 metre radius of the listing co-ordinates there are several examples of patterned ground such as circles of stones or stone stripes.
(a) Post a photo of one such example in your log with an object to give some idea of scale
(b) Explain briefly how such patterns could have been formed.
2) The jagged rocky summit of Mount Duse will hopefully be clearly visible off to the south east. You are just over half way up to the summit high and, if you have the ability, time & weather to climb it, you should be rewarded with a fabulous view straight down to King Edward Point. However, it is a tough climb as you would have to scramble up lots of loose, unstable scree where progress is the “two steps up, one step down” variety.
(a) You are standing at 870 feet (265 metres) above sea level and you should be able to see the base of the scree ahead and above you. The summit of Mount Duse is 1663 feet, 793 feet higher than you. Estimate how much vertical ascent would be up the scree if you were to climb to the summit.
(b) Explain what is meant by "desert varnish" and how it forms.
3) Most of the rock you have been walking across are the mudstones, shales & sandstones of the Cumberland Bay series of rocks. Go a little further on to S 54 16.600 W 36 29.370 or S 54 16.640 W 36 29.388 where you should see another obvious feature. Tell me what it is and what mineral is it likely to be.
I don´t need pages of description; just a sentence or two for each question would do!
Please log your “find” once you have emailed me your answers and I will get back to you as soon as possible to let you know if they are ‘spot on’ or if there are any errors.