Tasty tectonics: South Sandwich Islands

If you sail east through Drake's Passage at the southern end of South America, you enter the Scotia Sea. If you continue eastwards a few hundred kilometres further you will reach a string of volcanic islands, the South Sandwich Islands. These were named by Captain Cook in 1775; the same Cook who had a few years previously navigated around the Pacific and the Australian continent. They are a group of around 11 volcanic islands and seamounts that stretch over 400km, north-south. The island chain is a classic example of an intra-oceanic volcanic arc and the tectonics (and history!) of this region are fascinating.

A very abbreviated tectonic summary

In the Mesozoic, South America and Antarctica remained connected, retaining their position as part of Gondwana. By around 100 million years ago the two continents began to break apart, as a result of the opening of the Indian and Atlantic oceans. The tear between the two eventually caused the formation of a microplate - the Scotia Plate - that remains the substrate of the Scotia Sea, but whose tectonics became a lot more complicated as time progressed.

Importantly for future penguin populations, as South America was extended during the break up, blocks of South American crust were transported away from the main continent to form smaller blocks that remain on the edge of the Scotia Plate, such as the one that now hosts South Georgia island.

By the early Cenozoic, 60 million years ago time South America began to move westwards under the influence of rapid spreading at the Mid-Atlantic Ridge, which drove the continent westwards. Due to a series of major transform faults at the southern edge of the Mid-Atlantic Ridge, Antarctica was spared much of this movement and remained relatively stationary through the early Cenozoic.

In contrast, for the Scotia Microplate things had just started to get complicated.

The westwards motion of the southern Atlantic oceanic crust now abutted the microplate, and was forced downwards into a subduction system. By around 30 million years ago this subduction system was linked to a complex series of transform faults and back-arc spreading within the Scotia Microplate itself.

As a consequence of this, Drake's Passage began to open more rapidly and the two continents, who had maintained a stubborn connection until the Cenozoic, finally separated. This separation occurred at around the same time as the final breakup of Antarctica and Australia. As a consequence of these great continental separations by around 30 million years ago a major oceanic current had developed, the Antarctic Circum-Polar Current, which had a profound effect on global climate by limiting oceanic heat transfer from the south and contributed to ensuing glaciation and ice sheet development on Antarctica.

Since the middle part of the Cenozoic, around 30 million years ago, oceanic arc magmatism and associated wrenching has uplifted a series of volcanic edifices along the eastern edge of the microplate, forming the South Sandwich Islands.

Compressing nearly 200 million years into a few seconds we get this amazing animation courtesy of GPlates, a portal for which provides an amazing series of different animations of Earth's tectonics.

South Sandwich volcanism

The South Sandwich islands are like icebergs in the ocean - the exposed portions are a fraction of the total volume. The islands are the tips of volcanic edifices that extend several kilometres below the water to the ocean floor.

The South Sandwich Island volcanic rocks are mostly basalt and basaltic andesite, but there are also more silica-rich dacites as well. In terms of the geochemistry, the South Sandwich Island volcanic rocks are typical of oceanic arc magmas, with the basaltic endmembers having enrichment in incompatible trace elements, such as large-ion lithophile elements (e.g. Sr, Ba, K), and depletion in high-field-strength elements (e.g. Nb, Ta, Ti), reflecting their subduction-modified mantle origin.

Oblique view of the topography of the South Sandwich Islands and adjacent sea floor. SRTM15 Topography data accessed via GPlates portal.

Summary

As part of my ongoing effort to educate myself about the tectonics of various parts of the world, I've been poring over Google Earth recently and the South Sandwich islands are one of those places where the geometry of the plates is so intricate, so complex that I just had to do a deep dive into their geological setting. A fantastic special issue of the journal Global and Planetary Change edited by Maldonado et al. (2015) titled "Scotia arc evolution: global implications" provides a wealth of information.

The curvature of the South Sandwich Trench is spectacular and might be a consequence of subduction rollback. The trench is the locus for regular earthquakes, with the Earthquake app showing me that a few magnitude 4 and above earthquakes have happened in this region just in the last few days. Volcanic eruptions are ongoing in the region, and range from stratovolcanoes to volcanic degassing beneath the icy cover that most of the islands possess. The South Sandwich Islands are so remote that there are only a few people - mostly scientists and fisherfolk - that live out there in the roaring gales in the south Atlantic... hardy folk they must be!

The complex tectonic shuffling that occurred between South America and Antarctica as a result of the breakup of Gondwana formed the South Sandwich Islands as a result of the westwards subduction of the Atlantic oceanic crust beneath the oceanic crust beneath the Scotia Sea. The islands represent some of the most remote volcanic activity on the planet and a unique polar environment, a far flung outpost of life in the deep south of the Atlantic Ocean.

St. Andrews Bay, South Georgia. Nearly half a million king penguins breed here. Glaciers in the hinterland are retreating. Photo: Wuquan Cui, 2018. Courtesy EGU.