Deception Island is a volcanic caldera west of the Antarctica Peninsula. It is part of the volcanic South Shetland Islands and one of the two currently active volcanoes on the continent. Total land area of the island is just under 100 km2. Nearly 60% of the island is covered by glaciers. The caldera is sea-filled, with a narrow opening to the sea, making it a safe harbor from the surrounding Southern Sea.
There is a ring of hills around the caldera. The island has the only geothermal lagoon in the Antarctic. Deception Island has a protected interior port which has long been used as a safe harbor for visiting boats.
Climate is polar maritime, with mean annual temperatures of -3 C. Average yearly rainfall is 500 mm.
The island has unique flora, with 18 species of lichen and moss not found elsewhere. It is home to sea birds that breed on the island. It hosts the largest known colony of chinstrap penguins, with over 100,000 nesting pairs. Sea life includes kelp, sponges, and starfish. On writer describes an undersea wall below the island as “… one of the most beautiful underwater sites I have seen around the Antarctic Peninsula.”
Today, the island hosts a pair of scientific stations owned by Argentina and Spain. It is also one of the most visited tourist sites in Antarctica.
It was initially discovered by British sealers in early 1820, perhaps as early as 1819. An American sealing sloop visited that summer. Over a few years, a fur sealing industry grew in the South Shetlands, with nearly 100 ships active 1821 – 1822. Massive overhunting nearly drove the seals extinct by 1825 when Deception Island was first abandoned.
Whalers established a base of operations on the island in 1906. A fleet of ships operated until 1931 when massive overhunting of whales also made this business too uneconomic to continue (we humans keep on making the same mistakes). Whales were captured, killed, blubber processed for oil which was stored in tanks and shipped to the rest of the world. There is a cemetery with 35 whalers who perished during the 25 years of operations. It contains a monument to another 10 missing whalers. The boilers and storage tanks remain along with the remains of boats and buildings.
The British formally claimed the island in 1908.
Argentina, Chile and Britain argued about ownership of the island from 1940s – 1950s. In 1944, Britain took over the old whaling station, dumping Argentinian flags. The expressed concern was about Argentine sympathies with Germany during the war. The Germans never came, though the act did encourage Argentina and Chile to build bases nearby.
A series of volcanic eruptions forced the British and Chileans to abandon the island in 1967.
Over the years, Deception Island has turned into a significant tourist destination when visiting Antarctica.
South Shetland Islands
The South Shetland Islands are a group of volcanic islands 120 km north and west of the Antarctic Peninsula. They include 11 major islands and numerous minor ones stretching some 540 km. Total area is just under 3,700 km2, over 80% of which is permanently covered by glaciers. The sea around the islands is closed by ice April to December yearly.
The islands are subject to conflicting territorial claims, claimed by Great Britain since 1908, Chile since 1940, and Argentina since 1943. Today, they along with the rest of Antarctica are governed by the signatories to the Antarctic Treaty. There are 18 current research stations on the islands and another three field camps owned by 13 nations.
The largest island is King George followed in size by Livingston.
Elephant Island is a largish desolate island located some 180 km NE from King George Island. It does not support any native plant or animal life, though migratory seals and penguins may be found. There is no safe anchorage on the island. Climate is normally foggy with snow and winds can reach 160 km/hr.
It is this island that the crew of Ernest Shackleton found in 1916 after their ship, the Endurance was crushed by ice in the Weddell Sea. 28 men reached the island after spending months on drifting ice. Shackleton and five others sailed in an open lifeboat nearly 1,300 km to South Georgia over 16 days for help. They crossed the island’s interior to find a whaling station on the other side of the island where they organized a rescue party for the remaining crew on Elephant Island. The fourth attempt was successful.
South Shetland Islands Volcanoes
Progression of volcanic activity on the South Shetland Islands is generally from west to east. The main string of islands are all volcanic, with activity being ancient. For instance, volcanic debris on Livingston dates back to the Triassic. King George volcanic debris is similarly dated.
The three newest volcanoes of the South Shetland Islands are Bridgeman Island, Deception Island and Penguin Island. All sit to the east of the main island string. Two of these are recently active.
Bridgeman Island was constructed along the Bransfield Rift spreading center between the Shetland and Wedell microplates. It is a 240 m tall remains of a large submarine volcano layered with lava and pyroclastic flows. The island is surrounded with steep cliffs created by flank collapse episodes. Today is measures 0.9 x 0.6 km. It is classified as an inactive Pleistocene – Holocene stratovolcano. There are disputed reports of 19th Century fumarole activity that may refer to the much younger Penguin Island. There are no dates listed for a last known eruption. The volcano consists of andesites and basalts.
Penguin Island is much younger and located off the SE coast of King George Island. It measures 1.4 x 1.7 km and has a scoria cone named Deacon Peak measuring some 180 m high. The scoria cone was formed some 300 years ago. There is a 300 m wide maar that dates around 100 years ago. The last known eruption was 1905. While not currently erupting, volcanic activity on the island is sufficiently recent to consider it active. Primary eruptive products are basalts.
Deception Island is an active volcano, a restless caldera. The flooded floor of the caldera is one of the most protected ports in all of Antarctica. The island is approximately circular at 12 km in diameter. The highest peak is 542 m. The caldera, now called Port Foster, measures 9 x 6 km. The entrance is 230 m wide with known hazards in the middle of the channel.
There are black sand beaches around the interior of the caldera. The hydrothermal system powers active hot springs and warms the sands in places. There are a number of maars, cinder cones, and pyroclastic cones on the interior shore of the caldera.
Major eruptive products are mainly basalts with variations including trachybasalts, trachyandesite, and trachytes.
The caldera forming eruption took place some 10,000 years ago. It ejected at least 30 km3 DRE of magma. Some estimates are as large as 60 km3. The summit collapsed to form the Port Foster caldera. The eruption is classified as VEI 6.
Deception Island was initially thought to be a classic ring-fault caldera. More recently, the orientation of normal faults in the caldera have been explained by the extension of the Bransfield Strait rather than a typical ring fault collapse.
A study of penguin population dynamics on Ardley Island, some 120 km NW found three population growth phases terminated by sudden crashes, blamed on volcanic eruptions from Deception Island over the last 7,000 years.
There are numerous tephra layers documenting eruptions out of Deception with 5 taking place between 1641 – 600 AD, and another 10 between 100 AD – 3250 BC.
The volcano was particularly active in the 18th and 19th centuries with explosive eruptions.
There were a pair of eruptive periods in the 20th century in 1906 – 1910 and 1967 – 1970. The 1970 eruption was the most violent recent eruption on Deception Island. It began with a large phreatomagmatic eruption and continued from five marine craters and seven land vents. It took place in the same general location as the 1967 eruptions. The 1969 eruption opened a series of rifts beneath glacial ice. The rifts measured 4 km along the east interior of the caldera. The activity produced catastrophic lahars and several small cinder cones. Eruptions in the 20th century were all VEI 3 in severity.
There was a satellite image of a plume from Deception Island in 1987 extending 100 km SE. Local seismic activity was detected during this event but no evidence of an eruption was found during later field surveys. Phreatomagmatic explosion perhaps?
The period of enhanced seismic activity in 1992 included ground deformation and increased water temperatures in the hot springs.
The Spanish Antarctic Program has conducted monitoring at Deception Island during the summer period since 1989. During 2014 – 2015 the survey noted sustained deformation and increased seismicity. The survey showed an accumulated deformation of 10 cm. Seismic events during this period were an order of magnitude more numerous than previous years and the most recorded since 1999. Both tectonic and volcano-related earthquakes were common but too small to be recorded across the entire network.
Today the floor of the caldera is rising as geothermal activity continues. The Smithsonian GVP list of Deception Island eruptions suggests that there is no favorite location for new activity, with numerous eruptions on the north, south and west sides of the caldera. The volcano is classified as a restless caldera with significant volcanic risk.
There are maars on the shores of the caldera. The largest is about a km in diameter. The caldera floor is 190 m deep under the surface of the lagoon.
The South Shetland Islands are back arc volcanic islands formed by the last surviving portion of a subduction zone that extended along the western margin of the Antarctic Peninsula. There is a South Shetland Trench offshore to the west. That trench was formed by subduction of the extinct Phoenix Plate and is currently mobilized by a spreading ridge in the remains of that plate.
It is called a tectonic microplate bordered by the Antarctic and Scotia Plates with complex boundaries.
The tectonic region is unique in that there is no relative motion between the plate segments, as they are both part of the Antarctica Plate. The subduction zone shortened as the spreading ridge in the Phoenix Plate collided with the Antarctic Plate with the only remaining segments going inactive around 4 Ma.
The shape of the trench is similar to other active trenches, but has low levels of seismicity and no shallow thrust faulting earthquakes seen in most subduction zones. If subduction is still taking place, it should be equal to the spreading rate of the Bransfield Strait between the island chain and the Antarctic Peninsula as there is no relative movement between the two. Preliminary GPS measurements indicate a subduction rate around 1 cm/yr, placing the trench as an extreme example of hot subduction slow convergence of young oceanic lithosphere ( 14 Ma at the SW end to 23 Ma as the NE end of the trench).
Crustal thickness varies from around 10 km in the central portion of the Bransfield Basin to 15 km in the SW. The crust is undergoing extension but no new oceanic crust is being formed.
One of the things that happens with subducting slabs is that they heat up as they enter the mantle. Earthquakes disappear above a certain temperature. These are called transition temperatures, around 400 C for crustal rocks and 800 C for mantle rocks. Young slabs and slowly subducting slabs reach the transition temperatures at a shallower depth than a quickly subducting plate. As a comparison, Cascadia is subducting at 4 cm/yr.
An analysis of earthquakes in the late 1990s shows the distribution of earthquakes similar to the distribution of earthquakes in the Cascades. This distribution shows no clear rifting zones south of Deception Island, leading to a conclusion that the South Shetland microplate must be moving slowly NW relative to the rest of the Antarctic Plate.
There is enough magma available from the ongoing subduction to power a few active surface volcanoes and a number of submarine volcanoes.
The South Shetland Islands are one of the more inhospitable locations on the planet. Tectonic activity driving their movement and volcanic activity is complex, but very slow. As a result, volcanic activity and products behave a lot like intraplate volcanoes. I was struck by the ancient ages of the larger islands. Deception is the most recently active volcanoes in the islands. It is one of only two currently active volcanoes in Antarctica. Given its history, it would be a stretch to conclude that its eruptions have ceased or will cease any time soon.