Just when I think I will be finished with Antarctic volcanoes for a while, along comes a publication that posits one to several mantle plumes stretching from the Antarctic Peninsula to the Ross Sea. As such, it is probably time to discuss volcanoes in the Antarctic Peninsula.
The Antarctic Peninsula is the northernmost part of the Antarctic mainland. It extends some 1,300 km to the north. It ends some 1,000 south of Tierra del Fuego, the southernmost part of South America across the Drake Passage. The previously discussed South Shetland Islands (Antarctica 2) are located across the Bransfield Strait to the west. West Antarctica and Marie Byrd Land (Antarctica 3) is located to the south.
First sighting is disputed but most likely by a Russian ship in January 1820, followed a few days later by a British expedition. There are multiple claims of sovereignty, mostly by Chile, Argentina and Great Britain. These claims have been replaced by the Antarctic Treaty. There are multiple bases and research stations on the peninsula.
The peninsula is mountainous and has an ice cap. It is both the most northerly portion of Antarctica and as such, has the mildest climate on the continent. Warmest temperatures are on the western side. Coldest on the east, with the Weddell Sea and its associated ice shelves. The peninsula also gets significantly more precipitation than the greater portion of the continent.
The peninsula supports a good population of seals, penguins, sea birds. The oceans surrounding it have good amount of krill which in turn supports a population of whales. It is the most visited and most hospitable portion of the continent.
Beethoven Peninsula on Alexander Island
Alexander Island is the largest island in Antarctica. It is located on the south western corner of the peninsula. The surface is mostly ice covered. There are exposed nunataks.
There are two types of volcanic rocks found on Alexander Island. The first is dated in the late Cretaceous. They contain some andesites which were created by subduction of a neighboring mid-ocean ridge some 80 Ma. More recent activity is Neogene, alkaline volcanic rocks which formed after subduction stopped.
There are nine nunataks (basaltic tuyas) scattered over 2,000 km2 (possibly over 7,000 km2) on the Beethoven Peninsula. They are basaltic lava flows and associated hyaloclastites. Youngest rocks are less than a million years old.
There are similarities between volcanoes of the Beethoven Peninsula and those in Marie Byrd Land. There are hyaloclastites that are the result of interaction between basalts and water in a subglacial environment. There are also abundant pyroclastics. Changes from subaerial to aerial activity provides a record of glaciation on both Marie Byrd Land and Alexander Island. The ice sheet during this activity is estimated at greater than 400 m thick locally.
Neighboring Rothschild Island has two small basaltic occurrences with stratified water lapilli tuffs. There are four isolated basaltic outcrops that demonstrate sheet-like eruptive sequences under thin ice.
Southern Black Coast
The Southern Black Coast is on the eastern side of the Antarctic Peninsula. These are all part of the former Andean magmatic belt which was broken into two parts as the Antarctic Peninsula migrated south. The majority of surface volcanic activity took place in the Jurassic, no small amount of it before the breakup of Gondwana itself. These emplaced dikes, pyroclastic tuffs, lava flows on top of basement siltstone and slate. Magma production was due to subducting Pacific Plate underneath the peninsula from the west. There is a significant amount of emplaced magmatic plutons that did not erupt to the surface during the Cretaceous. There is a small intrusive mass of basalt in the Seward Mountains and a batholith in the Wegner range from recent extensional tectonic activity.
The Seal Nunataks are a group of at least 16 volcanic cones emerging from the Larsen Ice Shelf. The highest is 370 m. They are generally aligned along a NE striking line. They are 4 Ma to recent, with subaerial activity 700,000 years ago. Youngest dates measured are less than 200,000 years old. They were covered with glaciers in the past. There is some dispute whether they are separate volcanic islands or pyroclastic cones on a single large volcanic edifice. They are arranged in a generally E – W oriented box, 16 km by 80 km.
Fumarolic activity was observed in 1982. Fresh looking pyroclastics and a lava flow were observed in 1985, suggesting recent eruptions. More recent expeditions found that tephras was only found in ice-covered moraines, suggesting a glacial origin. The 1988 expedition did not observe any fumaroles, though there was some radiant heating of ice-cored moraines. These volcanoes are not visited very often, but generally present some combination of heat, heated fluid flow, fresh volcanic ash, fresh lava flows, and other volcanic debris to each new group of visitors.
There may have been an 1893 eruption of Lindenberg Island as a black ash flow was discovered in 1893 when the island was first visited. At the same time, Christensen Island had fumaroles and remains of a possible eruption. Dalman and Murdoch Islands had active fumaroles in 1982. Shackleton observed banded ice floes on the ice shelf in 1915 with Lindenberg suspected as the likely source.
Ice sheet thickness around the Seal Nunataks is estimated to be 500 – 600 m thick. Heat from volcanic activity has been blamed as the cause of the collapse of the Larsen Ice Shelf. In 2002, it appears that the Larsen B shelf received a massive injection of heat from whatever is powering volcanic activity from these nunataks.
James Ross Island Volcanic Group
The northern Antarctic Peninsula has had perhaps 10 Ma of eruptive activity by basalt volcanoes. Most of it has been subglacial. Lava-fed deltas, capped by basalt flows on top of glacial sediments are located on James Ross, Vega and neighboring islands. These are collectively known as the James Ross Volcanic Group.
Eruptions took place beneath thick ice (> 200 m) and under marine conditions. There are several tuff cone outcroppings. The Antarctic Peninsula Ice Sheet extended over James Ross and Vega Islands when the main volcanoes started to grow, before 6.2 Ma at the latest. Much of the subsequent growth of the volcanoes took place as eruptions interacted with local ice caps. There have been at least 50 manly effusive eruptions over that time. Ice coverage over the last 4.6 Ma has been relatively unchanged.
James Ross Island itself is a shield volcano along the Larsen Rift. It has younger flank tuff cones and other pyroclastic centers. The high point of the island is Mount Haddington, a 1,630 m shield volcano. Tuff and pyroclastic cones on the eastern flank below the summit ice cap are thought to be only a few thousand years old. It measures 76 km by 68 km and is surrounded by a number of related volcanic islands.
The island grew in five phases, the first of which was a single, major volcanic center that started erupting perhaps 10 Ma. It grew by successive emplacement of basalt / breccia units up to over 900 m above sea level. This indicates that at least 900 m of submergence took place. The top 600 m of the island are ice covered. The island re-elevated following stopping of most volcanic activity.
This volcanic group grew in much the same way as massive volcanic centers in Victoria Land, with interaction of basalts with water and ice creating massive piles of breccias and hyaoclastite debris. Like volcanoes in the Hallett, the island has a number of 300+ m high cliffs that stand in between valleys cut by glaciers from the ice sheet. There are a number of benches lower on the islands that are thought to mark fluctuating sea levels.
There are a number of magnetic anomalies west of Vega Island that are thought to reflect volcanic structures that have not yet emerged from the sea floor.
The Antarctic Peninsula was originally interpreted as a platelet created during the breakup of Gondwana that was jostled with other platelets and eventually assumed its final position. Recent papers make the case that it has been constructed by the accretion of terranes much like south central Alaska, with the Alaskan terranes being delivered by the Pacific Plate moving north.
The peninsula consists of six primary geologic units: metamorphic basement, Paleozoic to Triassic sedimentary rocks, Jurassic to Cenozoic sedimentary rocks, non-metamorphosed intrusive rocks, Jurassic to Paleogene volcanic rocks, Neogene to recent alkaline volcanic rocks. Oldest rocks were delivered to Gondwana as that continent was built. The mountains lying along the spine of the peninsula are described as an extension of the Andes that were carried southward when Gondwana broke up.
The Jurassic volcanics were associated with the breakup of the super continent and creation of the Chon Aike and Karoo – Ferrar large igneous provinces. Extension along the Weddell Sea and associated crustal thinning took place along the east part of the peninsula. Subduction along the peninsula waxed and waned over time, mostly stopping in the later Jurassic and magma emplacement switched to extension with corresponding mid-plate magmas produced.
The Cretaceous had significant intrusive magmatic intrusions beneath the peninsula. It peaked between 120 – 90 Ma. Much of it did not erupt. Back arc volcanic activity took place as some of the intrusions broke through to the surface as crustal thinning and extension took place. Subduction continued throughout the Cretaceous.
Magmatic activity decreased in the late Cretaceous and migrated northwards following active subduction. There were peaks of volcanic activity in the Eocene at Adelaide Island, Alexander Island and the South Shetland Islands reflecting a collision of a spreading ridge and a subduction zone. Extension between South America and Antarctica began around 50 Ma with the opening of the Drake Passage. Deep water opened up 34 – 30 Ma. Subsequent volcanism waned off until the scattered intraplate volcanism began 6.5 – 0.1 Ma. By the time the most recent volcanic activity started, subduction stopped.
The Antarctic Peninsula is heavily glaciated today. Earliest ice sheets started to develop around 35 Ma with mountain glaciation. This coincided with separation of the Andes and the Antarctic Peninsula, opening of the Drake Passage, and development of the Antarctic Circumpolar Current. Early ice sheets were thin and dynamic, fluctuating with variable climate due to Milankovitch cycles.
Between 23 – 2.5 Ma, ice sheets grew successively larger with periodic collapses during interglacials. When the larger sheets collapsed, there were a series of island ice caps left over.
Volcanic sequences beneath glacial ice on James Ross Island formed volcanic sequences of over 50 volcanic eruptions beneath glacial ice 9.9 – 2.6 Ma. These formed pillow lava and hyaloclastites. Depth of the ice were generally 250 – 300 m, occasionally reaching 850 m. Ice thicknesses increased toward the end of the Pliocene.
2.54 – 1.0 Ma, there appears to be an ice sheet dominated environment, with increasing grounding on the continental shelf. Ice streams started developing on the continental shelf1.00 – 0.20 Ma. Glacial cycles during the last 200,000 years started showing a 100,000 year cycle. Thick ice streams are abundant and grounded on the continental shelves around the peninsula.
There was a NASA study released in early November suggesting a mantle plume powering volcanic activity in western Antarctica. This in turn triggered the expected series of articles about volcanic activity in Antarctica. The source data for the study was heat output from bedrock underlying the ice cap. There were two locations of higher than normal heat flow from the bedrock in the Antarctic Peninsula. Both of these are in areas with recent volcanic activity. Whether this is due to a mantle plume or extension and decompression melting is not yet known.
Today, the Antarctic Peninsula is a pretty quiet location both volcanically and tectonically. Activity like we previously discussed in Victoria Land (Antarctica 4) is dominated by tectonic extension, with the most recent volcanoes being intraplate basalt edifices built by the interaction of basalt with liquid water and ice.
Today, the most recent activity appears to be out of the Seal Nunataks, emerging from the Larsen Ice Shelf. Nunataks on Alexander Island are perhaps as recent as a million years old. Finally, the significant activity in and around the James Ross Island Volcanic Group have been active for the last 10 Ma, with suspected below sea level hot spots for new island building in the vicinity of Vega Island. While currently quiet, the Antarctic Peninsula does not appear to be finished quite yet.