Antarctica has long been a land of mystery and heroic exploits made famous by the explorations of James Ross, Roald Amundsen, Robert Scott and Ernest Shackleton. A key piece of the puzzle for understanding global continental evolution, Antarctica contains examples that define the spectrum of Earth’s volcanic processes. Now, a joint study by the University of Utah and the University of Canterbury in New Zealand shows how CO2 deep subsoil helps magma avoid being trapped deep within the Earth and allows it to reach and accumulate on the surface.
The study, published in Nature Communication “expands our understanding of the sources and transport of various types of magma and volatile gases to the surface,” says Phil Wannamaker, the study’s second author and a geophysicist at the University of Utah’s Institute of Energy and Geoscience. .
“Mount Erebus is an example of CO2-dominated fault volcanoa complement to the more widely known arc volcanoes of the Pacific and elsewhere, dominated by H2O,” adds New Zealand co-researcher Graham Hill, lead author of the study.
“To understand both H2O and CO2 volcanoes is important for calculating the balance of these volatile gases deep within the earth, which involves injecting material into the earth’s mantle and returning it to the surface to start all over again,” says Wannamaker.
Wannamaker and Hill conducted the study with University of Utah alumni John Stodt and Michal Kordy and associate scientist Virginie Maris; geophysicists Paul Bedrosian of the United States Geological Survey, Martyn Unsworth of the University of Alberta and Yasuo Ogawa of the Tokyo Institute of Technology and senior volcanologist Phil Kyle of the New Mexico Institute of Mining and Technology. Other co-authors included Erin Wallin of the University of Hawaii and mountaineer Danny Uhlmann, who is currently studying geology at the University of Lausanne.
These other volcanoes
Mount Erebus is the only one in Antarctica active volcano. It and its dormant companion volcano, Mount Terror, were named after the exploration ships of Sir James Ross, who discovered them and the Transantarctic Mountains in 1841. Mount Erebus was first climbed by Sir Ernest Shackleton left in 1908.
Erebus illustrates a family of volcanoes with an alkaline chemical composition, with lava relatively rich in sodium, potassium and other elements including rare earths, while being relatively poor in silica.
Alkaline volcanoes are very different from volcanoes such as those in the Cascade Range stretching from northern California to Alaska to British Columbia. The Cascades are at a place where the Earth’s tectonic plates are pushing towards each other, with the ocean’s crust being forced under the continent’s crust. As this oceanic crust sinks into the Earth and partially melts, the water in the rocks becomes part of the melt and is the dominant “volatile”, or molecule that easily exudes, or bubbles out. of a solution like the fizz of a soft drink.
This evolving magma rises into and through the crust, but usually does not reach the surface because, as the pressure of the overlying crust decreases with ascent, water gushes out, sometimes explosively as in the case of Mount St Helens in 1980 or Mount Lassen in 1912. The remaining magma becomes immobilized and freezes in place, generally at a depth of about three miles (five kilometers).
But the Erebus volcano on Ross Island, Antarctica, is in a continental rift zone. Continental rifting occurs when the Earth’s crust and mantle separate. Western Utah is an example of a rift zone. It lies at the eastern edge of the geological province of the Great Basin, which is actively dividing, and slowly stretching from east to west. Erebus is found along the margin of the West Antarctic Rift System, which originated tens of millions of years ago and continues today.
Rift zone magmas also contain volatiles from the recycling of oceanic crust and sediments, but these are much older and are released to the surface by the rifting process. Instead of water, the volatiles of these magmas are CO2-dominated.
Erebus also has a persistent lava lake, a classic feature of an evolved CO2-rich rift volcano. But lava lakes, also exemplified by the Nyiragongo volcano in East Africa, and others, are not found in arc volcanoes like the Cascades and show that there must be something about volcanoes in the rift that allows magma to reach the surface relatively peacefully.
Imagine the magma in the Earth
It is impractical to collect rock samples from depths more than a few kilometers below the surface, so researchers rely on geophysical methods to infer structures and processes at greater depths. This is similar to the CT scan of the human body. The most widely applied and well-known geophysical techniques are seismic, where sound waves are used for internal imaging. This is widely applied, for example, in oil and gas exploration. However, natural seismic sources that can penetrate deep into the Earth are rare around Erebus Volcano, and images using them have only been derived at shallow depths.
Hill, Wannamaker and colleagues used a method called magnetotelluric sounding. Magnetotelluric sounding uses natural electromagnetic waves generated by the sun and lightning. Most of these waves travel through the air, but “some enter the Earth, scatter over rock structures of interest, and return to the surface, where we can measure them” using sophisticated “voltmeters” , explains Wannamaker.
As electromagnetic waves pass through the Earth’s interior, they travel faster or slower depending on how well the rock and other materials conduct or resist electricity. Magma is conductive, so it can be detected by this technique.
This is not the first geophysical foray into Antarctica for Wannamaker’s group. Together with U alum and co-author Dr John Stodt, they pioneered the technique of high-fidelity magnetotelluric measurements on polar ice caps. In addition to studies in West-Central Antarctica and the South Pole, Wannamaker conducted a multi-season campaign across the Central Transantarctic Mountains showing how these mountains were uplifted. Their technique is now used by other researchers in the Arctic and Antarctic polar regions.
From 2014 to 2017, Wannamaker and his colleagues made measurements at 129 magnetotelluric sites spanning Erebus’ Ross Island. Patterns of backscattered electromagnetic radiation from all stations were then assembled by a computer program to create an image of the Earth’s crust and upper mantle across the island and volcano to a depth of approximately 60 miles (100 kilometers).
Their magnetotelluric data show a steep conduit of low electrical resistivity coming from the upper mantle – the source of magma. But the conduit takes a sharp lateral bend in the deep crust before reaching the shallower magmatic storage and the summit lava lake. “We interpret the lateral bend as representing a structural ‘fault valve’ controlling the episodic flow of magma and CO2 gases, which replenish and heat the high-level phonolite magma evolution chamber,” says Wannamaker. Phonolite is the type of rock formed by the magma of Erebus.
This magmatic valve is probably formed by the intersection of north-south and east-west faults, since faults in the same east-west orientation are known to exist in the area around Erebus, and provide the path of magma to the surface.
Unlike the H2Cascades and Pacific O-rich arc volcanoes, CO2-dominated Erebus shows the structures that allow magma to rise up to the volcano’s lava lake, since the magma does not stop in the crust like water-dominated magmas.
“Understanding the controls and transport pathways for this type of volcano, dominated by CO2reveals to us the scales and volumes of volatile transport on Earth,” says Wannamaker. “These volcanoes elsewhere are important hosts for critical mineral deposits such as rare earths, which are increasingly important for the future resource needs of societies.
Transcrustal structural control of CO2-rich extensional magmatic systems revealed at Mount Erebus in Antarctica, Nature Communication (2022). DOI: 10.1038/s41467-022-30627-7
University of Utah
Quote: Antarctica’s only active volcano shows how CO2 enables volcanoes to form persistent lava lakes on the surface (2022, May 30) Retrieved May 31, 2022 from https://phys.org/news/2022- 05-antarctica-volcano-co2-volcanoes-persistent.html
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