Underwater volcanoes amaze geologists

Marie Byrd seamounts in the Antarctic emerged unlike any other fire mountains

3D illustration of Marie Byrd Seamounts © R. Werner, GEOMAR
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They do not fit into any common scheme: The Marie Byrd seamounts off the coast of Antarctica are indeed volcanoes. But they are neither at a plate boundary nor an active Mantelplume. But what created these underwater fire mountains then? Geologists have now elucidated this puzzle: remnants of hotspot material trapped beneath the earth's plate have melted the crust so far that these volcanoes were created. The volcanic formation is thus more varied than previously thought.

Snowstorms, pack ice and glaciers - these are the usual pictures that you associate with the Antarctic. But at the same time, it is also a region of fire. The Antarctic continent and the waters around it are peppered with volcanoes. Among them are active, but also extinct fire mountains. The latter group includes the Marie Byrd seamounts in the Amundsen Lake. Its summit plateaus are today in 2, 400 to 1, 600 meters of water. Because they are difficult to reach with conventional research vessels, they have hardly been explored so far. The Marie Byrd-Seamounts are fascinating formations: They do not fit into any of the usual models for the formation of volcanoes.

Neither plate boundary nor plume

Classically, volcanologists distinguish two types of fire mountains. One species arises where earth plates are contiguous. At these seams of the earth's crust magma can reach the surface particularly easily. The other species forms within the earth plates. "These intraplate volcanoes often overlap a so-called mantle plume, " explains Reinhard Werner from the GEOMAR Helmholtz Center for Ocean Research in Kiel. Hot magma rises from the deep mantle at these hotspots and melts through the earth's crust. For example, the Hawaiian Islands were created.

Rock samples from the Marie Byrd Seamounts are spotted, documented and packaged aboard the Polarstern. © F. Hauff, GEOMAR

But for the Marie Byrd-Seamounts both models do not fit. "There is no plate boundary in the vicinity and no plume in the ground, " says first author Andrea Kipf from GEOMAR. In order to clarify the origin of the Marie Byrd Seamounts, the Kiel scientists took part in 2006 in an expedition of the research icebreaker POLARSTERN in the Amundsen lake. They collected rock samples from the underwater mountains which, after returning to laboratories, were subjected to thorough geological, volcanological and geochemical investigations.

Plume relics trapped under the plate

Interestingly, we found chemical signatures typical of pluma volcanoes. And they resembled those of volcanoes in New Zealand and on the Antarctic continent, "explains geochemist Folkmar Hauff. Based on this finding, the scientists looked for an explanation. They found them in the history of earth plates in the southern hemisphere. About 100 million years ago, the remnants of the former supercontinent Gondwana were located in what is today Antarctica. A mantle plume melted through this continental plate and broke it open. display

Two new continents were born: the Antarctic and Zealandia, of which still today the islands of New Zealand testify. As the young continents drifted in different directions away from the mantle plume, large amounts of the hot plume material stuck to their bottoms. These formed reservoirs for later volcanic eruptions on the two continents. "This process explains why we also find plume material signatures on volcanoes that are not above plumes, " says Hauff.

56 million year old lava samples from Hubert Miller Seamount, the largest of the Marie Byrd Seamounts. H F. Hauff, GEOMAR

Dripped by molten streams

But that does not explain the Marie Byrd seamounts, because they are not on the Antarctic continent, but on the neighboring oceanic crust. Continental earth plates are thicker than the oceanic ones. This ensures, among other things, temperature differences in the substrate, "explains Werner. And as winds blow between warm air masses of different degrees, movements also occur under the earth's crust when there are temperature differences. So the plum material that once lay beneath the continent came under the oceanic plate.

As it was disrupted by further tectonic processes, there were cracks and fissures along which the hot material rose, turned into magma, and around 60 million years ago, the Marie Byrd seamounts grew. "This created islands that are comparable to today's Canary Islands, " says Kipf. At some point, however, the volcanoes went out again, wind and weather eroded the cones to sea level, geological processes then let the mountains sink even further. After all, the summit plateaus were at the level we know today.

Using the Marie Byrd seamounts, which had previously been barely studied, the researchers were able to show another example of how diverse and complex the processes that can cause volcanism are. "We are still a long way from understanding all these processes. But with the current study, we can contribute a small component to the overall picture, "emphasizes Werner. (Gondwana Research, 2013; doi: 10.1016 / j.gr.2013.1006.1013)

(Geomar, 14.10.2013 - NPO)