Hotspot? Not! Antarctic Volcanoes' Surprising Source

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Hotspot? Not! Antarctic Volcanoes' Surprising Source
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The mystery of how an Antarctic underwater volcano chain formed may have finally been solved.

The Marie Byrd Seamounts are a group of eight large volcanoes and many small cones spanning the seafloor offshore of West Antarctica.

To geologists, volcanoes lined up like Russian nesting dolls in the middle of a tectonic plate usually signal a mantle plume, a hot upwelling coming from the bowels of the Earth. As the planet's tectonic plates slowly trundle over the hot spot, magma punches through the crust, creating volcanoes. A classic example of this phenomenon is seen in the Hawaii and Emperor Seamount chain.  

But the first comprehensive survey to peel back the secrets of the Marie Byrd Seamounts failed to find any signs of a hotspot — even though the volcanoes contain geochemical tracers that point to a hotspot origin.

Yet researchers at the GEOMAR Helmholtz Centre for Ocean Research Kiel in Germany think they've solved the riddle of what formed the seamounts.

The submerged volcanoes formed when part of the Earth's crust pulled apart some 60 million years ago, according to the researchers' findings, published July 16 in the journal Gondwana Research. The deep fractures allowed "fossilized" mantle-plume material, frozen onto the bottom of the Antarctic continent during the Cretaceous period, to escape to the surface. [Infographic: Tallest Mountain to Deepest Ocean Trench]

"The Marie Byrd Seamounts are particularly interesting, since they represent an example for enigmatic intraplate volcanism which cannot be explained with the 'classical' model (i.e., hotspots or mantle plumes) for the origin of volcanism within the Earth plates and, therefore, requires alternative models," Reinhard Werner, a study co-author and geologist at the GEOMAR Helmholtz Centre, told LiveScience's OurAmazingPlanet in an email interview.

Jagged edge

To set the stage, here's a little geologic history: Before the Marie Byrd volcanoes appeared, Antarctica was part of a supercontinent called Gondwana. When Gondwana broke apart, Australia, India and Antarctica all cleaved from one another, creating the Southern Ocean. When continents rift, bits and pieces of crust hang on like a ragged cut. For example, Antarctica's boundary had a micro-continent called Zealandia (now New Zealand) and the temporary, tiny Bellingshausen tectonic plate.

The vast Marie Byrd seamount chain stretches across 500 miles (800 kilometers) and covers an area of 77,000 square miles (200,000 square km). It sits in the middle of this tectonic house of cards, where the shards of plates reshuffled their boundaries starting about 100 million years ago. (Zealandia went sailing off toward Australia, while the Bellingshausen plate was eventually absorbed back into Antarctica.)

In 2006, researchers aboard the RV Polarstern mapped the seafloor and dredged rock samples from five large Marie Byrd seamounts. The biggest peaks are comparable in size to Canary Islands volcanoes, such as El Hierro and La Palma, the researchers said.

Isotopic dating revealed that most of the lavas erupted between 57 million and 64 million years ago. The flat, eroded tops on the peaks signal the volcanoes once popped up above the ocean surface. But some samples were as young as 3 million years old, suggesting the volcanism is long-lived.

Hotspot puzzle

The discovery that volcanic eruptions took place across long time periods at the same spots rules out a hotspot source, the researchers said. Instead, Werner and his co-authors think extension in the crust freed molten rock.

"Tectonic processes in [this] area have significantly contributed to their origin," Werner said. "Extensional processes which occurred in the area of the present Marie Byrd Seamounts some 60 million years ago caused faults in the Earth's crust, which allowed melts to rise up through the crust and form the Marie Byrd volcanoes."

Yet geochemical studies show that more than one kind of magma fed the volcanoes. There are signs of a hotspot-type melt, but also ocean-island basalts, such as those found at the spreading ridges between oceanic crustal places. Other lavas from the Southern Hemisphere share this chemistry, placing the seamounts in a common "genetic" family that includes volcanic fields offshore New Zealand and on land, within the West Antarctic Rift.

Werner and his colleagues think the hotspot portion came from an ancient magma plume trapped beneath Antarctica. When the crust rifted in the spot where the Marie Byrd Seamounts emerged, parts of the plume were caught up in molten rock heading toward the surface.

"This process explains why we find signatures of plume material at volcanoes that are not on top of plumes," Folkmar Hauff, a study co-author and geochemist at GEOMAR, said in a statement.

Email Becky Oskin or follow her @beckyoskin. Follow us @OAPlanet, Facebook & Google+. Original article on LiveScience's OurAmazingPlanet.

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