OREGON STATE UNIVERSITY

Debate Over Geologic “Hotspots” Continues, as New Paper Traces Origins of Samoa

06/10/2008

CORVALLIS, Ore. – A new study that determines Samoa is indeed on the path of a geologic “hotspot” trail is adding fuel to a vigorous scientific debate over the origins of volcano chains – especially in the Pacific Ocean.

Results of the research were published in the June issue of the journal Geology.

The study, by Oregon State University marine geologist Anthony Koppers Anthony Koppers and colleagues, concluded that the age progression of Samoa and surrounding islands is clearly in line with scientific theory that the chain was created by a hotspot.

Hotspots are stationary, deep-seated mantle plumes that slowly rise from deep within the Earth to create seamounts. As the tectonic plate moves, these volcanic structures “migrate” along the path of this plate motion, creating a string of seamounts that can rise above the surface of the ocean, as in the case of the Hawaiian Islands, Samoa and the Society Islands in the Pacific.

But there also has been a vocal “anti-plume” faction in the scientific community that claims the hotspot theory doesn’t hold up under scrutiny. They advocate the idea that the world’s 50,000 seamounts – most of which have never been explored – may have been formed due to plate tectonic forces that locally have put the Pacific plate under stress and extension.

There is room for both theories, says Koppers, who was lead author for the Geology study.

“There are areas, including Samoa, where a clear linear age progression of the islands or seamounts indicates a hotspot trail, and there are areas where alternative theories make more sense,” said Koppers, an associate professor in OSU’s College of Oceanic and Atmospheric Sciences. “The whole debate underscores the need for more research, because these are fundamental questions about the processes that formed, and continue to form, the Earth’s crust.”

Samoa has been an enigma among scientists because the ages determined from lava sampled on the sub-aerial surfaces of its islands doesn’t seem to line up in a linear fashion with the movement of plates. Some scientists took that to indicate that the creation of the islands was related to magma seeping up through cracks in the seafloor in stressed fracture zones.

In their study, funded by the National Science Foundation, Koppers’ team found that previous age determination studies of the Samoan islands focused on land that had risen above sea level and was much “younger” geologically than older rocks at the base of the seamounts.

Past studies indicated that the island of Savai’i, of which only 3 percent is above water, was just 400,000 years old. If it truly was part of a primary hotspot trail, it should have been closer to 5 million years old, based on age progressions observed in other Pacific seamount trails, like the one starting at the “big island” of Hawaii.

Koppers and his colleagues – from the Woods Hole Oceanographic Institution and the Scripps Institution of Oceanography – sampled rocks deep in the flanks of the island then used radiometric age dating to determine that they were indeed 5 million years old.

“We’ve reinstated Samoa as a primary hotspot trail,” Koppers said, “but it seems that the young sub-aerial volcanism is more related to the stressed state of the Pacific plate. It doesn’t have to be an ‘either-or’ model. Samoa and others may be a hybrid of hotspot activity and stresses causing huge intra-plate fracture zones.”

Samoa is close to the Tonga Trench, Koppers said, where the Pacific plate submerges beneath another tectonic plate and bends sharply. “The resulting stress on the plate complicates the volcanic processes,” he said.

“As we get better data sets, we’ll gain a clearer understanding of the processes that are shaping the planet,” Koppers said. “Until those models are refined, we’ll have to collect data and evaluate each seamount trail on a case-by-case basis.”