Tamu Massif: World's Biggest Volcano Stripped of Title After Magnetic Anomalies Reveal Its True Nature

The world's biggest volcano has lost its title after scientists discovered it formed via seafloor spreading rather than a single eruption.

Tamu Massif is an extinct volcano in the Pacific Ocean, around 1,000 miles east of Japan. It covers an area of 120,000 square miles—roughly the size of New Mexico. It is part of the Shatsky Rise underwater mountain range, which formed about 145 million years ago during a series of volcanic eruptions. Previously, it was unclear if Tamu Massif was the result of multiple eruptions from different volcanoes, or one single, enormous volcano.

In 2013, researchers led by William Sager, from the University of Houston, announced the volcano was the result of one massive eruption at the center of the mountain—making it the world's biggest shield volcano, and one of the largest in the solar system. However, new research also led by Sager has now shown the volcano is not what it initially seemed.

"After our 2013 paper saying that Tamu Massif is the largest shield volcano in the world, there were still things about it that nagged me," he told Newsweek. Tamu Massif is located at a spreading ridge triple junction—the point where the boundaries of three tectonic plates meet. How the volcano interacted with this geological feature—also a type of volcano—was unclear.

Sager said they wanted to find out more about the volcano formation by looking at its magnetic signature. Earth is surrounded by a magnetic field that is constantly moving. Occasionally, the whole field flips—if you were to look at a compass during a period of reversal, north would point south and vice versa.

Magma contains magnetic minerals, so when it cools and solidifies, it aligns with the magnetic field—providing a record of its position at that point in time. Because of the way the Earth's crust divides, scientists can use this information to work out the overall magnetic field. "Subtract away the expected magnetic field value, and you have magnetic 'anomalies,' positive and negative," Sager explained.

tamu massif
Tamu Massif was thought to be the world's biggest volcano. William Sager/University of Houston

"I thought that the magnetic signature would be revealing because magnetic anomalies formed by spreading ridges surround Tamu Massif. I wanted to find out whether they just die out at the edges or how deep into the volcano they go."

Magnetic field reversals take place over thousands or millions of years, so if Tamu Massif had erupted in a single event, there would be no anomalies. However, an eruptive period that lasted far longer could have a record of a reversal.

To find out, the team went to Tamu Massif on the Schmidt Ocean Institute's research vessel Falkor to make a magnetic anomaly map. Their findings showed there were magnetic anomalies in the volcano that fit with the anomalies seen at its edges. "A huge shield volcano should not form magnetic stripes," Sager said. "If it formed in a short time (one polarity period), it will have a coherent overall anomaly. If it formed during multiple polarity periods, the anomaly should be irregular. Lava flows that travel long distances should destroy the magnetic stripes."

Publishing their findings in Nature Geoscience, the team says Tamu Massif appears to be an oceanic plateau that formed through a process called seafloor spreading. Instead of new material being added to the top of the volcano, it was built through the inflation and deflation of the crust as it grew thicker and thinner, forming at the spreading ridge crest. This means new material is added at the center and older material drifts away.

As a result of the findings, the title of the world's biggest volcano goes back to Hawaii's Mauna Loa. It also means Tamu Massif is a strange hybrid of volcanism and plate formation, and—as far as Sager is aware—is unique. "I am working on analyzing existing data in other oceanic plateaus to see if I can find more evidence of ridge formation," he said. "So far, I can't find an example of an oceanic plateau that did not form at a ridge. The problem is that existing data are poor (oceanic plateaus have found a good place to hide), so for many, I cannot conclusively say whether they formed at the ridge."

This article has been updated to include more information on the research ship used for the survey.