Big, dark ocean waves were rolling our research ship from side to side. The Falkor is 83 meters long and weighs more than 2,000 metric tons, but a storm from Siberia that had just missed us was still churning the seas. Sitting in the science lab on the main deck, I was trying to keep my coffee from spilling onto my map of the seafloor.

It was mid-October 2015, and we were in the northwestern Pacific Ocean, about 1,600 kilometers east of Japan. For the umpteenth time I was looking at a map that showed somewhat parallel stripes along the seafloor around Tamu Massif, an enormous, ancient volcano. Each stripe indicated how a band of seafloor was magnetized—positively or negatively—but the pattern did not agree with how I thought Tamu Massif had erupted.

A sudden wave hit the Falkor with a loud thud, jarring me, and just then I realized what I had been missing. I had been studying this volcano for more than two decades. I had published the definitive papers giving the volcano its name and explaining its history. So my insight was only partly a “Eureka!” moment; the other part was a Homer Simpson moment: “D'oh!” My old ideas, and everyone else's, about how this volcano formed had been wrong.

Tamu Massif is special. It is roughly 430 kilometers wide and 600 kilometers long, covering an area similar to that of New Mexico. It is more than 50 times bigger than Mauna Loa on the island of Hawaii by volume, yet it is essentially flat. Its broad slopes dip by about one degree from the middle toward the edges, whereas a typical undersea volcano has a decline of five to 10 degrees. Imagine an entire football field covered by a taut gray tarp, with a stick just 60 centimeters high propping it up at midfield.

The volcano is the main mountain in one of the largest oceanic plateaus on the planet: Shatsky Rise. Yet the peak is still about 1,980 meters below the sea's surface. Most oceanic plateaus are made of basalt, implying that great volumes of magma rose from Earth's mantle and moved through the crust, squeezing up through the seafloor and pouring outward. Although Tamu Massif's shape seems to reflect this eruption process, the data I have collected since 2015 show that this is not what happened.

This new insight means scientists have misunderstood how dozens of immense underwater volcanoes have created more than 5 percent of the planet's current seafloor. Indeed, we have stumbled on an entirely new type of volcano.

If Tamu Massif or one of its cousins erupted again, it could make the Pacific Ocean more acidic, killing all kinds of marine life. It could also release large amounts of greenhouse gases into the ocean and the atmosphere. When we look back through history, it appears that eruptions from a similar volcano, the Ontong Java Plateau in the southwestern Pacific Ocean, corresponded with widespread, low-oxygen ocean conditions. Finally, although I am inspired by the thought that we are rewriting our ideas about the seafloor's formation, I also have to accept a hard reality: Tamu Massif, which had been branded “Earth's largest shield volcano,” no longer deserves that title.

Piece of Cake

Tamu Massif formed gradually, over several million years, about 145 million years ago. During that period Earth's magnetic field reversed a couple of times at irregular intervals, leaving telltale magnetic stripes in the oceanic crust.

My first paper about Tamu Massif's magnetic history was published in 1993, when I was at Texas A&M University—the origin of “Tamu” (and massif means “massive” in French). In it, I concluded that the volcano must have formed from one eruption event in a short time: a huge blob of magma hundreds of kilometers in diameter rose through the mantle and spread out onto the seafloor. Massive eruptions caused floods of hot basalt to run down the accumulating slopes, building a broad, slightly domed layer of new earth. Subsequent eruptions would have added more layers, creating something like a layer cake, with the oldest basalt layer on the bottom and the youngest basalt layer at the top. On land, this is generally how shield volcanoes form. Other experts had similar thoughts about the world's largest oceanic plateaus: Ontong Java and the Kerguelen Plateau in the southern Indian Ocean.

Time line shows the irregular polarity-reversal pattern of Earth’s magnetic field: each change leaves a signature in seafloor rock
Credit: Jen Christiansen; Source: Timescales of the Paleomagnetic Field, by D. V. Kent and W. Lowrie. Edited by J.E.T. Channell et al. American Geophysical Union, 2004 (time line)

After more research expeditions to further map Tamu Massif and take samples of its basalts, in 2013 my colleagues and I published a paper in Nature Geoscience indicating that Tamu Massif was an enormous shield volcano. Soon enough the media declared that we scientists had discovered “the world's largest shield volcano.”

That superlative always made me cringe. I tried to tell journalists that we did not discover Tamu Massif (that happened in the early 20th century) and that there are larger oceanic plateaus. But something else bothered me: the pattern of magnetic stripes was odd for a broad shield that had formed like a layer cake.

If you looked down from space at the floor of the Pacific Ocean with glasses that revealed magnetism, you would see parallel stripes everywhere. But at a volcano, you would expect to see a big splatter mark because lava pouring out from the center would have interrupted that pattern. Not having such glasses, I had been collecting magnetic-field data at sea. My “Eureka!” moment on the Falkor happened when it became clear that there was indeed a wide, continuous stripe across Tamu Massif.

Tamu Massif formed at a triple junction—a place where three tectonic plates meet, like three huge wedges converging at a single point. When two of the plates spread apart, a crack opened along their boundary. Magma oozed up to fill the void and solidified as basalt. As the plates moved farther from the center, the new ribbon was torn along its axis and pulled apart, and newer magma filled the newer void.

This process repeated over and over. Tamu Massif was not built like a layer cake at all. Instead imagine a sheet cake being pulled apart horizontally, with new cake filling the crack that formed down the middle. That cake was subsequently pulled apart, newer cake filled the newer crack, and so on. If new ribbons of cake alternated between chocolate and vanilla, over time a pattern of stripes would be created. On Tamu Massif, positive and negative magnetic stripes correspond to this pattern.

There are two physical problems with this explanation, however. The stripes on Tamu Massif's southeastern quadrant turn 90 degrees counterclockwise. In retrospect, the reason for this seems somewhat obvious. As Tamu Massif erupted over time, a piece of the plate to the northeast broke off and moved, causing a segment near the triple junction to rotate 90 degrees. This segment is where Tamu Massif formed. Realizing that the stripe down Tamu Massif's back was a spreading magnetic anomaly was my “D'oh!” moment.

The second problem is that in the sheet-cake model, each newly formed ribbon of cake should have the same height as the existing cake being pulled apart. But Tamu Massif is thickest in the middle. I think this structure developed because the melting at the center increased for some time, forming a higher crust.

Tamu, Take Two

My colleagues and I have collected a lot of seafloor data and core samples drilled from area basalts that are helping us convince other scientists that our interpretation is correct. Our new understanding of Tamu Massif revolutionizes the view of how oceanic plateaus formed. Observations from a few other oceanic plateaus—those for which we have enough magnetic data to map the stripes—imply that many formed in a similar manner. Plateaus that developed where plates were diverging must be a new class of volcano. This means that the widely accepted assumption that oceanic plateaus are large shield volcanoes created by long basaltic lava flows is incorrect.

Why did we get the picture wrong before? And does it matter that Tamu Massif is not a classic shield volcano? We were wrong because submarine volcanoes hide under thousands of meters of water, so we cobbled together a picture from fragmental data. Imagine trying to reconstruct a dinosaur from just a tooth and a toe bone. You would attempt to connect them in a diagram based on what you know about other dinosaurs, but if your assumptions are incorrect, the picture will be incorrect, too. Tamu Massif is no longer the largest shield volcano on Earth, because it is not a shield volcano. We assumed that it formed like other volcanoes, but that was a bad assumption. Instead we found a new family of volcanic mountains—a new explanation for how giant features on Earth were formed. And there are dozens of them under the sea.

Scientists are always trying to understand how things came to be. That is our goal—even if it overrules our own prior findings. Our fresh understanding of Tamu Massif allows me to say, “We finally figured it out.” That may not be as headline-worthy as “world's biggest,” but I am happier with it.