A new study published in Nature Geoscience concludes that Tamu Massif, thought to be the largest volcano in the world, is a different breed of volcanic mountain. The study analyzed magnetic field data over Tamu Massif, finding that magnetic anomalies (perturbations to the field caused by magnetic rocks in the Earth’s crust) are like those formed at mid-ocean ridge plate boundaries. This finding led the authors to conclude that Tamu Massif formed by mid-ocean ridge “spreading”, the geologists’ term for creation of ocean crust at divergent plate boundaries, rather than as a shield volcano as previously thought. Tamu Massif is a hybrid of plate formation and extraordinary volcanism, creating a unique type of volcanic mountain.
The guiding question of the study was to understand how the massive Tamu Massif volcano, with an area close to that of New Mexico, formed near the nexus of three spreading ridges, which are themselves long, linear volcanoes. The key is magnetic anomalies. Mid-ocean ridges – plate boundaries where oceanic plates move apart – record distinctive linear magnetic anomalies, parallel to the ridge, as they form new crust. This is a result of lava flows and magma being concentrated near the ridge axis where the magnetic minerals in the new crust record reversals of the magnetic field polarity. Previously, linear magnetic anomalies formed by the three ridges were found all around Tamu Massif, but it was unclear where these linear anomalies stopped within the volcano. A paper published in 2013 (also in Nature Geoscience) by author Sager and colleagues concluded that Tamu Massif is an enormous shield volcano, formed by a pile of far-reaching lava flows emanating from its summit. Such a formation mechanism should create a single coherent anomaly, if the volcano was built in a short time (within one magnetic polarity period), or an irregular, radial pattern if its formation encompassed magnetic reversals.
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The study compiled a magnetic anomaly map over Tamu Massif, using 4.6 million magnetic field readings collected over 53 years along 72,000 kilometers of ship tracks. The data set was anchored by a grid of magnetic profiles collected in 2015 by the Schmidt Ocean Institute research ship Falkor, using modern navigation methods. The resulting map shows that linear magnetic anomalies around Tamu Massif blend into linear anomalies over the mountain itself. Although complicated by a 90° rotation by one ridge segment, coincident with Tamu Massif emplacement, the linear anomalies imply that Tamu Massif formed by anomalous mid-ocean ridge crustal formation that created its thick crust.
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Why is this finding important? It shows that Tamu Massif and other oceanic plateaus, large volcanic mountains in the ocean, are formed by a different process than previously thought. A widely-accepted model posits that a large blob of magma (a “mantle plume”) rises through the mantle and creates a massive volcano when it arrives at the surface. This eruption is thought to be analogous to massive eruptions on land, called “continental flood basalts”. The Deccan Traps in India and Siberian Traps in Russia are examples. This formation model implies that plume magmas penetrate the crust and form a stack of lava flows on top, i.e., a vertical age progression with the oldest on the bottom. By the ocean-ridge-spreading hypothesis, the age progression is lateral instead. New material is always added at the center of the ridge as older material drifts laterally away. An implication is that the slopes of Tamu Massif, which are very gentle at <1°, are not caused by lava flow shape, but instead by a gradual inflation and then deflation of ridge volcanism as the crust became thicker and then grew thinner. Another implication of this hypothesis is that another, similar mountain must have formed on the other side of the ridge. Plate movement must have carried this lost twin to the northwest where it collided with the western Pacific trenches and was incorporated into Asia. The new result also weakens the accepted analogy between eruptions of continental flood basalts and oceanic plateaus because the formation mechanisms are shown to be different.
Does this new picture say that Tamu Massif is not the biggest volcano in the world? It does indeed mean that the mantle of world’s largest shield volcano goes back to Mauna Loa, on the island of Hawaii. Tamu Massif is not a shield, but a different type of volcanic mountain. There are larger oceanic plateaus, Ontong Java in the Pacific Ocean and Kerguelen in the Indian Ocean, but their formation mechanisms are not known in detail. Tamu Massif is certainly one of the largest volcanic mountains in the world.
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Why was the previous story wrong? This is an example of the difficulty of studying oceanic volcanoes, which are inaccessible owing to being submerged in the middle of the ocean. The previous study relied on two findings: (1) thick lava flows were cored on the flanks by the Integrated Ocean Drilling Program (IODP) drill ship JOIDES Resolution in 2009 and (2) a subsequent research cruise by the US seismic research ship Marcus G. Langseth found parallel subsurface lava flows, traced back to the summit, suggesting the volcano consists of a vertical pile of flows. By analogy with continental flood basalt provinces, where single lava flows have been traced hundreds of kilometers, the scientists concluded that Tamu Massif formed in similar fashion. But this conclusion was akin to reconstructing a dinosaur from a tooth and leg bone, where the gaps must be filled by inference from other beasts. The magnetic data of the current study are critical because linear anomalies cannot be formed in a different way. Clinching evidence came from a paleomagnetic study of the basalt samples cored by IODP, which indicate that lava flows at two different locations on Tamu Massif have opposite magnetic polarities, in accord with the magnetic map interpretation.