lecting 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 sin-
gle point. When two of the plates spread apart, a crack opened
along their boundary. Magma oozed up to fill the void and solid-
ified 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, new-
er 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, how-
ever. 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 seg-
ment 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 new-
ly formed ribbon of cake should have the same height as the exist-
ing 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.TA MU, TAKE T WO
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 under-
standing of Tamu Massif revolutionizes the view of how oceanicplateaus 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 basal-
tic 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 assump-
tions are incorrect, the picture will be incorrect, too. Tamu Mas-
sif is no longer the largest shield volcano on Earth, because it is
not a shield volcano. We assumed that it formed like other volca-
noes, 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 find-
ings. 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.150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0
Millions of Years Ago
PresentEarth’s magnetic fieldNormal polarity Reversed polarityIllustration by Jen ChristiansenSOURCE: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)May 2020, ScientificAmerican.comMagnetic Flip-Flop
Magnetism provided a crucial clue to Tamu Massif’s cre-
ation. Earth’s magnetic field reverses polarity at irregular
intervals; each change leaves a signature in seafloor rock.
Tamu Massif began developing roughly 150 million to
144 million years ago, recording several stripes of polari-
ty on and around the volcano.MORE TO EXPLORE
An Immense Shield Volcano within the Shatsky Rise Oceanic Plateau, Northwest
Pacific Ocean. William W. Sager et al. in Nature Geoscience, Vol. 6, pages 976–981;
November 2013.
Oceanic Plateau Formation by Seafloor Spreading Implied by Tamu Massif Magnetic
Anomalies. William W. Sager et al. in Nature Geoscience, Vol. 12, pages 661–666;
August 2019.
FROM OUR ARCHIVES
The Secrets of Supervolcanoes. Ilya N. Bindeman; June 2006.
scientificamerican.com/magazine/sa