B
ig, dark ocean waves were rolling our research ship from side to
side. The Falkor is 83 meters long and weighs more than 2,000 met-
ric 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 north-
western 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 mag-
netized—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 larg-
est oceanic plateaus on the planet: Shatsky Rise. Yet
the peak is still about 1,980 meters below the sea’s sur-
face. Most oceanic plateaus are made of basalt, imply-
ing 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 pro-
cess, the data I have collected since 2015 show that this
is not what happened.
This new insight means scientists have misunder-
stood how dozens of immense underwater volcanoes
have created more than 5 percent of the planet’s cur-
rent 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 Pla-
teau in the southwestern Pacific Ocean, correspondedwith 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 lon-
ger 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 histo-
ry was published in 1993, when I was at Texas A&M Uni-
versity—the origin of “Tamu” (and massif means “mas-
sive” 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 diame-
ter 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 erup-
tions would have added more layers, creating some-
thing 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. Oth-
er experts had similar thoughts about the world’s larg-
est oceanic plateaus: Ontong Java and the Kerguelen
Plateau in the southern Indian Ocean.
After more research expeditions to further map Tamu
Massif and take samples of its basalts, in 2013 my col-
leagues and I published a paper in Nature Geoscience
indicating that Tamu Massif was an enormous shield vol-
cano. 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 both-
ered 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 col-IN BRIEF
New magnetic data
from the Pacific
Ocean seafloor
show that the enor-
mous Tamu Massif
volcano was not
formed the way
experts thought.
Rather than erupt-
ing like a volcanic
mountain, Tamu
Massif was created
by lava oozing up
between separating
tectonic plates.
It appears that doz-
ens of other seafloor
volcanoes formed
in this way—a new
explanation for how
giant Earth features
were created.William W. Sager
is a professor of
geophysics at the
University of Houston.
He has sailed on
46 oceanographic
expeditions and gave
the Tamu Massif
volcano its name.QQ group:1067583220