12321148 million years agoJapan Trench Magnetic Anomalies above SeafloorShatsky RiseTamu MassifJapan^ TrenchSeafloor Plates below seafloor149.5 million years ago
Newest seafloorOlder seafloorNew seafloor is pulled awayAs seafloor cools, it recordsPACIFIC OCEANTAMU^MASSIFTAMUMASSIFSeafloor todayNew View of Tamu
For years scientists thought Tamu Massif, on the Pacific Ocean’s
seafloor, was a classic shield volcano. But the pattern of magnetic
stripes across it does not reflect that view. Recent data about
how tectonic plates moved when Tamu Massif formed millions
of years ago reveal that magma built a new kind of volcano, like
a sheet cake torn in two.
HOW THE VOLCANO GOT ITS STRIPES
About 149 million years ago three tectonic plates under the
Pacific Ocean were separating from a triple junction ● 1 , near
where Tamu Massif would later arise. A widening crack was
filled with new crust. Roughly 148 million years ago, while
Tamu Massif was forming, the triple junc tion jumped nor theast,
and a bit of ridge from the Pacific-Farallon Plate boundary
rotated counterclockwise to become part of the Pacific-
Izanagi Ridge ● 2. This caused the stripes on Tamu Massif
to rotate. By 144 million years ago, after the volcano had
stopped erupting, the junction had moved again because
the Izanagi Plate had migrated farther ● 3. The wide stripe
down the volcano’s back indicates that most of the erupting
occurred during a single period of reversed polarity.Triple junction of
three tectonic platesFarallon PlatePacific PlateNewly formed, cooling
crust at this spreading
plate boundary retains
the magnetic signature
of the time periodFarallon PlatePacific PlateIzanagi PlateFractured plate edges cause a bit
of ridge along the Pacific-Farallon
boundary to rotate and become
part of the Pacific-Izanagi Ridge.
Tamu Massif forms here for
2.8 million years, during
several polarity periods.
New position
of triple
junctionIzanagi PlateNew crust