Scientific American - USA (2020-05)

Sea surface

3

144.5 million years ago

Magnetic Anomaly

(a function of Earth’s polarity)

Negative Positive

Magnetic Anomalies above Seafloor

Seafloor Plates below seafloor

Crust

Lithosphere

Cracks

Eruption 1 (oldest)Eruption 2

Eruption 3Eruption 4 (youngest)

Sea surface

Plate 1

Plate 2

Plate junction

Newest seafloor

Older seafloor

New seafloor is pulled away

from plate boundary

As seafloor cools, it records

the prevailing magnetic polarity

Magnetic stripe

Crust

T Lithosphere

AM

U^

M

AS

SI

F

Seafloor today

Warm mantle

Mantle plume

Maps by Craig Taylor, Mapzilla, Illustrations by Jen Christiansen

THE SEAFLOOR

TO DAY

A map of the seafloor’s mag-

netic signature around Tamu

Massif ( inset ) shows parallel

stripes to the west and east

of the volcano’s lower half,

running from northwest

to southeast ● 1. But the

lines on the central volcano

itself ● 2 are turned about

90 degrees counterclock-

wise and are about twice

as wide as the rest

because the volcano

formed where tectonic

plates were rotating

apart quickly.

OLD EXPLANATION: LAYER CAKE

A shield volcano—the original description of Tamu Massif—forms like a layer cake.

Warm mantle rises and melts underneath the cooler lithosphere. A vertical crack

allows hot magma from the plume to move up through the ocean crust and erupt,

flooding out across the seafloor and cooling as a low dome. Over time subsequent

eruptions add newer layers, building up the dome like a layer cake.

NEW EXPLANATION: SHEET CAKE

As tectonic plates separate, they pull the lithosphere and crust apart, opening a crack

along the boundary—as if a sheet cake were cut down the center and separated

horizontally. Hot magma oozes up to fill the void and solidifies as seafloor. As the plates

continue to diverge, they pull apart the newer seafloor, creating another crack that

fills with fresher magma. This process repeats; each time new rock cools, it takes on

the prevailing magnetic signature, creating magnetic stripes over time.

Pacific Plate

Izanagi Plate

Plate junction

continues to

migrate north