The Hawaiian archipelago travels thousands of miles from its
newest volcano off the Big Island to undersea peaks tens of millions of years
old near the Aleutians, where it they are called the Emperor Seamounts.
If you look at a map of the Pacific sea floor, you can see
the mysterious bend in the chain—the bend that separates the Hawaiian from the
Emperor groups. (Image: The ocean floor of the North Pacific, showing the Hawaiian-Emperor archipelago as light blue dots on the darker blue ocean floor. Credit: University of Sydney.)
The Hawaiian part of the chain is younger, and is mostly
islands. The Emperor seamounts are older and none reaches the surface.
They are clearly part of the same chain, but that bend has
been a mystery for decades.
Geologists believe the Pacific tectonic plate moves over a
hot plume, which occasionally pops through the crust to form volcanoes. Thus
the line of Hawaiian volcanoes can map the historical direction of the Pacific
Plate’s movement.
It has long been assumed that the bend in the chain has been
associated with a change in the direction of the Pacific Plate’s drift, from
generally north-northwestward more than 50 million years ago, to generally
west-northwestward during the past 50 million years.
We covered that in this column in 2008.
But science moves on constantly, and what was accepted
yesterday doesn’t necessarily get full credence today.
One thing that remained confusing was that other chains
on the Pacific Plate don’t show the same bend. So, if a change in the direction of the movement of
the Pacific Plate affected the Hawaiian-Emperor chain, why would it not also
have shown up on other long-lasting Pacific chains?
“The flow dynamics underlying the formation of the sharp
bend occurring only in the Hawaiian–Emperor hotspot track in the Pacific Ocean
remains enigmatic,” write the authors of a new study on the Hawaiian-Emperor
bend.
In the new assessment, researchers suggest that two things
were moving at once: The plate was on a generally unchanging west-northwest drift,
but the underlying plume of molten rock was also drifting—in this case
southward—until 50 million years ago.
And since then, the plume has been
generally stationary, allowing the plate drift alone to establish the pattern
of the Hawaiian Islands.
Rakib Hassan, of the University of Sydney's School of
Geosciences, was lead author in the study published in Nature, “A rapid burst
in hotspot motion through the interaction of tectonics and deep mantle flow.”
His co-authors are R. Dietmar Müller, Michael Gurnis, Simon E. Williams and
Nicolas Flament.
They are not the first to suggest that both the plume and
the plate were in motion, but they used the power of a supercomputer to
envision how it all works.
To make sense of this, it’s useful to think of the globe as
solid at the surface, molten in the middle and solid again at the core. Okay,
that’s an oversimplification of a very complex process. The mantle, which lies
below the surface crust of the Earth, is actually solid, but on geological time
scales, it flows. Plumes of molten rock, which can poke through the crust to
form volcanoes (Like Kilauea), rise from piles of rock deep in the mantle.
Here is how Science Daily described what Hassan and
co-authors believe happened: “Between 50-100 million years ago, the edge of the
pile under the north Pacific was pushed rapidly southward, along with the base
of Hawaii's volcanic plume, causing it to tilt. The plume became vertical again
once the motion of its base stopped; this dramatic start-stop motion resulted
in the seamount chain's sharp bend.”
This is difficult to envision. Imagine a kid flying a kite.
The kite is the surface volcano. The wind is the Pacific Plate. The kite string
is the plume and the kid is the base of the plume.
As long as the wind is steady and the kid is standing still,
the kite string bottom-to-top points straight downwind. But if the kid starts
running to the left, perpendicular to the wind, the kite string points in a
different direction—impacted both by the movement of the kid and the movement
of the wind. The kite string will now point downwind and to the right.
To understand the kite’s movement across the sky, you need
to understand both the wind, and the kid.
Or, as co-author Müller said, “It is
now clear that we first need to understand the dynamics of the deepest
'Underworld', right above the core, to unravel the history of volcanism at
Earth's surface.”
The researchers used a great deal of computing power to
model what they believe happened. A YouTube animation can be seen here.
© Jan TenBruggencate 2016
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