Monday, May 30, 2016

Archipelago turning: New understanding of the ‘bend’ in the Hawaiian-Emperor archipelago

 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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