A Pacific Ocean chain of islands and underwater mountains — including the Hawaiian Islands — has a distinctive bend. Scientists have puzzled for years about what might have caused the curve. A new analysis now blames it on the disappearance of a tectonic plate. Long ago, this plate slid into Earth’s interior.
Tectonic plates are the huge moving slabs of rock that cover Earth’s outer surface. They form the firm foundation on which the continents and oceans sit. In the mantle below them flows an ocean of molten rock. Over millions of years, the edges of some plates subduct — or slide down under a neighboring plate’s edge — and melt. At other sites, new plate material slowly rises up from the mantle and hardens.
A new analysis now suggests that as one plate started to subduct, the current of moving rock in the mantle below changed direction. The end result: a curved island chain.
“For the longest time scientists assumed that this prominent bend formed because the Pacific Plate changed direction,” says Dietmar Müller. He’s a geophysicist at the University of Sydney in Australia. He also was an author for two new studies on the subject. From their findings, he says: “We now can demonstrate that the mantle, not the plate, changed its direction of motion.”
Changing plate migrations
Müller’s team used a computer model to reconstruct the likely direction, some 50 million years ago, of the liquid rock flowing below the Pacific Ocean. Earlier, the Izanagi Plate near East Asia, had submerged. This, the new analysis showed, likely would have reversed the direction of the mantle’s flow.
The volcanic mountains in the area developed as plumes of magma — liquid rock — escaped from the mantle, through opening in or between the plates. And a U-turn in the mantle’s flow could have immobilized the volcanic plume of magma that built the mountain ranges.
As Earth’s crust moved westward over the magma plume, an abrupt stop in the mantle flow would explain the roughly 120-degree kink in the Hawaiian-Emperor seamount chain. (Seamounts are undersea mountains, usually created by volcanoes.)
Müller’s team offered its conclusions online March 24 in Geophysical Research Letters.
Until now, Earth scientists had attributed the bend to a sudden shift in the movement of the Pacific Plate over the magma-spewing hot spot. This would have occurred where the Emperor undersea mountain range connects to the Hawaiian one. But that plate didn’t significantly alter its course around the time that the seamount chain developed its curve. That’s the conclusion of a second paper by Müller’s team. They published it online March 27 in Geology.
The two findings together suggest that the bend’s initial origin story was off the mark, says Lijun Liu. He’s an Earth scientist at the University of Illinois at Urbana-Champaign. “These two papers might actually resolve this longstanding debate,” he says. “These are two powerful, independent pieces of evidence,” he says. They suggest the Hawaiian-Emperor bend comes from “a deep mantle process, rather than a surface plate motion.”
How the new story might read
A tube-shaped shaft in the West Pacific carries magma from near Earth’s core up to the surface. This site is known as the Hawaiian hot spot. On and off over more than 80 million years, molten rock has spewed up from this spot. All it takes is for the thinness of the plate above it to let this magma punch through. Then a volcanic mountain forms. One after another, mountain after mountain developed. Few broke the water’s surface. But those that have became islands.
Together this chain of peaks stretch more than 5,800 kilometers (3,600 miles) across the Pacific Ocean floor.
The chain grew south at first. Then, at some point it abruptly turned east. That was around 50 million years ago.
But a tectonic plate off the coast of East Asia met its doom some 10 million years before the seamount chain took that eastward turn. This Izanagi Plate completely slipped under another plate. And the mantle just gobbled it up.
Maria Seton works at the University of Sydney. Her team wondered how the plate’s plunge would have affected the nearby flow of hot rock in the mantle. Until then, the sinking rocky slab had acted like a wall extending down into the mantle. It had obstructed the flow of molten rock there.
Using a computer, her team calculated how the mantle and the sinking plate would have interacted. And their analyses now indicate that removing the mantle-blocking Izanagi Plate triggered a 7-million-year reorganization of mantle movements. Mantle flows did an about-face in their computer model. The flows had been moving south at about 0.5 to 1.7 centimeters (0.2 to 0.7 inch) per year. Now the flow crept north and a bit west at a sluggish 0.1 to 1.1 centimeters per year.
Because the Hawaiian hot spot sits within the mantle, the reversed mantle flow halted the hot spot’s southward drift. So it remained stationary as the Pacific Plate drifted westward above it. This caused the developing Hawaiian seamount chain to expand to the east.
(for more about Power Words, click here)
core In geology, Earth’s innermost layer.
crust (in geology) Earth’s outermost surface, usually made from dense, solid rock.
lava Molten rock that comes up from the mantle, through Earth’s crust, and out of a volcano.
magma The molten rock that resides under Earth’s crust. When it erupts from a volcano, this material is referred to as lava.
mantle (in geology) The thick layer of the Earth beneath its outer crust. The mantle is semi-solid and generally divided into an upper and lower mantle.
molten A word describing something that is melted, such as the liquid rock that makes up lava.
plume (in geology) Magma that rises into Earth’s crust.
seamount A mountain (usually formed by a volcano) whose entire structure sits below the surface of the ocean. If it broke the surface, it would become an island.
subductor subduction The process by which tectonic plates sink or slide back from Earth’s outer layer into its middle layer, called the mantle.
subduction zone A large fault where one tectonic plate sinks beneath another as they collide. Subduction zones usually have a deep trench along the top.
tectonic plates The gigantic slabs — some spanning thousands of kilometers (or miles) across — that make up Earth’s outer layer.
volcanic Having to do with volcanoes and volcanism.
volcano A place on Earth’s crust that opens, allowing magma and gases to spew out from underground reservoirs of molten material. The magma rises through a system of pipes or channels, sometimes spending time in chambers where it bubbles with gas and undergoes chemical transformations. This plumbing system can become more complex over time. This can result in a change, over time, to the chemical composition of the lava as well. The surface around a volcano’s opening can grow into a mound or cone shape as successive eruptions send more lava onto the surface, where it cools into hard rock.
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Original Journal Source: N.M. Wright et al. Revision of Paleogene plate motions in the Pacific and implications for the Hawaiian-Emperor bend. Geology. Published online March 27, 2015. doi: 10.1130/G36303.1.
Original Journal Source: N M. Seton et al. Ridge subduction sparked reorganization of the Pacific plate-mantle system 60–50 million years ago. Geophysical Research Letters. Published online March 24, 2015. doi: 10.1002/2015GL063057.