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The Honolulu Advertiser
Posted on: Monday, September 4, 2006

Hawaiian geology gets update

By Jan TenBruggencate
Advertiser Science Writer

RESEARCH UNVEILS SCIENCE MYSTERY

Synopsis: The Hawaiian Islands extend some 1,400 miles northwest from the Big Island to Kure Atoll. The chain continues beyond Kure in the form of a long line of undersea mountains called the Emperor Seamounts. They continue northwest to a point 2,300 miles from the Big Island, and then turn north and disappear at the Aleutian Islands.

Scientists believe the chain has been built by a volcanic "hot spot" that ruptures the floor of the Pacific Ocean as the vast Pacific Plate moves over it. As new volcanoes erupt at the hot spot, older volcanoes are carried away on the moving ocean floor.

A mystery for science has been a "bend" in the line of island and seamounts near Kimmei Seamount. New research suggests that the Hawaiian-Emperor Bend was caused by a major change in the direction of the Pacific Plate, and that this coincides with a series of other global geological upheavals including India crashing into Eurasia.

Definition: The Pacific Plate is one of about 10 major and several minor tectonic plates vast portions of the planet's crust that "float" on the liquid mantel beneath. Where plates meet are zones of earthquakes and often volcanoes. The more dense crust of one plate can be drawn under the less dense adjacent plate in a "subduction" zone.

Why should we care: It is important to understand plate dynamics because they are a key source of earthquakes that form tsunamis in the Pacific. Many of the major tsunamis that have hit Hawai'i have been generated at plate boundaries. The 2004 Indian Ocean tsunami was generated by movement at a plate boundary.

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Geologists have established a new date for a sharp "bend" in the line of seamounts at the western end of the Hawaiian archipelago to 50 million years ago, which links it with the start of extended volcanic activity near the Marianas Islands, and possibly with the collision of India into Eurasia.

The work of David Clague and Warren Sharp brings a key feature of Hawaiian geology into global focus. In a paper published in Thursday's issue of Science magazine, the scientists say that a series of massive geological events all appeared about the same time, and may all have been connected:

  • The dogleg, known as the Hawaiian-Emperor Bend, in the line of islands and undersea mountains that extend from the Big Island to the Aleutian Islands.

  • A line of volcanoes in the western Pacific, called the Izu-Bonin-Mariana Arc, which abruptly started erupting just about 50 million years ago at a time when the Pacific Plate began being pulled down and under the adjacent Australian Plate. Scientists have dated the oldest rocks there to establish their age.

  • A possible trigger for both of those changes a geological "lockup" that happened about 50 million years ago when India crashed into Eurasia and dramatically slowed its own movement. Clague said the change could have caused a general "reorganization" of plate movement

    NEW DATING TECHNIQUES

    Geologist Clague, a former staffer at the Hawaiian Volcano observatory, works with the Monterey Bay Aquarium Research Institute and geochemist Sharp at the Berkeley Geochronology Center. They used modern radio-isotope dating techniques to establish the ages of volcanic rocks collected on seamounts on both sides of the Hawaiian-Emperor Bend.

    They dated the Suiko Seamount at 60.9 million years ago, the north side of Koko Seamount at 52.6 and the south side of Koko at 50.4 million years ago. Koko is just north of the bend. To the southeast of the bend is Kimmei Seamount at 47.9 million years ago and southeast of it, Daikakuji at 46.7.

    HAWAIIAN HOT SPOT

    All these seamounts and the Hawaiian Islands were formed according to geological theory by the same feature. The Hawaiian Hot Spot is believed to be a generally stationary hot plume that delivers molten rock from deep in the earth to the surface. As the Pacific Plate moves over it, older islands are carried away from the hot spot the farther away an island or seamount is, the older it is.

    The rocks in the region of the Hawaiian-Emperor Bend had been estimated at 43 million years old. That created a mystery because there wasn't much else happening geologically at that time that would have explained the change in direction. One suggestion floated by wondering geologists was that perhaps the plate didn't change direction so much as the Hawaiian Hot Spot itself was moving.

    The new dating data seems to create a package that makes geological sense and attributes the bend to the movement of the Pacific Plate rather than the hot spot.

    And it suggests that the whole thing took its time.

    "India's collision into Asia went on for millions of years," Sharp said.

    And the rock dates suggest the Pacific Plate changed direction just as slowly less like a sports car and more like a city bus.

    "The bend was not a quick event," Clague said.

    "Our new ages show that the (Hawaiian-Emperor) bend formed over about 8 million years, much longer than previously believed," Sharp said.

    Does any of this matter? In an age in which tsunamis and earthquakes are regular features in the public perception, it does, he said.

    "Overall, our understanding of how plates move, broadly, is going to help us understand what the earthquake hazards are," he said.

    Reach Jan TenBruggencate at jant@honoluluadvertiser.com.

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