The Hawaiian Island chain, as most people know it, runs from Hawai'i to Kaua'i and Ni'ihau, but the islands are only the tail of a string of volcanoes that runs all the way to the Aleutians. What baffles science is why this long string makes a distinct turn midway.
(Image: The Hawaiian-Emperor Chain extends 3,700 miles northwest from the main Hawaiian Islands, and then makes a right turn beyond Midway and Kure, continuing as a line of seamounts that eventually disappears at the northern edge of the Pacific tectonic plate. The blue line follows the course of the chain and the red arrow marks the right turn. Credit: Modified from Google Earth.)
Current geological thinking is that the massive Pacific plate, which forms part of the Earth's crust, is constantly moving, its edges sliding along or under or over other plates, or being shoved away from others by volcanic activity. And a feature called the Hawaiian hot spot punches volcanoes up through the plate.
Like a pencil marking a line of dots on a page, the hot spot under the moving plate leaves a line of volcanoes.
But what could have caused the bend in the line? Some movement of the hot spot? A dramatic change in the direction in which the plate moves? The question was tackled in a recent paper in Science Magazine by geologists David Clague, former director of the Hawaiian Volcano Observatory now working with the Monterey Bay Aquarium Research Institute, and Warren Sharp, of the Berkeley Geochronology Center.
They conclude that the bend in the 129-volcano Hawaiian-Emperor Chain (the name for the combination of the Hawaiian Archipelago south of the bend and the Emperor Seamounts north of it), occurred about 50 million years ago.
They carefully determined the ages at which rocks in the chain were created, using the latest dating techniques available. In many cases, they were using rock samples collected in the 1960s, but whose ages had been established years ago with less effective equipment. With the new technology, many dates changed.
They found that the rocks from Kimmei seamount, in the bend and 2,270 miles from the active Kīlauea volcano on the Big Island, are 47.9 million years old, give or take a couple of hundred thousand years. That's several million years older than earlier estimates.
Samples from seamounts to the north and south show a continual progression in age as the chain goes north. When they compared ages of rocks to distance from Kīlauea, Sharp and Clague found that the Pacific Plate appears to move steadily, although with some speed-ups and slow-downs.
And they concluded that the change in direction, as represented by the Hawaiian-Emperor Bend (HEB), didn't happen suddenly.
“The new ages reveal that the HEB formed over a period of several million years...Initiation of the HEB occurred north of Daikakuji, near Kimmei seamount, where the chain's trend rotates from nearly due south to southeasterly,” they write.
Elsewhere in the paper, they say the bend took as much as 8 million years.
So, what was going on in and around the Pacific 50 million years ago that might have been associated with the change?
There was new volcanic activity along more than 1,300 miles of the plate's western edge, an area called the Izu-Bonin-Mariana arc. Rocks from that area also date to 50 million years ago.
That new activity may have been part of the change in the planet's geology that allowed the plate to slide more westward than southward.
The association of the Hawaiian-Emperor Bend with the Izu-Bonin-Mariana activity has previously been discounted, because earlier dates of the rocks suggested they happened several million years apart. The new dates from the bend suggest the bend was happening at the same time the activity at the western end of the plate took place, and that the one may have helped cause the other.
As followers of murder mysteries know, just a slight change in the timeline can turn someone with an airtight alibi into a prime suspect.
In geology, the same thing can take place.