Hot Spot That Spawned Hawaii Was on the Move, Study Finds
for National Geographic News
|August 14, 2003|
A recent discovery that the so-called "fixed" hot spot, which created the Hawaiian Islands, actually drifted southward between 81 and 47 million years ago is causing geologists to revise their descriptions of the interior workings of the Earth.
The findings stem from analysis of ancient lava flows found on four seamounts, or undersea mountains, in the Pacific Ocean. Rock samples from the seamounts were gathered during a two-month expedition to the Pacific Ocean in 2001. The project was co-led by John Tarduno, a researcher at the University of Rochester in New York.
Hot spots have long been defined as stationary plumes of molten material that sit beneath tectonic plates. As these plates pass by, molten material periodically spews through the crust to create a chain of volcanic mountains.
Since hot spots are easiest to locate where the crust is thinnest, they are commonly found on seafloors. There they create strings of seamounts and volcanic islands that are believed to tell the story of how the overlying plates moved over time.
A well-known example of this phenomenon is the Hawaii-Emperor Seamount chain that stretches some 3,000 miles (5,800 kilometers) along the floor of the Pacific Ocean from the Big Island of Hawaii to Alaska's Aleutian Trench.
William Siesser, a geologist at Vanderbilt University in Nashville, Tennessee, who is familiar with the work by Tarduno and colleagues, said the evidence clearly indicates the Hawaii hot spot moved between 81 and 47 million years ago.
"Recognition that the Hawaii hot spot has moved suggests the possibility that other hot spots may have moved as well," he said. "If so, this means that all estimates of global plate motion based on hot spot trails would have to be recalculated for the movement of the hot spots themselves."
Hawaii Hot Spot
The prevailing theory among geophysicists is that the Hawaii-Emperor Seamounts formed as the Pacific Plate moved over a fixed hot spot that today is located beneath the island of Hawaii and is responsible for the world's most active volcano, Kilauea.
The segment known as the Hawaiian Ridge, which includes the main Hawaiian Islands and a chain of islands, atolls, and seamounts known collectively as the Northwestern Hawaiian Islands, extends some 1,800 miles (3,000 kilometers) northwest across the Pacific.
At the end of the Northwestern Hawaiian Islands, the chain turns sharply northward and becomes the Emperor Seamounts, which stretch to their intersection with the Aleutian Trench at the Kamchatka Peninsula in Siberia.
According to most researchers, this sharp bend represents a rapid change in direction of the Pacific Plate as it passed over the fixed hot spot 47 million years ago.
The research by Tarduno and colleagues proves this notion wrong by showing that the bend is at least partly the result of the movement of the hot spot plume within the mantle itself.
"The Hawaii bend was used as a classic example of how a large plate can change motion quickly. You can find a diagram of the Hawaii-Emperor bend entered into just about every introductory geological textbook out there. It really is something that catches your eye," said Tarduno.
The new findings were published late last month on Science Express, the online research journal of the American Association for the Advancement of Science, and will be published in an upcoming issue of the print journal Science.
Michael Manga, a geologist at the University of California at Berkeley, said the findings by Tarduno and colleagues are believable and help resolve how the Hawaii bend was made possible.
"They've shown the way the Earth works is not inconsistent with the way we expect it to work," he said.
For years geologists have struggled to explain the dynamics of plate tectonics that accounted for the observed Hawaii bend. Given a fixed hot spot, models never could accurately explain the phenomenon, added Manga.
On the Move
To test the theory that the hot spot was on the move before the Hawaii bend, Tarduno and colleagues embarked on a two-month excursion July 1, 2001, aboard the research vessel JOIDES Resolution to collect samples of solidified lava flows from four seamounts. Back in their laboratories, the researchers analyzed the samples.
"The test is straightforward," said Tarduno, "You need to know how old the rocks are and where they formed."
Study co-author Robert Duncan, a geophysicist at Oregon State University in Corvallis, and colleagues looked at the decay of the element potassium to argon, which occurs at a constant rate, and determined that the seamounts located before the bend formed between 81 million and 47 million years ago.
Tarduno and his colleagues determined where the seamounts formed by analyzing a mineral called magnetite in the rock samples. As hot magma from an erupting volcano cools, magnetite's natural magnetization aligns with the Earth's magnetic pole, much like a compass needle. Once the magma hardens, this magnetization is locked in place, becoming a permanent record.
"Magnetite is like a little compass, it tells us the direction to the pole, and it tells us how far from the pole," said Tarduno.
If the hot spot had been fixed for the past 80 million years, the latitude as determined by the magnetite should be the same for each sample and should also be the same latitude as the current Big Island of Hawaii, according to the researchers.
Tarduno and colleagues found that the hot spot actually crept southward at a rate of about 1.6 inches (40 millimeters) per year between 81 and 47 million years ago. They write in their paper that the finding changes "our understanding of terrestrial dynamics."
Forty-seven million years ago, the southerly movement of the hot spot slowed drastically and maybe even stopped, said Tarduno. The result is the bend seen in the seamount chain, which until now was considered proof that the Pacific Plate itself had changed direction.
Given the fact that the Hawaii-Emperor Seamounts have been used as an example of plate motion change, Tarduno says the findings question whether or not major plates can actually undergo rapid changes in direction.
As well, if hot spots are not fixed, then they cannot be used as a fixed frame of reference to track things such as climate change over time. Additionally, the finding may force geologists to re-think how the tectonic plates have moved over time.
"It means you can't ignore the mantle in thinking about how plates are moving and thinking about dynamics of the plates," said Tarduno.
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