Results from an expedition to the sea floor near the Hawaiian Islands show
evidence that the deep Earth is more unsettled than geologists have long believed.
A new University of Rochester study suggests that the long chain of islands
and seamounts, which is deemed a "textbook" example of tectonic plate
motion, was formed in part by a moving plume of magma, upsetting the prevailing
theory that plumes have been unmoving fixtures in Earth's history. The research
will be published in the August 22 issue of Science.
"Mobile magma plumes force us to reassess some of our most basic assumptions about the way the mantle operates," says John Tarduno, professor of earth and environmental sciences at the University. "We've relied on them for a long time as unwavering markers, but now we'll have to redefine our understanding of global geography."
Traditionally, the islands were thought to have formed as the massive Pacific plate, the largest single section of Earth's crust, moved sluggishly between the Americas and Asia. A plume, or "hot spot," brought super-heated magma from deep in the Earth to close to the crust, resulting in concentrated areas of volcanic activity. As the Pacific plate moved across this hot spot, the plume created a long series of islands and subsurface mountains. Though this chain of seamounts seemed like a perfect record of Pacific plate movement, a strange bend in the chain, dated at about 47 million years ago, troubled some geologists. To most, however, this bend was taken as the classic example of how plates can change their motion. In fact, a figure of the bend can be found in nearly all introductory text books on geology and geophysics.
Tarduno and an international team spent two months aboard the ocean drilling ship JOIDES Resolution, retrieving samples of rock from the Emperor-Hawaiian seamount chain miles beneath the sea's surface. Rocks retrieved in 1980 and 1992 hinted that the seamounts were not conforming to expectations. The team started at the northern end of the chain, near Japan, braving cold, foggy days and dodging the occasional typhoon to pull up several long cores of rock as they worked their way south. Using a highly sensitive magnetic device called a SQUID (Superconducting Quantum Interference Device), Tarduno's team discovered that the magnetism of the cores did not fit with conventional wisdom of fixed hotspots.
The magnetization of the lavas recovered from the northern end of the Emperor-Hawaiian chain suggested these rocks were formed much farther north than the current hotspot, which is forming Hawaii today. As magma forms, magnetite, a magnetically sensitive mineral, records the Earth's magnetic field just like a compass. As the magma cools and becomes solid rock, the compass is locked in place. Measuring the angle that this magnetism records relative to the Earth's surface allows geophysicists to determine the latitude at which magma solidified: Near the equator the angle is very small while nearer the poles, the angle is near vertical. If the Hawaiian hot spot had always been fixed at its current location of 19 degrees north, then all the rocks of the entire chain should have formed and cooled there, preserving the magnetic signature of 19 degrees even as the plate dragged the new stones north-westward. Tarduno's team, however, found that the more northern their samples, the higher their latitude. The northern-most lavas they recovered were formed at over 30 degrees north about 80 million years ago, nearly a thousand miles from where the hot spot currently lies.
"The only way to account for these findings is if the Pacific plate was almost stationary for a time while the magma plume was moving south," says Rory Cottrell, research scientist and coauthor of the paper. "At some point about 45 million years ago, it seems that the plume stopped moving and the plate began."
At the mysterious bend in the chain the magnetite latitude readings level off to 19 degrees, suggesting that for some reason the magma plume stopped dead in its tracks.
"Why the hot spot stopped moving south, and whether this is related to the Pacific plate suddenly moving, is something we'd all like to discover," says Tarduno. "There's been a quiet controversy about hot-spot motion for 30 years because some people thought the accepted theory wasn't adding up. This study answers a lot of questions."
Aside from shedding light on tectonic motion, the findings will likely prove a boon for climatologists studying the ancient Earth. Climate changes are recorded in rocks such as those on the Pacific ocean floor, but in order to accurately judge ancient climate, the scientists must know at what latitude the rocks were at a given time in the past. Measuring against the bent Hawaiian-Emperor chain would yield results that would misplace those rocks and so throw off scientists' picture of early Earth's climate. The study also vindicates the work of some mantle modelers who have never had a problem with moving hot spots and who did not like the idea that a crustal plate as large as the Pacific could make a nearly right-angle bend in just a million years or so.
A meeting this month in Iceland, beneath which a hot spot is thought to currently reside, will focus heavily on the state of knowledge about plumes including the new idea that they are not stationary. As Tarduno says, "We're all just swaying around in the mantle wind."
This research was funded by the National Science Foundation.