Return to Previous Press Release
Enter your name and a friend's email address in the fields below and click "Submit" to email this Press Release to a friend.
Your message will look like this:
[YOUR NAME HERE] thought you might be interested in this story from the University of Rochester.
MEDIA CONTACT: Jonathan Sherwood 585.273.4726
August 18, 2003
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.
The University of Rochester (www.rochester.edu) is one of the nation's leading private universities. Located in Rochester, N.Y., the University gives students exceptional opportunities for interdisciplinary study and close collaboration with faculty through its unique cluster-based curriculum. Its College of Arts, Sciences, and Engineering is complemented by the Eastman School of Music, Simon School of Business, Warner School of Education, Laboratory for Laser Energetics, Schools of Medicine and Nursing, and the Memorial Art Gallery.
PR 53, MS 0