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PROVIDENCE, R.I. - One great beauty of plate
tectonics theory is that it explains so many geological phenomena at
one time. But plate tectonics could not explain the location of many
volcanic islands – Hawaii, the Azores or the Galapagos Islands, often
called “hotspots” – far from the edge of tectonic plates. To deal with
those observations, geologists invoked the concept of “plumes” – areas
where buoyant sections of mantle material rose, melted and developed
into concentrated upwellings of magma, forming seamounts and island
chains.
A running battle has evolved over the last 30 years
concerning hotspots: One camp claims it is not necessary to invoke
mantle plumes to explain such volcanic islands, and the other camp – a
sizeable portion of the geological community – supports mantle plumes
as the most internally consistent explanation for a wide variety of
data.
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Raising the bar for mantle plume opponents
This trail of underwater mountains, known as the
Hawaiian-Emperor Seamount Chain or the Emperor Seamounts, was
created as the tectonic plate moved across the Hawaii hotspot over
the course of millions of years. A study of uranium isotopes
strongly supports upwelling of mantle material as the original
source of these islands.
Image: NOAA |
A study published this week in the
journal Nature raises the bar for plume opponents by finding a
close correlation between modeled and observed ratios of
uranium-series isotopes across eight island locations. The study
strongly supports upwelling of mantle material as the source of these
islands. Moreover, the detailed data allow researchers to estimate the
change in temperature, speed and size of mantle plumes at the
locations studied.
Alberto Saal, assistant professor of
geology at Brown University, contributed data from the Galapagos
Islands, complementing information from researchers working in Hawaii,
Pitcairn, the Azores, the Canary Islands, the Afar region and Iceland.
With such a breadth of data in hand, lead author Bernard Bourdon,
professor at the Swiss Federal Institute of Technology in Zürich (formerly
at the Institut de Physique du Globe in Paris), was able to build
robust correlations between the ratios of isotopes in the actinide
series and the flux of material needed to build the observed islands.
“What’s exciting about this,” says
Saal, “is that it allows us to make inferences about physical
conditions based on chemical measurements.” While it is impossible to
visit the boundary of the mantle to make the physical measurements, it
is possible to collect the chemical evidence that has been brought all
the way to the surface.
When mantle rocks melt, the ratio of
uranium isotopes to their decay products changes dramatically, then
moves back to equilibrium at a steady, predictable rate. Using this
change in ratios, the researchers were able to determine how quickly
and completely the material melted. This also allowed them to estimate
the difference in temperature between the mantle and the plumes, which
determines the speed and size of the upwellings.
Beyond adding to the general evidence
for mantle plumes, the study allows researchers to generate some
numbers that could potentially be tested. “We think we can provide
some extra constraints on these parameters that are generally poorly
known,” says Bourdon.
Their estimates of temperature
differences ranged between 50 and 200 degrees C with the larger
differences seen in areas believed to have stronger plumes – such as
Hawaii and the Galapagos. Assuming symmetrical plumes, Bourdon and his
colleagues were also able to make estimates of the radius of mantle
plumes at each location that roughly fit with estimates of plume
diameters from seismological sources.
The more researchers can make the
notion of hotspots concrete, the better chance they have to prove it
right – or wrong. |
