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Hotspots or Not? Isotopes Score One for Traditional Theory
December 07, 2006PROVIDENCE, 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.
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.
Brown University
Plate Tectonics Current Events and Plate Tectonics News RSSNew Method to Measure Snow, Soil Moisture With GPS May Benefit Meteorologists, Farmers
A research team led by the University of Colorado at Boulder has found a clever way to use traditional GPS satellite signals to measure snow depth as well as soil and vegetation moisture, a technique expected to benefit meteorologists, water resource managers, climate modelers and farmers.
New Celestial Map Gives Directions for GPS
Many of us have been rescued from unfamiliar territory by directions from a Global Positioning System (GPS) navigator. GPS satellites send signals to a receiver in your GPS navigator, which calculates your position based on the location of the satellites and your distance from them.
Extraterrestrial platinum was 'stirred' into the Earth
Report author CSIRO Minerals Down Under Flagship researcher Dr Stephen Barnes said the study group collected a large body of data on the platinum content of lava flows called komatiites, which host some of the world's major nickel deposits.
Jade sheds light on Guatemala's geologic history
A new analysis of jade found along the Motagua fault that bisects Guatemala is underscoring the fact that this region has a more complex geologic history than previously thought.
New map hints at Venus's wet, volcanic past
Venus Express has charted the first map of Venus's southern hemisphere at infrared wavelengths. The new map hints that our neighbouring world may once have been more Earth-like, with both, a plate tectonics system and an ocean of water.
New definition could further limit habitable zones around distant suns
As astronomers gaze toward nearby planetary systems in search of life, they are focusing their attention on each system's habitable zone, where heat radiated from the star is just right to keep a planet's water in liquid form.
Asteroid attack 3.9 billion years ago may have enhanced early life on Earth, says CU-Boulder study
The bombardment of Earth nearly 4 billion years ago by asteroids as large as Kansas would not have had the firepower to extinguish potential early life on the planet and may even have given it a boost, says a new University of Colorado at Boulder study.
Bent tectonics
More than 80 undersea volcanoes and a multitude of islands are dotted along the Hawaii-Emperor seamount chain like pearls on a necklace. A sharp bend in the middle is the only blemish.
African initiative trains students, explores geophysical mysteries
Earthquakes, volcanoes and the African superplume are only some of the phenomena under investigation through AfricaArray, a program that establishes geophysical observatories, trains African and American students and examines geophysical phenomena on the African continent.
Decline of carbon dioxide-gobbling plankton coincided with ancient global cooling
The evolutionary history of diatoms -- abundant oceanic plankton that remove billions of tons of carbon dioxide from the air each year -- needs to be rewritten, according to a new Cornell study.
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