Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

A fiery debate about volcanoes

09.05.2003


In a Perspective in the May 9 issue of Science, geochemist Don DePaolo and geodynamicist Michael Manga defend a fundamental assumption of Earth science, the mantle plume model of hotspots, against an outbreak of seismic skepticism.


The premier example of a hotspot is Hawaii, which exudes lava at a higher rate per unit area than any other place on Earth.


The Hawaii Scientific Drilling Project has collected core samples whose chemistry establishes the deep mantle origin of the lava that built the islands.



DePaolo and Manga are members of Lawrence Berkeley National Laboratory’s Earth Sciences Division and the University of California at Berkeley’s Department of Earth and Planetary Science. DePaolo studies the chemical signatures of geological structures like the lava beneath the island of Hawaii. Manga studies volcanism and other fluid processes using a variety of techniques, including physical models.

The premier example of a hotspot is Hawaii, which exudes lava at a higher rate per unit area than any other place on Earth.


On Earth, most volcanoes are tectonic in origin -- they occur where crustal plates are spreading apart or colliding. This isn’t true for all volcanoes on this planet, however, and it’s an unlikely explanation for the active volcanism on other planets and moons of our solar system.

An additional volcanic mechanism, the mantle plume model, was proposed over 30 years ago and is almost as widely accepted as plate tectonics itself. A mantle plume is a source of extra-hot rock rising from deep in the planet -- probably from the base of the mantle itself, where it meets the iron-rich liquid core.

Mantle plumes are thought to underlie "hotspots" far from plate boundaries, like those under the Hawaiian Islands or Yellowstone National Park. There also appears to be a source of hot rock under Iceland, lying much deeper than the source of melt that erupts at the mid-Atlantic ridge. Estimates of the number of hotspots in the world range from a handful to more than a hundred, but most scientists agree to a list of several dozen.

While plate movements can be observed directly, however, plumes under hotspots must be inferred from seismology. So when seismologists like Gillian Foulger of the University of Durham, United Kingdom, report no evidence for plumes under Yellowstone and Iceland, it’s a short step to the claim that plumes don’t exist at all. (Foulger is an author of a Perspective in the same May 9 Science presenting the "antiplume" view.)

"It seems to me that the critics don’t understand some aspects of the basic problem," says DePaolo. "For example, they claim that the mantle beneath Iceland is not hot, and the mantle beneath Yellowstone is not flowing upward. My view of the data in the papers they cite is that it leads straight to the conclusion that the rock is hot and flowing upward, just as we expect for mantle plumes."

DePaolo and Manga cite the strong evidence for deep plumes under at least a few hotspots. When a mid-ocean ridge moves right across a hotspot, as the Reunion hotspot in the Indian Ocean has done, the ridge doesn’t excavate the hotspot; instead, volcanism jumps from one plate to the other. Thus the hotspot source must lie deeper than the region of upwelling beneath the spreading plates.

In Hawaii, far from a plate boundary, so much magma is produced in a tight circle less than 100 kilometers across that it must be moving upward at some 50 centimeters a year -- ten times faster than the movement of crustal plates (which move at roughly the speed of growing fingernails). The lithosphere under Hawaii is some 80 kilometers thick. For melted rock to exist that deep, the temperature must be 200 to 300 degrees Celsius hotter than the surrounding mantle.

Measurements of chemistry and isotope ratios by DePaolo and others also point to the deep origin of the lavas of Hawaii and other hotspots -- an origin different from that of the relatively shallow-melt basalts of the mid-ocean ridges. Especially telling is the high ratio of rare helium 3 to common helium 4, a greater ratio characteristic of helium from the deep mantle. Similar information comes from isotopes of the elements neodymium, strontium, lead, and hafnium.

The evidence goes to "one of the central problems in Earth science," says Manga, and not only because plume origins are key to making connections "between seismologically imaged regions and geochemically inferred reservoirs." He explains that "because Earth’s core cools through plumes, understanding plumes is essential to understanding the energetics and evolution of the core."

Manga’s studies of fluids, some involving what he calls "neat stuff in the lab," like a heated fish tank filled with motor oil and soy oil, have shown how plumes can form from a superhot layer of rock at the core-mantle boundary. Like a gigantic growing mushroom, a plume breaks off and slowly rises through the mantle. At the surface it can burst out in titanic flood basalts like those seen in India, Siberia, and west of Yellowstone.

Most important, once a plume reaches the surface, its thin tail can stay in place for 100 million years or more -- forming an open pipe from the core-mantle boundary, through which hot rock continues to rise. A plume’s persistence, once formed, is a natural physical explanation for the stability and longevity of hotspots.

The stationary Hawaii hotspot, for example, has traced out a chain of islands and seamounts 6,000 kilometers long as the Pacific Plate moves slowly across it. A symmetrical 500-kilometer-wide bulge in the sea floor beneath the island’s huge lava mass, lifted by the upwelling plume beneath, verges on direct, seeing-is-believing evidence for the existence of the plume.

DePaolo and Manga do not insist that all hotspots are caused by deep mantle plumes. The evidence is strong for half a dozen hotspots, but others may owe their persistent existence to different mechanisms.

Yet the poor resolution of present-day seismological images, and the many uncertainties involved in extracting information on density, temperature, and viscosity of deep rock from seismic wave paths, make for weak arguments against the existence of plumes.

"Many natural phenomena were well understood before instruments became sensitive enough to measure them directly," says DePaolo -- and that may be the case with seismology. DePaolo and Manga illustrate their Science Perspective with high-resolution seismic images, made with a new tomography technique recently developed by Guust Nolet and others at Princeton University, which map a thin column of high temperature in the mid-Pacific all the way down to the core-mantle boundary, 2,900 kilometers beneath the active volcanoes of Hawaii that sit atop it.

The Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California. Visit our website at http://www.lbl.gov.

Paul Preuss | DOE/Lawrence Berkeley National L
Further information:
http://www.lbl.gov/Science-Articles/Archive/ESD-volcanic-debate.html
http://www.lbl.gov

More articles from Earth Sciences:

nachricht Ice cave in Transylvania yields window into region's past
28.04.2017 | National Science Foundation

nachricht Citizen science campaign to aid disaster response
28.04.2017 | International Institute for Applied Systems Analysis (IIASA)

All articles from Earth Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Making lightweight construction suitable for series production

More and more automobile companies are focusing on body parts made of carbon fiber reinforced plastics (CFRP). However, manufacturing and repair costs must be further reduced in order to make CFRP more economical in use. Together with the Volkswagen AG and five other partners in the project HolQueSt 3D, the Laser Zentrum Hannover e.V. (LZH) has developed laser processes for the automatic trimming, drilling and repair of three-dimensional components.

Automated manufacturing processes are the basis for ultimately establishing the series production of CFRP components. In the project HolQueSt 3D, the LZH has...

Im Focus: Wonder material? Novel nanotube structure strengthens thin films for flexible electronics

Reflecting the structure of composites found in nature and the ancient world, researchers at the University of Illinois at Urbana-Champaign have synthesized thin carbon nanotube (CNT) textiles that exhibit both high electrical conductivity and a level of toughness that is about fifty times higher than copper films, currently used in electronics.

"The structural robustness of thin metal films has significant importance for the reliable operation of smart skin and flexible electronics including...

Im Focus: Deep inside Galaxy M87

The nearby, giant radio galaxy M87 hosts a supermassive black hole (BH) and is well-known for its bright jet dominating the spectrum over ten orders of magnitude in frequency. Due to its proximity, jet prominence, and the large black hole mass, M87 is the best laboratory for investigating the formation, acceleration, and collimation of relativistic jets. A research team led by Silke Britzen from the Max Planck Institute for Radio Astronomy in Bonn, Germany, has found strong indication for turbulent processes connecting the accretion disk and the jet of that galaxy providing insights into the longstanding problem of the origin of astrophysical jets.

Supermassive black holes form some of the most enigmatic phenomena in astrophysics. Their enormous energy output is supposed to be generated by the...

Im Focus: A Quantum Low Pass for Photons

Physicists in Garching observe novel quantum effect that limits the number of emitted photons.

The probability to find a certain number of photons inside a laser pulse usually corresponds to a classical distribution of independent events, the so-called...

Im Focus: Microprocessors based on a layer of just three atoms

Microprocessors based on atomically thin materials hold the promise of the evolution of traditional processors as well as new applications in the field of flexible electronics. Now, a TU Wien research team led by Thomas Müller has made a breakthrough in this field as part of an ongoing research project.

Two-dimensional materials, or 2D materials for short, are extremely versatile, although – or often more precisely because – they are made up of just one or a...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Fighting drug resistant tuberculosis – InfectoGnostics meets MYCO-NET² partners in Peru

28.04.2017 | Event News

Expert meeting “Health Business Connect” will connect international medical technology companies

20.04.2017 | Event News

Wenn der Computer das Gehirn austrickst

18.04.2017 | Event News

 
Latest News

Wireless power can drive tiny electronic devices in the GI tract

28.04.2017 | Medical Engineering

Ice cave in Transylvania yields window into region's past

28.04.2017 | Earth Sciences

Nose2Brain – Better Therapy for Multiple Sclerosis

28.04.2017 | Life Sciences

VideoLinks
B2B-VideoLinks
More VideoLinks >>>