ScienceWeek

GEOPHYSICS: VOLCANIC HOTSPOTS AND PLATE MOTIONS

In general, the term "hotspot" refers to an area of high volcanic activity.

The term "seamount" refers to a submarine peak (mountain) which does not rise above sea level, and which is often found in chains presumably resulting from the lithosphere migrating over a hotspot.

The following points are made by Joann Stock (Science 2003 301:1059):

1) Volcanic hotspots such as Hawaii and Iceland are believed to be caused by fixed volcanic sources deep in Earth's mantle. As a tectonic plate drifts over such a hotspot, age-progressive island chains and seamounts -- such as the Hawaiian-Emperor seamounts --are created. But how do we know that the hotspots are fixed relative to one another and that their tracks reflect only the plate motions above them? Recent paleomagnetic data from drill holes into the Emperor seamounts demonstrate that the story is not that simple.

2) The ages of the Emperor seamounts indicate rapid (>5 cm/year) relative motion between the plate and the hotspot source from 81 to 47 million years ago. This could mean that the Pacific plate was moving northward over a stationary hotspot source, carrying the volcanic record of the Emperor seamounts with it. Alternatively, the Pacific plate could have been stationary, underlain by a southward-moving source of hotspot volcanism.

3) Paleomagnetic data can reveal the original latitude of the seamount eruptions and the history of latitudinal motion of the Pacific plate. These data can thus distinguish between these two extreme possibilities, which have very different implications for mantle dynamics. If all Emperor seamounts were at 19ºN when they erupted, this would suggest that the hotspot source was fixed at 19ºN with respect to the North Pole and that there was northward motion of the Pacific plate. Conversely, if all seamounts had paleolatitudes equivalent to their modern latitudes, the Pacific plate would have been stationary, but the hotspot plume would have moved southward.

4) Recent paleomagnetic results demonstrate that the truth lies somewhere in between. The latitude of the volcanic centers was not constant with time, but was south from their modern latitudes for seamounts north of Koko Seamount. Earlier data showed that the Pacific plate had a northward component of motion. The recent Emperor Seamount drilling results reveal that there was also southward drift of the hotspot plume beneath the Pacific plate, contrary to the fixed-hotspot hypothesis.

Science http://www.sciencemag.org

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GEOLOGY: ON THE ORIGIN OF HOTSPOTS

The following points are made by D.J. DePaolo and M. Manga (Science 2003 300:920):

1) The workings of the hot interiors of the rocky planets of the solar system are most dramatically expressed by the size and arrangement of their volcanoes. Most volcanoes on Earth are a result of plate tectonics. At mid-ocean ridges, the spreading of the ocean floor generates upward flow of hot mantle rock beneath the ridge. This flow generates magma as a result of adiabatic decompression. At subduction zones, plates returning to the depths of the mantle carry water down in hydrous minerals. The water, when released by metamorphism, causes already hot rock material beneath island arcs to melt.

2) But not all volcanoes on Earth are located at mid-ocean ridges or subduction zones. "Hotspots" -- regions with particularly high rates of volcanism -- are not necessarily associated with plate boundaries. Hawaii, the premier example, is thousands of kilometers from the nearest plate boundary yet exudes lava at a higher rate per unit area than at any other place on Earth. The Hawaiian volcanic anomaly has remained mostly stationary for tens of millions of years and produced a 6000-km-long chain of islands and seamounts. This phenomenon is not explained by plate tectonics. It requires a separate mantle process that can account for narrow, long-lived upwellings of unusually hot mantle rock.

3) Shortly after the discovery of plate tectonics in the late 1960s, Morgan (1972) proposed that hotspots represent narrow (100 km diameter) upwelling plumes that originate within the lower mantle. Since that time, evidence from geophysics, fluid dynamics, petrology, and geochemistry has supported if not required the existence of mantle plumes. For many geoscientists, the mantle plume model is as well established as plate tectonics.

4) Nonetheless, it is reasonable that we should want to verify the model by direct observation. The only way we can "see" into the deep Earth is with seismology. This endeavor has so far not produced the rubber stamp that most thought it would. Seismological studies of the Yellowstone hotspot found no clear evidence for a lower mantle source, while evidence of a deep plume beneath the Iceland hotspot remains equivocal. Does the model need rethinking, or are the seismological tools still not quite up to the task, or perhaps both?

Science http://www.sciencemag.org

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