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ScienceWeek
EARTH SCIENCE: MANTLE MELTING AND MID-OCEAN RIDGES
The following points are made by Mark Wilson (Physics Today 2005 January):
1) The mid-ocean ridge is a 75,000-kilometer network of undersea mountain chains that, like the seam of a baseball, winds around the globe from the Arctic Ocean to the Atlantic, around Africa, Asia, and Australia, under the Pacific, and skirts the west coast of North America. This fundamental feature of Earth's superstructure delimits the intersection of tectonic plates and is the source of 85% of Earth's volcanism. As plates separate and cracks form, the mantle wells up to fill the gap. Partial melting of the solid mantle produces magma that percolates upward through pores and grain boundaries in the rock and then crystallizes into basalt -- new layers of Earth's crust. The process prompted oceanographer Bruce Heezen to characterize the ridge system as "the wound that never heals".
2) But if the mechanics of tectonic fracture were all that controlled the accretion of crust, one might expect uniform depth everywhere as the plates spread. In fact, the topography of the ridge is rich and varied, with frequent discontinuities and offsets, ranging in length from a few kilometers to hundreds of kilometers, that partition it into segments. Ridge geometry can be complicated, in part because the segments behave like giant, mobile cracks that may lengthen or shorten over time. Nevertheless, they appear to be closely linked to melting in Earth's mantle, based on well-known variations in the petrology, magnetization, and, in some places, the thickness of crust formed beneath the deep oceans along the ridge.
3) Exactly what accounts for those variations has puzzled researchers for years. The problem of determining how the production and transport of heat and magma from the mantle influence ridge topography (and vice versa) is not new, but the inaccessibility of Earth's deep interior has limited how researchers could address it. In the deep oceans, kilometers of basaltic crust cover the residual mantle rock that melted to produce the crust. Seismic imaging can remotely determine the amount of molten material below the crust, and thereby indicate the distribution of magma chambers that feed the ridge. Rocks coughed out of volcanoes and dredged from the sea floor provide a range in composition data, but only indirect information about the mantle from which they came.
4) Another approach has been to analyze the topography in the ridge and variations in the geochemistry of basalt that crystallized from the melt and erupted onto the sea floor. The basalt studies have shown a correlation between composition and segmentation in some places but not in others. The melt's percolation to the surface along with fractional crystallization and mixing in magma chambers obscures mantle processes.[1-4]
References:
1. L. Le Mee, J. Girardeau, C. Monnier, Nature 432, 167 (2004)
2. See, for example, A. Nicolas, F. Boudier, B. Ildefonse, E. Ball, Mar. Geophys. Res. 21, 147 (2000)
3. E. M. Klein, C. H. Langmuir, J. Geophys. Res. 92, 8089 (1987)
4. P. B. Kelemen et al., Phil. Trans. R. Soc. London A 355, 283 (1997)
Physics Today http://www.physicstoday.org
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EARTH SCIENCE: ON THE ARCTIC MID-OCEAN RIDGE
The following points are made by Emily M. Klein (Nature 2003 423:932):
1) In 1893, Fridtjof Nansen (1861-1930) and members of his expedition set out to conduct a remarkable scientific study in the Arctic. Their ship, the Fram, was designed to become encased in pack ice and to move with it, and over a three-year period the Fram's stately motion demonstrated the drift of the polar ocean current. Just over a century later, a fresh chapter in the scientific study of the northern high latitudes opened with a two-ship, international expedition to study the ocean floor beneath the Arctic Ocean. Some results from this study -- AMORE, for Arctic Mid-Ocean Ridge Expedition -- appeared early this year(1), and others are described by Michael et al(2) and Jokat et al(3). The Arctic Ocean is covered by sea ice, so the studies were carried out on scientifically equipped icebreakers, the US Coast Guard Cutter Healy and the German PFS Polarstern.
2) At mid-ocean ridges, partial melting of upwelling mantle from deep within the Earth produces magmas that erupt and cool to become basalt lavas. This magmatic activity creates new ocean crust that spreads on either side of the ridge, at different rates depending on the location. Ridges may also be sites of intense hydrothermal activity, as sea water penetrates the crust, becomes heated by its proximity to hot magma at depth, and re-emerges into the water column through vents as element-rich fluids.
3) The ridge beneath the Arctic Ocean is the 1800-km-long Gakkel ridge, and it has some unique characteristics. Given these characteristics, studies of other parts of the global mid-ocean-ridge system allowed certain predictions to be made about what would be found there. For instance, almost three decades ago aeromagnetic surveys had shown that the Gakkel ridge is spreading at the slowest rate of any mid-ocean ridge(4), with a "full rate" (that is, accounting for motion on either side) of about 0.3-1.0 cm per year in the eastern part. That compares with about 6 cm per year for "intermediate-spreading" ridges elsewhere. Spreading rate had been shown to correlate in a general way with a host of ridge characteristics, such as the balance between magmatism and tectonic faulting, the composition of the lavas, the architecture of the ocean crust, and the nature and distribution of hydrothermal activity.
4) From these correlations, it was predicted that the Gakkel ridge would have generally "anemic" magmatism that would progressively decrease in volume as the spreading rate decreased towards the east, where there is a thicker, conductively cooled cap on mantle upwelling and melting(5). In addition, the extrapolation from fast-spreading to slow-spreading rates suggested that there would be little or no hydrothermal activity along ultraslow-spreading sections such as the Gakkel ridge, because of decreased magmatism and heat (which together drive hydrothermal circulation).
5) Surprisingly, both of these predictions were found to be incorrect. Based on their spectacular bathymetric and tectonic maps of the western 1000 km of the ridge, as well as the amounts of lava recovered by dredging, Michael et al.(2) show that the eruption of magma is surprisingly robust in the western zone, only sparse in the central zone, and robust again in the eastern zone, where the spreading rate is slowest. Furthermore, hydrothermal plume activity turns out to be among the most vigorous of any ocean ridge yet studied.
References (abridged):
1. Edmonds, H. N. et al. Nature 421, 252-256 (2003)
2. Michael, P. J. et al. Nature 423, 956-961 (2003)
3. Jokat, W. et al. Nature 423, 962-965 (2003)
4. Vogt, P. R., Taylor, P. T., Kovacs, L. C. & Johnson, G. L. J. Geophys. Res. 84, 1071-1089 (1979)
5. Reid, I. & Jackson, H. R. Mar. Geophys. Res. 5, 165-172 (1981)
Nature http://www.nature.com/nature
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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
ScienceWeek http://scienceweek.com
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