Under the oceans, mountains to climb

Mid-Ocean Ridges
January 5, 2001

It is no exaggeration to say that the mid-ocean ridges are the defining features of the earth. Mars may have volcanoes of spectacular proportions, and Venus has experienced active tectonics and faulting. But, as far as we can tell, no other planet exhibits anything quite like the active volcanic ridges that snake around the Earth. They represent the smoking gun of geological activity on the planet, at which heat generated deep within the mantle provides the energy for ocean crust formation, for venting of hot, mineral-enriched water, and for the development of communities of unique and highly adapted organisms.

These processes have been the subjects of intensive study for ten years by national and international programmes of multidisciplinary research, following the example of the US Ridge (Ridge Inter-Disciplinary Global Experiments) initiative. Exploration of the sea bed using manned and remotely operated submersibles and the development of new techniques for imaging and sampling the sea floor and seawater are rapidly improving the picture of the workings of our planet.

This volume is the proceedings of a Royal Society discussion meeting, under the same title, held in March 1996. The format gathers key researchers in "hot" areas of science over a couple of days, with ample time for comment and questioning. It is common practice for papers presented at such meetings to be published in the Philosophical Transactions of the Royal Society , which used to be produced in large format on high-quality paper. These thematic sets of papers were also published separately. Following recent practice, Cambridge University Press published this collection in normal book format. This is a shame, as it would benefit from high-quality reproduction.

All but one of the 13 papers were published in Philosophical Transactions in 1997, so it is hard to understand why it took three years to issue this book. The publishers' claim that the book "will be of importance to specialists and reseachers (sic) wishing to be informed of the latest developments in all aspects of the science of mid-ocean ridges" is less convincing now than it was three or four years ago. There have been advances in almost all aspects of mid-ocean ridge science since 1996.

Nevertheless, the book has considerable merits, not least that of bringing together aspects of mid-ocean ridge research that would otherwise be scattered through journals ranging from geophysics to zoology. Contributions focus on processes rather than observations and cover melt production and migration, the production of ocean crust at fast-spreading and plume-affected ridges, hydrothermal circulation and formation of sulphide deposits, and water column and biological consequences of hydrothermal activity.

Melt production and migration are dealt with by P. D. Asimov and others, and by Peter Kelemen and others. The first uses a high-temperature and pressure thermodynamic model to predict the amount of melt produced by upwelling mantle peridotite as melting proceeds. Generally speaking, and perhaps counter-intuitively, the authors conclude, melt production should increase with progressive melting. Tectonic styles at the fast-spreading East Pacific Rise and at the slow-spreading, plume-affected Reykjanes Ridge on the Mid-Atlantic Ridge are discussed by Marie-Helne Cormier and by Nicky White. Cormier considers the evidence for two-dimensional (sheet upwelling) versus three-dimensional (plume-like upwelling) flow of mantle rock beneath the East Pacific Rise, and plays safe by advancing a model with elements of both. White relates the occurrence or absence of fracture zones in oceanic crust to variations in mantle temperature.

One of the more observational contributions is the paper by Martin Sinha and colleagues, who report on a beautifully designed and executed experiment, incorporating seismic and electromagnetic techniques, that provides convincing evidence of an ephemeral magma lens beneath the Reykjanes Ridge, part of the slow-spreading Mid-Atlantic Ridge. Despite the spirited argument that the presence of a crustal magma body here is related to a particularly active stage in the magma supply cycle, one would like similar evidence from a ridge segment far from any plume influence before accepting this as a general model for slow-spreading ridges. The results of this work were published in full in the open literature in 1998.

Adam Schultz and Harry Elderfield (not Henry, as printed) give a valuable review of hydrothermal circulation, showing that diffuse, low-temperature flow is the dominant mode of heat and water flux into the oceans, even at the ridge axis, where the high-temperature black and white smoker systems occur. D. A. Butterfield and others present a chemical analysis of fluids issued from diffuse vents at the CoAxial site off the coast of Washington, collected within a few weeks of an eruption, detected in 1993 by a US military hydrophone array.

The only previously unpublished contribution is that of Rachel Mills and Maurice Tivey, which presents results from Ocean Drilling Program Leg 158 on the Trans-Atlantic Geotraverse hydrothermal mound in the north Atlantic. Recently published analyses for strontium, calcium and magnesium in anhydrite samples and fluid inclusion evidence are used to establish the temperature distribution within the mound, and to evaluate the importance of seawater mixing. The results have a direct bearing on the interpretation of massive sulphide deposits in ophiolites in which anhydrite is absent because of dissolution on cooling. A useful overview of the locations of major hydrothermal sulphide deposits is provided by Yves Fouquet, who points out that the best analogues for massive sulphide deposits on land may be those on ridges associated with volcanic arcs, such as those in the western Pacific.

Biological contributions are limited to two articles. The first, by Kim Juniper and Verena Tunnicliffe looks at how the stability and variety of vent fauna habitats is linked to spreading rate, providing some useful ideas despite the paucity of observational evidence from much of the world's ridge system. The second, by Holger Jannasch, deals with the potential roles of enzymes produced by microbial processes at high temperatures.

The overall quality of the contributions is high, but production and design are less than inspiring. The volume would have benefited from a short introductory chapter, showing the global distribution of known vent sites, together with some examples of black-and-white smokers and vent communities.

To get the most out of this collection, a high level of background knowledge of the subject is demanded. This book would probably be of most use to graduate students or established researchers needing a snapshot, albeit a little faded now, of progress towards understanding how the whole mid-ocean ridge system works. For observational evidence, including detailed maps of ridges and vent fields, pictures of the vents themselves, black smokers and vent fauna, look elsewhere.

Roy A. Livermore is a marine geophysicist, British Antarctic Survey.

Mid-Ocean Ridges: Dynamics of Processes Associated with the Creation of New Oceanic Crust

Author - J. R. Cann, H. Elderfield and A. S. Laughton
ISBN - 0 521 58522 8
Publisher - Cambridge University Press
Price - £55.00
Pages - 301

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