Mantle Plumes and Their Record in Earth History
By Kent C. Kondie, 2001, Cambridge University Press, 302 pp, £27.99 (paperback) ISBN 0521014727
Kent Condie is Professor of Geochemistry at New Mexico Institute of Mining & Technology, and has published a number of books and articles on the origin and evolution of continents and the early history of the Earth. Both plate tectonics and mantle plumes emerged as essential to understanding geological forces in the 1960s, but attention focused on the exciting developments in plate tectonics. Three developments in the 1990s turned attention to mantle plumes: increased accuracy and speed of computers enabled numerical modelling and laboratory experiments to study viscosity, density and thermal gradients of more complex mantle processes; detailed images of Mars & Venus surfaces by Pathfinder and Magellan missions showed no evidence of plate tectonics but suggested gigantic mantle plumes; and increasingly precise seismic tomography showed that the mantle is inhomogeneous and that plumes really do have very deep roots. There is apparently still a small pocket of flat-earthers who claim all the features ascribed to mantle plumes can be explained by other processes, so the evidence mustered takes nothing for granted.
This book aims to summarise the enormous amount of data that has been published recently, and to raise questions such as whether the Earth could have been more like Mars and Venus during the Archaean, and cooled principally by rising mantle plumes rather than subduction as today; what were the consequences for the atmosphere, oceans and biosphere of large mantle plume events; and what role they played in the growth of continents. Hotspots and large igneous provinces (LIPs) provide an observable database, and upwellings and superswells, giant dyke swarms, large layered intrusions, kimberlites, uplift, deformations and subsidence, wrinkle ridges – all features not wholly explained by plate tectonics – are examined.
Plate tectonic and mantle plumes at times interact: mantle upwellings/geoid highs are broad (sometimes up to 10 000 km in extent), warm regions of return flow balancing the cool downwellings of subducted slabs and giving rise to volcanism and mafic underplating of the crust. There are only two today, centred beneath the Pacific and the African plates, and containing most of the Earth's hotspots. There is a fascinating discussion of the possibility that subducted slabs accumulate at the 660 km discontinuity and every now and then break through it in a slab avalanche causing thermal instability at the core-mantle boundary which gives rise to upwellings and plumes. The role of plumes in supercontinent cycles is also examined, but it is always stressed that these hypotheses are based on modelling and interpretation of seismic tomography, and that evidence on the ground is harder and harder to find further back in geological time than Pangaea/Gondwana and Rodinia.
The book is extremely well-organised and clearly written, with short, headed sections which make for easy reference. Spoilt by the OU however, I noticed many figures were lacking in explanatory keys. Perhaps there are American conventions in this field which everybody understands, but publication by the CUP seems to indicate world-wide usage.
It is complementary to S339 in many ways, especially in the discussion on the Archaean to Book 5. Professor Condie is also a greenstone specialist and in this, as in other areas, amplifies and clarifies for those interested in understanding more about mantle plumes. It repays re-reading, though perhaps not for time-stressed continuing students, but is also useful for dipping into and as a reference book.