A Public Lecture by Professor Gillian Foulger (University of Durham).
Two radically different views exist of the origin of large-volume magmatism (as exemplified by flood basalts), and persistent, low-volume magmatism (as exemplified by volcano chains). These are the Plate-, and the Plume hypotheses. The Plume hypothesis envisages a large, thermal diapir that rises from the core-mantle boundary, actively penetrates the lithosphere, and causes surface volcanism. It is envisaged to be independent of shallow structures and processes, and to be driven by thermal energy from Earth’s core.
The Plate hypothesis is the conceptual inverse. It envisages magmatism to be driven by shallow processes that ultimately draw their driving forces from plate tectonics [http://www.mantleplumes.org/]. Magmatism is envisaged to be as a passive reaction to lithospheric extension, and its quantity and chemistry to reflect source fusibility and composition. Thus, “anomalous” magmatism, is expected to occur preferentially near extensional plate boundaries, e.g,, the mid-Atlantic ridge, and continental rift zones. Where volumes are large, the chemical fingerprints of high source fusibility are expected.
Hypothesis-testing normally comprises testing predictions against observations. The conventional Plume hypothesis predicts a) surface uplift tens of millions of years before flood volcanism, b) flood volcanism lasting a few tens of millions of years, c) a “plume tail” extending from the surface to the core-mantle boundary, d) a time-progressive volcanic chain, and e) high source temperatures. These predictions are rarely confirmed with confidence, and have never all been confirmed at a single volcanic province. The Plume hypothesis has undergone extensive ad hoc elaboration over the years to accommodate this quandary, including proposals that plumes can arise from almost any depth and that plume material can flow sideways for thousands of kilometres.
The Plate hypothesis predicts a) that volcanism is associated with extension, and b) that large-volume magmatism is related to source fusibility. Prediction a) is confirmed at some volcanic provinces, e.g., Iceland, and in the East African Rift, though observations are lacking from many, less accessible regions, e.g., in the remote interiors of oceanic plates. Prediction b) is manifest in the “ocean island basalt” chemical signature of many lavas, which indicates a component of recycled, near-surface, fusible material in the magma source.
For further reading, see Plates vs. Plumes: A Geological Controversy, Gillian R. Foulger, published by Wiley-Blackwell 2010.
Professor Gillian Foulger is primarily an earthquake seismologist with a strong interdisciplinary ethos. She studied geothermal earthquakes in Iceland, the USA and Indonesia, discovering that they may be caused by cracks opening. She pioneered the use of GPS to study plate kinematics, conducting continental-scale surveys in Iceland, the USA and Turkey, discovering the best example of post-tectonic visco-elastic relaxation ever documented. She applied teleseismic tomography to study the mantle beneath Iceland, discovering that the observations did not support the mantle plume model, which led to her current leadership of the global debate over whether mantle plumes exist or not (www.mantleplumes.org). She is currently transferring her microearthquake expertise to seismicity induced by hydrofracturing. She has published three books and over 100 peer-reviewed papers. In 2005 she was made a Fellow of the Icelandic National Academy of Sciences and awarded the Price Medal of the Royal Astronomical Society.