Plates vs Plumes
July 21 2105 ” Plates vs. Plumes: A Geological Controversy”
Presented by Dr. Gillian Foulger, Professor, Durham University, UK.
Dr. Gillian Foulger is one of the world’s preeminent scientists studying what drives plate tectonics, basically laboring to understand how our earth works. She will describe the two radically different views that exist to explain the mechanisms of plate tectonics – of which a key aspect to understand is the origin of large-volume magmatism (as exemplified by flood basalts), and persistent, low-volume magmatism (as exemplified by volcano chains). The two differing explanations are the “Plate”, and the “Plume” (“deep mantle plumes”) hypotheses. The Plume hypothesis envisages a large, thermal diapir that rises from the earth’s 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 [see http://www.mantleplumes.org/]. Magmatism is envisaged to occur as a passive reaction to lithospheric extension, and its quantity and chemistry to reflect source fusibility (rock melting temperature) 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 during scientific study 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 (melting temperature). 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.
Dr. Foulger will expand on why she strongly believes that the”Plates” hypothesis is correct and the “Plumes” hypothesis does NOT match global data sets, is NOT a viable hypothesis.