Download Solid-state convection in Earth`s deep interior and the origin of

Solid-state convection in Earth's deep interior and the origin of volcanic
islands
Gillian R. Foulger
Univ. Durham, Science Labs., South Rd., Durham DH1 3LE, U.K.
email: [email protected]
The Earth’s mantle–the region between depths of about 30 and 3,000 km–is made up of an
assemblage of minerals such as peridot and garnet that deforms slowly at the temperatures of the
interior of the Earth. It is this ability to deform that permits the surface tectonic plates to move and for
continental drift, earthquakes, and volcanic eruptions to occur. The properties of the mantle thus
affect human beings more than many people may suspect!
The radical effect that mantle convection has on Earth’s surface came into focus in the mid-1960s and
was subsequently subject to much research. Despite this, many aspects of the dynamic behaviour of
the mantle are still controversial. Key questions that are the subject of current cutting-edge research in
Earth science include how molten material is distributed in the mantle, whether the entire mantle
convects vigorously or only the uppermost part, and the chicken-and-egg question of whether tectonic
plate movements drive deformation of the mantle or vice versa.
In recent years, these unknowns have given rise to a major controversy in Earth science. This is the
cause of volcanic islands and volcanic fields in the interior of the continents. Such places include the
islands of Hawaii and Iceland, and areas such as the Siberian Traps and the volcanoes of Italy and
East Africa.
Two radically different views have emerged, known as the Plate-, and the Plume hypotheses. The
Plume hypothesis views Earth’s hot core as comprising a major thermal reservoir that inputs heat into
the mantle above. In this view, large thermal diapirs rise from the bottom-most mantle, where it
touches the core, and actively rise, punch through the crust, and cause volcanism. Plumes, and the
volcanoes they spawn, are thus envisaged to be driven by thermal energy from Earth’s core alone.
The Plate hypothesis is the conceptual inverse. It envisages the main driver of magmatism to be heat
loss from the surface. This is the ultimate driver of plate tectonics, which is responsible for essentially
all surface volcanism [http://www.mantleplumes.org/]. Magma is envisaged to leak out of the mantle
as a passive reaction to surface, plate-tectonic-induced stretching, e.g., at mid-ocean ridges and rift
valleys.
Hypothesis-testing normally comprises testing predictions against observations. If the predictions of
an hypothesis are consistently falsified, the hypothesis is abandoned. Unfortunately, in the case of the
Plate/Plume controversy this scientific method has become corrupted and workers have found it very
difficult, in practice. to abandon failing hypotheses. Part of the reason for this is the increasing
specialization of Earth scientists, which has led to a “blind-men-and-the-elephant” situation where
each worker only understands a small subset of the data in existence. Another is the “bandwagon”
problem where an hypothesis, assumed to be correct by the mainstream scientific machine comprising
scientists, students, grant-awarding authorities, journals and reviewers, has become “too big to fail”.
This is not the first time this has happened in Earth science. A strong scientific case was built for
continental drift by Alfred Wegener in 1915, but not accepted by mainstream Earth science for over
fifty years.
In my talk I shall aim to touch on all these aspects of this multi-faceted and never-dull subject.
Questions and input on any aspect, throughout my presentation, will be welcome.
Further reading
Foulger, G. R. (2010), Plates vs Plumes: A
Geological Controversy, Wiley-Blackwell,
Chichester, U.K., xii+328 pp.
http://www.mantleplumes.org
Morgan, W. J. (1971), Convection plumes in the
lower mantle, Nature, 230, 42-43.
Oreskes, N. (1999), The Rejection of Continental
Drift: Theory and Method in American Earth
Science, Oxford University Press, ix + 420 pp.
Wegener, A. (1915), Die Entstehung der
Kontinente und Ozeane, Friedrich Vieweg und
Sohn, Braunschweig.
http://www.mantleplumes.org