Australia: The Land Where Time Began

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Superplumes – Structure of the Earth – Thermal and Compositional

A major revolution has taken place over the past 2 decades concerning the understanding of the way the Earth, as well as the other planets, might have evolved. There 2 discoveries that are the most influential driving forces that form the basis of this revolution, high-resolution of the deep interior of the Earth, mainly by seismology, and studies of high-pressure mineral studies. It has been established by high-resolution seismological studies that deep in the interior of the Earth there are large heterogeneities that include very large low velocity anomalies in the mantle beneath Africa and the South Pacific, which are sometimes referred to as Superplumes.

The 2 superplumes have been shown by Zhao in 3-D P-wave tomographic images, as well as several other plumes, with or without tails, at the core-mantle boundary (CMB). It is also common to find stagnant slabs in most subduction zones throughout the world at a depths range of 410-660 km, the mantle transition zone, by this improved tomographic method, which is primarily based on P-waves as well as other body-wave phases.

The key to inferring the origin of a superplume is an understanding of the post-perovskite phase change in the lower mantle. The phase relations in both pyrolite and MORB compositions from the surface to CMB conditions was summarised by Hirose with emphasis on the post-perovskite phase, that was discovered recently, which is stable in the D” layer. Also, the solubility of light elements such as Si, O, Mg and Fe liquid is reviewed.

It is necessary to calculate the Clapeyron slope by using first-principles quantum mechanical calculations because of uncertainties in the pressure-scale under conditions of extremely high pressure (Hirose et al., 2006). A critical review of this debate has been presented (Tsuchiya, et al.), and recommendation for a value of 7.5 MPa/K, though there are still large variations resulting from the particular method used in these calculations.

If low-temperature slabs that are stagnant collapse catastrophically at the mantle transition zone, then move into the lower mantle the core dynamics must also be strongly affected by a cold slab avalanche due to its refrigerating effect on the liquid core. Flow direction in the outer core is from north to south in a bundle of convective columns, which may cause the preferential growth at the equatorial zone of the inner core, which is solid, to the polar regions (Yoshida et al., 1996) and the convective plastic flows in the solid inner core are controlled by this. The concepts are introduced of an upper outer core that is isotropic and a lower inner core that is anisotropic, as well as an argument of a possible phase transition of Fe.

The recent progress on core dynamics is reviewed by M. Ishii, as well as her own recent works. Seismology can be used to determine the structure of the solid inner core in greater detail. The solid inner core has a rich structure that potentially contains important clues to an understanding of the Earth’s evolution and dynamics, unlike the liquid outer core. A good guidance to the use of the seismological literature on the core is provided by Ishii’s paper in order to understand the dynamics and evolution of the Earth.

Sources & Further reading

  1. Yuen, D.A., Maruyama, S, Karato, Shun-ichiro & Windley, B., (Eds), 2007, Superplumes: Beyond Plate Tectonics, Springer

Author: M. H. Monroe
Last updated: 14/07/2014
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