From: Andy Soos, ENN
Published June 1, 2010 04:16 PM

The Not So Solid Earth

The interior of the Earth, similar to the other rocky or terrestrial planets, is divided into layers. The mantle is a highly viscous layer between the crust and the outer core. Earth's mantle is a rocky shell about 1,800 miles thick that constitutes over 80% percent of the Earth's volume (The part of the Earth best known to us humans.). Two thousand miles beneath our feet, the Earth's solid rock — known as the mantle — gives way to the swirling liquid iron of the outer core. The last few hundred miles of the lowermost mantle is also known as D” (pronounced dee-double-prime).  D" is one of the most enigmatic parts of the Earth which scientists have struggled to understand for decades; it can only be measured remotely, using seismic waves from earthquakes.


The @100 mile thick layer of the lower mantle directly above the boundary is referred to as the D. The D name originates from the mathematician Keith Bullen's designations for the Earth's layers. His system was to label each layer alphabetically, A through G, with the crust as A and the inner core as G. In his 1942 publication of his model, the entire lower mantle was the D layer. In 1950, Bullen found his D layer to actually be two different layers. The upper part of the D layer, about 1800 km thick, was renamed D” and the lower part (the bottom 200 km) was named D".

In simple language the outer crust is what humans see and use the most of. Below this is the outer mantle which is rigid. The lower mantle, though still consisting of the same basic materials, is not as solid and can be described as a plastic flowing material. The inner core is the heart of the world and is liquid and correspondingly hot.

Geoscientists from the University of Bristol and University College London, have just reported in the journal Nature that some of the poorly understood properties of  D" might be explained by a sudden softening of the main mineral which makes up the mantle, due to the enormous pressures and temperatures near the Earth’s core.

By using computers to make quantum mechanical simulations of the atoms which make up the mineral, the team showed that a recently discovered change in its crystal structure makes it orders of magnitude weaker and therefore much easier to deform and flow.

Dr James Wookey, an author on the paper from the University of Bristol's Department of Earth Sciences, said: "This softened mineral would allow the material in the lowermost mantle to flow much more easily along the surface of the iron core — rather like toothpaste being squeezed out of a tube. This zone of flow would have a profound effect on our understanding of the dynamic processes at work at the core mantle interface."

The importance of the lowermost mantle (D") stems from its role as an interface between the hot liquid core and relatively cooler solid outer mantle. The inner core is always rapidly churning its liquid iron. This is the engine which drives the Earth's magnetic field. The transmission of heat from the inner core to the outer mantle through the D" layer is what drives continental plate tectonics and volcanism. Tectonic action forms and destroys the earth's surface slowly as well as causes the phenomena of continental drift.

Understanding how and why the D" layer functions is key to the long term understanding of the earth's continental evolution as well as the indirect effects on climate from volcanism.

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