Australia: The Land Where Time Began

A biography of the Australian continent 

Anomalous Zones the Size of Continents of Low Seismic Velocity at the Base of the Mantle

It has been revealed by seismic images of the interior of the Earth that 2 massive zones of slow seismic wave travel are located at the base of the mantle, above the core. Surrounding these anomalous regions are mantle materials that are believed to be composed of cooler rocks associated with downward advection of former tectonic plates. It is uncertain what the composition of these anomalous provinces is. These zones have been depicted for a long time as mantle materials that are related to convective upwelling that are warmer than the average mantle materials. According to Garnero et al. they may also be chemically distinct from the mantle that surrounds them, and potentially they may be partly composed of subducted or primordial material, and they have therefore been termed thermochemical piles. The emerging view, based on seismic, geochemical and mineral physics data, is that these thermochemical piles appear to be denser than the surrounding mantle materials, and they are dynamically stable and long-lived, and large scale mantle flow has shaped them.  The composition of the piles is modified over time by stirring as well as by chemical reactions with material from the surrounding mantle, the underlying core, and potentially from volatile elements that have been transported to the deep Earth by subducting plates, whether they are remnants of a primordial layer or more dense materials that accumulated later. It is suggested that upwelling mantle plumes may originate from the thermochemical piles, therefore the unusual chemical composition of the piles could possibly be the source of distinct signatures of trace elements that have been observed in lavas from hotspots.

Sources & Further reading

1. Garnero, E. J., A. K. McNamara and S.-H. Shim (2016). "Continent-sized anomalous zones with low seismic velocity at the base of Earth's mantle." Nature Geosci 9(7): 481-489.