Tribology, water and plate tectonics
R. David Whitby | TLT Worldwide July 2017
Scientists find evidence of water in the Earth’s mantle and in pockets of magma.
It is possible that the Earth had water from the very beginning, in the dust and rocks from which it was first formed.
© Can Stock Photo / artjazz
IN 2009 SCIENTISTS AT OREGON STATE UNIVERSITY created the first global 3D map of electrical conductivity in the Earth’s mantle. Their model, based on the results of electromagnetic induction soundings of the mantle, suggested the enhanced conductivity in certain areas of the mantle might indicate the presence of water. Additionally, these areas appeared to coincide with tectonic plate subduction zones.
Many geophysicists had thought that tectonic plates were not likely to carry much, if any, water deep into the Earth’s mantle when they are being subducted. Most evidence suggested that subducting rocks initially held water but it was released as the rocks heated up. The Oregon State University scientists acknowledged that there may be other explanations for the enhanced electrical conductivity, but the presence of water is the simplest explanation for the close association between subduction zones and high conductivity. An alternative explanation is an elevated iron content or carbon in the mantle.
Then in 2014 scientists at Northwestern University and the University of New Mexico found seismic evidence of water, stored in the crystal structures of high-pressure minerals, in pockets of magma around 400 miles (644 km) beneath North America. The water was located in the transition zone between the upper mantle and the lower mantle, which varies in depth between 250 and 410 miles (400 and 660 km) beneath the surface. It is believed that the transition zone could, theoretically, hold about 1% of its total weight as water.
In 2016 research, again at Northwestern University, found infrared spectrographic evidence of water inside a diamond that had been ejected about 90 million years ago by a volcano near the São Luís River in Juina, Brazil. The ferropericlase inclusion inside the diamond contained iron and magnesium oxide, together with chromium, aluminium and titanium. The presence of these last three elements indicated that the diamond must have formed under the intense temperature and pressure conditions in the lower mantle, around 620 miles (1,000 km) beneath the surface. Also, since the diamond was formed at least 90 million years ago, the water must have been this deep in the mantle for at least this long. It is worth noting that the distance down is almost a third of the way to the center of the Earth.
The inescapable conclusion from these findings is that water has a role in the Earth’s plate tectonics. It also has implications for the origin of water on the planet, as it is possible that the Earth had water from the very beginning, in the dust and rocks from which it was first formed. However, it is not clear how the water got so far down into the mantle and exactly how long it has been there.
The research work may help to explain why the Earth is the only planet we currently know to have plate tectonics. The scientists at Northwestern University note that water mixes with ocean crust and is subducted at convergent plate boundaries. Introducing water into the mantle promotes melting and weakens rocks. They speculated that the water is aiding the motions of the plates, much like grease.
It has been known for some time that water interacts with minerals differently at different depths and that small amounts of water can change the physical properties of rocks, alter the viscosity of materials in the mantle and assist the formation of rising plumes of molten rock.
Earlier this year, computer simulations of reactions in the upper mantle, performed at the University of Saskatchewan, confirmed 2014 Japanese studies of the reaction between liquid hydrogen and silica (silicon dioxide), at temperatures of 2,552 F (1,400 C) and pressures of 20,000 atmospheres, to form liquid water and silicon hydride. These pressures and temperatures are typical of those in the upper mantle between 64 and 250 miles (40 and 400 km) beneath the surface.
Several geophysicists believe that these two sources of water could help to explain very deep earthquakes and the apparent ease with which tectonic plate subduction occurs. Tribology seems to be happening deep beneath our feet.
David Whitby is chief executive of Pathmaster Marketing Ltd. in Surrey, England. You can reach him at pathmaster.marketing@yahoo.co.uk.