Earth's Mantle below the Oceans [Preview]

Samples collected from the ocean floor reveal how the mantle's convective forces shape Earth's surface, create its crust and perhaps even affect its rotation

Where does the water that produces mantle metasomatism come from? One possible source of this water is the sinking slabs of old oceanic lithosphere in subduction zones at the margin of the oceans. This process recycles water into the mantle. Water could also be released in the upper mantle during degassing processes of the deep mantle. In addition, water molecules can be stored in the actual structure of mantle minerals.

Consider the mineral perovskite, a silicate of magnesium and iron that constitutes the main component of the lower mantle and is therefore the most abundant mineral on Earth. Perovskite can contain water in concentrations up to 1 percent. A lower-pressure form of perovskite, called wadsleyite, prevails in the zone of the mantle at a depth of between 660 and 450 kilometers and can contain water up to concentrations of about 1.5 percent. Totaling up all these water molecules, we can speculate that the total amount of water in Earth's mantle could be equivalent to that of several oceans. Much of this water is probably primordial, captured in Earth's mantle at the time of its formation over four billion years ago. The presence of water molecules dispersed in mantle minerals has important consequences. For example, it significantly lowers mantle viscosity, facilitating convective motions that cause the movement of lithospheric plates and the drifting of continents.

Uneven mantle tilts earths axis
OUR STUDIES OF MANTLE PERIDOTITES from the Mid-Atlantic Ridge suggest that some areas with cooler mantle temperatures may represent the return strokes of the convection cycle in the mantle--that is, the downwelling regions. To understand this notion, we must look south of the Azores region, to the equatorial zone where the Mid-Atlantic Ridge lies deeper than the ridge at higher latitudes. The mineral composition of peridotites recovered from the equatorial Atlantic indicates that they underwent little or no melting, which implies that the mantle temperature was exceptionally low. Schilling and Nadia Sushevskaya of the Vernadsky Institute of Geochemistry of the Russian Academy of Sciences reached similar conclusions after studying basalts from the equatorial Atlantic. In addition, Yu-Shen Zhang and Toshiro Tanimoto of Caltech found that the velocity of the seismic waves is faster in the upper mantle below the equatorial Mid-Atlantic Ridge than at higher latitudes. These observations imply a denser, colder upper mantle below the equatorial region of the Atlantic. The temperature of the upper mantle there may be up to 100 degrees C lower than the mantle temperatures elsewhere below the ridge.

A plausible explanation for the relatively cool and dense equatorial upper mantle is that it results from downwelling mantle currents. Hot mantle plumes upwelling in the northern and southern Atlantic mantle may flow toward the equator, giving up their heat to their cooler surroundings and then sink.

The equatorial position of the "cold" Atlantic mantle belt may not be arbitrary. It is possible that Earth's rotation and convection in the mantle are intimately connected phenomena. In the late 1800s George Darwin (the second son of Charles) pointed out that the distribution of large masses on the surface (such as continents) affects the position of Earth's axis of rotation. Several scientists since then have investigated how density inhomogeneities within the mantle cause true polar wander (that is, the shifting of Earth's axis of rotation relative to the mantle). The wander results from the natural tendency of a spinning object to minimize the energy spent for its rotation.

The redistribution of mass inside Earth may be recorded in the mantle. The late H. William Menard and LeRoy M. Dorman of the Scripps Institution of Oceanography suggested that the depth of mid-ocean ridges generally depends on latitude: ridges become deeper toward the equator and shallower toward the poles. Moreover, gravity measurements revealed that an excess of mass sits below the equatorial areas, at least in the Atlantic. These data suggest that abnormally cold and dense masses exist in the equatorial upper mantle.

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