Study Sheds Light on Earth's Chemically, Thermally Active Interior

New research from Arizona State University's (ASU's) School of Earth and Space Exploration (SESE) shows that the interior of the Earth is not a homogeneous zone of thermal driven events but also a chemically diverse zone where events are chemically complex.

Nicholas Schmerr, a Ph. D. candidate at ASU's SESE, came to a new understanding of how the inner Earth functions because of his study of the phase boundary in the mantel beneath South America. According to temperature based, or thermal, models of the structure of Earth's interior, the phase boundaries, which are transition zones between Earth's mantel and the underlying tectonic plates, should be "up-warped," or varied toward the surface.

The up-warping is predicted to be caused by extreme cold beneath South America. It was formerly the site of oceanic crust and the underlying tectonic plate at this location sinks into Earth's interior as a complement to the topography of South America's western coast subductive region. However, when Schmerr and Edward Garnero, an advising professor at ASU's SESE, analyzed the seismic recordings of energy reflected off the phase boundary beneath South America, they found data indicating that the boundary is not up-warped but rather deflecting downward: deepened.

According to Garnero, this data contradicts previously accepted models that predict phase boundary variations being determined by hot and cold temperatures resulting from subducting plates or hot rifts. Tectonic plates that are drawn downward or overriden by another plate because of a plate collision. The opposite of cold subduction zones are zones that are hot and upwelling like the area called the mid-Atlantic rift (fissure).

Phase boundaries are areas in the Earth's mantel where chemicals undergo a rearranging of their atoms as a result of increasing degrees of temperature and pressure in a fashion similar to that which makes diamonds of graphite. Schmerr's objective is to establish an alternate model that recognizes that the interior of the Earth has regions that are thermally different (temperatures are different) and areas that are chemically different. This is a divergence from the standard model of a homogeneous interior that has varying thermal properties and various thermal anomalies, i. e., inconsistencies and incongruities.

There are three phase boundaries: at 410 kilometers, where olivine compresses under temperature and pressure to wadsleyite; 520 kilometers, wadsleyite compresses to ringwoodite; 660 kilometers, ringwoodite compresses to perovskite and mangnesiowustite. The phase boundaries vary in depth depending on the presence of hot or cold territory in the Earth's inner mantel.

In the cold territory of the South American subduction zone, the 410-kilometer phase boundary should be "upwelling" and not tending downward as it would do in a heat zone. Schmerr and Garnero say that something other than thermal influence (temperature) is causing, or modulating, the deviation in the 410-kilometer phase boundary behavior. That something, they are suggesting, is a chemical interaction that significantly modifies the effects of temperature. They specifically pinpoint the chemical hydrogen or iron.

Schmerr and Garnero acknowledge that they are not the first to suggest chemical modulation of phase boundaries. They are, however, the first to suggest that the specific chemical element is hydrogen or iron. Hydrogen could be introduced into Earth's crust from ocean waters. These hydrogen molecules could bond with minerals subsequently drawn down to the phase boundary where it creates hydrated transition minerals effecting the modulation of the 410-kilometer phase boundary.

Iron could be introduced as a modulating factor from the surface of the Earth as iron-poor materials are drawn down creating compositions that are stable to greater depths, thus modulating a downward deflection of the 410-phase boundary. Schmerr and Garnero liken Earth's interior to a living organism with active chemical interactions that alter interior and have effects on exterior conditions.

To make their observations, Schmerr and Garnero used the USArray, a collection of 500 seismometers "deployed in a movable grid across the United States," said Schmerr. "It's an unheard of density of seismometers."

"ASU study depicts Earth as living organism," Arizona State University.