FIGURE 2.20 Predicted atomic-scale structure of a model magma (MgSiO3 composition) showing that the large compressibility of liquids is caused by rearrangement of the structure from an open configuration near zero pressure (left) to a much more compact and highly coordinated structure at the pressure of the core-mantle boundary (right). Silicon-oxygen coordination polyhedra are shown in blue and magnesium ions in yellow. SOURCE: Stixrude and Karki (2005). Reprinted with permission from the American Association for the Advancement of Science (AAAS).

small number of atoms can be modeled, which means that it is not yet possible to use this approach to explore how trace elements behave.

Can seismic waves be used to uniquely determine mantle mineralogy? Material properties and seismology are interdependent in a fundamental way. Seismologists can measure the speed at which seismic waves traverse the mantle and use this information to construct pictures of the deep Earth in a process analogous to a medical CAT scan. At the same time, pictures of the deep mantle cannot be interpreted without information about mantle minerals and rocks, just as radiologists need to know how bone and other types of tissue transmit X-rays. The changes in seismic wave velocity through different structures in the deep Earth are small—about 1 percent—so the elastic properties of the minerals need to be known precisely to interpret the changes. As these properties become better known, geologists hope to use seismic images to map the temperature and composition variations in the mantle and perhaps even the pattern of convection. The latter is possible because seismic wave velocity is dependent on direction, or anisotropy, and can be related to flow patterns if there is sufficient knowledge of the elasticity of minerals and the mechanisms by which they deform (Karato, 1998). A striking example of anisotropy inside Earth may be seen in the inner core, where longitudinal seismic waves travel 3 percent faster along the rotational axis than in the equatorial plane. This difference may be due to alignment of iron crystals in the core, although the mechanism for producing the alignment is still uncertain (Stixrude and Brown, 1998). Understanding the origin of this alignment is likely to tell us a great deal about the dynamics at Earth’s center, the history of the core, and the origin of the geomagnetic field.

How Much Water Is in the Solid Earth?

Earth is unique in the Solar System for its abundant surface water, and most models for the early Earth suggest that the source of this water was the mantle via volcanic eruptions. Based on recent research, it seems likely that the interior continues to be a major reservoir of both water and carbon dioxide (Williams and Hemley, 2001). Earth is so massive that if the mantle is only 0.03 percent water, it would hold the equivalent of all the water in the modern oceans. Upwelling mantle material at midocean ridges appears to contain about this much water, so at present Earth’s interior has at least one ocean’s worth of water. How much more it might have and how this amount has changed over Earth’s history are outstanding questions.

We do not know whether Earth has always had the present amount of water at its surface, but the answer has implications for a variety of processes. To reach the answer, we need a deeper understanding of where water and carbon dioxide are stored in the mantle. We know of two potential reservoirs of water: hydrous phases, such as clays that contain predictable amounts of water within their crystal structures, and nominally anhydrous phases, such as olivine (the most abundant mineral in the upper mantle), which include hydrogen as defects (Figure 2.21). Knowing more about these reservoirs may frame our view of the long-term evolution of the hydrosphere, including formation of the oceans (Question 2). Understanding the evolution of the deep hydrosphere is also central to our view of mantle dynamics, since even small amounts of hydrogen can change the viscosity of the mantle by orders of magnitude and the melting temperature of rocks by hundreds of degrees (Question 4). For example, if the mantle has more water, it might convect faster and produce more volcanism, by which it loses water to the

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