quantitatively predict the dependence of erosion rate on other variables and the strength and deformation properties of rocks in the lower continental crust and the upper mantle.

QUESTION 6:
HOW ARE EARTH PROCESSES CONTROLLED BY MATERIAL PROPERTIES?

Geology is founded on the central insight that rocks can be read as a record of Earth’s history. Rocks and minerals are produced and altered by geological processes—melting, eruption, weathering, erosion, deformation, and metamorphism. Therefore, deciphering the secrets of the rock record begins with an understanding of large-scale geological processes. The keys to understanding these processes are the basic physics and chemistry of the materials that make up the planet. Scientists now recognize that macroscale behaviors—plate tectonics, volcanism, and so on—arise from the microscale composition of Earth materials and indeed from the smallest details of their atomic structures. Understanding materials at this microscale is essential for comprehending Earth’s history (NRC, 1987) and making reasonable predictions about how things may change in the future.

The high pressures and temperatures of Earth’s interior, the enormous size of Earth and its structures, the long expanse of geological time, and the vast diversity of materials and properties present challenges to investigation. Moreover, minerals are complicated solids that generally contain not only their essential chemical constituents but also trace amounts of almost every element known in nature. Although we can learn much about Earth from the study of pure compounds that approximate real minerals, we also know that even minute amounts of other chemical elements can radically change a mineral’s behavior.

Fortunately, the surge of interest in understanding Earth materials at the atomic level has been accompanied by rapid development of new tools, including new synchrotron sources that bring the ability to probe the atomic structure of minerals and liquids (Figure 2.16); high-pressure devices to simulate the distortion of atomic arrangements under huge pressures; and advanced quantum mechanical theory, which promises major advances in our understanding of physics and chemistry at the extreme conditions of planetary interiors and at the smallest scales of mineral surfaces and nanoparticles. Advances at the other end of the spectrum, when the scale is extremely large and/or the processes are extremely slow, will require advances in experiment, theory, computation, and observation. Only the combination of all four is likely to bring progress.

What Minerals Comprise Planetary Interiors?

As noted in Questions 4 and 5, the nature of the convection and deformation that affect Earth’s mantle and crust, and hence models for plate tectonics and Earth’s temperature history, depends directly on the material properties of rocks and minerals at the high temperatures and pressures of planetary interiors. The pressure is 136 GPa (1.36 million atmospheres) at the base of the mantle and 364 GPa at Earth’s center, while the temperature reaches 4000 K at the base of the mantle and 6000 K at Earth’s center (similar to the temperature at the surface of the Sun; Figure 2.17).


Phase transformations. The pressure in Earth’s interior is so enormous that it alters the fundamental properties of elements; for example, it can convert insulators to metals and cause magnetism to collapse (Figure 2.18). Such changes occur because pressure compresses and distorts the electron orbitals, thereby changing the most basic properties of the materials. Changing pressures bring about many kinds of phase transformations. The most familiar of these are melting and freezing, but many more complex phase transformations have been identified. Structural phase transitions are also common. The transition from graphite to diamond is well known, but more important for Earth processes is how mantle olivine and pyroxenes change at high pressure.

High-pressure mineral transformations, and their dependence on temperature, allow us to estimate the temperature of the deep Earth and provide constraints on how mantle convection works. Temperatures inside Earth can be estimated by comparing the pressure and temperature conditions at which mineral transformations occur in the laboratory to the depths at which sudden changes in the physical properties of the mantle and core occur (Figure 2.19). We know, for example, that the boundary between the liquid outer core and the



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