FIGURE 2.24 Photograph (upper left) and thin section (upper right) of the Punch Bowl fault in southern California. The principal slip surface (pss) is thought to have accommodated several kilometers of slip. The slip is localized to a 1-mm (white) region, including a microshear zone with more intense shearing (dark) occurring within a few hundred microns. SOURCE: (Upper left) Chester and Chester (1998). Copyright 1998 Elsevier, reprinted with permission. (Upper right) Courtesy of Judith Chester, Texas A&M University. (Bottom) Results of experiments on fault slip in natural rocks showing that the friction coefficient depends on slip velocity and nearly vanishes for slip velocities similar to those of earthquakes (1 m/s). SOURCE: Di Toro et al. (2004). Reprinted by permission from Macmillan Publishers Ltd.: Nature, copyright 2004.

timescales, fluids behave like solids and rupture on short timescales. When magma is deformed very rapidly—for example, during an eruption—it may fracture. Understanding this behavior is helping us sort out the dynamics of volcanic eruptions (Question 9) and how these depend on features such as magma composition (e.g., Gonnermann and Manga, 2003).

Summary

Understanding how Earth works depends on knowledge of the properties of rocks and minerals. After a period of steady progress, breakthroughs are now at hand because of new analytical tools provided by advanced radiation sources (e.g., synchrotron, neutron, and laser facilities) and advanced computing. Much of



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