Nanoindentation is a commonly used technique in materials science that allows for the determination of the mechanical properties (e.g. hardness, elastic modulus, and creep) of single crystals and polycrystalline aggregates, but it has been severely underutilized for the study of geologic materials. In this dissertation, we demonstrate the effectiveness of nanoindentation for measuring the rheological behavior of a wide range of geologic materials. For all materials tested, including single crystals, polycrystalline samples, and two different natural fault surfaces, we identify an ‘indentation size effect,’ whereby the strength of the material increases with decreasing size of the indent. We explore the implications of this size effect for the scale-dependent surface roughness of faults and the plastic deformation of asperities in Chapter 2, and extrapolate the size effect to the relatively large grain size of the mantle to predict the peak strength of oceanic lithosphere in Chapter 3. In Chapter 4, we investigate the effect of relative humidity on the room temperature deformation of quartz in order to constrain the physical mechanism that gives rise to time-dependent increases in the static friction of quartz rocks. Our results demonstrate that there is no effect of relative humidity on the yield stress or creep behavior of quartz, in stark contrast with observations from macroscopic friction experiments on quartz rocks that show a dramatic effect of humidity on friction. This contrast demonstrates that asperity creep, the canonical explanation of the increase in static friction with time, is incorrect. Finally, in Chapter 5, we present a detailed nanoindentation study of the hardness and long duration creep behavior of halite. We demonstrate that creep parameters derived from our nanoindentation tests, when properly treated, are in quantitative agreement and can be directly compared to creep parameters derived from more traditional experimental geometries. This dissertation demonstrates the utility of using nanoindentation to determine physical properties and rheological behavior of rocks and minerals.