The Earth's dynamics change with depth due to changes in composition and increasing temperature and pressure. Earth's rheological profiles are often depicted through brittle behavior at shallow depths (low P-T) and plastic creep at larger depths (high P-T), overlooking the mechanism of low-temperature plasticity (LTP). Deformation by LTP occurs by dislocation glide in regions with high stresses, either in the strong lower crust or locally with relevance to microphysical processes such as the interaction between asperities during frictional sliding. The first results of LTP properties of hornblende (Ca-rich amphibole), an important constitute of the continental lower crust and subduction zones, are presented here. We use the nanoindentation technique, often used in material science, to derive materials’ plastic and elastic properties. The tests use a pyramid-shaped nano-scale diamond tip, which induces extremely high stresses (~2-20 GPa) yet with local high confining pressures, sufficient to inhibit fracture initiation even at room temperature. Over 300 nano-indentation tests were performed on three different hornblende planes (normal to a and c axes, and intermediate oblique orientation) under various loads (1-500 mN) and temperatures (25-200 C°). While hardness (plastic resistance to penetration) exhibits low variations for different crystal planes, interestingly, the elastic modulus varies significantly between the crystal plane normal to the a-axis and the plane normal to the c-axis, with the oblique-oriented crystal falling between. Additionally, we demonstrate that hardness is inversely proportional to temperature and the applied load. The observed load-dependence is a known phenomenon in nanoindentation tests with several possible origins. We interpret the observed size effect as resulting from the the change of volume beneath the indenter that influences the ease in which dislocations nucleate and glide. Finally, we discuss the rheological contrast of possible Moho boundary containing amphibole-rich lower crust and olivine-rich mantle.