Hydrogen (reported as H2O) affects the physical properties of silicate materials profoundly. Silicates (with Mg and Fe) dominate a significant portion of the earth’s crust and mantle (0-2900 km depth). The melting point of the silicate assemblage in the upper mantle (olivine dominated) is decreased by up to 400 degrees C at 3 GPa with the addition of 0.14 wt.% H2O. The strength of olivine is also reduced by two orders of magnitude with the incorporation of small amounts of water (e.g., 0.001 wt.%). H occurs in the earth’s upper mantle (30-670 km depth) in trace proportions (0.05-0.2 wt.% H2O equivalent). Over the depth range of the upper mantle there is a two orders of magnitude increase with depth in the ability of the dominant mineral (olivine) to accommodate hydrogen. This suggests that silicates derived from deeper (minerals or partial melts of those minerals) may be more hydrous. There is some speculation that Mg-perovskite (stable at 670-2900 km depths) may also accommodate significant hydrogen.