Although fluid flows in response to differences in hydraulic head, chemical reactions occur in response to changes in fluid pressure. Depending on the flow direction, cross-sectional area, and hydraulic conductivity of the system, flow of fluids through porous media may lead to increases and/or decreases in fluid pressure along the flow path. At high temperatures corresponding to those in the supercritical region of the system H2O, mass transfer among minerals and aqueous fluids in hydrothermal systems is sensitive to changes in fluid pressure as well as temperature. As a consequence, mineral solubilities may both increase and decrease in the direction of fluid flow, causing development of isolated channelways and a heterogeneous distribution of secondary porosity and alteration products. For example, vertical downward flow is accompanied by increasing fluid pressure if the Darcy velocity is less than the hydraulic conductivity of the medium. Hence, for a constant flow rate an abrupt increase in cross-sectional area may lead to dissolution of quartz, but precipitation of sulphides.