Abstract Magmas emplaced into the upper portions of the earth’s crust are accompanied by extensive hydrothermal activity. Hydrothermal activity is represented as a system of coupled processes that dissipate thermal, mechanical, and chemical energy into the magma’s lithocap, primarily by convection of H 2 O-rich fluids. To investigate dynamical behavior of the system, a serial experiment was undertaken in which T( t) and P( t) values are computed for a pluton location during the time the region was subjected to near-critical hydrothermal convective flow. The consequent evolution of fluid buoyancy, ∇ xρ f, ion stability, ΔḠ°, and fracture extension, δ L/ L 0 during this time indicates that variations in density gradients increase smoothly until 70,000 yr then burst into frequent, ≈100-yr oscillations. Oscillations first increase in magnitude then decrease. Oscillatory behavior of state conditions derived from numerical experiments illustrate resonant effects in chemical equilibrium and fracture extension processes and show the sensitivity of the stable mineral assemblage to either of the competing chemical and mechanical transport processes. An oscillatory zoned tourmaline that formed at near-critical conditions of H 2 O from the Geysers Geothermal deposit appears to provide evidence of nonlinear systematics in hydrothermal activity. Mathematical analogs to this system demonstrate that processes in this system record their dynamical behavior in the supercritical region and suggest that alteration events are generated by the complex, “chaotic” behavior of these processes. This type of behavior appears to be further augmented by strong divergence of H 2 O-fluid properties toward ± infinity at commonly encountered state conditions in the shallow reaches of magma-hydrothermal activity. System behavior elucidated here arises from affording for connectivity of processes by numerical experiments of hydrothermal activity for a region near the contact of a magma and its lithocap. The cumulative data from numerical experiments, equation-of-state (EOS) relationships, geologic and geochemical observations support the proposition that magma- hydrothermal processes should be thought of as complex dynamical systems whose behavior at state conditions near the supercritical region of the fluid is likely chaotic.