**8. Conclusions**

The rate at which heat is removed from crustal magma accumulations determines whether they grow into persistent magma bodies or instead form an incrementally emplaced pluton. Hydrothermal convection is far more efficient than conduction at moving heat to the surface, and regional heat flow in many geothermal areas is so high that convection must be the dominant mode of heat transfer. However, convection can only occur if there is sufficient water in the shallow crust to sustain it, and if self-sealing of fractures is overcome by extensional faulting.

The necessity of water recharge for convection suggests that long-term aridity could force systems into conduction, dramatically slowing removal of heat and promoting growth of zones of partially molten rock. This process may explain why many of Earth's greatest silicic ignimbrite provinces, such as the central Andes, Oligocene Great Basin, and Cretaceous Paraná-Etendeka, developed in arid and hyperarid areas. Many geothermal areas that are currently in arid regions sit among sinter and

travertine deposits that indicate significantly greater activity in the past, presumably during wetter and cooler glacial cycles when groundwater was more abundant.

Arid conditions are clearly not a requirement for formation of silicic calderas as counterexamples are abundant, but lack of permeability caused by self-sealing may shut down convection in areas not undergoing extension. This could explain why caldera formation appears to be suppressed in areas undergoing extension or in arcs with backarc extension.

#### **Funding:** This research received no external funding

**Acknowledgments:** Ken Wohletz and his Heat3D software were important to early development of the ideas in this paper. I thank all the scientists who worked on the Coso geothermal system under the aegis of the Geothermal Program O ffice and Frank Monastero, and their presentations at numerous Coso meetings, for opening my eyes to the links between petrology, structural geology, hydrology, and geophysics. Chuck Stern enlightened me about the Andes. Craig Magee, John Eichelberger, and two anonymous reviewers provided constructive and challenging reviews that significantly improved the presentation, and John Bartley provided an early review and contributed to this study in many ways.

**Conflicts of Interest:** The authors declare no conflict of interest.
