Thermally induced differential expansion between fluid in isolated pores and enclosing minearls decreases effective pressure at the pore-mineral interface. With a sufficient increase in temperature, effective pressure becomes equal to the tensile strength, and the rock fractures. The ratio f the coefficient of thermal expansion to thecompresibility for the pore fluid defines the nature of this process. Finite element computations indicate that thermally induced hydraulic fracturing of host rocks is inevitable in hot pluton environments. A fracturing front propagates away from cooling plutons at a rate of 100 cm/yr for the first 400 years and at 1 cm/yr at 105 years. Fluid energy release upon fracturing can produce microearth-quakes of measurable magnitude. Predicted frequencies of microearthquakes vary from 4¿103 events/day at 5-103 years to 500 events/day at 105 years. Estimates of ambient pore fluid pressures and effective pressures show that effective pressure continually increase with depth for geothermal gradients less than 10¿C/km. For larger geothermal gradients the effective pressure first increase but eventually decreases.