Microcracks in quartz ∼ 100 μm in length and < ∼ 10 μm in width heal in 4h at 600 °C and water pressure of 200 MPa (fluid pressure [Pf] = confining pressure [Pc]). Healing is thermally activated; the activation energy is estimated to be between 80 and 35 kJ/mol, depending on the model assumed. Rates also show dependence on fluid pressure, chemistry, and crack dimensions. Faster healing rates are observed in smaller cracks. Thus, when new cracks are not being produced in rocks at elevated temperatures and pressures, fractures will have a vast range of lifetimes: macrofractures transport most of the fluid volume and seal relatively slowly, whereas microcracks allow pervasive penetration of fluid into the rock mass but heal quickly.