In analogy to high-temperature sintering of ceramics and metal powder compacts, the formation of opening-mode fractures in siliceous mudstone during natural in-situ combustion of hydrocarbons is attributed to contractile surface forces between mineral grains and an interstitial melt phase. A comparison between bulk density increase during sintering and created fracture space indicates that fracturing resulted from contraction of the rock matrix due to porosity reduction, grain-scale mass transfer, and high-temperature mineral formation. It is suggested that contractile surface forces between mineral grains and between mineral grains and pore fluid contribute to subcritical fracture formation under a wide range of subsurface conditions. ¿ 2001 American Geophysical Union