Abstract: Fractures are initiated or extended within an earth formation from a well which includes a tubing string extending to a wellbore space adjacent the fracture zone from a conventional wellhead. Carbon dioxide, nitrogen or a similar highly expansible fluid is pumped into the wellbore space and/or at least a portion of the tubing string at a pressure greater than the fluid critical pressure and greater than the fracture initiation or extension pressure required in the formation zone. A perforating gun is fired or a shear disk is actuated to release the expansible fluid to flow into the formation at an initial velocity and kinetic energy which substantially exceeds that which is obtained with water or similar conventional fracturing fluids so as to initiate or extend hydraulic fractures with a minimum radius of curvature with respect to the wellbore.