A buoyancy model for half graben development, based on the stability of floating bodies, assumes parallel, high-angle normal faults which penetrate the brittle crust and define half graben parallelograms. Buoyancy forces balance the weight of the half grabens, and torques generated by buoyancy forces are balanced by opposing torques arising from crustal stresses. Model results indicate that widths of developing half grabens depend on the state of stress in the brittle crust, thickness of the brittle crust, and spacing of boundary faults. For a given thickness of elastic crust in a constant state of stress, narrower half grabens will experience greater rotation. Increasing deviatoric tension and/or thickening of the brittle crust will enable rotation of wider half grabens. The effective coefficient of friction across the fault must be ≤0.1--0.2 for half graben rotation to occur; and probably ≤0.1 for wider half grabens to form or any half grabens to rotate through large angles. These conclusions contribute to an explanation of half graben formation in the Socorro, New Mexico, area, where decreasing heat flows and associated thickening of the brittle crust allowed for development of wider half grabens as rifting progressed. ¿ American Geophysical Union 1992