Cracks were observed forming at the surface of Makaopuhi lava lake during the March 1965 Kilauea eruption, and were studied by repeated mapping and observations of this lake; the 1963 Alae lava lake was similarly studied. Cracks open within a minute after molten lava is exposed at the surface, and form either random or oriented orthogonal networks which outline large plates of unjointed crust. Within a few hours, additional cracks subdivide the plates into polygons averaging 15 ft in width. The accumulation of gases trapped beneath the crust near centers of polygons, and the escape of gases from marginal parts, cause upbowing of polygon centers and downsagging of margins. As the crust cools and increases in thickness following stagnation of the lake, existing cracks extend downward and new cracks open. Some new cracks subdivide pre-existing polygons, and short cracks of shallow depth form near polygon centers. Still other cracks apparently open at depths of tens of feet within the crust, propagate upward, and finally feather out near the surface into short cracks which are concentrated in long swarms that cross several polygons. Cracks initiate at temperatures ranging from ambient to about 900°C, and propagate downward into parts of the crust near 1000°C. Cracking results from stresses induced largely by thermal contraction, but also by differential subsidence of the crust. Seismic recordings of shock vibrations due to cracking of Kilauea Iki lava lake indicate a diurnal variation in frequency, with a maximum around midnight and a minimum around noon.