Several underground coal gasification (UCG) experiments have been conducted in the Hanna No. 1 coal seam. During the fall of 1980 the Laramie Energy Technology Center performed a post-burn field study of the Hanna II, Phases 2 and 3 experiment at the Hanna UCG site. The field work consisted of high resolution seismic, drilling, coring, and geophysical logging. The Department of Earth Resources, Colorado State University, contributed to the post-burn study by doing laboratory work on the cores and geophysical logs. The purpose of the laboratory work was to provide an estimate of the temperatures and chemical conditions reached during the conversion experiment by studying the mineralogical and textural characteristics of thermally altered and unaltered overburden. In the vicinity of the burn cavity, overburden rocks have been subjected to high temperature pyrometamorphism during the Hanna II Phases 2 and 3 UCG experiments. Paralava rocks, buchites and paralava breccias containing glass and various high temperature minerals such as oligoclase, clinopyroxene, ferrocordierite, mullite, cristobalite, magnetite, and tridymite formed. Textures of some of these minerals suggest crystallization directly from a melt. Mineralogy and melting relations of the paralavas, ash fusion temperatures, and thermocouple measurements made during the experiment suggest that temperatures in excess of 1200/sup 0/C were attained. Rock color and the presence of reduced iron bearing minerals and blebs of native iron indicate that the experimental burn and the product gases in the area of paralava formation were reducing.