Landsat TM data of active lava flows from Kilauea Volcano Hawaii (7/23/91) and forest fire within Yellowstone National Park (9/8/88) are compared to show the differences in the spectral and spatial distribution of radiance. At visible wavelengths, smoke from forest firest obscures terrain features, while active eruptions show little degassing at breakouts (ruptured lava tubes). Lava flows exhibit a gradual increase in infrared radiance (>2 &mgr;m) from the edge of an emplaced flow to the central breakout. The hottest parts of the forest fire (which radiate similar amounts of energy to flow breakouts) are located within 90 m of the edge of the burn scar with the interior portions exhibiting sharp decreases in infrared radiance. The radiance measured at 10.4--12.4 &mgr;m relative to that at near-infrared wavelengths (2.08--2.35 &mgr;m) suggests that the surface cracks exposing molten lava anneal proportionately with the progressive thicknening of the flow crust, while burn scars, produced by forest fires, cool to pre-burn background temperatures rapidly with only small areas of high-temperature embers remaining. These differences will be important for the implementation of an automatic thermal-anomaly detection system using satellite data. ¿ American Geophysical Union 1995.