Theoretically- and empirically-derived cooling rates for active pahoehoe lava flows show that surface cooling is controlled by conductive heat loss through a crust that is thickening with the square root of time. The model is based on a linear relationship that links log(time) with surface cooling. This predictable cooling behavior can be used assess the age of recently emplaced sheet flows from their surface temperatures. Using a single thermal image, or image mosaic, this allows quantification of the variation in areal coverage rates and lava discharge rates over 48 hour periods prior to image capture. For pahoehoe sheet flow at Kilauea (Hawai'i) this gives coverage rates of 1-5 m2/min at discharge rates of 0.01-0.05 m3/s, increasing to ~40 m2/min at 0.4-0.5 m3/s. Our thermal chronometry approach represents a quick and easy method of tracking flow advance over a three-day period using a single, thermal snap-shot.