The Jovian moon Io hosts the most powerful persistently active volcano in the Solar System, Loki Patera(1,2). The interior of this volcanic, caldera-like feature is composed of a warm, dark floor covering 21,500 square kilometres(3) surrounding a much cooler central 'island' 4. The temperature gradient seen across areas of the patera indicates a systematic resurfacing process(4-9), which has been seen to occur typically every one to three years since the 1980s(5,10). Analysis of past data has indicated that the resurfacing progressed around the patera in an anti-clockwise direction at a rate of one to two kilometres per day, and that it is caused either by episodic eruptions that emplace voluminous lava flows or by a cyclically overturning lava lake contained within the patera(5,8,9,11). However, spacecraft and telescope observations have been unable to map the emission from the entire patera floor at sufficient spatial resolution to establish the physical processes at play. Here we report temperature and lava cooling age maps of the entire patera floor at a spatial sampling of about two kilometres, derived from ground-based interferometric imaging of thermal emission from Loki Patera obtained on 8 March 2015 ut as the limb of Europa occulted Io. Our results indicate that Loki Patera is resurfaced by a multi-phase process in which two waves propagate and converge around the central island. The different velocities and start times of the waves indicate a non-uniformity in the lava gas content and/or crust bulk density across the patera.