Abstract The Near Infrared Mapping Spectrometer (NIMS) has obtained spectra of volcanoes on the surface of the jovian satellite Io. Fits to data using a silicate cooling model allow us to constrain lava eruption rates and eruption age. The thermal signatures of the hot spots are indicative of active and cooling silicate lava flows. For large, active hot spots maximum ages of flow surfaces detected by NIMS typically range from days to months, although in one case it is nearly 30 years. Mass eruption rates for the main hot spots are in the range 7 m 3 s −1 to 79 m 3 s −1, using an average flow thickness of 1 m. These mass eruption rates are orders of magnitude less than those implied for large thermal outbursts on Io: the eruptions observed during June 1996 are most likely of a different eruption style, on a much smaller scale. The observation analyzed here may be representative of the current background level of volcanic activity on the anti-jove hemisphere of Io. The global mass eruption rate from these “non-outburst” hot spots is calculated to be 43 km 3/year, equivalent to a global resurfacing rate of about 1 mm/year, about 10% of the minimum required global resurfacing volume. The total observed energy output from the 14 hot spots analyzed is 3.6×10 12 W. Normalized globally, these hot spots contribute approximately 10% of Io's radiometric heat flux.