Galileo Near Infrared Mapping Spectrometer (NIMS) data of volcanic thermal emission are analyzed to determine the power output of a number of Ionian volcanoes, from which are calculated volumetric eruption rates. A two-temperature model is used to determine surface temperatures and power output. Portions of Prometheus and six other volcanoes are found to be in excess of 1100 K: areas of Prometheus are at temperatures close to 1500 K. These are minimum eruption temperatures and are consistent with basaltic silicate volcanism. For the 14 volcanoes in G1INNSPEC01, volumetric eruption rates (E) are constrained between 3 and 300 m3/s for basaltic and ultramafic compositions. Actual E values lie between these limits. The small crack fractions (hot area to warm area ratio) implied from these fits are indicative of a relatively quiescent eruption style, such as the emplacement of a pahoehoe or a-a flow field. The volumetric eruption rates derived from the NIMS data are greater than those observed in similar styles of volcanism on Earth. It is apparent that for similar eruption styles, the areal extent of activity is greater on Io than on Earth. Derived flow thicknesses range from 0.1 to 13 m.