Galileo data and numerical modeling were used to investigate the summer 1997 eruption at Pillan Patera on Io. This event, now defined as Pillanian eruption style <Keszthelyi et al., this issue>, included a high-temperature (>1600 ¿C), possibly ultrabasic, 140-km-high plume eruption that deposited dark, orthopyroxene-rich pyroclastic material over >125,000 km2, followed by emplacement of dark flow-like material over >3100 km2 to the north of the caldera. We estimate that the high-temperature, energetic episode of this eruption had a duration of 52--167 days between May and September 1997, with peak eruption temperatures around June 28, 1997. Galileo 20 m pixel-1 images of part of the Pillan flow field show a widespread, rough, pitted surface that is unlike any flow surface we have seen before. We suggest that this surface may have resulted from (1) a fractured lava crust formed during rapid, low-viscosity lava surging, perhaps including turbulent flow emplacement; (2) disruption of the lava flow by explosive interaction with a volatile-rich substrate; or (3) a combination of 1 and 2 with or without accumulation of pyroclastic materials on the surface. Well-developed flow lobes are observed, suggesting that this is a relatively distal part of the flow field. Shadow measurements at flow margins indicate a thickness of ~8--10 m. We have modeled the emplacement of putative ultrabasic flows from the summer 1997 Pillan eruption using constraints from new Galileo data. Results suggest that either laminar sheet flows or turbulent channelized flows could have traveled 50--150 km on a flat, unobstructed surface, which is consistent with the estimated length of the Pillan flow field (~60 km). Our modeling suggests low thermal erosion rates (20 m) erosion channels was unlikely, especially distal to the source. We calculate a volumetric flow rate of ~2--7¿103 m3 s-1, which is greater than those for typical Mauna Loa/Kilauea flows but comparable to those for the (1783) Laki eruption and the inferred flow rates of the Roza flows in the Columbia River flood basalts. The differences in ultrabasic eruption styles on Earth and Io appear to be controlled by the different eruption environments: Plumes at sites of ultrabasic eruptions on Io suggest strong magma-volatile interactions on a low-gravity body lacking an atmosphere, whereas the geology at sites of komatiite eruptions on Earth suggest mostly submarine emplacement of thick flows with a pronounced lack of subaerial explosive activity. ¿ 2001 American Geophysical Union