Pele is the first volcanic eruption discovered on Io. It was viewed by Voyager 1, but had returned to a dormant state by the time Voyager 2 passed Io four months later. While it erupted, however, it sprayed molten sulfur and other material in a plume which reached 300 km above the surface. In some ways, the eruption of volcanoes on Io is more like terrestrial geysers than terrestrial volcanoes. This is due to the lower density of the "lava", lower surface gravity, and lower atmospheric pressure on Io. For comparison, however, if "Old Faithful" in Yellowstone Park could erupt in an environment like that of Pele, its plume would only reach 35 km high. Therefore it is not just the conditions of Io that allow such large eruptions--the force driving the volcanoes must be much stronger than that which causes terrestrial volcanoes.
