I refer to de Kleer at al. 2017 , doi:10.1038/nature22339 :
"...The Jovian moon Io hosts the most powerful persistently active volcano in the Solar System, Loki Patera...The interior of this volcanic, caldera-like feature is composed of a warm, dark floor covering 21,500 square kilometres surrounding a much cooler central island..."
"....Loki Patera is the largest volcanic depression on Jupiter's moon Io, 202 kilometres (126 mi) in diameter. It contains an active lava lake, with an episodically overturning crust. The level of activity seen is similar to a superfast spreading mid-ocean ridge on Earth. Temperature measurements of thermal emission at Loki Patera taken by Voyager 1's Infrared Interferometer Spectrometer and Radiometer (IRIS) instrument were consistent with sulfur volcanism.
Io's lava lakes such as Loki Patera are depressions partially filled with molten lava covered by a thin solidified crust. These lava lakes are directly connected to a magma reservoir below. Observations of thermal emission at several of Io's lava lakes reveal glowing molten rock along Loki Patera's margin, caused by the lake's crust breaking up along the edge of the patera. Over time, because the solidified lava is denser than the still-molten magma below, this crust can founder, triggering an increase in thermal emission at the volcano. At sites such as Loki Patera, this can occur episodically. During an overturning episode, Loki can emit up to ten times more heat than when its crust is stable. During an eruption, a wave of foundering crust spreads out across the patera at the rate of about 1 kilometre (0.6 mi) per day, until the crust of the lake has been resurfaced. Another eruption would begin once the new crust has cooled and thickened enough for it to no longer be buoyant over the molten lava.
Loki Patera is located at 13°N 308.8°WCoordinates: 13°N 308.8°W. It is named after the Norse god Loki. Amaterasu Patera is located to the north and Manua Patera to the northwest..."