The Galileo spacecraft caught Io in the act of an active volcanic eruption on Februrary 22, 2000. Tvashtar Catena is a chain of calderas, collapse pits formed by volcanic eruptions. The active site of the eruption is visible on the left edge of the image, where infrared imaging sees the glow of a hot lava flow more than 60 kilometers long.
This picture is about 250 kilometers across. North is toward the top and illumination from the Sun is from the west (left).