Tvashtar is one of the largest active volcanoes in the known solar system. It resides on Jupiter's moon Io, one of the few solar system bodies to have active magma volcanoes. While Io is close in size the the Earth's moon, Jupiter's immense gravitational pull on Io causes a tidal bulge in the moon's surface. This bulge causes frictional heating great enough to provide a molten core -- and incidentally, a magnetic field.
Io has hundreds of active volcanoes, and we have a limited ability to observe them. Over time it has emerged that Tvashtar is actually a large region covered in a series of volcanic depressions, a.k.a. patera, and as such is often referred to as Tvashtar Paterae. These depressions are filled with lava lakes and are usually covered by a crust of cooled lava. When erupting it can send a plume hundreds of kilometres above the surface; in 1999 the Galileo spacecraft observed plume of volcanic gasses rising 385 kilometres (239 mi) above the surface (the lava itself rose 1- to 2-kilometres above the surface, travelling laterally up to 700 kilometres). New Horizons observed a similar plume when it flew by Io in 2007.
While Tvashtar is an impressive volcanic formation, it is largely noteworthy because it is one of the few active extraterrestrial volcanic features we have been able to observe over a fairly long period of time. A 2006 eruption event was observed by the Keck Observatory for 530 days, making it the longest tracked eruption on Io. Tvashtar doesn't hold the record for the biggest plumes on Io, though -- that honor belongs to Pele, with observed plumes of 500 kilometres (310 mi). Nor is Tvashtar the largest volcanic depression on Io; that would be Loki, producing about 25% of the heat blasting off of Io's surface even on a slow year.
Tvashtar is named after the Vedic blacksmith god, Tvashtr.
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