active flow fields more than long have been observed on io at amirani and masubi. a relatively inactive flow field named lei-kung fluctus covers more than , an area slightly larger than nicaragua. the thickness of flow fields was not determined by galileo, but the individual breakouts on their surface are likely to be thick. in many cases, active lava breakouts flow out onto the surface at locations tens to hundreds of kilometres from the source vent, with low amounts of thermal emission observed between it and the breakout. this suggests that lava flows through lava tubes from the source vent to the breakout.