 There are over 100 calderas with abundant and widely spread radiating lava flows.And the composition of the lava is interpreted to be mainly sulfur and silicates from the high eruption temperature of at least 1200 K. 

In addition to extensive volcanism, mountain ranges are the second observation on Io's surface.Io has 100~150 mountains with mean height of 6 km and a maximum height of 17 km.