Lava is molten rock expelled by a volcano during a non-explosive eruption. The temperature and viscosity of lavas determine how the lava will behave. Silica-poor basaltic magmas with high temperature and low viscosity are very fluid and develop a range of fluidal shapes, while cooler viscous rhyolitic flows often break up into blocks which are then reincorporated into the lava flow again, a process called autobrecciation.