Molten rock which erupts onto the earths surface. Viscosity is dependent on temperature and silica content. Low silica hot ( 1200 C ) lavas ie.basalt flow quickly cooler higher silica lavas such as dacite and rhyollite have a higher viscosity. Basaltic lavas produce lava fountains or prolonged effusive eruptions. High silica lavas can form lava domes and tend to erupt explosively. See lava types
The most common lava on the planet is basalt which is a mafic basalt woth less than 52% silica. Basalt is a low viscosity lava which erupts at temperatures of arond 1000 C . It can form rivers of flowing lava,pahoe hoe flows,lava tubes,fountains and pillow lavas. It forms the floorsof the oceans after being eupted at mid ocean ridges.
More viscous lavas such as dacite ( 63-68%) silica form extruded domes of lava hich can collpase to form pyroclastic flows.