Brief explanation:

Lava fountains are sustained jets of (usually very) fluid lava into the atmosphere. Lava fountains occur commonly on basaltic volcanoes such as Kilauea, or Etna.
The fountain gains its momentum by the expansion of gas bubbles that dissolve from the magma as pressure falls while it is rising in the conduit.
Heights, appearance, duration and erupted volumes of lava fountains can vary greatly. Strong fountains can reach several hundreds of meters. Lava fountains of Vesuvius in 1779 have even reached an estimated height of 3 km !