Molten rock is called lava once it erupts onto Earth’s surface.
Below the surface, molten or partially molten rock is called magma. When that material is expelled through a volcanic vent, fissure or underwater opening and reaches the surface, it is called lava. The change in name reflects its location, not a completely different substance.
Lava commonly erupts at temperatures of roughly 800–1,200 °C, depending on its composition. Basaltic lava is relatively fluid and can travel considerable distances, while silica-rich lava is more viscous and tends to move slowly or build thick domes. As lava cools, it develops a solid crust and eventually becomes igneous volcanic rock.
A frequent mistake is treating magma, lava, ash and tephra as interchangeable. Magma is underground melt; lava is surface-extruded melt. Ash consists of very fine volcanic particles, while tephra is the broader term for fragmented material thrown into the air. Explosive eruptions can produce abundant ash and tephra, whereas effusive eruptions are especially associated with lava flows.
The word “lava” became widely associated with volcanic eruptions after descriptions of Mount Vesuvius in the eighteenth century.