Full glossary

Molten rock, called magma, is called lava when it reaches the surface during a volcanic eruption. Depending on how the magma erupts, it can form lava flows, lava fountains, lava lakes, or be fragmented into scoria and ash during explosive eruptions.

In a wider use of the term, also the cooled and solidified products of erupted magma are called lava (in form of coherent lava flows, or fragmented tephra).

Magma when it erupts as lava is almost never a completely molten rock, but contains 3 phases: liquid, solid mineral crystals (xenoliths) and gas bubbles, mainly water and carbon dioxide.
The temperature of erupted lava ranges from around 1200 deg C for basaltic lavas erupted at hot spot volcanoes to less than 600 deg C for the extreme end-member of natrocarbonatite lava.
The chemical compositions of lava encompass a wide range - most importantly, determined by the silica (SIO2) content, normally ranging from about 50 to 75 % for basaltic to rhyolite magma. The more silica lava contains and the cooler it is, the greater its viscosity (resistance to flow), because the silica molecules will try to form clusters that resist movement. Basaltic magmas, with high temperature and low silica content can therefore form fluid lava flows and travel great distances, while the opposite type of magma, rhyolite, tends to form solid plugs that block the vent, and erupt either in form of slow-creeping lava domes, or explode into pumice and ash (if it contains sufficient gas as well).
Along with temperature and composition, the gas content is a key factor to determine the style of an eruption. Dissolved gases such as water vapor and carbon dioxide can form up to 5-7 % of the magma, and when magma approaches the surface, will try to form bubbles and increase the partial pressure. If high viscosity prevents the gasses from erupting during lava fountains or mild strombolian activity, violent explosion takes place in which the magma is fragmented. More explosive eruptions are the result.

These and other factors (e.g. magma supply rate, thickness of crust, presence of ground water etc.) result in an incredibly wide range of physical properties and eruptive behaviors that ultimately determine the individual character of each single volcano.

The word "Lava" comes from the Italian word for stream, which is derived from the verb lavare to wash.