Magma vs Lava

People often use the words magma and lava as if they meant the same thing, but they are technically different. Both are molten rock: rock that has been heated until it melts. The difference is just where it is.

  • Magma: molten rock underground, still inside the Earth.
  • Lava: molten rock that has reached the surface, either flowing out of a volcano or out of a crack in the ground.

The same molten rock becomes magma when it is below the ground and lava the moment it appears at the surface. Their chemistry is identical at the moment of the transition; only the location changes.

  • Magma temperature700 to 1,300 degrees CDepends on type
  • Lava temperature700 to 1,250 degrees CCools rapidly at the surface
  • Hottest lavaBasaltUp to 1,250 degrees C, runny
  • Coolest lavaRhyoliteApproximately 700 degrees C, very thick
  • Underground depth of magmaOften 3 to 50 kmMagma chambers
  • Fastest lava flows60 km/hEspecially on steep slopes

What is magma made of?

Magma is a hot mixture of:

  • Melted rock (mostly silica, plus other minerals).
  • Dissolved gases, mostly water vapour, carbon dioxide and sulphur dioxide. These dissolved gases drive most eruption behaviour.
  • Suspended crystals that have already started to form as the magma cools.

The three main types of magma (and lava)

  • Basaltic: low in silica, runny, hot (1,000 to 1,250 degrees C). Erupts gently. Forms shield volcanoes. Mostly found in mid-ocean ridges and hot spots like Hawaii.
  • Andesitic: medium silica, medium viscosity, medium temperature (800 to 1,000 degrees C). Common in subduction zones. Forms stratovolcanoes. Can erupt either gently or explosively.
  • Rhyolitic: high silica, very thick, lower temperature (700 to 900 degrees C). Almost always erupts explosively, sometimes catastrophically. Forms lava domes and supervolcanoes.

What happens to lava once it erupts

Lava can flow out gently (called effusive eruption) or be thrown out violently with rock fragments and ash (called explosive eruption). Once it is in the air or on the ground, it starts cooling immediately. The result depends on the type:

  • Runny basalt lava: can flow for tens of kilometres before cooling. Forms smooth ropy flows called pahoehoe (Hawaiian) or rough jagged flows called a'a.
  • Sticky andesite or rhyolite lava: barely flows at all. Often forms steep-sided lava domes or piles up around the vent.
  • Air-cooled lava: thrown high enough into the air to cool while falling, lava forms small lumps called cinders or fine particles called volcanic ash.
  • Lava bombs: blobs of lava the size of a football or larger, ejected by explosive eruptions.
  • Volcanic glass (obsidian): lava that cooled so fast that no crystals had time to form.
Fact The hottest lava ever measured was approximately 1,250 degrees C: hot enough to melt copper and aluminium. To put that in perspective, a typical pizza oven runs at around 250 degrees C, and a hot wood fire at around 1,000 degrees C. Lava is glowingly hot enough to set anything flammable on fire instantly and can melt through thin metal sheets.

How fast can lava move?

Most lava flows move slowly: usually 1 to 10 km/h, slow enough that humans can outrun them. But under the right conditions, lava can be terrifyingly fast. The fastest recorded lava flow was at Nyiragongo in the Democratic Republic of Congo in 2002, where extremely runny lava on a steep slope reached 60 km/h: faster than a sprinting Olympic athlete. Several towns were overwhelmed before people could escape.

Did you know? The longest known lava flow in history was the Laki eruption in Iceland in 1783 to 1784, which produced over 14 cubic kilometres of lava from a 27-km-long fissure. The eruption also released huge amounts of sulphuric gas that caused crop failures and famine across Europe, and may have contributed to the unrest that led to the French Revolution.
Deeper dive: why some lava flows like water and others crawl like toothpaste

The single most important property of lava is its viscosity: how thick and sticky it is. Viscosity is what determines almost everything about a volcanic eruption.

Viscosity depends on two main things.

  • Temperature: hotter lava is runnier. Cooler lava is stickier.
  • Silica content: high-silica lavas (rhyolite, dacite) form long chains of molecules that lock together and resist flow. Low-silica lavas (basalt) have shorter molecular chains and flow more easily.

The contrast is dramatic. Basalt at 1,200 degrees C has the viscosity of thick honey: it pours and flows. Rhyolite at 700 degrees C has the viscosity of cold toffee or putty: it barely flows at all, just piles up.

This in turn affects how the volcano erupts. Runny basalt lava lets gas escape easily, so eruptions are gentle (Hawaiian-style fountains). Sticky rhyolite lava traps gas inside until pressure builds up to catastrophic levels, leading to explosive eruptions (Mount St Helens-style). Andesite is in between, which is why subduction-zone volcanoes (which mostly produce andesite) can have both kinds of eruption.

This is also why the most dangerous volcanoes (Vesuvius, Pinatubo, Toba) are nearly always those with thick high-silica magma, while the famous "tourist volcanoes" (Hawaii, Iceland) are mostly the runny basalt kind. Knowing the magma chemistry of a volcano tells you a lot about what to expect when it erupts.

For more, see what is a volcano, types of volcano and volcanic eruptions.