Magma and lava

On Kilauea, a geologist pushed a thermocouple into lava breaking out 3 km from its vent and read 1,100 °C.

Same rock, two names

Molten rock below the surface is called magma. Once it erupts from a volcano, the same material is called lava. USGS describes magma as a mixture of molten rock, crystals and dissolved gas, and suggests picturing an unopened bottle of soda with grains of sand inside. The molten part is made mostly of oxygen and silicon, with aluminium, iron, magnesium, calcium, sodium, potassium, titanium and manganese. When it cools, crystals of different minerals grow until it becomes solid igneous rock. More than 80 percent of the Earth's surface, above and below sea level, is of volcanic origin.

Magma forms many tens of kilometres down. It is lighter than the solid rock around it, so buoyancy pushes it upward, and the gas dissolved in it adds pressure, like the fizz in that shaken bottle. It forces its way through weak zones and cracks, often pausing in a storage area, a magma chamber, a few kilometres below the volcano. If it finds a way out, an eruption begins. Sometimes the molten rock simply pours out as lava flows. Sometimes it shoots into the air as clouds of rock shards, called tephra, and gas. The finest pieces, ash, are about as thick as a strand of hair and can be carried around the world by high winds.

How hot is lava?

USGS lists lava lakes, lava flows, lava domes and pyroclastic flows as features that can reach about 1,200 °C. Measuring them is dangerous, so scientists only push a thermocouple into lava where the area is safe to stand in for several minutes; on Kilauea one such reading gave 1,100 °C. For comparison, a kitchen oven tops out near 250 °C, and steam vents on a volcano run from boiling up to about 400 °C. Lava glows because it is so hot. As its surface cools it turns dark, even while the inside is still molten.

Runny or sticky

The most important property of magma for the shape of a volcano and the style of its eruptions is viscosity, how sticky it is. BGS explains that runny basalt lava flows great distances before it cools, so it builds broad, gentle slopes. Sticky magma, such as the andesite common at volcanoes above subduction zones, does not flow far. It piles up near the vent, and gas trapped in it cannot escape easily. Pressure builds until it escapes violently, which is why sticky magma tends to mean explosive eruptions. Picture blowing through a straw into water, which makes gentle bubbles, and then into thick syrup, where the bubbles burst out with a pop.

Where does magma form? Mostly in three places. At mid-ocean ridges hot mantle rises into the gap between separating plates. Above subduction zones, water released from the sinking plate helps the mantle above it melt. And at hotspots such as Hawaii, the Hawaiian Volcano Observatory says heat partially melts mantle rock at depths starting about 200 to 400 km. The rest of this course follows those three settings.