Drillers have hit live magma only three times, twice in Iceland
Almost everything known about magma comes from studying it after it erupts. Only three drilling projects have ever struck it underground: two geothermal wells in Iceland and one in Hawaii. Deep down, it can hold over 10 percent dissolved water, a gas load that shapes how volcanoes behave.
The word comes from the Greek for a thick ointment. Magma is molten or partly molten rock, usually carrying suspended crystals and gas bubbles, and it is the parent of every igneous rock. It forms when mantle or crust melts at subduction zones, continental rifts, mid-ocean ridges and hotspots, then rises and is stored in chambers or crystal-rich mush zones. While stored, it can change through crystallisation, mixing with other melts, absorbing surrounding crust and losing gas. Some eventually erupts as lava; the rest freezes underground as dikes, sills, plutons or vast batholiths.
Gas is central to its character. Water vapour is typically the most plentiful magmatic gas, followed by carbon dioxide and sulfur dioxide. How much gas stays dissolved depends on pressure, temperature and composition. Under great pressure a magma may hold over a tenth of its weight as water, but by the time it pours out as lava it is almost bone dry, because the gas escapes as pressure falls. Carbon dioxide dissolves far less readily and often forms separate fluid even at depth, which is why tiny bubbles of it are trapped inside crystals grown far underground.
Water also thins magma. Silica atoms link into chains and sheets that stiffen the melt, and geologists express this linkage with a ratio called NBO/T: basalts typically score 0.6 to 0.9, rhyolites only 0.02 to 0.2. Dissolved water breaks those links and sharply lowers viscosity. Across common lavas, viscosity spans seven orders of magnitude.
A few magmas are truly strange. Ol Doinyo Lengai in Tanzania is the only active volcano erupting carbonatite, lava made mostly of carbonates rather than silicates. Iron oxide magma is thought to have formed the ore at Kiruna in Sweden, and at Lastarria in Chile melted sulfur has flowed in streams up to 250 metres long at temperatures as low as 113 degrees Celsius.
Source: Magma
