Most lava flows about as thickly as ketchup, yet travels for kilometres
Molten basalt is roughly as viscous as ketchup, tens of thousands of times thicker than water. It still travels huge distances because its surface quickly hardens into a crust that insulates the liquid inside. The secret to how any lava behaves lies mostly in one ingredient: silica.
Lava is magma that reaches the surface of a planet or moon, emerging from volcanoes or cracks on land or under the sea, usually at 800 to 1,200 degrees Celsius. The word is Italian, probably from the Latin labes, a fall or slide. One early use in the volcanic sense appears in Francesco Serao's account of the 1737 eruption of Vesuvius, where he compared the fiery flow to the torrents of water and mud that poured down the mountain after heavy rain.
On Earth, lava is overwhelmingly made of silicate minerals, dominated by oxygen and silicon. Silicon atoms lock onto four oxygen atoms in tetrahedra, and when oxygen links two silicon atoms the melt starts forming chains and clumps. This partial polymerisation makes the liquid stiff, so the more silica a lava contains, the more viscous it is. Iron, calcium and magnesium work the other way, loosening the structure.
Geologists therefore sort lavas by silica content. Felsic types such as rhyolite, above 63 percent silica, are so sticky that they usually erupt explosively, though they sometimes squeeze out as domes and spines often laced with obsidian. Intermediate andesites, between 52 and 63 percent, build steep volcanoes like those of the Andes and flow with the consistency of smooth peanut butter. Mafic basalts, with 45 to 52 percent, erupt hottest among common lavas and spread into broad shield volcanoes and flood basalts.
At the extreme are ultramafic komatiites, with under 45 percent silica and more than 18 percent magnesium oxide. They are thought to have erupted at about 1,600 degrees Celsius and flowed almost like light motor oil. None erupt today: nearly all date from the Proterozoic or earlier, because Earth's mantle has since cooled too much to produce them.
Source: Lava
