Rhyolite, the stickiest lava on Earth, tends to explode rather than flow
Rhyolite holds more silica than any other volcanic rock, and that makes its molten form thick enough to trap gas until it bursts. So it usually leaves the ground as ash and pumice, not rivers of lava. Only four rhyolite eruptions have been recorded since 1900, including the biggest eruption of the whole 20th century.
Chemically, rhyolite is granite that reached the surface and cooled fast. Its main minerals are quartz, sanidine and plagioclase, with few of the dark iron- and magnesium-rich minerals, so it is usually pale and so fine-grained or glassy that crystals are hard to see. Geologists define it by the share of quartz and feldspar, or, when the grains are too tiny, by its silica and alkali content on a standard chemical chart. The German geologist Ferdinand von Richthofen named it in 1860 from a Greek word for a stream of lava.
It erupts relatively cool, around 800 to 1,000 °C, compared with 1,100 to 1,200 °C for basalt, and its viscosity favours violent blasts. Rhyolitic ash-flow tuffs are the only volcanic deposits that approach flood basalts in sheer volume. Novarupta in Alaska in 1912 began explosively and later built a rhyolite dome in its vent; the other modern eruptions came at St. Andrew Strait in Papua New Guinea and at Chaitén and Cordón Caulle in Chile. Unusual peralkaline rhyolites, rich in alkali metals, can run 10 to 30 times more fluidly.
Most rhyolite forms along subduction zones, especially beneath continents, where thick crust gives rising magma time to evolve and melt surrounding rock. Water helps too, lowering the melting point; some rhyolitic magmas hold 7 to 8 percent water by weight. Ocean islands rarely produce it: about 8% of Iceland's volcanic rock is rhyolite, yet Hawaii has none known.
The rock's textures depend on cooling. Chill it too quickly for crystals and you get obsidian, the volcanic glass that takes an edge sharp enough for prehistoric blades and has been tested for surgical scalpels. Gas-filled froth becomes pumice, used as an abrasive, in concrete and in soil. Indigenous peoples of the Northeastern Woodlands quarried rhyolite at Carbaugh Run in Pennsylvania from 11,500 years ago and traded it for spear points, and ancient Romans built with rhyolitic tuff.
Source: Rhyolite
