Volcanoes!

There's a new volcanic eruption going on in Hawaii, and a Reuters video popped up at Scientific American. Although they can look scary, Hawaiian-style eruptions are basically gentle compared to other types. For example, note that in this video, the lava is fairly fluid- that is, it flows easily- and that in the shots of the lava lake, there are some bursts of gas, but they don't splash all that much:

Basaltic lava tends to have low viscosity (resistance to flow) and low gas content- on the order of a few tenths of a percent by weight. In contrast, more silica-rich lavas, such as rhyolite or dacite, tend to have a high viscosity- almost to the point of being solid- and high gas content, on the order of 3-4 percent. This means that the vapor pressure is extremely high, and the confining pressure is also high. If you've ever taken a small amount of gunpowder and lit it, you know it makes a nice little ball of fire and sparks. But that's very different from taking the same amount of gunpowder confined in a shell and firing it: in the latter situation it creates a substantial explosion, not just a pretty little fire. The confinement of gas (mostly water) in volcanism is what leads to the the most violent, dangerous eruptions. They're just steam explosions. Steam explosions that loft millions of tons of incandescent semi-molten rock into the air, yes, but simple steam explosions just the same.

The May 18, 1980 Mt. St. Helens eruption is undoubtedly the best-known eruption to most Americans, though as big eruptions go, it really wasn't extraordinary. The 1991 eruption of Pinatubo in the Philippines was about ten times larger, but I couldn't find a good video focusing on the great eruption. Probably the most famous eruptions of all were the 79 AD eruption of Vesuvius that destroyed Pompeii and Herculaneum, and the eruption of Krakatoa in 1883. The biggest eruption I'm aware of in the Geologic record was the eruption at Yellowstone about 600,000 years ago, which some have estimated to have produced 600 cubic miles of ejecta- over a thousand times the amount produced by Mt. St. Helens.