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You probably didn’t notice, since nobody but me has bothered to point it out, but August 27 was the 103rd anniversary of the eruption of Krakatoa, an active (obviously) island volcano located in the Sunda Strait, south of Sumatra and west of Java. In 1883 it blew apart in the most violent explosion on Earth in modern times (and yes, that’s including nuclear explosions), loud enough to be heard in the Indian Ocean, 4,800 kilometres away, and equivalent to 200 megatons of TNT. It ejected 21 cubic kilometres of dust into the atmosphere, caused 12-story waves and killed 36,000 people.

If you’re a typical small boy, as I was (and still am, in many ways), these kinds of statistics make you go, “Wow! Cool!” (or whatever is the appropriate expression for your generation). They also help seal your life-long fascination with volcanoes.

We don’t often think about it, but beneath the relatively thin crust of the Earth lies a vast expanse of molten rock called magma, kept hot by the decay of naturally occurring radioactive material and possibly by heat left over from the planet’s formation. Volcanoes occur whenever there’s a fissure in the crust that allows this magma to leak out, a process that can be either gentle (as in the case of the famous volcanoes of Hawaii, whose eruptions are tourist attractions) or extremely violent (witness Krakatoa).

Generally, these fissures occur in places where the various moving plates of the Earth’s crust are either running into each other or pulling apart. The greatest volcanic activity occurs at the latter locations, but they’re mostly out of sight on the ocean floor. (Iceland is an exception.) As well, the lava flow there tends to be a nice quiet oozing.

By contrast, volcanoes that arise where two plates are crashing into each other (if you can call something happening at five centimetres a year crashing) are among the most violent. The Andes Mountains and the Aleutian Islands are examples.

Finally, there are volcanoes that arise well away from the edges of the plates, over “hot spots,” such as the Hawaiian Islands and the Yellowstone volcanic field.

Hawaiian-type volcanoes are called “shield” volcanoes because they look like the shields of medieval warriors. They’re shaped that way because the magma that erupts from them (called lava once it’s above-ground) emerges quietly and flows gently away from the vent, creating mild slopes. (Gentle it may be, but it’s still nothing to mess with: lava emerges at 850 to 1,250 degrees, hot enough to burn practically anything.)

Some magmas have high gas contents and are far less fluid. As a result, they don’t erupt as smoothly, and are often blown sky-high during an eruption, falling as solid “volcanic bombs,” ranging from fine ash to house-sized blocks. The smaller fragments tend to accumulate right around the vent, forming small, steep-sided “cinder cones.”

The third type of volcanoes, Mt. St. Helens and its snow-covered ilk, are composite volcanoes or “stratovolcanoes.” They go through cycles of quiet eruptions of fluid lava followed by explosive eruptions of less-fluid lava. The fluid lava creates a shell over top of the debris from the explosion, forming tall, steep-sided volcanic cones.

The different kinds of eruptions have different names. The most gentle eruptions of fluid lava are termed Hawaiian. Next up the scale of violence are Strombolian eruptions, named after a volcano in the Aeolian Islands off the coast of Sicily. Strombolian eruptions feature moderately fluid lava flows, violent lava-fountaining and lots of bombs.

Vulcanian eruptions have nothing to do with people with pointy ears. The best-known examples are the eruptions of Mt. Vesuvius, infamous for burying Pompeii in 79 A.D. Vulcanian eruptions feature more viscous lava that forms short, thick flows around the volcanic vents, from which fragments are violently ejected.

Pelean eruptions are similar to Vulcanian ones but have even more viscous lava, which forms domes over the vents. They get their name from Mt. Pele on the West Indian island of Martinique, which erupted in 1902, destroying the city of St. Pierre and killing more than 30,000 people. In a Pelean eruption pressure builds up beneath the dome until it either blows explosively or the side of the mountain gives way, in which case the force of the explosion goes sideways. A particularly nasty phenomenon associated with Pelean eruptions is known as the “nue ardente” or “glowing cloud.” This is a cloud of superheated air and ash, so dense that it flows along the ground like a fluid. It travels down the volcano at hurricane speed, killing everything in its path.

Finally, the most violent eruptions of all, such as that of Krakatoa, are called Plinian eruptions after Pliny the Elder, who died in the Vesuvius eruption of A.D. 79. They include the ejection of a huge amount of volcanic ash (68 years before Krakatoa the eruption of Tambora injected an incredible 100 cubic kilometres of rock into the atmosphere), followed by the collapse of the central part of the volcano. Mt. St. Helens was a Plinean eruption, but on a very small scale, as such things go.

What’s small-scale volcano-wise, however, is pretty big-scale human-wise. We’re a long way from Mt. St. Helens, but even here volcanic ash dusted our cars in 1980. After Krakatoa blew up, sunsets were red worldwide for a year, and the eruption of Mount Pinatubo in the Philippines a couple of years ago was blamed for cooling the Earth’s climate.

Which makes the ever-present possibility of a really BIG eruption even more worrisome, if you really need something else to worry about.

What’s a really BIG eruption? Multiply Mt. Pinatubo a thousand times. Multiply its cooling effect a thousand times, too. The last time Earth suffered a really BIG eruption was an eruption at Toba in Indonesia 70,000 years ago. It triggered an ice age.

What’s a bit worrisome is that, as volcano expert Professor Bill McGuire pointed out at a science festival in Birmingham, England, last week, Earth usually has a couple of these massive eruptions every 100,000 years, which means that, statistically, we’re overdue. And of the 500 or 600 active volcanoes known to exist around the world, only about 100 of them are even being monitored–which means a huge eruption could happen without any advance warning at all.

And yet, I confess, the possibility of such a global disaster being caused by a volcano only makes them more interesting to me. As much as I like humans (I’m one myself, you know), I have to say that as a species we can be just a tad overconfident about our place on this planet and our ability to wrestle its forces into submission.

Nobody, but nobody, wrestles with a volcano.

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