In 1960, Anak Krakatau had a minimum diameter of about one mile and was 545 feet tall. The 2,000-foot-diameter crater on the south side contained the growing cinder cone seen just below the ash column.

Image courtesy of Volcano World. © Robert W. Decker, used with permission.

Krakatau lofted the equivalent of 20 cubic kilometers of rock into the atmosphere, sending a column of smoke and ash 26 kilometers above the Earth's surface. Movement of the seafloor caused tsunamis (defined) that killed 36,000 people and destroyed 160 villages; remnants of the giant waves were measured in the English Channel, on the far side of the Earth. In other words, that rock was no punk!

In a textbook example of the power lurking beneath Earth's quiescent surface, the eruption obliterated two-thirds of the 11-kilometer-long island of Krakatau (until lately written "Krakatoa" by Westerners), darkened the skies for dozens of kilometers around, and deposited a red-hot layer of ash 30 meters thick (and up to 60 meters in some places) on the one-third of Krakatau that remained, and on neighboring islands. Eventually, in 1930, resurgent volcanic activity from the same magma source produced another cone: Anak Krakatau ("Child of Krakatau" in the Indonesian language).

Ash cloud has a silver lining
The Krakatau explosion was soon recognized as a giant natural experiment in colonization -- the movement of species to new habitat. In other words, by sterilizing itself and neighboring islands Krakatau became a biologists' bonanza (see "Krakatau: the Destruction and Reassembly" in the bibliography).

The first problem in island colonization, notes Australian zoologist and Krakatau expert Ian Thornton, is transportation: How do species get to a sterile island? [This problem is much more severe in Hawaii, a series of volcanic islands that formed thousands of kilometers from the nearest source of propagules (defined) ].

Here are the options, transport-wise:

Air: through passive transport, as a light orchid seed or fern spore, or through flight, as a bird or insect.

Sea: either passively floating, linked to a larger mass of debris like a log, or swimming.

Animal: (The hitchhikers' express). Plant and animal propagules may travel in or on long-distance animals. Some mollusks, for example, use a sticky goo to grab a moving vehicle. Likewise, seeds may have adhesives or barbs for the same purpose. Fruit eaters may carry seeds in their gut and if these are not destroyed by the digestive process, viable seeds may be deposited on the island.

Krakatau, located at a funnel between the Pacific and Indian Oceans, collected floating crud from an enormous region. In general, as expected, the first species to arrive were those that disperse easily. If the natural experiment lasts long enough, the rate of extinction will start to equal the rate of immigration -- meaning that the species count will reach an equilibrium.

The experiment could be voided, however, if Anak Krakatau heeds its explosive heritage and departs the scene as its noisy parent did. There's no way of predicting when or if that will happen, but Anak Krakatau is erupting periodically -- not sleeping as peacefully as a baby.

Indeed, people have a mistaken assumption that just because they haven't seen a gargantuan eruption, it won't happen. The problem could arise at El Popo in Mexico, Long Valley, or elsewhere, says Stanley Williams of Arizona State University. Mt. St. Helens, which emitted one cubic kilometer of ash, was the kind of thing that happens every decade or so. Krakatau, which put out about 20 times as much stuff, can happen roughly once every century. But 75,000 years ago, Mt. Toba, also in Indonesia, blew out 3,000 cubic kilometers, an eruption that must have had global climatic implications.

The moral? Gargantuan eruptions are rare, but they do happen. Did you miss our coverage of flood basalts)? Earth is not, geologically speaking, past its unstable youth.

The Why Files asked Prof. Ian Thornton, who wrote the book (see "Krakatau: the Destruction and Reassembly..." in the bibliography) on the ecological aftermath at Krakatau, to explain what the sterilization and recovery of an island ecosystem tells us about colonization (biologically speaking).

He wrote us a great letter.