Tom Simkin (–2009)

This is a compendium made by the Smithsonian for the 100th anniversary of the eruption, gathering a number of studies and eye-witness reports. It’s excellent as a reference; not to read through in a sitting but to dive into from time to time. There are a large number of contemporary and near-contemporary maps, ship’s logs, a reproduction of the Batavia gasworks strip chart (mentioned by KGB) that shows the barometric pressure waves from the eruptions, and so on. A couple of things (not already covered) that interested me: as of the date of publication, the jury was still out on exactly what caused the tsunami that wiped out the west coast of Java. The suspects were:


(1)The effect of the explosion itself;
(2)The gravitational collapse of the ejecta column, dropping several cubic kilometers of ejecta into the ocean;
(3)The collapse of more than half of the island of Krakatau into the sea after the supporting magma was ejected;
(4)The impact of a low-angle pyroclastic flow.


Option 3 was the most common for years, reinforced by the discovery, shortly after the eruption, that a 300-fathom deep sea lead could not find bottom in the former location of an island peak originally several hundred feet high (by the time modern depth-sounding equipment came along subsequent activity and continued slumping had left the area much shallower). Further corroboration came from a ship only 12 miles from the eruption that reported a current toward the volcano so strong (crew estimated it at 12 to 16 knots) that it was feared the vessel’s bow would be pulled off by the strain on the anchors.


Option 2 seems to be currently favored, as more information has become available about the nature of pyroclastic flows (alas, the information usually came the hard way for people downstream from the same).


However, there’s some interesting support for Number 4, as well. Careful examination of tide gauge records showed that the center of the wave pattern was not centered on Krakatau Island but some distance to the northeast. At the time of the original eruption the idea of directional pyroclastic flows was not understood; it was believed that all volcanoes erupted straight up (reinforced by observations of Vesuvius, which normal erupted this way). Even after obvious directional flows at Mt. Pelee, Mt. Lassen, and Mt. St. Helens the average mental image of a volcano is probably something that shoots straight up in the air.


I suspect the actual cause is, as usual, a combination of several above.


Another intriguing fact was the discovery that air waves from the eruption could couple with sea waves, intensifying both and perhaps partially explaining why sounds and sea movement from the eruption were noted so far away. Based on this coupling, the total energy released was between 100 and 150 megatons.


There are some nice color plates, including illustrations from the 1884 expedition, drawings of atmospheric phenomena, and more recent photographs. There’s also, surprisingly, a photograph of Krakatau in eruption in 1883 – not the August cataclysm but an earlier eruption in May.
… (altro)