John suggested that, contrary to convential wisdom, Plinian eruptions do not require viscous magma. "Basalt erupts this way," he said, "Water erupts this way." To underscore his new way of thinking about the topic, John did the funniest thing I've seen yet at GSW: He showed a photograph of two dimes (10-cent coins) lying on a black background. Then he showed another, similar photograph, but in the second one, the dimes had been moved to the left by an inch or so. His explanation of this apparent act of performance art? "A paradigm shift!" (pair... of... dimes...) It got big laughs; We're geeks.

John then started a detailed discussion of the inner workings of a $9 coffee maker he had bought at Wal-Mart. He introduced the idea of a "magma table" akin to a water table, and showed how the relationship between density, pressure, gravity, and the height of the water table determined how coffeemaker water 'erupts' out of a conduit (black in the image above, stolen from the "How Coffeemakers Work" page at howstuffworks.com), in spite of the conduit's greater height. It has to do with lowering the density of the material in the conduit by heating it to be partially steam. He suggested that this is akin to how a geyser erupts out of a conduit, as sufficient heating lowers the density, which lowers the pressure on the water below, which flashes to steam, which lowers its density, which lowers the pressure on the water below that, and so on. This chain reaction propagates downward, and it keeps working until the geyser's subterranean reservoir is emptied. (Note that the same principle applies to coffeemakers: they use up all the water inside, and make it into coffee -- there's nothing left sloshing around in there when it's done.) John returned to volcanoes when he invoked the same process to explain Plinian eruptions.

It was a lively, thoughtful presentation that emphasized simple physical relations and familiar analogies to explain one of the most distinctive phenomena of our planet. Thumbs up!