(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

Replica of a 19th century soda bottle that was sealed with a blue marble (visible in lower crimp of the bottle). Bottles like this played a part in creating the bottled soda analogy of how and why volcanoes erupt. The Halema‘uma‘u Overlook gas plume is visible in the background. USGS photo.

Bottled soda, when opened suddenly after shaking, is commonly used as an analogy for volcanic eruptions. Perhaps the first use of this analogy for Hawaiian volcanoes was during the 1899 Mauna Loa eruption, when Sereno Bishop, a missionary with an interest in science, suggested the idea in a letter to the Hawaiian Star newspaper on July 20.

The 1899 eruption started with explosions and high lava fountains visible tens of miles from the eruption site on the Northeast Rift Zone of Mauna Loa. In his letter, Reverend Bishop asked the question, “Whence and what is the force which ejects such enormous quantities of molten rock from the earth’s interior with such stupendous explosions?”

James Dwight Dana, an American volcanologist, favored steam as the propellant for all volcanic eruptions. “Steam is generated by the contact of the interior molten masses with surface water or sea water percolating down through the intervening rocks.” But it was difficult to visualize how this worked for all eruptions—how surface or sea water could get under or into magma before expanding into steam to propel it out of the earth—although it has worked for some events, like the 1924 Halema‘uma‘u eruption.

Bishop favored another theory: “The whole of the superheated magma or lava of the interior, is saturated with condensed gases under pressure in liquid or solid form, but ready to expand and effervesce when the superincumbent pressure is removed. This condition is analogous to that of the water in a soda bottle. It is heavily charged with carbonic acid gas, but looks like simple water. Remove the stopper, and the liquid particles of carbon dioxide instantly fly into vapor with explosive force. Just so, the various gases included in solid or liquid form throughout the molecules of the hot magma, fly into vapor wherever an exit is opened.”

Soda is carbonated by dissolving carbon dioxide gas into it under pressure. To remain carbonated, the soda must be stored under pressure. In the 19th century, soda was generally available at soda fountain shops, but bottling it for sale was rapidly becoming common. Bottles were blown by hand inside molds, which resulted in irregularities in bottle wall thickness, and were sealed with corks or marbles. But in the 1890s, as bottled soda was gaining in popularity, several accidents in which soda bottles exploded, severely injuring the thirsty bottle-holders, were reported in Hawai‘i newspapers.

No doubt, missionary Bishop took note of these accidental explosions, as well as the general tendency of soda to erupt from the bottle when it was opened, and saw similarities to eruptive behavior at Mauna Loa. But Bishop made one mistake in his analogy statement. He hypothesized that the carbon dioxide was pressurized into a liquid or solid before being combined with water in a carbonated drink. Another letter writer, using the pen name of Scribendi Caccoethes (Latin for “insatiable desire to write”), took him to task for this error and pointed out that the carbon dioxide gas was dissolved into water.

The discussion between Sereno Bishop and Scribendi Caccoethes consisted of eight letters. Afterward, the Pacific Commercial Advertiser labeled it “A Painful Controversy” and suggested that a well be drilled in Punchbowl Crater on O‘ahu “10,000 feet to the locality of the earth’s hot innards” where the truth will be found. They also suggested that if the well were to allow lava to come up to the surface and fountain, it would act as “volcanic fly paper which will attract and catch all of the tourists of the world.”

This controversy has been largely forgotten, but the soda bottle analogy for erupting volcanoes has endured the test of time. Other methods of simulating an eruption by mixing two ingredients can be found online—for example, Mentos® candy and diet cola or baking soda and vinegar—but carbonated soda is a better analogy because only the release of the confining pressure is needed for an “eruption.”

On active volcanoes, as magma rises and pressure drops, dissolved gases come out of solution to effervesce, expand, and drive lava out of the ground in fountains and flows, just as soda effervesces and sometimes foams out of an opened bottle. Both eruptions can be delightful, but both can also pose some danger.