While volcanoes are common under the sea, nobody dreamed that in some places, vents erupt not lava but asphalt. That's what a 2003 research expedition found in the Gulf of Mexico on a seafloor hill the scientists named Chapopote, the Mexican Spanish name for tar. It was the world's first known asphalt volcano. There are more being found all the time.
The geologic setting at the site, west of the Yucatán in 3000 meters of water, is a field of salt domes called the Campeche Knolls. These tall, steep hills grow as ductile salt bodies rise into the overlying seafloor rocks; as is common around the Gulf, oil and gas leak upward with the salt.
The Asphalt Volcano Locality
The researchers, a team led by Ian MacDonald of Texas A&M University's Corpus Christi campus, dangled a remote-controlled camera off the German ship RV Sonne to the seafloor far below. Even with this short-range visual instrument they documented one square kilometer of tar flows, some of them 20 meters across.
Besides asphalt, the expedition found places soaked with petroleum and others with cold, white layers of methane hydrate. Like cold seeps elsewhere on the world's seafloor, all of these localities supported colonies of chemical-eating organisms. Bunches of tubeworms were found growing in and around the tar flows. Apparently something makes the asphalt attractive to life, but no one is sure yet how the biogeochemistry works.
At Chapopote the tar seems to have come out of the ground hot, but like undersea lava flows, it quickly hardens in the cold seawater. In fact it forms asphalt "aa" and "pahoehoe" just like what you find in Hawaiian basalt. In another parallel with ordinary volcanoes, the warm asphalt turns delicate icy layers of methane hydrate into bursts of free gas, just as hot rock lava causes explosions by flashing groundwater into steam — phreatomagmatic eruptions. (But I don't know what you'd call a tar/hydrate eruption in scientific Latin.)
A Supercritical Hypothesis
In 2005 the team reported more details, and a provocative theory. Examining samples from the tar flows, the researchers found abundant small pores lined with various minerals: sulfates, chlorides and carbonates. They theorized, in the 18 October 2005 Eos, that the energy source involves a special substance: supercritical water.
Supercritical water is water at such high pressures and temperatures (300 times atmospheric pressure and 400 degrees C) that it is above the "critical point," neither a gas nor a liquid but a searing combination. It is about one-third the density of liquid water and is a nonpolar fluid capable of dissolving hydrocarbons, unlike surface water with which oil cannot mix.
Such fluid could form deep in the crust, insulated under the seafloor sediments, just as it does beneath black smokers. If a suitable passage connected it to the surface — and a salt dome is a perfect example — then this sort of water magma could rise bearing a heavy load of dissolved minerals and hydrocarbons. As the water cools and the dissolved load precipitates, a shell of tar would form protecting the hot fluid inside, analogous to lava tubes, and the fluid would eventually reach the sea floor. There the more volatile parts of the "lava" would enter the seawater while the heavy asphalt remains.
Supercritical water could exist at depths as shallow as 2800 meters. Because the flows of Chapopote are deeper than that, the theory checks out so far. This is not the first time supercritical fluids have been suspected in the Earth: they are theorized in large mud volcanoes and in catastrophic landslides.
Are There More Asphalt Volcanoes?
Surely there's a lot more asphalt in the Campeche Knolls and elsewhere. In fact MacDonald, in the 14 May 2004 Science, pointed out that tar flows had been photographed 200 km to the north of Chapopote in 1971. He suggested that others might locate more occurrences by doing what his team did: looking for oil slicks in satellite images of the sea surface.
In 2007, researchers first reported extinct asphalt volcanoes in the waters off Santa Barbara, California that stand 30 meters high above the seafloor (Nature published their paper in 2010). And in 2014 "tar flowers" were documented south of Galveston, Texas in the northern Gulf of Mexico. Each new deep-sea oasis we find—joining black smokers, carbonate smokers, cold seeps and whale falls — makes the ocean floor a still more lively place.
PS: I like what MacDonald told a university journalist: "Chapopote is more evidence that this planet is alive from top to bottom. There is no place on Earth where life is absent; wherever life gets the slightest foothold, it will adapt and blossom."