On the outside, our planet seems pretty calm. Aside from the extreme weather that whips things up, the actual surface of the Earth is relatively stable. But underneath that serene demeanor, Earth is quite lively; and whenever a volcano erupts, we get a peek at the furious energy harbored within.

The planet's solid rock crust accounts for only 1 percent of the Earth's mass, below that is a roiling body of hot rocks that exists, on average, 25 miles beneath our feet at temperatures ranging from 2,000 to 10,000 degrees Fahrenheit. Along the edges of tectonic plates, volcanoes form, allowing what's inside this orb to escape. More than 80 percent of the Earth’s surface, above and below sea level, is volcanic in origin.

When we think of volcanoes, we generally think of cone-shaped mountains spewing ash and hot glowing lava; a scene that is at once as beautiful as it is terrifying. There’s something poetic about the Earth slowly turning itself inside out – and running streams of vibrant molten rock are a striking sight.

But not all volcanoes are as majestic at first glance. Case in point: The asphalt volcano.

Beneath the ocean on the seafloor, asphalt volcanoes ooze gasses and thick, gooey oil into the water. 

The asphalt of these volcanoes is produced by the same processes that generate oil and gas, explains the National Oceanic and Atmospheric Administration (NOAA), but it goes through a geological transformation. Some oil in undersea reservoirs is so old that is has been “cooking” for millions of years, resulting in evaporated volatile compounds and a thick sticky remnant. When oil erupts from a submarine volcano, it mixes with sediments and cools as it hits the water, resulting in huge ropey masses and mounds.

The first discovery of asphalt volcanoes was in 2004 when scientists on the German research vessel F/V Sonne reported asphalt volcanoes 10,000 feet deep in the southern Gulf of Mexico. Since then, further reports have confirmed more asphalt volcanoes off the coast of California and West Africa. And more recently, NOAA explorers investigating a suspected shipwreck off the coast of Galveston found not sunken treasure, but more asphalt volcanoes, expanding the number of known sites and confirming the presence of an asphalt ecosystem across the Gulf. (Don't miss this incredible footage of the Galveston asphalt volcano.)

Yet despite the name, which brings to mind the strong stench of bubbling tar and roads being paved, the formations are quite beautiful in their own way. They evoke lava flows and flowers, and play home to incredible displays of marine life.

Asphalt volcano

Photo: MARUM - Center for Marine Environmental Sciences

The asphalt volcanoes found in 2007 near Santa Barbara off the coast of California are in mound form; the largest, affectionately named Il Duomo, is the size of two football fields and the height of a six-story building. The scientists from Woods Hole Oceanographic Institution who explored the sight explained their theory on the formation:

“What we think happened here is that as oils worked their way up and became exposed at the seafloor, little organisms and pieces of sediment began accumulating in the oils and making them heavier. Also some of their lighter, more gasoline-like compounds quickly dissolved into the ocean or wafted away, leaving behind the heavier compounds. The material became heavier than the seawater and began to settle back down to the seafloor. At that point, it began to flow downslope in a way that looks very much like a Hawaiian lava flow.”
The scientists describe its appearance as something fluid, frozen in place, and covered in an array of sea life.

Meanwhile, the asphalt volcanoes found off Galveston have been given a much more euphonic moniker: “tar lilies.” And indeed, the Galveston formations shoot up from the sea floor like palm fronds and petals. Here, a massive plug was forced out at the seafloor, and the tar flowed in separate extensions which became brittle and cracked apart. Once the “petals” of the giant lilies were in place, vibrant and graceful species of marine life – from corals and barnacles to anemones and fish – moved in and made it home.

While the NOAA science team was able to give the newly discovered formations a relatively young age (tens to hundreds of years old), the volcano appears to be dormant for now. But the size of the extrusions suggests that there may be more asphalt below that may get squeezed out in the future, continuing the planet’s profound cycle of turning itself inside-out. In this case, Mother Nature paving the seafloor, over and over again.

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