Asphalt volcano
Encyclopedia
An asphalt volcano is a rare type of submarine volcano
(seamount
) first discovered in 2003. Several examples have been found: first, along the coasts of America and Mexico
, and, recently, all over the world; a few are still active. Resembling seamounts in structure, they are made entirely of asphalt
, and form when natural oil
seeps up from the Earth's crust
underwater.
during an expedition of the research vessel SONNE, led by Gerhard Bohrmann of the DFG Research Center Ocean Margins. On these volcanoes a previously unknown highly diverse ecosystem at a water depth of 3,000 meters was discovered. The discovery was reported in Science
.
The first asphalt volcanoes were discovered in 2003 by a research expedition to the Gulf of Mexico
. They are located on a seafloor hill named "Chapopote," Nahuatl
for "tar." The site is located in a field of salt dome
s known as the Campeche Knolls, a series of steep hills formed from salt bodies that rise from underlying rock, a common feature in the gulf. The research team documented tar flows as wide as 20 m (66 ft) across. Also discovered alongside the asphalt were areas soaked with petroleum
and methane hydrate, also spewed from the volcano. This kind of an environment proves attractive to chemical-loving bacteria
and tubeworms, although the exact biogeochemical relationship is not yet known.
The tar is relatively hot when it comes out of the seafloor, but just like undersea lava
flows, it is quickly cooled by the much colder seawater around it. This produces forms similar to the distinctive A'a and pahoehoe types of basalt
lava flow seen in places like Hawaii
. Another similarity is that the tar heats methane hydrate and causes it to explode into a free gas, similar to the action hot lava has on groundwater
in phreatomagmatic eruption
s.
The team proposed an asphalt volcano formation theory in a paper published in Eos
. The article suggested that water heated past the critical point
underneath the seafloor found a passageway to the surface, most likely a salt dome
, and carried with it a heavy load of hydrocarbon
s and dissolved minerals. A special property of such critically heated water is that it can mix with oils, whereas normal water cannot. The same process is attributed to the formation of black smokers. Once the water reaches the surface, it cools, and its carrying capacity drops. The lighter compounds in the mixture escape to the surface, while the tar and other heavier materials remain on the seafloor, eventually building up the asphalt volcano's structure.
In 2007, seven more such structures were discovered off the coast of Santa Barbara, California
. The largest of these domes lies at a depth of 700 ft (213 m). The structures were larger than a football field and about as tall as a six-story building, all made completely out of asphalt. The unusual features were first noted by Ed Keller on bathymetric surveys
conducted in the 1990s, and first viewed by a team led by David Valentine in 2007, utilizing DSV Alvin
. Samples were brought up for testing at the university campus and the Woods Hole Oceanographic Institution
.
Two further dives with DSV Alvin in 2009 and a detailed photographic survey of the area by the autonomous underwater vehicle Sentry showed many similarities to volcanic flows, including flow texture and cracking of the asphalt layers. Carbon dating puts the structures at between 30 and 40 thousand years old. They had at one time been a prolific source of methane
. The two largest structures, less than 1 km (0.621372736649807 mi) apart, are pocked by pits and depressions, a sign of methane gas bubbling up long ago. Although the structures are still emitting residual gas, at present the amounts are too small to have any effect. The amount of crude oil in the largest of the structures alone is "enough to fuel my Honda Civic for about half a billion miles. [However] the quality of the material is very poor...It's not worth something like light sweet crude," said Valentine. The petroleum in the structure is more viscous than that which is usually found in underground wells. This is because it has had less time to "bake" under the Earth's heat before being released. In addition, as much as 20% of its mass is made of "junk"—microscopic organisms, sand, and miscellaneous materials that gradually accumulated in the oil.
Analysis of the samples collected from the mounds suggest that they required several decades, even centuries, to build up their current bulk, and that the volcanoes last erupted around 35,000 years ago. In addition they may account for a mysterious spike in oceanic methane
concentrations around 35,000 years ago. Methane forms naturally alongside the petroleum underneath the structure, and while petroleum flows have long abated, some residual methane continues to bubble up. This burst of methane would have caused a rapid increase in the population of methane-eating bacteria, which in turn caused a decrease in oxygen in the water, possibly causing a dead zone
, in addition to the large amounts of crude oil released into the environment.
Ecologically speaking, the presence of these structures provides a hard surface on which life can grow, as the surrounding ocean floor is generally muddy. This is similar to what happens on seamount
s, resulting in their place as an ecological "hub."
Submarine volcano
Submarine volcanoes are underwater fissures in the Earth's surface from which magma can erupt. They are estimated to account for 75% of annual magma output. The vast majority are located near areas of tectonic plate movement, known as ocean ridges...
(seamount
Seamount
A seamount is a mountain rising from the ocean seafloor that does not reach to the water's surface , and thus is not an island. These are typically formed from extinct volcanoes, that rise abruptly and are usually found rising from a seafloor of depth. They are defined by oceanographers as...
) first discovered in 2003. Several examples have been found: first, along the coasts of America and Mexico
Mexico
The United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of...
, and, recently, all over the world; a few are still active. Resembling seamounts in structure, they are made entirely of asphalt
Asphalt
Asphalt or , also known as bitumen, is a sticky, black and highly viscous liquid or semi-solid that is present in most crude petroleums and in some natural deposits, it is a substance classed as a pitch...
, and form when natural oil
Petroleum
Petroleum or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. Petroleum is recovered mostly through oil drilling...
seeps up from the Earth's crust
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...
underwater.
Formation and distribution
Asphalt volcanoes are ocean floor vents that erupt asphalt instead of lava. They were discovered in the Gulf of MexicoGulf of Mexico
The Gulf of Mexico is a partially landlocked ocean basin largely surrounded by the North American continent and the island of Cuba. It is bounded on the northeast, north and northwest by the Gulf Coast of the United States, on the southwest and south by Mexico, and on the southeast by Cuba. In...
during an expedition of the research vessel SONNE, led by Gerhard Bohrmann of the DFG Research Center Ocean Margins. On these volcanoes a previously unknown highly diverse ecosystem at a water depth of 3,000 meters was discovered. The discovery was reported in Science
Science (journal)
Science is the academic journal of the American Association for the Advancement of Science and is one of the world's top scientific journals....
.
The first asphalt volcanoes were discovered in 2003 by a research expedition to the Gulf of Mexico
Gulf of Mexico
The Gulf of Mexico is a partially landlocked ocean basin largely surrounded by the North American continent and the island of Cuba. It is bounded on the northeast, north and northwest by the Gulf Coast of the United States, on the southwest and south by Mexico, and on the southeast by Cuba. In...
. They are located on a seafloor hill named "Chapopote," Nahuatl
Nahuatl
Nahuatl is thought to mean "a good, clear sound" This language name has several spellings, among them náhuatl , Naoatl, Nauatl, Nahuatl, Nawatl. In a back formation from the name of the language, the ethnic group of Nahuatl speakers are called Nahua...
for "tar." The site is located in a field of salt dome
Salt dome
A salt dome is a type of structural dome formed when a thick bed of evaporite minerals found at depth intrudes vertically into surrounding rock strata, forming a diapir....
s known as the Campeche Knolls, a series of steep hills formed from salt bodies that rise from underlying rock, a common feature in the gulf. The research team documented tar flows as wide as 20 m (66 ft) across. Also discovered alongside the asphalt were areas soaked with petroleum
Petroleum
Petroleum or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. Petroleum is recovered mostly through oil drilling...
and methane hydrate, also spewed from the volcano. This kind of an environment proves attractive to chemical-loving bacteria
Extremophile
An extremophile is an organism that thrives in physically or geochemically extreme conditions that are detrimental to most life on Earth. In contrast, organisms that live in more moderate environments may be termed mesophiles or neutrophiles...
and tubeworms, although the exact biogeochemical relationship is not yet known.
The tar is relatively hot when it comes out of the seafloor, but just like undersea lava
Lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...
flows, it is quickly cooled by the much colder seawater around it. This produces forms similar to the distinctive A'a and pahoehoe types of basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
lava flow seen in places like Hawaii
Hawaii hotspot
The Hawaii hotspot is the volcanic hotspot that created the Hawaiian Islands in the central Pacific Ocean, and is one of Earth's best-known and most heavily-studied hotspots....
. Another similarity is that the tar heats methane hydrate and causes it to explode into a free gas, similar to the action hot lava has on groundwater
Groundwater
Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...
in phreatomagmatic eruption
Phreatomagmatic eruption
Phreatomagmatic eruptions are defined as juvenile forming eruptions as a result of interaction between water and magma. They are different from magmatic and phreatic eruptions. The products of phreatomagmatic eruptions contain juvenile clasts, unlike phreatic eruptions, and are the result of...
s.
The team proposed an asphalt volcano formation theory in a paper published in Eos
Eos (journal)
Eos, Transactions, American Geophysical Union, a publication of the American Geophysical Union, is a weekly newspaper of geophysics that carries refereed articles on current research and on the relationship of geophysics to social and political questions, news, book reviews, AGU journal and meeting...
. The article suggested that water heated past the critical point
Critical point (thermodynamics)
In physical chemistry, thermodynamics, chemistry and condensed matter physics, a critical point, also called a critical state, specifies the conditions at which a phase boundary ceases to exist...
underneath the seafloor found a passageway to the surface, most likely a salt dome
Salt dome
A salt dome is a type of structural dome formed when a thick bed of evaporite minerals found at depth intrudes vertically into surrounding rock strata, forming a diapir....
, and carried with it a heavy load of hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
s and dissolved minerals. A special property of such critically heated water is that it can mix with oils, whereas normal water cannot. The same process is attributed to the formation of black smokers. Once the water reaches the surface, it cools, and its carrying capacity drops. The lighter compounds in the mixture escape to the surface, while the tar and other heavier materials remain on the seafloor, eventually building up the asphalt volcano's structure.
In 2007, seven more such structures were discovered off the coast of Santa Barbara, California
Santa Barbara, California
Santa Barbara is the county seat of Santa Barbara County, California, United States. Situated on an east-west trending section of coastline, the longest such section on the West Coast of the United States, the city lies between the steeply-rising Santa Ynez Mountains and the Pacific Ocean...
. The largest of these domes lies at a depth of 700 ft (213 m). The structures were larger than a football field and about as tall as a six-story building, all made completely out of asphalt. The unusual features were first noted by Ed Keller on bathymetric surveys
Bathymetry
Bathymetry is the study of underwater depth of lake or ocean floors. In other words, bathymetry is the underwater equivalent to hypsometry. The name comes from Greek βαθύς , "deep", and μέτρον , "measure"...
conducted in the 1990s, and first viewed by a team led by David Valentine in 2007, utilizing DSV Alvin
DSV Alvin
Alvin is a manned deep-ocean research submersible owned by the United States Navy and operated by the Woods Hole Oceanographic Institution in Woods Hole, Massachusetts. The vehicle was built by General Mills' Electronics Group in the same factory used to manufacture breakfast cereal-producing...
. Samples were brought up for testing at the university campus and the Woods Hole Oceanographic Institution
Woods Hole Oceanographic Institution
The Woods Hole Oceanographic Institution is a private, nonprofit research and higher education facility dedicated to the study of all aspects of marine science and engineering and to the education of marine researchers. Established in 1930, it is the largest independent oceanographic research...
.
Two further dives with DSV Alvin in 2009 and a detailed photographic survey of the area by the autonomous underwater vehicle Sentry showed many similarities to volcanic flows, including flow texture and cracking of the asphalt layers. Carbon dating puts the structures at between 30 and 40 thousand years old. They had at one time been a prolific source of methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
. The two largest structures, less than 1 km (0.621372736649807 mi) apart, are pocked by pits and depressions, a sign of methane gas bubbling up long ago. Although the structures are still emitting residual gas, at present the amounts are too small to have any effect. The amount of crude oil in the largest of the structures alone is "enough to fuel my Honda Civic for about half a billion miles. [However] the quality of the material is very poor...It's not worth something like light sweet crude," said Valentine. The petroleum in the structure is more viscous than that which is usually found in underground wells. This is because it has had less time to "bake" under the Earth's heat before being released. In addition, as much as 20% of its mass is made of "junk"—microscopic organisms, sand, and miscellaneous materials that gradually accumulated in the oil.
Analysis of the samples collected from the mounds suggest that they required several decades, even centuries, to build up their current bulk, and that the volcanoes last erupted around 35,000 years ago. In addition they may account for a mysterious spike in oceanic methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
concentrations around 35,000 years ago. Methane forms naturally alongside the petroleum underneath the structure, and while petroleum flows have long abated, some residual methane continues to bubble up. This burst of methane would have caused a rapid increase in the population of methane-eating bacteria, which in turn caused a decrease in oxygen in the water, possibly causing a dead zone
Dead zone (ecology)
Dead zones are hypoxic areas in the world's oceans, the observed incidences of which have been increasing since oceanographers began noting them in the 1970s. These occur near inhabited coastlines, where aquatic life is most concentrated...
, in addition to the large amounts of crude oil released into the environment.
Ecologically speaking, the presence of these structures provides a hard surface on which life can grow, as the surrounding ocean floor is generally muddy. This is similar to what happens on seamount
Seamount
A seamount is a mountain rising from the ocean seafloor that does not reach to the water's surface , and thus is not an island. These are typically formed from extinct volcanoes, that rise abruptly and are usually found rising from a seafloor of depth. They are defined by oceanographers as...
s, resulting in their place as an ecological "hub."