ASPHALT VOLCANISM
Gerhard Bohrmann
MARUM-Center for Marine Environmental Sciences,
University of Bremen, Bremen, Germany
Definition
Asphalt volcanism is a type of hydrocarbon seepage associated with submarine mounds found in the Gulf of Mexico and the Santa Barbara Basin. The term asphalt
volcanism was introduced because of the lavalike appearance of the asphalt flows and several other indications,
which resemble magmatic eruptions.
Asphalt seepage versus asphalt volcanism
Although asphalt deposits have been described from several places of the ocean floor, the term asphalt volcanism
has been introduced as a novel type of hydrocarbon seepage by MacDonald et al. (2004) after an area of approximately 1 km
2 solidified asphalt was found on top of one
of the Campeche Knolls in the southern Gulf of Mexico.
The knoll was subsequently named Chapopote which is
the Aztec word for tar. Those knolls in the Gulf of Mexico
are clearly associated with salt tectonism which controls
the development of hydrocarbon reservoirs and faults that
allow oil and gas to escape at the seafloor. Guided by satellite data that showed evidence of persistent oil seepage
in the region, the seafloor of Chapopote was mapped and
investigated by MacDonald et al. (2004). Visual surveys
revealed extensive surface deposits of solidified asphalt
and light crude oil, emanating from sites along the southern rim of a crater-like structure in 3.000 m water depth.
Some of the asphalt flows were measured to be at least
15 m across and extended far down the slope. In some
places the surface appearance of the asphalt deposits was
blocky or ropy (Figure 1) similar to pa’hoehoe lava flows
of basalt. Furthermore, large areas of the asphalt deposits
were colonized by vestimentiferan tubeworms, bacterial
mats, and other biological communities. Also discovered
alongside the asphalt were locations of sediment/gas
hydrate interlayering associated with emanating gas and
oil bubbles from the seafloor.
Based on the collected data and observations, MacDonald et al. (2004) postulated repeated, extensive eruptions
of molten asphalt under conditions which could destabilize gas hydrates on the seafloor. A violent destabilization
of hydrates could contribute to slope failures and mass
wasting mapped on Chapopote Knoll, as well as
documented on other Campeche Knolls. Based on the idea
that the asphalt on Chapopote was molten during extrusion, Hovland et al. (2005) argued for a model that relies
on supercritical water being transported vertically upward
through a suspected internal conduit within the salt diapir,
from near the base of the sedimentary column at perhaps
13 km depth. Organic material including bitumen should
have been transported upward together with
“hydrothermal-like” components as a hot substance. At
the summit of the Chapopote structure, a hot slurry flowed
out onto the seafloor where bitumen and asphalt
devolatilized rapidly, eventually building up the asphalt
volcano’s structure.
The Chapopote Knoll was surveyed in greater detail by
Brüning et al. (2010) using the MARUM ROV QUEST.
The results support the concept that the asphalt deposits
on Chapopote originate from seepage of heavy oil with a
density slightly greater than water, which leads to
remaining petroleum and oil residues on the seafloor. During extrusion of the heavy petroleum, the viscosity
increased due to the loss of volatiles, and the heavy petroleum forms the lavalike flow structures along the distance
where continuous solidification occurs. The investigations
Asphalt Volcanism, Figure 1 Seafloor images from asphalt flows of Chapopote Knoll/Southern Gulf of Mexico taken by MARUM ROV
QUEST. (a) Vestimentiferan tubeworms and galatheid crabs colonizing the surface of the asphalt. (b) Several asphalt layers of ropy
surface piled up above the seafloor.
24
ASPHALT VOLCANISM
Gerhard Bohrmann
MARUM-Center for Marine Environmental Sciences,
University of Bremen, Bremen, Germany
Definition
Asphalt volcanism is a type of hydrocarbon seepage associated with submarine mounds found in the Gulf of Mexico and the Santa Barbara Basin. The term asphalt
volcanism was introduced because of the lavalike appearance of the asphalt flows and several other indications,
which resemble magmatic eruptions.
Asphalt seepage versus asphalt volcanism
Although asphalt deposits have been described from several places of the ocean floor, the term asphalt volcanism
has been introduced as a novel type of hydrocarbon seepage by MacDonald et al. (2004) after an area of approximately 1 km
2 solidified asphalt was found on top of one
of the Campeche Knolls in the southern Gulf of Mexico.
The knoll was subsequently named Chapopote which is
the Aztec word for tar. Those knolls in the Gulf of Mexico
are clearly associated with salt tectonism which controls
the development of hydrocarbon reservoirs and faults that
allow oil and gas to escape at the seafloor. Guided by satellite data that showed evidence of persistent oil seepage
in the region, the seafloor of Chapopote was mapped and
investigated by MacDonald et al. (2004). Visual surveys
revealed extensive surface deposits of solidified asphalt
and light crude oil, emanating from sites along the southern rim of a crater-like structure in 3.000 m water depth.
Some of the asphalt flows were measured to be at least
15 m across and extended far down the slope. In some
places the surface appearance of the asphalt deposits was
blocky or ropy (Figure 1) similar to pa’hoehoe lava flows
of basalt. Furthermore, large areas of the asphalt deposits
were colonized by vestimentiferan tubeworms, bacterial
mats, and other biological communities. Also discovered
alongside the asphalt were locations of sediment/gas
hydrate interlayering associated with emanating gas and
oil bubbles from the seafloor.
Based on the collected data and observations, MacDonald et al. (2004) postulated repeated, extensive eruptions
of molten asphalt under conditions which could destabilize gas hydrates on the seafloor. A violent destabilization
of hydrates could contribute to slope failures and mass
wasting mapped on Chapopote Knoll, as well as
documented on other Campeche Knolls. Based on the idea
that the asphalt on Chapopote was molten during extrusion, Hovland et al. (2005) argued for a model that relies
on supercritical water being transported vertically upward
through a suspected internal conduit within the salt diapir,
from near the base of the sedimentary column at perhaps
13 km depth. Organic material including bitumen should
have been transported upward together with
“hydrothermal-like” components as a hot substance. At
the summit of the Chapopote structure, a hot slurry flowed
out onto the seafloor where bitumen and asphalt
devolatilized rapidly, eventually building up the asphalt
volcano’s structure.
The Chapopote Knoll was surveyed in greater detail by
Brüning et al. (2010) using the MARUM ROV QUEST.
The results support the concept that the asphalt deposits
on Chapopote originate from seepage of heavy oil with a
density slightly greater than water, which leads to
remaining petroleum and oil residues on the seafloor. During extrusion of the heavy petroleum, the viscosity
increased due to the loss of volatiles, and the heavy petroleum forms the lavalike flow structures along the distance
where continuous solidification occurs. The investigations
Asphalt Volcanism, Figure 1 Seafloor images from asphalt flows of Chapopote Knoll/Southern Gulf of Mexico taken by MARUM ROV
QUEST. (a) Vestimentiferan tubeworms and galatheid crabs colonizing the surface of the asphalt. (b) Several asphalt layers of ropy
surface piled up above the seafloor.
24
ASPHALT VOLCANISM