Encyclopedia of Marine Geosciences

Living Edition
| Editors: Jan Harff, Martin Meschede, Sven Petersen, Jörn Thiede

Asphalt Volcanism

  • Gerhard BohrmannEmail author
Living reference work entry
DOI: https://doi.org/10.1007/978-94-007-6644-0_1-1

Keywords

Cold Seep Salt Diapir Heavy Petroleum Salt Tectonism Asphalt Deposit 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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 km2 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 (Fig. 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.
Fig. 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

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 of Brüning et al. (2010) documented that the asphalt is subject to sequential alteration. While fresh asphalt was gooey, older asphalt appeared fragmented and brittle. Highly altered asphalt was often colonized by further chemosynthetic fauna like mytilid clams and others. The change in the consistency of the asphalts goes along with a change in the geochemical composition and microbial signatures (Schubotz et al., 2011). Besides the unusual asphalt formation, the putative “volcanic structure” is representing a very interesting seepage area which extended our knowledge about the broad spectrum of seafloor venting phenomena.

Beside the Gulf of Mexico, asphalt volcanoes are known from the Santa Barbara Basin, California, in much shallower water depths close to the coast. Seven of those morphological structures were described as extinct asphalt volcanoes by Valentine et al. (2010). Radiocarbon dating of carbonate layers intercalated with the asphalt deposits indicated formation of two of the volcanoes between 44 and 31 kyr ago. Based on quantitative assumptions and the geochemistry of samples taken from the volcanoes, the authors estimated the amount of oil and accompanied methane gas, which are emitted at the sites where the residues of the hydrocarbon seepage (i.e., the asphalt) currently are deposited. Since the amount of greenhouse gas (in this case methane) emissions is not known during former times, the study is of great value to reveal estimates of former seepage rates.

Cross-References

Bibliography

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Copyright information

© Springer Science+Business Media Dordrecht 2014

Authors and Affiliations

  1. 1.Fachbereich Geowissenschaften, Center for Marine Environmental SciencesUniversity of Bremen, MARUMBremenGermany