Encyclopedia of Marine Geosciences

1529 Pages·2015·82.143 MB·English

by  Jan Harff, Martin Meschede, Sven Petersen, Jörn Thiede (eds.)

EncyclopediaofMarineGeosciences
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Asphalt Volcanism
GerhardBohrmann*
FachbereichGeowissenschaften,CenterforMarineEnvironmentalSciences,UniversityofBremen,MARUM,Bremen,
Germany
Definition
Asphaltvolcanismisatypeofhydrocarbonseepageassociatedwithsubmarinemoundsfoundinthe
GulfofMexicoandtheSantaBarbaraBasin.Thetermasphaltvolcanismwasintroducedbecauseof
thelavalikeappearanceoftheasphaltflowsandseveralotherindications,whichresemblemagmatic
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 wordfor tar. Those knolls in the Gulf of Mexico areclearlyassociated with salt
tectonism which controls the development of hydrocarbon reservoirs and faults that allow oil and
gastoescapeattheseafloor.Guidedbysatellitedatathatshowedevidenceofpersistentoilseepage
intheregion, theseafloorofChapopote was mapped andinvestigated byMacDonald etal.(2004).
Visualsurveysrevealedextensivesurfacedepositsofsolidifiedasphaltandlightcrudeoil,emanat-
ing from sites along the southern rimof acrater-like structure in 3.000m water depth. Some ofthe
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 emanat-
ing gas and oil bubbles from the seafloor.
Based on the collected data and observations, MacDonald et al. (2004) postulated repeated,
extensiveeruptionsofmoltenasphaltunderconditionswhichcoulddestabilizegashydratesonthe
seafloor. Aviolent destabilization of hydrates could contribute to slope failures and mass wasting
mappedonChapopoteKnoll,aswellasdocumentedonotherCampecheKnolls.Basedontheidea
thattheasphaltonChapopotewasmoltenduringextrusion,Hovlandetal.(2005)arguedforamodel
that relies on supercritical water being transported vertically upward through a suspected internal
conduitwithinthesaltdiapir,fromnearthebaseofthesedimentarycolumnatperhaps13kmdepth.
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.
*Email:[email protected]
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Fig. 1 Seafloor images from asphalt flows of Chapopote Knoll/Southern Gulf of Mexico taken by MARUM ROV
QUEST.(a)Vestimentiferantubewormsandgalatheidcrabscolonizingthesurfaceoftheasphalt.(b)Severalasphalt
layersofropysurfacepiledupabovetheseafloor
TheChapopoteKnollwassurveyedingreaterdetailbyBr€uningetal.(2010)usingtheMARUM
ROVQUEST.TheresultssupporttheconceptthattheasphaltdepositsonChapopoteoriginatefrom
seepageofheavyoilwithadensityslightlygreaterthanwater,whichleadstoremainingpetroleum
andoilresiduesontheseafloor.Duringextrusionoftheheavypetroleum,theviscosityincreaseddue
tothelossofvolatiles,andtheheavypetroleumformsthelavalikeflowstructuresalongthedistance
wherecontinuoussolidificationoccurs.TheinvestigationsofBr€uningetal.(2010)documentedthat
theasphaltissubjecttosequentialalteration.Whilefreshasphaltwasgooey,olderasphaltappeared
fragmentedandbrittle.Highlyalteredasphaltwasoftencolonizedbyfurtherchemosyntheticfauna
likemytilidclamsandothers.Thechangeintheconsistencyoftheasphaltsgoesalongwithachange
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, Califor-
nia,inmuchshallowerwaterdepthsclosetothecoast.Sevenofthosemorphologicalstructureswere
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
44and31kyrago.Basedonquantitativeassumptionsandthegeochemistryofsamplestakenfrom
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.Sincetheamountofgreenhousegas(inthiscasemethane)emissionsisnotknownduring
former times, the study is of great value to reveal estimates of former seepage rates.
Cross-References
▶Cold Seeps
▶Gas hydrates
▶Mud volcano
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Bibliography
Br€uning,M.,Sahling,H.,MacDonald,I.R.,Ding,F.,andBohrmann,G.,2010.Origin,distribution,
and alteration of asphalts at the Chapopote Knoll, Southern Gulf of Mexico. Marine and
Petroleum Geology, 27(5), 1093–1106, doi:10.1016/j.marpetgeo.2009.09.005.
Hovland,M.,MacDonald,I.R.,Rueslatten,H.,Johnsen,H.K.,Naehr,T.,andBohrmann,G.,2005.
Chapopote asphalt volcano may have been generated by supercritical water. EOS, Transactions,
86(42), 397–402, doi:10.1029/2005EO420002.
MacDonald,I.R.,Bohrmann,G.,Escobar,E.,Abegg,F.,Blanchon,P.,Blinova,V.N.,Brueckmann,
W., Drews, M., Eisenhauer, A., Han, X., Heeschen, K. U., Meier, F., Mortera, C., Naehr, T.,
Orcutt, B., Bernard, B., Brooks, J., and de Farágo, M., 2004. Asphalt volcanism and chemosyn-
thetic life, Campeche Knolls, Gulf of Mexico. Science, 304(5673), 999–1002, doi:10.1126/
science.1097154.
Schubotz, F., Lipp, J. S., Elvert, M., Kasten, S., Mollar, X. P., Zabel, M., Bohrmann, G., and
Hinrichs, K. U., 2011. Geochimica et Cosmochimica Acta, 75(16), 4377–4398, doi:10.1016/j.
gca.2011.05.025.
Valentine, D. L., Reddy, C. M., Farwell, C., Hill, T. M., Pizzarro, O., Yoerger, D. R., Camilli, R.,
Nelson,R.K.,Peacock,E.E.,Bagby,S.C.,Clarke,B.A.,Roman,C.N.,andSoloway,M.,2010.
Asphalt volcanoes as a potential source of methane to late Pleistocene coastal waters. Nature
Geosciences, 3, 345–348, doi:10.1038/NGEO848.
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Axial Volcanic Ridges
IsobelYeo*
GEOMARHelmholtzInstituteforOceanResearchKiel,Kiel,Germany
Synonyms
Neo-volcanic ridge
Definition
Axial volcanic ridges (AVRs). Composite volcanic edifices, comprising an elongate, typically
spreading-normal orientated topographic high, produced within the inner valleys of mid-ocean
ridges, usually those that are slow spreading.
Introduction
The surface expression of volcanic activity is extremely variable at different spreading rates (see
“▶SpreadingRatesandRidgeMorphology”),asaresultofthedifferingeruptionstylesassociated
with each. Axial volcanic ridges (AVRs), sometimes called neo-volcanic ridges, have been recog-
nized at many slow-spreading Mid-Atlantic Ridge (MAR) spreading segments (e.g., Ballard and
Van Andel, 1977; Karson et al., 1987; Smith and Cann, 1992; Parson et al., 1993; Sempere et al.,
1993; Lawsonet al., 1996; Bideau etal., 1998; Gracia et al., 1998; Navin et al., 1998; Briais et al.,
2000;PeirceandSinha,2008;Searleetal.,2010)andsomeultraslow-spreadingsegmentselsewhere
(Mendel et al., 2003). AVRs are elongate, composite volcanoes, typically with a ridge parallel
orientation (Fig. 1). They vary in size, although are typically a few kilometers wide, are tens of
kilometers long, and reach heights of several hundreds of meters above the surrounding seafloor.
AVRs are usually found in the middle of spreading segments and are often associated with
hourglass-shaped axial valleys. They may extend all the way from the center of the valley to the
base of the bounding axial valley wall faults or be surrounded by areas of flatter seafloor. Where
present, an AVR usually represents the largest volume magmatic structure on the segment.
AVR Eruption Style and Volcanic Architecture
AVRs themselves are built almost entirely of agglomerations of volcanic hummocks (Smith and
Cann, 1990; Yeo et al., 2012), which are circular or subcircular, probably monogenetic, volcanic
cones, or domes 50–500 m in diameter with heights of tens to hundreds of meters (Fig. 1 inset).
Hummocks are constructed of a combination of pillow, elongate pillow, and lobate lavas (see
“▶Lava Types”), which are erupted from a central vent and flow outwards and down the sides of
the hummock. These hummocky structures are responsible for the rough, lumpy surface texture of
*Email:[email protected]
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Fig. 1 EM120 bathymetry gridded at 50 m showing the axial volcanic ridge at 45(cid:1)N on the Mid-Atlantic Ridge as
surveyed by Searle et al. (2010). The prominent 22-km-long AVR is shown by the gray-shaded area and lies almost
paralleltotheridgestrikeinthecenterofthehourglass-shapedinnervalley.Thespreadingdirectionisshownbythe
white arrows. The northern end (north of 45(cid:1)33) appears more tectonized than the southern end. The rough surface
textureisaresultofthehummocksthatcoveritssurface.Inset:TOBIside-scanmosaicofthehummockyterraininthe
areacoveredbythedashedboxlabeledSSonthemainfigure(Dataisinsonifiedtothenorth)
AVRs in multibeam data (Fig. 1). Several hummocks may be produced in one eruption, often, but
not always, distributed along an eruptive fissure forming a hummocky lineament on the seafloor
(Searleetal.,2010).Thefeederdykesfortheseeruptionsareunlikelytobeactiveformorethanone
eruptionasthetypicalmid-oceanridgedykethicknessoflessthan2m(QinandBuck,2008)issuch
that it will solidify before the next predicted dyke emplacement (Head et al., 1996). Such predom-
inantly ridge parallel fissure eruptions are thought to be the dominant eruption style on AVRs. The
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individualhummocksareformedasaresultofpointfocusingdowntoanumberofindividualvents
thatfeeddiscrete,roundedifices(SmithandCann,1992;Headetal.,1996;SmithandCann,1999).
Hummocks may coalesce together to form larger hummocky ridges or mounds (Smith and Cann,
1993;Smithetal.,1995;Headetal.,1996;Lawsonetal.,1996;Briaisetal.,2000),whicharesimilar
tolargepillowmounderuptionsonintermediate-spreadingrateridges(Yeoetal.,2013).Hummocks
commonlycollapsedowntheAVRflanks,convertingaround12%ofthelavaseruptedontheAVR
to talus, which probably also form a component of the AVR structure (Yeo et al., 2012). Rare flat-
topped seamounts and small areas of smoother lava flows may also form small parts of the AVR
structure.
The dominant rock type found on AVRs is normal mid-ocean ridge basalt (N-MORB), with
variations due to differences in the degree of partial melting or heterogeneities in the source (see
“▶Mid-Ocean Ridge Magmatism and Volcanism”).
Formation and Growth
Meltsupplyatslow-spreadingmid-oceanridgesisirregularinspaceandtime,and,atatypicalslow-
spreadingridge,meltproductionistoolowtosustainlarge,steady-statemagmachambersanywhere
along the segment (Forsyth, 1992; Lin and Morgan, 1992; Sinton and Detrick, 1992; Magde et al.,
2000). Therefore at slow- and ultraslow-spreading ridges, volcanism must be episodic. AVRs lie
entirelywithintheBrunheschronandthereforearedifficulttodate;however,anumberofAVRlife
cyclesasaresultofsuchepisodicmagmatismhavebeenproposed.Estimatesofthelengthsofthese
cyclesarehighlyvariable,rangingfromseveraltensofthousandsofyears(e.g.,10kyr(Bryanand
Moore,1977),20kyr(Sinhaetal.,1998),and25kyr(BallardandVanAndel,1977))tomuchlonger
periods (e.g., 600 kyr (Searle et al., 1998)) on the Mid-Atlantic Ridge and 400 kyr–2.4 Myr on the
SouthwestIndianRidge(Mendeletal.,2003).Additionally,whereavailable,theagesmeasuredfor
AVRs–10kyr(Sturmetal.,2000)and~12kyr(Searleetal.,2010)–aremuchyoungerthantheage
ofthecrustcalculatedbasedonspreadingrate.This,combinedwiththesimilarityofestimatedages
for lava flows all over an AVR (Yeo and Searle, 2013), suggests that AVRs are the product of
episodes of higher than normal volcanic activity.
Suchalifecycleisprobablycomprisedofatleastonevolcanicphase,followedbyanamagmatic
phaseinwhichtheAVRisbrokenapartandpossiblyriftedoffaxisbytectonicactivity(Parsonetal.,
1993;Mendeletal.,2003;PeirceandSinha,2008).Thelengthofthesevariousphasesandtheextent
to which rejuvenation may occur during periods of predominantly tectonic extension are poorly
constrained. In the extreme, this could, if periods of tectonic extension were insufficient to destroy
the AVR between rejuvenation episodes, actually result in an almost steady-state AVR, where
a bathymetric high is present nearly all the time, maintained by regular episodic volcanism.
However, evidence from the RAMASSES experiment conducted on the Reykjanes Ridge (Sinha
et al., 1998) suggests that magma chambers may only exist beneath an AVR on a slow-spreading
ridge for around 10 % of the cycle.
Summary
AVRsarelarge,constructional,volcanicfeaturesformedpredominantlyofvolcanichummocksthat
areverycommonlyfoundonslow-andultraslow-spreadingridges.Theytypicallylieinthemiddle
ofasegmentandarethefocusofvolcanicactivityandthereforeprobablyuppercrustalconstruction.
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Due tothe irregular magmasupply to slow- and ultraslow-spreading ridges, volcanism onAVRs is
almost certainly episodic although the timings and durations of magmatic episodes are currently
poorly constrained.
Cross-References
▶Crustal Accretion (incl. Gabbro Glacier Model)
▶Lava Types
▶Mid-Ocean Ridge Magmatism and Volcanism
▶Oceanic Rift System
▶Oceanic Spreading Center
▶Spreading Axis
▶Spreading Rates and Ridge Morphology
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Axial Summit Trough
SamuelAdamSoulea*andMichaelPerfitb
aWoodsHoleOceanographicInstitution,WoodsHole,MA,USA
bDepartmentofGeologicalSciences,UniversityofFlorida,Gainesville,FL,USA
Synonyms
Axial summit collapse trough; Axial summit graben; Cleft
Definition
An axial summit trough (AST) is a narrow trough or volcanically modified graben that develops at the
crest of a midocean ridge and typically is the locus of volcanic and hydrothermal activity.
Introduction
Atthesummitofmagmaticallyrobustmidoceanridges(MORs),anarrowtroughmaydevelopthatmarks
thelocationofthedivergentplateboundaryandisthelocusofmostmagmaticandhydrothermalactivity
(Macdonald and Fox, 1988). This axial summit trough (AST) is typically less than 500 m in width and
50mindepthandmaybediscontinuousalongtheridgeintermsofitswidth,depth,strike,andcontinuity.
ASTsarecommonalongfast-andintermediate-spreadingrateridges(e.g.,EastPacificRise,JuandeFuca
Ridge, Galapagos Spreading Center), and magma-rich, inflated portions of the slow-spreading
Mid-Atlantic Ridge and Lau Basin back-arc spreading center.
Development of the Axial Summit Trough
TheASTisavolcanicallymodifiedtectonicgraben.Thegrabenformsinresponsetodeformationabove
magmaticdikesoriginatingattheaxialmagmalenslocated1–3kmbeneaththeridgecrest(Chadwickand
Embley, 1998; Fornari et al., 1998). Models of dike intrusion into a homogeneous elastic medium with
physical properties similar to oceanic crust produce horizontal stresses at the seafloor, which reach their
maximumatdistancesfromthedikecenterlineof~1.5timesthedepthtothediketip(RubinandPollard,
1988; Fig. 1). Slips along normal faults between these symmetric zones of dilation and the dike tip
produce grabens with widths many times greater than depths. The typical depth and width of ASTs on
ridgesarelargerthancouldbeexpectedfortheintrusionofasingle1-mwidedike,thenominaldikewidth
on midocean ridges (Qin and Buck, 2008). Thus, it is assumed that the AST represents accumulated
deformation over many dike intrusion cycles. In some instances, an AST may evolve into a much wider
(~1–2km)anddeeper(~50–100m)axialvalleybythisprocess(Carbotteetal.,2006;Souleetal.,2009).
Other contributions to graben subsidence may include magma withdrawal from subridge melt lenses
(Carbotte et al., 2003).
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