JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 95, NO. B8, PAGES 12,689-12,696, AUGUST 10, 1990 Axial Seamount' An Active Ridge Axis Volcano on the Central Juan De Fuca Ridge H. PAUL JOHNSON Schoolof Oceanography,University of Washington,Seattle ROBERT W. EMBLEY NOAA, PacificMarine EnvironmentalLaboratory, Hatfield Marine ScienceCenter Newport, Oregon INTRODUCTION Juande FucaRidge can be dividedinto six 50- to 100-km-long AxialSeamount (some of themanuscripts in this special segments, with the Axial Segment being the third segment sectionrefer to theedifice with the more precise name of northof theBlanco Fracture Zone (Figure 2). "AxialVolcano"), a large ridge axis volcano, islocated on the AxialSeamount represents a large mass excess associated centralsegment ofthe Juan de Fuca Ridge approximately 250 with a "hotspot"-influencedspreading center. The term nauticalmiles west of the Washington/Oregon/British"hotspot" is usedin quotationshere since geochemical Columbiacoast. Currentlyboth volcanically and hydro- evidence (Desonie and Duncan, this issue; Rhodes etal., this thermallyactive, Axial lies directly at the intersection of the issue)argues that Axial is nota geochemicalhotspot in the Cobb-EickelbergSeamount Chain and the Juan de Fuca Ridge traditional Hawaiian or Icelandicsense. Examination of the (Figure1). Thevolcanic activity associated with the seamount older, more distal portion of the Cobb-Eickelberg (C-E) chain formationstrongly interacts with, and is affectedby, the (Figure1) showsthat it is a relativelythick "band" of normalseafloor spreading processes at the intersection.seamounts, rather than a singlebathymetric lineation. Becauseof thisunique geologic setting, its proximity to west Riddihough etal. [1983]and Davis and Karsten[1986] coastports and oceanographic institutions, and its shallowobserved that almost all of theseamounts formed at theC-E depth,Axial hasbecome the focusof a largenumber of hotspotand smaller seamounts found in proximityto theJuan scientificinvestigations over the past decade. deFuca Ridge north and south of Axiallie onthe Pacific plate AxialSeamount became an early target for study using the to thewest, rather than on the smaller Juan de Fuca plate to newly developedswath-mapping tools of Sea Beam,the east [Riddihough et al., 1983;Davis and Karsten, 1986]. SeaMARC I andII, andGLORIA. During the entire decade Thegeneral setting of Axial Seamount inthe eastern Pacific, of the1980s, geologists and geophysicists from a widevariety and the overall morphology of the edifice, can be seenin of institutions,including the U.S. Geological Survey, NOAA, Figures1 and 2, andin thecolor Plates A andB of thisissue. the PacificGeoscience Centre, as well as the Universitiesof To thesouth, structures of theVance Segment overlap with the Washington,Victoria, and British Columbia, and Oregon State South Rift Zone of Axial and interact with the seamount in the University,conducted extensive surveys and detailed sampling SE quadrant. This zone of interaction between the Vance and programsof boththe summit and flanks of thevolcano. Since Axial segmentsis quiteextensive, with at least30 km of 1984,the NOAA VENTS Program has devoted amajor surface overlap. The farthest northward projection of spreading from shipand submersible effort to high-densitystudies of the theVance Segment is a zoneof intensefissuring (observed in summitarea and its hydrothermal systems. The shallow 1500- SeaMARC I andII data)on the outer flank of thevolcano m summitdepth of theseamount has allowed access with [Johnsonand Holmes, 1989; Appelgate, this issue]. Piscessubmersibles (both Canadian and Russian) as well as Interactionofthe Axial Segment with Northern Symmetrical Alvin.This extensive sea-going effort has made Axial one of (alsonamed Cobb) Segment to thenorth is morecomplex. themost intensely studied and best characterized areas of Bothmagnetics and morphology data show that the Axial seafloorin the world. Segmentforms a "bentspreading center" connection zone betweenthe two segments,an unusualintrasegment connection GEOLOGICALSETTING regionthat is apparentlynonoverlapping at the present [Johnsonand Holmes, 1989]. An elevated saddle of basement Axial Seamountis locatedastride the centralsegment of the rock connectsthe west flank of Axial with the easternmost Juande FucaRidge, an intermediatespreading-rate ridge with flank of Brown Bear Seamount(Plate A), the next oldest a total openingvelocity of 6 cm/yr. The Juande Fuca Ridge seamount in the C-E chain. The Son-of-Brown Bear is boundedon the southby the BlancoFracture Zone and on Seamount,lying directlyeast of Axial Volcano,and three the northby a triple junctionformed by the ridge, the left- smallervolcanos just to the northand south (Plate A) arefour lateral Nootka fault and the SoyancoFracture Zone [Atwater, members of the rare class of volcanos that formed near the 1970;Hyndman et al., 1979]. Like the EastPacific Rise, the ridgeaxis and ended up on the Juande Fucaplate, rather than the Pacific plate. Copyright 1990 by the AmericanGeophysical Union. STRUCTuRAL COMPONENTS OF THE VOLCANO Paper number 90JB01037. Structurally,the edifice of Axial Volcanocan be interpreted 0148-0227/90/90JB-01037502.00 to represent a complex interplay of three usually distinct 12,689 12,690 JOHNSONAND EMBLEY.' •NTRODUCTION t50 ø t40 ø 430 ø •20 ø 60 ø ' I ' I CANADA /røo/•/r .. PRATT ' •'0, ":"'' •'•__ •,• W,•. '.-..... •1•'e• OH41N .'..'.' • ß • • •:..... •'•,• • . WELKER ,._ • "''. • WILSON • KNOLLS VANCOUVER 50 ø T.ZO ' •/c,•4 ß • ISLAND EICKELBERG COBB PACIFICPLATE • ß THOMPSON JUAN DE FUCA PLATE I 0 km 500 BROWN ,• I BEAR U.S.A. at 42øN a:: RDA <z PLATE ß . .. 40 ø i ! Fig. 1. Seamountsand spreadingcenters in the northeastPacific, adaptedfrom Riddihoughet at. [1983]. Thick hachured lines are subductionzones. Note that the Cobb-Eickelberg Chain has a finite width and actually consistsof a band of seamounts rather than a linear chain. geologicalprocesses: seafloor spreading, seamount formation, and movement of the water column above Axial; and (4) andHawaiian-style rifting. The elongatestructure of theentire specific experimentsand studiesthat focus on the active edifice (PlateA) appearsto representthe distributionof an hydrothermalsystems. "excess"of magmatic activity along the entire Axial Segment. The elongateNorth and South Rifts show evidenceof "rifting" Axial as a Volcano or, more correctly phrased,longitudinal growth due to along- The large-scale context of Axial is presentedin the papers strike, near-surface magma transport. Finally, the dome- by Desonie and Duncan and by Rhodes et al. Desonie and shapedsummit and caldera argue for seamountlikeformation Duncanexamine the age andchemistry of the Cobb-Eickelberg processes,including eruption from (and subsequentcollapse (C-E) seamounts,using dredgesamples collected mostly from into) localized magma reservoirs. the distal members of the chain, and find a westward age The summit of Axial, with its well-defined, three-sided progressionaway from the spreadingcenter, consistentwith caldera(Figure 3 and PlateB) and associatedactive the general assumption thatAxial is the youngest member of hydrothermalsystems, hasprovided the focus of many of the theprogressively older (to the west) seamount chain. The studiesinthis volume. Floored with very recent lava Hows, same study also shows aneastward decrease inage difference someof which are young enough toonlap actively venting between theseamounts andthe crust onwhich they are formed, hydrothermal deposits, this caldera is a potential natural arguing that the spreading center and the "mantle source" laboratory. Because of the shallow 1500-m depth of the responsible for the seamountshave been approaching each calderafloor, high-temperature hydrothermal systems can other as the Juan de Fuca migrates toward the fixed C-E undergo"phase separation," a term used by water chemists "hotspot." Desonie andDuncan, however, findthat there isnot when they are discussing"boiling" of the hydrothermalHuid a clear, mantle plume signatureto the geochemistryof the (Butterfieldetal., this issue). The influence ofthis "phase surface rocks of these outer seamounts andcall upon mixing separation" on hydrothermal systems is profound and is a between a weak mantle plume and normal spreading-center themethat runs through several of themanuscripts in this magmas toexplain the observed compositions. specialsection. Rhodesetal., on the other hand, concentrating onsamples from the summit and flanks of Axial volcano itself, find no THISISSUE evidenceat all of any mantleplume influence in the The manuscriptsin this issuecan be divided into four broad geochemistryof the rocks. These authorsargue that although categories: (1) the large view studiesthat look at Axial as an the geochemicaldata (as well as the morphologicalevidence) active submarine volcano; (2) the more focused summit appearto favor a very activeand extensive magmatic plumbing geologicaland geophysical studies; (3) studiesof thechemistry systemfor Axial Seamountand one that is separateand distinct JOHNSONAND EMBLEY: INTRODUCTION 12,691 t30 ø 126 ø 122 ø .. ... ... ß .. 52 ø 52 ø ß . .ß. ß ß ß ß ß o ß.Van.couver ß SFZ •eattle 48 ø 48 ø ß ß ß "NORTH • NSR CCobb) oo AMERICAN •" AXIAL PLATE ••. DEJUAN FUCA PLATE 44 ø 44 ø PACIFICPLATE ß o (,• 40 ø 40 ø 130ø 126ø 122ø Fig.2. Schematicdiagram of theridge segments of the Juan de Fuca Ridge, in thenortheast Pacificß Thick lines indicate thespreading center segments; thinner lines indicate fracture zones and the location of thecontinental