Cruise Report for Leg 6 of the Drift Expedition

Aboard the R/V Revelle

Operated by Scripps Institution of Oceanography


Title of NSF Project:

Assessing Hotspot Fixity in the Pacific Basin


Chief and Co-chief Scientists:

David F. Naar (USF)

Kevin Johnson (Bishop Museum)

Doug Pyle (OSU)

Paul Wessel (UH)


And CO-PI’s

Robert A. Duncan (OSU)

John Mahoney (UH)


Callao, Peru to Hanga Roa, Rapa Nui, Chile

November 5, 2001 to December 14, 2001


December 14, 2001
Table of Contents




Introduction and Methods


Tectonic Setting






Cruise Participants


Appendix A: Waypoint and Dredge Site List


Appendix B: Dredge Log


Appendix C: Maps, Descriptions, and Photos

For each volcano there is generally:

1)     color-shaded post-processed multibeam bathymetry merged with bathymetry predicted from altimetry gravity data and positions of dredge site(s) on the volcano

2)     a 2-D map of the same post-processed data of the volcano

3)     a  3-D perspective view of the volcano

4)     post-processed backscatter data of the volcano

5)     ship position during a specified dredge(s) with 25 meter contours

6)     raw backscatter overlain by 20 m contours and a straight line showing the start and stop of each dredge(s)

7)     description for each specified dredge – including one or more photographs of dredges (if volcanic rock was obtained)









We have completed a mapping and sampling survey of the longest volcanic chain on the Nazca Plate to investigate if hotspots are fixed and to what degree.  This 2900 km chain includes the Nazca Ridge and the Easter and Salas y Gomez Islands and seamounts.  It extends from the superfast seafloor spreading East Pacific Rise all the way to the Peru-Chile subduction zone.  We collected 38 days of 12 kHz multibeam, 3.5 kHz sub-bottom, and gravity data.  We also collected magnetic data at the beginning and at the end of the cruise.  We obtained 33 dredged samples that are suitable for geochemical data analysis, 24 of which are suitable for age dating.  These samples extend over a majority of the chain and form a grid-like pattern in the complicated zone of volcanoes in the “elbow” of the chain.  This broad zone of volcanoes in the “elbow” area appear to be a result of the hotspot’s interaction with one or more fracture zones.  Our sampling distribution will allow us to investigate if the volcanoes along the fracture zone traces have a similar geochemical pattern to what appears to be the main volcanic chain.   As expected, it was more difficult to obtain suitable volcanic rocks in the older volcanoes along the Nazca Ridge because of Mn coating and pelagic sediment cover.  However, our persistence paid off and we were able to obtain samples along the southern portion of the Nazca Ridge, an area where other suitable samples had not been obtained.  After geochemical data analyses and 40Ar-39Ar radiometric age measurements are made, we will be able to test several specific Nazca-hotspot plate motion models, including the Steinberger and O’Connell (1998) model which predicts a 20 mm/yr convergence rate between the Easter and Hawaiian hotspots (i.e., 600 km of convergence over 30 Ma).     Moreover, the compositional record of the hotspot will be extended back into time beyond 25 Ma.  Finally, we have obtained variable multibeam coverage over 46 volcanoes, which display a wide range of shapes, sizes, and geomorphologies including massive landslides.  

Introduction and Methods:


          In this field study, we have obtained the necessary volcanic rocks samples to test hotsot fixity by comparing the motion of the Hawaiian hotspot (Pacific plate) and the Easter/Salas y Gomez hotspot (Nazca plate) over the last 25-30 Ma.  The study area we have chosen is the best place on Earth to carry out such a test, because the seafloor spreading rate between these two plates and the predicted convergence rate between these two hotspots is the fastest anywhere (Steinberger and O’Connell, 1998).


          Historically, the hotspot reference frame, which assumes hotspot fixity, has been attractive because it is simple to understand and easy to use to measure plate motions – one need only map the geometry and age progression within the volcanic tracks of hotspots.  Independent arguments from geochemistry, seismic tomography, and modeling indicate that hotspots are maintained by deep mantle convective plumes embedded within the lower mantle, which convects much more slowly than the upper mantle.  However, many aspects of this appealing scenario have been questioned, including the existence of mantle plumes themselves (e.g., Anderson, 1996, 1999; Smith 1993).  The controversy surrounding the notion of mantle plumes and their fixity has stimulated a great deal of recent work because the plume paradigm is central to ideas about planetary heat loss and planetary plate tectonics.


          Leg 6 of the Drift Expedition aboard the R/V Revelle, informally known as RAPA NUI 2001, set out to map and collect volcanic rocks samples to test several hypotheses regarding hotspot fixity and mantle convection.  The survey was conducted in the southeast Pacific between Peru and Easter Island, located near 27o 10’ S and 109o 30’ W (Figure 1). We mapped and then conducted 68 dredges over the majority of the Nazca Ridge and Easter and Salas y Gomez seamount chain over a 38-day period. 


There was some risk of not obtaining suitable volcanic rocks for 40Ar-39Ar radiometric age dating from the older volcanoes due to Manganese coating and sediment cover (see orange dots in Figure 2).  However, with persistence, we obtained 33 successful dredges that contained volcanic rock, of which, at least 24 contained samples suitable for 40Ar-39Ar radiometric age dating, including the southern portion of the Nazca Ridge (see red dots in Figure 2).  We also collected a grid-like pattern of samples in the broad “elbow” area where the Nazca Ridge intersects the Easter/Salas y Gomez seamount chain (Figure 2).


We used the newly installed Kongsberg Simrad EM120 multibeam bathymetry and backscatter system.  Except for some problems at the beginning of the cruise, such as a frozen heading (an irretrievable error that required manual heading entries until the problem was corrected), and zeroed out installation parameters (a retrievable error that was corrected in post-processing), the system worked very well.  The only outstanding problem with the system is some kind of “wobble” or “warble” in the beam pattern, especially in the outer beams (see SMT001 bathymetry in Appendix C or the color version on the CD-ROM or website). This artifact was first noted in January 2001, during a test cruise Naar participated on.  Since that time, there have been many ideas on the cause of this pattern and many more additional tests to identify the cause of it.  Presently, at the writing of this report, recent data collected on a short transit from Costa Rica to Peru is being analyzed at Kongsberg Simrad in Norway.  The latest speculation is that there may be some kind of artifact being made by the “beam former” in its effort to stabilize pitch and/or yaw.  However, this is still speculative and until analysis is complete, we will not know for sure.  We attempted to keep this artifact in mind when selecting dredge targets, but at times during a dredge, we obtained a pinger trace we did not expect.  This was only a minor problem.


We found the Kongsberg Simrad software very useful, especially for quick gridding of both the bathmetry and backscatter data to allow dredge selection.  We normally displayed 20 m contours over the raw backscatter data and selected start and end waypoints using an interactive planning module in the Neptune post-processing data (see raw backscatter images in Appendix C or  on the CD-ROM or website).  The entire process of transferring data files from the acquisition computer to the post-processing computer, gridding them, displaying the contours over backscatter data, and selecting the dredge start and stop position, azimuth of dredging, and distance in dredging took on average ten to twenty minutes.  This was done directly at the workstation without having to make any paper plots.  This online method a provided great deal of flexibility to look at the volcano at different scales and also to use artificial illumination on the bathymetry data to cast shadows that emphasize the fine-detail structure such as roughness or hummocky surfaces.





Tectonic Setting:


          The eastern South Pacific seafloor has been formed by the world’s fastest seafloor spreading system and has been overprinted by abundant hotspot volcanic activity leaving trails of volcanoes on both the Pacific and Nazca plate, radiating from an area near Rapa Nui, also known as Easter Island, Chile (Figure 1; Hey et al., 1985, 1995).  Magnetic anomalies across this area have been identified from the East Pacific out to anomaly 3 and also from anomaly 7 to anomaly 13 (Figure 1; Handschumacher, 1976; Mayes et al., 1990; Liu, 1996).  Between anomaly 3 and 7 there are few magnetic anomalies identified, partially due to the tectonic complexities that appear to have occurred when the Farallon Plate reorganized into the Cocos and Nazca plates at about 25 Ma.  This caused the Pacific-Farallon plate boundary to change orientation and velocity (both speed and direction) until it became the Pacific-Nazca plate boundary, including the formation of several propagating rifts and the Mendoza microplate (Lonsdale, 1989; Liu, 1996; Figure 3; see reconstruction animation slide in PowerPoint presentation on CD-ROM or website).  Although the volcanoes that exist along the volcanic chain (Figure 2) preclude useful magnetic anomaly data from being collected in determining seafloor magnetic isochrons, the ages to be determined from radiometric age dating will be useful in the plate reconstructions and hotspot backtracking.




          Our primary product at the end of this long cruise is a web-based summary of our data collection, including tables, figures, photographs, and the text of this report.  Until radiometric age dating and geochemical data analyses are performed, we will not have any results related to the primary objectives of this expedition.  We have observed a multitude of volcanic geomorphologies including the shallow plateau structure of the Nazca Ridge, individual conical volcanoes, broad ridge-like structures apparently formed by coalesced volcanoes, razor-back like volcanic ridge structures apparently tectonically controlled, very large volcanoes and guyots some of which appear to have had large landslides, and low-lying volcanic fields similar to those observed near the East Pacific Rise (Hagen et al., 1989; Naar and Hey, 1991; Naar et al., 1993; Rappaport et al., 1997; Liu et al., 1997a, 1997b) and very large isolated volcanoes that appear to have had extensive landslides.

Multibeam bathymetry and backscatter data, gravity data, magnetic data, and 3.5 kHz data have all been recorded digitally and will be used in studying the geomorphology of the volcanic chain, sediment thickness where possible, and the effective elastic thickness of the lithosphere where possible.  Comparison of depths between the multibeam data and the bathymetry predicted from altimetry gravity data will also be conducted.  Preliminary estimates have shown that the summit depths have been within about 200 meters, but full three-dimensional analysis will be necessary to characterize the actual differences between the smoother predicted bathymetry from the more detailed multibeam bathymetry data.


          Keeping in mind that a picture is worth a thousand words, this cruise report is primarily a printout of the various data tables and images related to the dredged volcanoes.  Additional data files, photographs, tables, and color versions of the black and white images found in the appendices can be viewed from the CD-ROM (included with this report) or on our websites.  The primary website for this project will be maintained at the University of Hawaii at: http://www.soest.hawaii.edu/wessel/drft06rr at a location to be announced and monitored by Paul Wessel.  Links to this website and potential mirror websites will be found at http://www.marine.usf.edu after clicking on “Geology” and then “Rapa Nui 2001 (Leg 6 of Drift Expedition –DRFT06RR)”.  Other links will most likely set up at http://www.coas.oregonstate.edu and http://www.bishopmuseum.org.


          Appendix A contains our waypoints, dredge locations, coordinates that define the general boundaries of the volcano or volcanic ridge being dredged, and a simple key describing the outcome of the dredges.  Appendix B contains a digital version of the handwritten dredge log.  Appendix C contains: 1) a GMT image (Wessel and Smith, 1991) showing location of dredge locations on color-shaded post-processed multibeam bathymetry merged with bathymetry predicted from altimetry gravity data (Sandwell and Smith, 1997); 2) a two-dimensional map of the same post-processed data of the volcano generated by CARIS software; 3) a three-dimensional perspective view generated by CARIS software; 4) post-processed backscatter data using the Kongsberg Simrad Poseidon software which removes the high-backscatter strip along nadir for the same volcano; 5) another GMT image displaying the ship position during a specified dredge with 25 meter contours (there will be one or more dredges per each volcano); 6) raw backscatter overlain by 20 m contours and a straight line showing the start and stop of each dredge using the Kongsberg Simrad Neptune software (these images are what we quickly generated on a second computer workstation to select dredges start and stop times, azimuth of dredging, and length of dredge in meters); 7) Dredge Rock Database Summary for each specified dredge – including one or more photographs of dredges (if volcanic rock was obtained).


          A post-cruise meeting will be scheduled for summer or fall of 2002 to integrate results and prepare for presentation of results at the December 2002 AGU meeting in San Francisco, California.  Several manuscripts related to this project will be underway after data analysis is complete and will focus on the geochemistry, geophysics, geomorphology, the tectonic reconstructions, and a final synthesis manuscript describing our test of hotspot fixity and the Steinberger and O’Connell (1998) prediction (after the radiometric age dates have been measured).




          We thank Captain Desjardins and the crew of the R/V Revelle for their efforts in making our cruise successful and safe.  It was clear that the crew were interested in the science program and always willing to help and offer support when needed.  We are especially grateful for the efforts of Liz Brenner and Rose Dufour of the Scripps Institution of Oceanography Ship Scheduling Office prior to and during our cruise.  We thank the Chilean government and the Chilean Navy Hydrographic and Oceanographic Service for granting clearance to map and sample within 200 nm of Chilean coastlines.  We are most grateful for the tireless support and positive attitude provided by Eugene Pillard, the Resident Technician who was up for every single dredge over the entire cruise.  We were honored to be out to sea with him, at the 20th year anniversary of his employment with Scripps Institution of Oceanography. We are grateful for the dedicated work ethic of all the scientific party, including the volunteers who stood watch throughout the entire cruise.   This work was funded by the National Science Foundation. 










Science Party Listed in Alphabetical Order:


Gregory A. Berman

Marine Geologist

College of Marine Science

University of South Florida

St. Petersburg, Fl, 33731




Kate Ciembronowicz

Geologic Oceanographer

Univ. of South Florida

140 Seventh Avenue South

St. Petersburg, Fl 33713

email: kciembro@usgs.gov


Brian Donahue

Marine Technician / Research Assistant

University of South Florida

College of Marine Science

Center for Coastal Ocean Mapping

140 7th Avenue South

St. Petersburg, Fl 33701


Phone: 727-553-1121

Fax: 727-553-1189



Leigh Ann Elgin


College of Marine Science

University of South Florida

140 Seventh Avenue South

St. Petersburg, FL 33701-5016

Phone: 727-822-5344

Email: laelgin@hotmail.com




Yasushi Harada

Visiting Assistant Researcher


Dept. of Geology & Geophysics

SOEST, University of Hawaii at Manoa

1680 East-West Rd. POST 813, Honolulu, HI 96822 USA

TEL: 808-956-4043    FAX: 808-956-5154

E-mail: harada@soest.hawaii.edu


Kevin T. M. Johnson


Bishop Museum

1525 Bernice St.

Honolulu, HI 96817

tel: 1-808-848-4124

fax: 1-808-847-8252

email: kevinj@soest.hawaii.edu


David F. Naar


College of Marine Science

University of South Florida

140 Seventh Avenue South

St. Petersburg, FL 33701-5016

Phone: (727) 553-1637

Fax: (727) 553-1189



Douglas Pyle


College of Oceanic and Atmospheric Sciences

Oregon State University

Ocean Admin. Bldg. 104

Corvallis, OR 97330

PHONE: 541-737-8285

FAX: 541-737-2640

EMAIL: pyle@coas.oregonstate.edu




Jyotiranjan S. Ray

Postdoctoral Fellow

Department of Geology and Geophysics

606B POST Building

SOEST, University of Hawaii

1680 East-West Road

Honolulu HI 96822

Phone: 808-956-3444

Fax: 808-856-5512

E-mail: jsray@soest.hawaii.edu


Chris J. Russo

Igneous Petrologist/ Graduate Research Assistant

College of Oceanic and Atmospheric Sciences

Oregon State University

Ocean Admin. Bldg. 104

Corvallis, OR 97330

PHONE: 541-737-2649

FAX: 541-737-2640

EMAIL: crusso@coas.oregonstate.edu


Hetu C. Sheth

SOEST Young Investigator (Assistant Researcher)

POST Bldg. 605, Department of Geology and Geophysics,

School of Ocean and Earth Science and Technology (SOEST),

University of Hawaii at Manoa,

Honolulu, HI 96822

Email: sheth@soest.hawaii.edu / hetusheth@yahoo.com

Fax: 808-956-5512, Phone: 808-956-9543

Discipline: Geology, geochemistry and igneous petrology (principal

research interest: flood basalts, particularly Deccan Traps of India)


Paul Wessel, geophysicist

Department of Geology & Geophysics

School of Ocean and Earth Science and Technology

University of Hawaii

1680 East-West Road

Honolulu, HI 96822

1-808-956-4778/5154 (ph/fax)



Amy K. Wright

College of Marine Science

University of South Florida

St Petersburg FL, 33701

E-mail: awright@seas.marine.usf.edu


SIO/MARFAC Shipboard Support:

Pillard, Eugene, Resident Technician

Jacobson, Dan, Computer Technician

Foley, Steve, Multibeam Data Processor



Desjardins, Thomas, Master

Widdrington, Rob, 1st Mate

Ferris, Joseph, 2nd Mate

Kramer, David, 3rd Mate

Pearson, James, Bosun

Lewis, Stephen, A.B.

Black, Donald, A.B.

Hughes, Christy, A.B.

Allison, Lorna, O.S.

Mauricio, Paul, Chief Engineer

Hand, Charles, 1st Asst

Saint-Martin, Steven, 2nd Asst

Funk, William, 3rd Asst

Luallin, Alden, Electrician

Esteban, Reynaldo, Oiler

Thant, Kyaw, Oiler

Carter, Todd, Oiler

Angeles, Eduardo, Oiler

Schaum, Harold, Wiper

Adapon, Jockie, Cook

Lamp, Stacey, Cook










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Hey, R.N., D.F. Naar, M.C. Kleinrock, W.J. Phipps Morgan, E. Morales, and J.G. Schilling, Microplate tectonics along a superfast seafloor spreading system near Easter Island, Nature, 317, 325-331, 1985.


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