jed_roberts_aag_2007 by liwenting

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									Morphological Interpretation of
Seamounts in American Samoa:
 Inferring Genesis Mechanisms through
     Shape and Distribution Analysis




               Jed Roberts
     Master’s Candidate in Geography
       Department of Geosciences
          Oregon State University
     AAG San Francisco - April 19, 2007
Presentation Overview
        Study Area
    Research Questions
     Data Description
      Shape Statistics
    Distribution Analysis
Morphological Interpretation
        Future Work
    Acknowledgements
       Study Area




Image produced by the U.S. National Park Service
                   Study Area




Western Volcanic Province        Eastern Volcanic Province
        (Samoa)                     (American Samoa)


            Image produced by the U.S. National Park Service
   Why This Study Area?

    Data availability

   Intrigue of controversy
 regarding volcanic regime

 No previous comprehensive
       investigation of
geomorphology in the eastern
     volcanic province
Tectonic Setting




 Image modified from Sandwell and Smith
                             Controversy




Artwork by Jayne Doucette,
        Woods Hole
 Oceanographic Institution
                                     Artwork by Naoto Hirano, Scripps Institution of
                                                   Oceanography




    Hart et al. suggest              Natland suggests
    primary volcanic               lithospheric flexure
     mechanism is a                at plate boundary
     mantle plume                     results in shallow
         (hotspot)                 magma upwelling
    Research Questions
Will shape and distribution analyses
reveal new clues about seamount
       origin in the absence of
corresponding geochemical data?

   Will the findings support one
volcanic regime, both, or neither?

   How will predicted seamount
distributions compare with previous
               studies?
          Data Description

  Multiple datasets collected during separate
          research cruises (1999-2005)

  Cruises operated by Scripps Institution of
Oceanography, HURL, Oregon State University,
       and University of South Florida

Data collected by various shipboard multibeam
      sonar systems with differing quality

Data has been merged at a resolution of 210m
  with depths of up to 6 km below sea level
    covering an area of 27,181 square km
              Multibeam Data
Merged with Sandwell and Smith 1km resolution predicted bathymetry




                   Image created using Fledermaus
                  Data source: The Seamount Catalog
                           www.earthref.org
Multibeam Data
With 210m resolution swaths isolated




   Image created using Fledermaus
  Data source: The Seamount Catalog
           www.earthref.org
Methods | Identifying Seamounts

   Create slope surface for multibeam data

       Candidate seamounts are visually
           circumscribed by slope

 Avoid island and large seamount flanks, select
      seamounts near or on abyssal plain

 100 meters or more in height, due to resolution
                  constraints


           Completeness of data
Methods | Identifying Seamounts
                 Slope Surface




             Map created in Fledermaus
         Data source: The Seamount Catalog
                  www.earthref.org
Methods | Identifying Seamounts
            51 Seamounts Selected




             Map created in Fledermaus
         Data source: The Seamount Catalog
                  www.earthref.org
Methods | Characterizing Seamounts




     Assume an elliptical base and summit

 Approximate seamount shape as a conical frustum
Methods | Characterizing Seamounts
      Plan View




   Cross-sectional View




                          Images created in Fledermaus
Methods | Characterizing Seamounts




                             Azimuth Angle




                            Summit Width


       Slope Left                            Slope Right
                                  Height



                             Base Width                    Base Depth
                    Images created in Fledermaus
Methods | Seamount Statistics
    Base and Summit Areas
                    Height
                     Slope
               Base Depth
                   Flatness
          (ratio of summit to base area)

                Elongation
    (ratio of base minor axis to base major axis)

                   Volume
Results | Seamount Statistics
------------------------- Mean St. Dev.   Min.     Max.      Total

Base Area (km2)      6.7633   5.5800      1.7064   36.5213   344.9299

Summit Area (km2)    0.0891   0.2828      0.0044   2.0487    4.5453

Height (m)           323      152         105      850       N/A

Slope (%)            13.3     3.3         5.9      19.7      N/A

Base Depth (mbsl)    -4245    738         -2640    -5380     N/A

Flatness             0.0118   0.0161      0.0014   0.1021    N/A

Elongation           1.28     0.24        1.00     2.10      N/A

Volume (km3)         1.01     1.58        0.09     10.76     51.73
Results | Relational Statistics
Results | Relational Statistics
Results | Relational Statistics
Results | Relational Statistics
Results | Relational Statistics
Results | Relational Statistics
Results | Relational Statistics
Results | Relational Statistics
Methods | Distribution Analysis

 Negative Exponential Distribution
         (from Smith and Jordan [1988])

    Distribution of seamounts is modeled as:
              v(H) = v0exp(-ßH)
 Where v(H) is the # of seamounts per unit area with a
height greater than H, v0 is the total # of seamounts per
 unit area, and ß is the negative of the slope of the line
                fitting ln(v(H)) against H

   The characteristic height of the
seamount sample is equal to negative
          reciprocal of ß
Methods | Distribution Analysis
   Define appropriate sample
100 meter height bins containing at least
    three seamounts were included
48 seamounts in all, within 100-600 meter
             height range

 Define appropriate areal value
   Total area of data set is 27,181 km2
Reduced to 22,745 km2 by including only
          depths below -2640 m
 This area approximates only the near-
        lithosphere abyssal plain
Methods | Distribution Analysis


      100-600 m range
Methods | Distribution Analysis
   Define appropriate sample
100 meter height bins containing at least
    three seamounts were included
48 seamounts in all, within 100-600 meter
             height range

 Define appropriate areal value
   Total area of data set is 27,181 km2
Reduced to 22,745 km2 by including only
          depths below -2640 m
 This area approximates only the near-
        lithosphere abyssal plain
Methods | Distribution Analysis
        Calculation of Area by -2640 m Cutoff




     Total area before depth cutoff: 27,181 km2
       Total area after depth cutoff:
                22,745 km2

               Map created in Fledermaus
           Data source: The Seamount Catalog
                    www.earthref.org
Results | Distribution Analysis




    ν0 = 2.6 ± 0.2 (per 1000 km2)
    ß-1 = 138 m
        Results | Distribution Analysis
             Comparison with previous studies
Study                      Region (Latitude)            Height Range (m)   Seamount Density     Characteristic
                                                                           (per 103 km2) [v0]   Height (m) [ß-1]
This Study                 ASSC (13º-15ºS)              100 – 600          2.6 ± 0.2            138

Jaroslow et al. (2000)     MAR (25º-27ºN)               70 – 350           58.3 ± 1.6           92

Rappaport et al. (1997)    ESC (27º-29ºS)               200 – 1000         2.7 ± 1.5            308

Schierer et al. (1996)     Southern EPR (15º-19ºS)      200 – 1200         4.8 ± 0.2            421

Magde and Smith (1995)     Northern MAR (57º-62ºN)      50 – 250           310 ± 20             68

Schierer and MacDonald     Northern EPR (8º-18ºS)       200 – 800          1.9 ± 0.2            240
(1995)
Kleinrock and Brooks       Galapagos (2ºN, 95ºW)        50 – 350           370 ± 30             29
(1994)
Bemis and Smith (1993)     Southern Pacific (9º-22ºS)   300 – 700          13 ± 2               233

Smith and Cann (1990,      MAR (24º-30ºS)               50 – 210           195 ± 9              58
1992)
Abers et al. (1988)        Southern Pacific (7º-22ºS)   100 – 1000         12.6 ± 0.8           174

Smith and Jordan (1987),   Eastern Pacific              400 – 2500         5.4 ± 0.7            285
and Smith (1988)


ASSC is the American Samoa Seamount Chain, MAR is the Mid-Atlantic Ridge, ESC is
the Easter Seamount Chain, EPR is the East Pacific Rise
    Results | Interpretation
    Relational shape statistics are in
   agreement with those observed in
            previous studies
  Elongation and azimuth reveal slight
  directional trends that may support
           lithospheric flexure
   Distribution analysis demonstrates
seamount population densities typical of
             southern Pacific
Small seamount chains trend northeast-
 southwest, while large seamounts and
        islands trend east-west
Results | Interpretation
            Directional Trends




         Map created in Fledermaus
     Data source: The Seamount Catalog
              www.earthref.org
          Significance
 Lithospheric flexure is not ruled out as
volcanic mechanism for production of
           small seamounts

   Initial identification of seamounts


Volume and other shape statistics never
         before calculated

Locations and distribution of seamounts
  important for biological studies and
          habitat protection
            Future Work
    Re-grid dataset at slightly higher
               resolution
Add data collected by NOAA in 2006 to
    regional dataset compilation

Examine shape statistics and distributions
 based on natural geographic partitions

   Submit seamount locations and
morphologies to the Seamount Catalog

  Compare findings with forthcoming
      geochronological data
     Acknowledgements
Dr. Dawn Wright, Oregon State University
             Graduate Advisor
Dr. Anthony Koppers, Oregon State University
       Seamount Catalog Webmaster
Scripps Institution of Oceanography, Hawaii
   Undersea Research Lab, Oregon State
 University, and University of South Florida
                 Data Sources
       Dr. Deborah Smith, Woods Hole
          Oceanographic Institution
Dr. Thomas Jordan, Massachusetts Institute of
                  Technology
        Distribution Analysis Methods
You can download this presentation here:
http://oregonstate.edu/~robertje/projects/aag2007

     Contact me via e-mail at:
 jed.roberts@geo.oregonstate.edu
                             References
Abers, G. A., Parsons, B., and Weissel, J. K. 1988. Seamount abundances and distributions in
      the southeast Pacific. Earth and Planetary Science Letters. 87: 137-51.

Bemis, K. G., and Smith, D. K. 1993. Production of small volcanoes in the Superswell region of
      the South Pacific. Earth and Planetary Science Letters. 118: 251-62.

Hart, S. R., Staudigel, H., Koppers, A. A. P., Blusztajn, J., Baker, E. T., Workman, R., Jackson, M.,
       Hauri, E., Kurz, M., Sims, K., Fornari, D., Saal, A., and Lyons, S. 2000. Vailulu'u undersea
       volcano: The New Samoa. Geochemistry Geophysics Geosystems. 1(12):
       2000GC000108.

Hart, S. R., Coetzee, M., Workman, R. K., Blusztajn, J., Johnson, K. T. M., Sinton, J. M.,
       Steinberger, B., and Hawkins, J. W. 2004. Genesis of the Western Samoa seamount
       province: age, geochemical fingerprint and tectonics. Earth and Planetary Science
       Letters. 227: 37-56.

Hirano, N., Takahashi, E., Yamamoto, J., Abe, N., Ingle, S.P., Kaneoka, I., Hirata, T., Kimura, J.,
      Ishii, T., Ogawa, Y., Machida, S., and Suyehiro, K. 2006. Volcanism in Response to Plate
      Flexure. Science. 313: 1426-28.

Jaroslow, G. E., Smith, D. K., and Tucholke, B. E. 2000. Record of seamount production and
      off-axis evolution in the western North Atlantic Ocean, 25º25'-27º10'N. Journal of
      Geophysical Research. 105(B2): 2721-36.

Kleinrock, M. C., and Brooks, B. A. 1994. Construction and destruction of volcanic knobs at
       the Cocos-Nazca spreading system near 95ºW. Geophysical Research Letters. 21(21):
       2307-10.

Magde, L. S., and Smith, D. K. 1995. Seamount volcanism at the Reykjanes Ridge:
    Relationship to the Iceland hot spot. Journal of Geophyical Research. 100(B5): 8449-68.
                           References
Natland, J. H. 1980. The progression of volcanism in the Samoan linear volcanic chain.
      American Journal of Science. 280-A: 709-35.

Natland, J. H. 2004. The Samoan Chain: A Shallow Lithospheric Fracture System.
      www.mantleplumes.org (last accessed March 11, 2006).

Rappaport, Y., Naar, D. F., Barton, C. C., Liu, Z. J., and Hey, R. N. 1997. Mophology and
    distrubution of seamounts surrounding Easter Island. Journal of Geophysical Research.
    102(B11): 24,713-28.

Scheirer, D. S., and Macdonald, K. C. 1995. Near-axis seamounts on the flanks of the East
      Pacific Rise, 8ºN to 17ºN. Journal of Geophysical Research. 100(B2): 2239-59.

Scheirer, D. S., MacDonald, K. C., Forsyth, D. W., and Shen, Y. 1996. Abundant Seamounts of
      the Rano Rahi Seamount Field Near the Southern East Pacific Rise , 15º S to 19º S.
      Marine Geophysical Researches. 18: 13-52.

Smith, D. K. 1988. Shape analysis of Pacific seamounts. Earth and Planetary Science Letters.
       90: 457-66.

Smith, D. K., and Jordan, T. H. 1988. Seamount Statistics in the Pacific Ocean. Journal of
       Geophysical Research. 93(B4): 2899-918.

Smith, D. K., and Cann, J. R. 1990. Hundreds of small volcanoes on the median valley floor of
       the Mid-Atlantic Ridge at 24º-30º N. Nature. 348: 152-5.

Smith, D. K., and Cann, J. R. 1992. The Role of Seamount Volcanism in Crustal Construction at
       the Mid-Atlantic Ridge (24º-30ºN). Journal of Geophyical Research. 97(B2): 1645-58.
                           References
Walker, G. P. L., and Eyre, P. R. 1995. Dike complexes in American Samoa. Journal of
     Volcanology and Geothermal Research. 69: 241-54.

Workman, R. K., Hart, S. R., Jackson, M., Regelous, M., Farley, K. A., Blusztajn, J., Kurz, M., and
     Staudigel, H. 2004. Recycles metasomatized lithosphere as the origin of the Enriched
     Mantle II (EM2) end-member: Evidence from the Samoan Volcanic Chain.
     Geochemistry Geophysics Geosystems. 5(4): 2003GC000623.

								
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