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

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

Eastern Volcanic Province (American Samoa) Western Volcanic Province (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 primary volcanic mechanism is a mantle plume (hotspot) Natland suggests lithospheric flexure at plate boundary results in shallow 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 ( ) 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

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

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

Map created in Fledermaus Data source: The Seamount Catalog Methods | Identifying Seamounts Slope Surface

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

Assume an elliptical base and summit Approximate seamount shape as a conical frustum Methods | Characterizing Seamounts

Images created in Fledermaus Methods | Characterizing Seamounts Cross-sectional View Plan View

Slope Left Base Width Slope Right Summit Width Height Images created in Fledermaus Methods | Characterizing Seamounts Base Depth Azimuth Angle

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 Methods | Seamount Statistics

Results | Seamount Statistics MeanSt. Dev.Min.Max.Total Base Area (km 2 ) Summit Area (km 2 ) Height (m) N/A Slope (%) N/A Base Depth (mbsl) N/A Flatness N/A Elongation N/A Volume (km 3 )

Results | Relational Statistics

Methods | Distribution Analysis Negative Exponential Distribution (from Smith and Jordan [1988]) Distribution of seamounts is modeled as: v( H ) = v 0 exp(-ß H ) Where v( H ) is the # of seamounts per unit area with a height greater than H, v 0 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

Define appropriate sample 100 meter height bins containing at least three seamounts were included 48 seamounts in all, within meter height range Define appropriate areal value Total area of data set is 27,181 km 2 Reduced to 22,745 km 2 by including only depths below m This area approximates only the near- lithosphere abyssal plain Methods | Distribution Analysis

m range

Define appropriate sample 100 meter height bins containing at least three seamounts were included 48 seamounts in all, within meter height range Define appropriate areal value Total area of data set is 27,181 km 2 Reduced to 22,745 km 2 by including only depths below m This area approximates only the near- lithosphere abyssal plain Methods | Distribution Analysis

Map created in Fledermaus Data source: The Seamount Catalog Calculation of Area by m Cutoff Total area before depth cutoff: 27,181 km 2 Methods | Distribution Analysis Total area after depth cutoff: 22,745 km 2

Results | Distribution Analysis ν 0 = 2.6 ± 0.2 (per 1000 km 2 ) ß -1 = 138 m ν 0 = 2.6 ± 0.2 (per 1000 km 2 ) ß -1 = 138 m

Results | Distribution Analysis Comparison with previous studies StudyRegion (Latitude) Height Range (m)Seamount Density (per 10 3 km 2 ) [v 0 ] Characteristic Height (m) [ß -1 ] This StudyASSC (13º-15ºS)100 – ± Jaroslow et al. (2000)MAR (25º-27ºN)70 – ± Rappaport et al. (1997)ESC (27º-29ºS)200 – ± Schierer et al. (1996)Southern EPR (15º-19ºS)200 – ± Magde and Smith (1995)Northern MAR (57º-62ºN)50 – ± 2068 Schierer and MacDonald (1995) Northern EPR (8º-18ºS)200 – ± Kleinrock and Brooks (1994) Galapagos (2ºN, 95ºW)50 – ± 3029 Bemis and Smith (1993)Southern Pacific (9º-22ºS)300 – ± 2233 Smith and Cann (1990, 1992) MAR (24º-30ºS)50 – ± 958 Abers et al. (1988)Southern Pacific (7º-22ºS)100 – ± Smith and Jordan (1987), and Smith (1988) Eastern Pacific400 – ± 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

Map created in Fledermaus Data source: The Seamount Catalog Directional Trends Results | Interpretation

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

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References Natland, J. H The progression of volcanism in the Samoan linear volcanic chain. American Journal of Science. 280-A: Natland, J. H The Samoan Chain: A Shallow Lithospheric Fracture System. (last accessed March 11, 2006). Rappaport, Y., Naar, D. F., Barton, C. C., Liu, Z. J., and Hey, R. N Mophology and distrubution of seamounts surrounding Easter Island. Journal of Geophysical Research. 102(B11): 24, Scheirer, D. S., and Macdonald, K. C Near-axis seamounts on the flanks of the East Pacific Rise, 8ºN to 17ºN. Journal of Geophysical Research. 100(B2): Scheirer, D. S., MacDonald, K. C., Forsyth, D. W., and Shen, Y Abundant Seamounts of the Rano Rahi Seamount Field Near the Southern East Pacific Rise, 15º S to 19º S. Marine Geophysical Researches. 18: Smith, D. K Shape analysis of Pacific seamounts. Earth and Planetary Science Letters. 90: Smith, D. K., and Jordan, T. H Seamount Statistics in the Pacific Ocean. Journal of Geophysical Research. 93(B4): Smith, D. K., and Cann, J. R Hundreds of small volcanoes on the median valley floor of the Mid-Atlantic Ridge at 24º-30º N. Nature. 348: Smith, D. K., and Cann, J. R The Role of Seamount Volcanism in Crustal Construction at the Mid-Atlantic Ridge (24º-30ºN). Journal of Geophyical Research. 97(B2):

References Walker, G. P. L., and Eyre, P. R Dike complexes in American Samoa. Journal of Volcanology and Geothermal Research. 69: Workman, R. K., Hart, S. R., Jackson, M., Regelous, M., Farley, K. A., Blusztajn, J., Kurz, M., and Staudigel, H 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): 2003GC