The Marine Geomorphology of American Samoa:

Shapes and Distributions of Deep Sea Volcanics

Jed Roberts

Master’s Candidate in Geography Department of Geosciences Oregon State University Thesis Defense – November 20, 2007

Presentation Overview

Study Area Significance of Research Geomorphologic Background Research Questions Multibeam Compilation Shape Statistics Distribution Analysis Morphological Interpretation Conclusions Future Work Acknowledgements

Study Area

Image produced by the U.S. National Park Service

Study Area

Western Volcanic Province (Samoa)

Eastern Volcanic Province

(American Samoa) Image produced by the U.S. National Park Service

Why This Study Area?

Data availability

Intrigue of controversy regarding volcanic origins

No previous comprehensive investigation of geomorphology in the eastern volcanic province

Significance of Research

Identification and characterization of new seamounts First mapping of major geomorphologic features Part of U.S. exclusive economic zone (EEZ): provides for inventory of potential fish habitat, mineral deposits, and geologic hazards Compilation of all available multibeam bathymetry in American Samoa

Early Exploration

First Mapping of the Samoan Islands Woodcut of Tafua Crater on Upolu James Dwight Dana, Geologist and Mineralogist

With the U.S. Exploring Expedition, Dana (1849) was the first geologist to visit the Samoan Islands He noted voluminous and recent volcanism on Savai’i and Upolu, and suggested a west-to-east age-progression

Hotspot Evolution

Evolution of hotspot volcanoes as observed in Hawaii and other Pacific hotspot chains

Anomalous Samoa?

As Dana (1849) and others note, the Samoan Islands appear to progress from west-to-east

Tectonic Setting

Image modified from Bird (2003)

Mechanism 1| Lithospheric Flexure

Image from Natland (1980) Natland (1980) suggests lithospheric flexure at plate boundary results in shallow magma upwelling

Radiometric Age Dating

Image modified from the U.S. National Park Service Age data from McDougall (1985), Natland and Turner (1985), and Duncan (1985) Ages of shield volcanism approximately in line with plate speed Natland maintains that a mantle plume in this location would be “fortuitous”

Mechanism 2 | Mantle Plume

Modified from artwork by Jayne Doucette, Woods Hole Oceanographic Institution McDougall (1985) suggests primary magmatic source is a mantle plume (hotspot)

Vailulu’u | The Smoking Gun

Image from Hart et al. (2000) First-time mapping and sampling of Vailulu’u by Hart et al. (2000) reveal active volcanism

What of Rose Atoll?

Image modified from the U.S. National Park Service Age data from McDougall (1985), Natland and Turner (1985), Duncan (1985), and Hart et al. (2000) Based on its geochemical signature, Rose Atoll is not a part of the Samoan “pedigree” (Rodgers et al. 2003) Geomorphologic evidence suggests Rose Atoll is much older than Samoan Islands

Recent Volcanism in Western Samoa?

Rejuvenated volcanism on Savai’i and Upolu occurred after a period of erosion and is geochemically distinct when compared to shield volcanism

Plausibility of Lithospheric Flexure?

Abbott and Fisk (1986) claim stress associated with the “corner” of the Tonga Trench would not cause lithospheric deformation outside of 200 km

Geodetic Reconstruction

Image modified by Hart et al. (2004) from Ruellan et al. (2003)

Geodetic Reconstruction

Image modified by Hart et al. (2004) from Ruellan et al. (2003)

Geodetic Reconstruction

Image modified by Hart et al. (2004) from Ruellan et al. (2003)

Lithospheric Stress at Present

Savai’i Upolu Tutuila 200 km radius

Image modified by Hart et al. (2004) from Ruellan et al. (2003)

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 mechanism, both, or neither?

How will predicted seamount distributions compare with previous studies?

Primary Data Acquisition

Image from Lost City Expedition (2003) Hi-Resolution Multibeam Sonar!

Data Description

Multiple datasets collected during separate research cruises (1984-2006) Cruises operated by numerous institutions with a variety of scientific objectives Data collected by various shipboard multibeam sonar systems with differing quality Data has been merged at a resolution of 200m with depths of up to ~6 km below sea level covering an area of 28,446 km 2

Data Description by Expedition

Expedition Year

Marathon Papatua Roundabout Boomerang Kiwi AVON Cook Drift ALIA HURL 1984 1985-86 1989 1996 1997 1999 2001 2002 2005 2005-06

Institution(s)

Scripps Institution of Oceanography Scripps Institution of Oceanography Scripps Institution of Oceanography Oregon State University, Scripps Institution of Oceanography Scripps Institution of Oceanography Scripps Institution of Oceanography, Woods Hole Oceanographic Institution University of Rhode Island Scripps Institution of Oceanography, University of South Florida Woods Hole Oceanographic Institution

Vessel

R/V Thomas Washington R/V Thomas Washington R/V Thomas Washington R/V Melville R/V Revelle R/V Melville R/V Melville R/V Revelle R/V Kilo Moana Hawaii Undersea Research Lab R/V Ka’imikai-O-Kanaloa

Multibeam Swaths by Expedition

Multibeam Compilation

Methods | Identifying Seamounts

Creation of slope surface using multibeam compilation Candidate seamounts are visually circumscribed by slope Avoid island and large seamount flanks, select seamounts on or near abyssal plain 100 meters or more in height, due to resolution constraints Completeness of data!

Un-treated Multibeam Compilation

Slope of Bathymetric Surface

51 Seamounts Identified!

Methods | Characterizing Seamounts

Assume an elliptical base and summit Approximate seamount shape as a “conical frustum”

Methods | Seamount Parameters

Basal and Summit Areas Height Slope Basal Depth Azimuth Flatness (ratio of summit to basal area) Elongation (ratio of basal minor axis to basal major axis) Volume

Methods | Characterizing Seamounts Plan-View Cross-sectional View

Images created in Fledermaus

Methods | Characterizing Seamounts Azimuth Angle Slope Left Summit Diameter Height Slope Right Basal Diameter

Images created in Fledermaus

Base Depth

Results | Seamount Statistics

------------------------- Mean St. Dev. Min.

Basal Area (km 2 )

6.67

5.58

1.71

Summit Area (km 2 )

0.090

Height (m)

325 0.283

152 0.004

105

Slope (%) Basal Depth (mbsl)

13.4

-4208

Flatness Elongation Volume (km 3 )

0.0120

1.28

1.01

3.5

727 0.0161

0.24

1.58

5.9

-2640 1.00

0.09

Max.

36.52

2.049

850 20.1

-5205 0.0014

0.1021

2.10

10.76

Total

340.51

4.584

N/A N/A N/A N/A N/A 51.38

Results | Relational Statistics

Basal Area vs. Height

900 800 700 600 500 400 300 200 100 0 0 5 10 15 20

Basal Area (km 2 )

25 30 35 40 Height and basal area share a strong positive correlation, in agreement with Smith (1988)

Lobate Seamounts

Results | Relational Statistics

Flatness vs. Height

900 800 700 600 500 400 300 200 100 0 0.00

0.02

0.04

0.06

Flatness

0.08

0.10

0.12

As height increases, the factor of flatness decreases, though flatness is highly variable at heights below 400 m

Results | Relational Statistics

Average Slope vs. Height

900 800 700 600 500 400 300 200 100 0 5 7 9 11 13

Average Slope (%)

15 17 19 21 There is a diffuse but slightly positive relationship between slope and height, as observed for small seamounts (Smith 1988)

Results | Relational Statistics

Volume vs. Height

900 800 700 600 500 400 300 200 100 0 0.0

2.0

4.0

6.0

Volume (km 3 )

8.0

10.0

12.0

Volume and height demonstrate a positive relationship akin to the relationship between basal area and height

Results | Relational Statistics

Flatness vs. Basal Depth

0.04

Flatness

0.06

0.08

-2000 0.00

-2500 -3000 -3500 -4000 -4500 -5000 -5500 0.02

0.10

0.12

The flattest seamounts occur at intermediate basal depths

Results | Relational Statistics

Azimuth of Major Basal Axis

A cluster is observed at N0°E amidst an otherwise diffuse distribution of directionalities

Methods | Distribution Analysis

Negative Exponential Distribution

(from Smith and Jordan [1988]) Distribution of seamounts is modeled as:

v(H) = v

height greater than H, v

0 0 exp(-ßH)

Where v(H) is the # of seamounts per unit area with a 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 28,446 km 2 Reduced to 21,158 km 2 by omitting area occupied by large seamounts This area approximates only the near lithosphere abyssal plain

Methods | Distribution Analysis

Histogram Seamounts included in distribution analysis 10 8 2 0 6 4 18 16 14 12 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 100 300 400 500 Height Bins (m) 600 700 800 900

Skewed distribution of heights illustrates the suitability of an exponential model

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 28,446 km 2 Reduced to 21,158 km 2 by omitting area occupied by large seamounts This area approximates only the near lithosphere abyssal plain

Methods | Distribution Analysis

Results | Distribution Analysis

Characteristic Height (ß -1 ) = 139 m Seamount Density (v 0 ) = 2.8 ± 0.2 per 1,000 km 2

Results | Distribution Analysis

Study This Study

Jaroslow et al. (2000) Rappaport et al. (1997) Schierer et al. (1996) Magde & Smith (1995) Schierer & MacDonald (1995) Kleinrock & Brooks (1994) Bemis & Smith (1993) Smith & Cann (1990, 1992) Abers et al. (1988) Smith & Jordan (1988)

Region (Latitude) ESVP (13º-15ºS)

MAR (25º-27ºN) ESC (27º-29ºS) Southern EPR (15º-19ºS) Northern MAR (57º-62ºN) Northern EPR (8º-18ºS) Galapagos (2ºN, 95ºW) Southern Pacific (9º-22ºS) MAR (24º-30ºS) Southern Pacific (7º-22ºS) Eastern Pacific

H Range 100 – 600

70 – 350 200 – 1000 200 – 1200 50 – 250 200 – 800 50 – 350 300 – 700 50 – 210 100 – 1000 400 – 2500

v 0 2.8 ± 0.2 ß -1 139

58.3 ± 1.6 92

2.7 ± 1.5

4.8 ± 0.2

308 421

310 ± 20 1.9 ± 0.2

370 ± 30 13 ± 2 195 ± 9

12.6 ± 0.8 174

5.4 ± 0.7

347 68 240 29 233 58 Comparison with previous studies. H range is height range, v seamount density per 1,000 km 2 , ß -1 0 is characteristic height. is Studies in Atlantic Ocean , studies in Pacific Ocean , and anomalous results for Pacific Ocean .

Results | Distribution Analysis

Anomalous density for Galapagos likely due to complexity of triple-junction and slow spreading rate Spreading rate and seamount density have an inverse relationship Most studies are near spreading ridges, none represent hotspots Distribution analysis demonstrates seamount densities typical of southern Pacific

Interpretation of Major Features

Interpretation | Tutuila

Interpretation | Papatua

Interpretation | Muli

Interpretation | Tulaga

Interpretation | Malumalu

Interpretation | Ofu and Olosega

Interpretation | Ta’u

Interpretation | Vailulu’u

Interpretation | Tama’i and Soso

Interpretation | Tutuila-Soso Relationship

Interpretation | Malulu

Interpretation | Rose Atoll

En Echelon Lineaments

1.0 Ma

Inferred Age-Progressions

0.3 Ma 0.1 Ma

???

Large Off-Axes Seamounts

???

0.3 Ma 0.1 Ma 1.0 Ma ???

???

The “Tutuila Event”

Image from Walker and Eyre (1995) Walker and Eyre (1995) note the departure of the main Tutuila rift zone from the Samoan island chain trend and suggest an extension of the North Fiji Fracture Zone is the cause

The “Tutuila Event”

0.3 Ma 0.1 Ma 1.0 Ma 1.5 Ma

Disturbance at 1.5 Ma and then divergence to a bifurcated hotspot trace after 1.0 Ma

Small Scale Post-Erosional Volcanism?

Possible Post-Erosional Mechanisms Magma Chamber

Image modified from Binard et al. (2004) 1) Binard et al. (2004) suggest large seamounts weigh upon underlying magma chamber and push magma out laterally along preferential weaknesses in the oceanic crust 2) Lithospheric flexure proposed by Natland (1980)

Possible Post-Erosional Mechanisms

3) Slab rollback into the mantle plume, causing decompressional melting (Hart et al. 2004)

Summary of Findings

Shape statistics of newly identified seamounts agree with previous studies, but some outliers may be indicative of low-viscosity flow types Density of small seamounts in the ESVP is in line with others observed in the Pacific, though it is the only hotspot environment studied thus far En echelon lineaments occur after the “Tutuila Event” (1.0 Ma) Apparent small scale post-erosional volcanism superimposed on large seamount flanks and observed in off-flank regions Post-erosional volcanism can be attributed to one of three mechanisms or combinations of those mechanisms

Future Work

Submit seamount locations and shape characteristics to the Seamount Catalog Name new seamounts!

Add new shallow bathymetry from NOAA to multibeam compilation Compare findings with forthcoming geochemical and geochronological data

Selected References

Abbott, D. H., and Fisk, M. 1986. Tectonically controlled origin of three unusual rock suites in the Woodlark Basin. Tectonics. 5(7): 1145-60.

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.

Binard, N., Hekinian, R., Stoffers, P., and Cheminée, J. L. 2004. South Pacific Intraplate Volcanism: Structure, Morphology and Style of Eruption. In Oceanic Hotspots: Intraplate Submarine Magmatism and Tectonism. Edited by Hekinian, R., Stoffers, P. and Cheminée, J. L. New York, NY. Springer-Verlag Press: 157-207.

Bird, P. 2003. An updated digital model of plate boundaries. Geochemistry Geophysics Geosystems. 4(3): 1027. doi:10.1029/2001GC000252.

Dana, J. D. 1849. US Exploring expedition during the years 1838-1842 under command of Charles Wilkes, USN. Geology. 10: 307-36.

Duncan, R. A. 1985. Radiometric ages from volcanic rocks along the New Hebrides-Samoa lineament. Circum-Pacific Council for Energy and Mineral Resources Earth Science Series. 3: 67-76.

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.

Selected References

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.

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.

McDougall, I. 1985. Age and Evolution of the Volcanoes of Tutuila, American Samoa. Pacific Science. 39(4): 311-20.

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., and Turner, D. L. 1985. Age progression and petrological development of Samoan shield volcanoes: evidence from K-AR ages, lava compositions, and mineral studies. Circum-Pacific Council for Energy and Mineral Resources Earth Science Series. 3: 139-71.

Selected References

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.

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.

Acknowledgements

Dr. Dawn Wright

Graduate Advisor

Dr. Anthony Koppers

Committee Member

Dr. Randy Keller and Dr. Gary Klinkhammer

Committee Members

Scripps Institution of Oceanography, Woods Hole Oceanographic Institution, Oregon State University, University of South Florida, University of Rhode Island, NOAA Pacific Islands Fisheries Science Center, and Hawaii Undersea Research Lab

Data Sources

Questions?

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http://oregonstate.edu/~robertje/projects/thesis

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