Transcript Slide 1

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
(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 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 (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 Right
Slope Left
Height
Base Width
Images created in Fledermaus
Base Depth
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 nearlithosphere 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 nearlithosphere 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
(per 103 km2) [v0]
Characteristic
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
(1995)
Northern EPR (8º-18ºS)
200 – 800
1.9 ± 0.2
240
Kleinrock and Brooks
(1994)
Galapagos (2ºN, 95ºW)
50 – 350
370 ± 30
29
Bemis and Smith (1993)
Southern Pacific (9º-22ºS)
300 – 700
13 ± 2
233
Smith and Cann (1990,
1992)
MAR (24º-30ºS)
50 – 210
195 ± 9
58
Abers et al. (1988)
Southern Pacific (7º-22ºS)
100 – 1000
12.6 ± 0.8
174
Smith and Jordan (1987),
and Smith (1988)
Eastern Pacific
400 – 2500
5.4 ± 0.7
285
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 northeastsouthwest, 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:
[email protected]
References
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