Earrh and Planetary Science Letters, 87 (1988) 137-151
Elsevier Science Publishers B.V., Amsterdam
- Printed
137
in The Netherlands
Seamount abundances and distributions
Geoffrey
’ Department
A. Abers
ofEarth,Atmospheric,
’ Lumont-Doherty
‘, Barry
Parsons
in the southeast Pacific
‘,* and Jeffrey
K. Weissel
2
and Planetary Science, Massachusetts Institute of Technology, Cambridge MA 02139 (U.S.A.)
Geological Observatory of Columbia University, Palisades NY 10964 (U.S.A.)
Received
March 25, 1987; revised version received August
19, 1987
Sea Beam bathymetry
was recorded for 17,277 km of ship track in the southeast Pacific and has been analyzed for
seamount population
characteristics.
All the ship tracks are over the Pacific plate and most fall along 7 lines parallel to
the East Pacific Rise between 7 o S and 22O S. The lines fall into three age categories: one line is over 0.5-2 Ma crust,
three are over S-10 Ma crust, and three are over 30-40 Ma crust. Seamount locations were recorded, diameters were
manually estimated,
and heights were measured if the swath crossed the seamount
center. Over 382 features were
counted along the entire ship track with heights ranging from 50 m to over 2500 m, with sampling most consistent at
heights between 100 m and 1000 m. Height-to-radius
ratios vary considerably,
suggesting that seamount shapes do not
scale to a single parameter.
The observed variety of morphological
forms demonstrates
that there is a wide range of
seamount
shapes for small features. The distribution
of sizes for the population
was approximated
by exponential
dependence
and by power-law
dependence
using methods developed
by Smith and Jordan [17]. The power-law
distribution
overestimates
abundances
at the smallest size ranges and both distributions
fail to predict abundances
of
large seamounts determined
from wide-beam data [17]. Size distributions
were also determined
for seamounts in each
seafloor age category. Almost all small seamounts appear to be produced on crust younger than 0.5-2 Ma, while the
number of larger seamounts increases to 5-10 Ma. For all size ranges observed here the number of seamounts at 5-10
Ma and at 30-40 Ma is roughly identical. These observations
suggest that most small seamounts are formed on very
young, thin lithosphere
that permits the passage of small volumes of magma. Small volumes would cool in older,
thicker lithosphere
before reaching the surface although larger magma bodies might not. Numerous
small sources of
melt must exist near the ridge crest to supply the small seamounts,
probably
trapped remnants
of the large-scale
upwelling.
1. Introduction
cated top sometimes containing
craters (e.g. [9]).
Small or young volcanoes
may form as fairly
simple cones or elongated domes, but as volcanoes
grow larger circumferential
feeders
and flank
eruptive centers become common [1,3]. Flat tops
are often observed to develop, presumably
by the
filling in of a central depression
built by caldera
collapse and circumferential
growth along ringfracture
conduits
[1,3,10].
Larger
and
older
seamounts
show a wide variety of morphologies,
including
both flat and domed tops [4], multiple
summits
and flank rift zones [S], and possible
coalescing of adjacent volcanic centers [3]. Craters
may form intermittently
on the flat or bulging top
as eruptions
occur. It is possible
that large
seamounts
are active over periods of millions of
years [ll]. The presence of numerous
seamounts
near the ridge crest suggests that smaller features
In the past twenty years much attention
has
been given to features which are obviously related
to first-order
plate-tectonic
processes,
such as
mid-ocean
ridges, fracture
zones, and hot-spot
volcanoes.
Only
recently
have high-resolution
mapping
instruments
become available
to allow
smaller features such as seamounts
to be studied
in detail (e.g. [l-S]). The advent of these technologies has allowed small-scale submarine
volcanism
and other second-order
marine processes to be
understood
as well as more fundamental
features.
Seamounts
are usually defined as roughly circular steep-sided features, with a conical or trun* Present address: Department
of Earth Sciences,
of Oxford, Parks Road, Oxford OX1 3PR, U.K.
0012-821X/88/$03.50
University
0 1988 Elsevier Science Publishers
B.V.
138
can grow over time scales of hundreds of thousands of years (e.g. [1,3]). However, few seamounts
have well-documented histories.
Estimates for total abundance of seamounts
vary over an order of magnitude from study to
study [9,12-18]. Most of this uncertainty results
from basing abundance estimates on map counts
in regions of irregularly and often poorly sampled
seafloor bathymetry; for example the region investigated in this study has areas of several
hundred square kilometers with no bathymetric
information at all. Also, when a seamount is
located by only a single track of wide-beam data it
is not known whether or not the true top of the
seamount is observed, so sizes are frequently underestimated.
Rather than counting features off maps to
estimate abundances [12-15] an alternative approach is to treat seamounts identified on individual depth profiles as samples from a random
distribution on the seafloor and to statistically
derive areal distributions [16-18]. Statistical studies based on wide-beam echo-sounder records
]16,17] assumed that each seamount was a truncated cone with a fixed flatness and height-toradius ratio. These assumptions were necessary to
relate the statistical distribution of apparent
volcano heights on sonar records to the distribution of true volcano height abundances. These
studies indicated that in the east Pacific seamounts
cover 6% of the seafloor and comprise 0.4% of the
total crustal volume [16]. Hence, seamount
volcanism is a significant contributor to the oceanic crust.
It is well known that large seamounts are more
numerous on older seafloor (e.g. [11-14]). Smaller
seamounts, however, do not seem to increase much
in abundance with crustal age and may in fact
decrease in number [13-17]. The increase in the
total number of seamounts is easily explained by
the existence of off-ridge volcanism, but the change
in size distribution is harder to understand. Increasing plate thickness is often hypothesized as
causing the size of seamounts produced to increase (e.g. [13,14]) but few mechanisms have been
proposed. Menard [9] suggested the increase in
average volcano size is simply due to continued
growth as the seamount moves away from the
ridge. Vogt [19] and Gorodnitskiy et al. [14] used
isostatic mass balance to argue for seamount height
increasing as the square-root of crustal age, proportional to lithospheric thickness. Increasing
sediment thickness with age could cause burial of
smaller seamounts in such a way that the observed
ratio of large to small seamounts would increase
[13].
Temporal variability in seamount production at
mid-ocean ridges could also explain the observed
abundance variations. There is much evidence for
a mid-late Cretaceous episode of extensive intrusive igneous activity and increased seamount production, especially for larger seamounts [9,20].
This evidence consists of extensive 115-70 Ma
sills and extrusive basalts in the central Pacific
[20], flexural signatures for 90-120 My crust indicative of anomalously high ridge-crest volume
[21,22], anomalous seamount abundances in this
age range [17], and abnormally thick crust in
Cretaceous volcanic plateaus. Other peaks in
global magma productivity have been suggested as
well, such as during the mid-Miocene [23] or
Eocene [13], but these are not easily seen in
seamount abundance variations (e.g. [17]).
The proposed mechanisms for control of
seamount locations and distributions generally fall
into two classes: crustal variations that constrain
magma migration paths, and source variations that
control where magma is produced (e.g. mantle
plumes). Many workers suggest the distribution of
seamounts is non-random (e.g. [1,6,14,15]) and is
controlled by features like fracture zones [1,24],
overlapping spreading centers [5], and other
mid-ocean ridge irregularities (e.g. [3,25]). However. such an observation is hard to quantify as
correlations and lineations are often identifiable in
any random areal distribution. Furthermore, many
near-ridge seamounts cannot be obviously associated with any such feature. Many small seamounts
are found near ridge crests so that magma
processes associated with seafloor spreading are
probably important for seamount production.
Magnetic and geochemical evidence suggests that
some small seamounts form away from the ridge
crest [26], but the volcanoes studied were still on
fairly young ( < 10 Ma) crust.
Mechanisms for lithospheric control of off-ridge
seamount locations are somewhat more enigmatic,
although pre-existing fractures in the crust have
been suggested as primary conduits [1,24,27].
Heavily fractured crust south of the Eltanin frac-
139
ture zone has been associated with anomalously
high seamount
abundances
[17] possibly indicating that crustal fractures provide easy pathways
for magma to ascend. At some scale it seems likely
that the mantle thermal regime controls intraplate
seamount
production.
Hot-spot
traces are certainly
well documented
sources
of seafloor
volcanism but are usually associated with chains
of ocean islands or large guyots. It is not clear that
the concept of mantle plumes is relevant to the
more numerous
smaller seamounts.
It is not the
aim of this paper to discuss the origin of large
volcanoes possibly of hotspot origin; rather, we
wish to characterize
the population
of smaller
seamounts
that are probably not associated with
isolated, individual
upwellings.
This study takes advantage of the high resolution of modem bottom-mapping
technology (Sea
Beam) to characterize many seamounts in one part
of the Pacific. Most of the data (45667 km* of Sea
Beam swath) is confined to a relatively homogeneous region of the crust distributed
over several
age ranges, allowing for good resolution of population variations
with age. The coverage of highresolution bathymetry
is well suited for constraining the gross properties of smaller seamounts
(<
500 m in height) that conventional
echo-sounders
cannot resolve. Accurate estimation of basal diameters, heights, slopes, and volumes are made. It is
almost always possible to tell if the summit of a
seamount
was ensonified
by Sea Beam, unlike
wide-beam sonar, so that population
statistics can
be generated
without relying on an a priori assumed shape for the features. A primary goal of
this study is to determine
whether or not small
seamounts form away from the ridge crest.
2. Observations
Over 17,000 km of multi-narrow-beam
sonar
(Sea Beam) was recorded in the eastern Pacific on
R/V “Robert D. Conrad” cruise RC2608 in 1985
(Fig. 1). Height resolution
for Sea Beam is nominally about 10 m [28] although scatter between
successive
soundings
can be somewhat
greater.
Systematic artifacts can be quite large, especially
in regions of rough topography, but most are easy
to identify since they produce distinctive patterns
in the bathymetry
[29]. To reduce the random
fluctuations
depths for each beam were averaged
over several (usually 5) soundings
during postprocessing.
Averaging
produced
a set of depth
values that were roughly
evenly spaced every
100-200
m both along-track
and across-track.
After the data was collected, it was remerged with
corrected navigation
(Global Positioning
System
for 6-8 hours/day,
transit satellite fixes, and dead
reckoning
between fixes), providing
an approximate grid of ocean depth values 17,277 km long
and averaging 2.6 km wide. Total swath width,
approximately
75% of the water depth, varied
from 2000 m near the ridge to 3000 m over older
seafloor.
Seamounts
were located and their sizes were
measured
interactively
with computer-generated
swath charts (Fig. 2). The charts were created so
that the ship track is straight and passes horizontally from left to right, with depths calculated by
linearly interpolating
between recorded Sea Beam
depth values. Actual
cross-track
distances
determined by the Sea Beam system were retained,
as were the locations of data gaps, to minimize
distortions
and extrapolations.
The diameter of each seamount was estimated
by visually fitting a circle to its base, defined to be
the contour
where the feature becomes
indistinguishable
from the surrounding
terrain. A height
was calculated
for all seamounts
whose centers
were crossed by the Sea Beam swath by subtracting the minimum
water depth in the central half
of the circle from a basal depth at a point picked
on the perimeter. The radius and height comprise
the basic size parameterization
of each seamount.
Diameters
were estimated
for 523 features,
of
which 382 had centers which fell on the swaths.
Parameters for all the seamounts crossed are summarized in Table 1.
Cross-sectional
profiles were constructed
for all
seamounts whose centers were crossed by averaging the depth values recorded by Sea Beam in
each of a series of concentric
annuli around the
center of the seamount.
Volumes were estimated
for the seamounts by integrating
the cross-sections
around
the assumed
center,
assuming
the
seamounts
possessed axial symmetry. Other variables were tabulated
to describe the morphology
of the seamount
such as a top radius (for flattopped features) and existence of lineations,
satellite cones, and craters. Since it is not always clear
whether or not a feature is a seamount
(i.e. a
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qg. 1. Bathymetric map for the ,study area. Ship-tracks used in the Sea Beam data set (dashed lines) are also shown. Pacific plate motion in the hot-spot frame of reference is
hown by large one-headed arrows, and relative plate motion is shown by the two-headed arrows. Small squares are earthquake epicenters.
5C
,o
I0
~0
141
Fig. 2. Some examples of seamounts on the Sea Beam data. Color contours change every 40 m. Horizontal scale bar has tics every 5
km; horizontal and vertical scales are the same. The ship-track passes from left to right across the center of each figure. A. Large
regular, conical seamount with a flat top, estimated height = 912 m and radius = 2973 m. B. Small conical seamount, height = 330 m,
radius = 1316 m, and h / r = 0.251. C. Small seamount similar in area to B but much flatter, height = 231 m, radius = 1490 m, and
h / r = 0.155. D. Seamount elongated parallel to the structural trends, estimated height = 475 m and radius = 1978 m. E. Irregular
structure assumed to be volcanic, with approximated height = 502 m and radius = 2280 m. B and C illustrate typical variation in h/r
for small seamounts. D and E are examples of non-conical morphologies.
TABLE 1
Summary of study regions
Region
All data
All data b
On ridge
0 - 2 M a crust
5 - 1 0 M a crust
3 0 - 4 0 M a crust
Track
Area
Number of seamounts:
Total
length
(km)
covered
(km2)
total
identified
center
on swath
volume
(km3)
17277
17277
- 800
1508
4964
4526
45667
45667
- 2000
3330
12577
14055
523
324
0
45
143
129
382
233
0
35
95
99
1082.0
534.5
0
66.1
434.9
298.4
a For seamounts with centers on the Sea Beam swath.
b Regular edifices only, assigned highest-quality rating.
a
Radius a
Height a
Seamounts
min.
(m)
max.
(m)
min.
(m)
max.
(m)
with
craters a
292
332
.
423
342
292
10790
9052
.
.
5363
9052
10790
49
64
.
70
49
64
2394
2394
.
1382
2394
1325
40
35
14
11
4
142
volcanic edifice) levels of certainty were also attached to the identifications. Axially symmetric
cones with simple, regular shape were assigned to
the highest confidence level (e.g. Fig. 2A-C), while
more irregular features or elongate features associated with seafloor lineations were given a lower
confidence rating (e.g. Fig. 2D, E). Often the
largest features were assigned low confidences because these features were rarely simple cones and
it was difficult to tell if the center was crossed. A
tabulation describing all the seamounts is available from the authors.
The technique for fitting radii assumes all
seamounts are circular in plan view, which was
not always true. For elliptical features (Fig. 2D) or
irregular features (Fig. 2E) we attempted to fit a
circle to the base of the seamount such that it
covered approximately the same area as the actual
feature. The radius then should provide a robust
estimate of the seamount size. Diameters were
extrapolated for larger features that extend off the
Sea Beam swath.
Although the base picked for the seamount
circumference is used in the height determination,
the actual diameter has little influence on relative
depth values (if slopes near the base are small) so
the height and radius measurements have been
treated as being independent. For most features
the basal depth is fairly clear from the regional
bathymetry and a reference height is easily established. Heights were only estimated for features
whose centers clearly lie on the swath so that no
extrapolations were necessary to define the shape
of seamounts. Minimum water depth is then usually adequate to estimate height for most
seamounts, which have a well-defined peak or are
flat-topped. A problem exists in estimating height
for irregular features (Fig. 2E), where it is difficult
to be sure that the true summit was crossed and
not just some flank peak. Sediment thicknesses,
determined from the 3.5 kHz echo-sounder, were
almost always less than 100 m and should not
significantly affect the height distributions.
3. Height-to-radius ratios
The homogeneity of the region surveyed allows
systematic properties of the shapes of seamounts
to be investigated. It has been suggested (e.g.
height vs. radius
2000
1500
l
A
E
1000
500
.~,~%,~~
0
'
40'00
80'00
rad;us (m)
Fig. 3. Scatter plot of height vs. base radius. Only the seamounts
whose centers were crossed and which were designated regular,
conical volcanoes are plotted.
[10,16,30]) that the height-to-radius ratio is constant for most submarine volcanoes, implying that
constructional processes are fairly uniform. For
wide-beam parameter estimation, it is necessary to
assume values for some of these relationships to
relate apparent height observations to areal abundances [16,17]. In order to see if this parameter is
useful for describing the shapes of small seamounts
measurements were compared using only the 233
features described as regularly shaped seamounts
and whose centers were crossed. Sample size is less
important here than measurement quality since
the inferences concern shapes, not abundances.
Plotting height (h) against radius (r) for the
volcanoes shows that there is a wide range in
seamount shapes for the smaller features (also
compare Fig. 2B and 2C), but for larger seamounts
height increases roughly with radius (Fig. 3). The
average value for h/r is 0.221 +_ 0.091 for 233
seamounts smaller than h = 2400 m, compared to
a value of 0.214_+ 0.006 obtained by Smith [18]
for 85 Pacific seamounts smaller than h = 3800 m.
Smith's estimate of h/r is based on many features
that are larger than those from our study and has
less scatter. Comparing her results with ours suggests that larger seamounts are more regular in
shape than small ones. There appears to be an
upper limit on the h/r values (Fig. 3), with many
measurements bunching up near the maximum
observed slope.
143
4. Size distributions
their notation to facilitate comparison with their
results. All seamounts with a basal radius in the
range r k + A r / 2 were counted to obtain the number n k of seamounts in radius interval k, where
rk = (k - 1 / 2 ) A r for k = 1, 2 . . . . , N. The n k form
a set of differential abundances. The cumulative
abundance ~k of all seamounts with radius greater
The variation in abundances of seamounts with
height and radius was calculated by making histograms of the seamount size measurements, following the methodology of Jordan et al. [16] and
Smith and Jordan [17]. We will attempt to follow
( XPONENTIAL
Height range 100- lO00m
Radius range 5 0 0 - 6 0 0 0 m
10 3
103
x. x
b~
102
102
©.(3
0
bOO.
101
I00
O',
o
10-1
i
i
I
i
400
0,,(3
,,x
",O "O
0
"ocoXXo 0 (30
I 0o
10-1
i
i
I
o
800
'
2000
(m)
height
..
o %0, %~
C~ "x
6
o
,~X~x
O" O
c
"q. o o~-~.
.x
%,
101
"'~,x
o-.
•
o
"~.x
o
"~.x
"©
x'x
O-.O
O" "O
~ 40'00
radius
'
60'00
'
(m)
POWER-LAW
Height
Radius range 500-6000m
100- lO00m
range
10 3
10 3
"x-x
10 2
x~
x-. x -x
10 2
0'.0
°.%
0
o
101
Oo
(3OO
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06"oo
101
oo
©
©(1N~
10 0
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~
".0
'-OO
I 0o
10-I
I0-1
10 2
height
10 3
(m)
10 2
103
radius (m)
10 4
Fig. 4. Semi-log (top) and log-log (bottom) abundance histograms for height (left) and radius (right) measurements. The circles are
the observed number of seamounts in each size interval, and " × " designates the cumulative number of all seamounts with sizes
greater than the height or radius of the × . Only seamounts whose centers were crossed are used in all the abundance diagrams. The
dashed lines show the best-fitting exponential (top) and power-law (bottom) population models. The size ranges used in the
maximum-likelihood estimation are shown above each figure.
144
than rk - Ar/2 was counted from the differential
abundances, and similar statistics were generated
f o r t h e h e i g h t m e a s u r e m e n t s a t sizes h j + A h / 2 ,
j = 1, 2 . . . . . M . Size r a n g e s A r a n d A h w e r e t a k e n
t o b e 250 m a n d 5 0 m , r e s p e c t i v e l y , f o r r a d i i f r o m
500 t o 6 0 0 0 m a n d h e i g h t s f r o m 100 t o 1000 m.
Abundance histograms were generated for the
382 s e a m o u n t s w h o s e c e n t e r s w e r e c r o s s e d a n d f o r
t h e s u b s e t o f 233 f e a t u r e s t h a t w e r e a s s i g n e d t h e
h i g h e s t q u a l i t y level. O t h e r t h a n i n t o t a l n u m b e r s
no obvious distinction was found between the
distributions of the two groupings. Thus the larger
set o f o b s e r v a t i o n s , w h i c h i n c l u d e s i r r e g u l a r l y
shaped seamounts, was used in making comparisons.
The abundance of seamounts increases drastic a l l y w i t h d e c r e a s i n g size in a m a n n e r t h a t h a s
b e e n d e s c r i b e d as e i t h e r e x p o n e n t i a l o r p o w e r - l a w
d e c a y [17]. T h e c u m u l a t i v e e x p o n e n t i a l
tion has the form:
distribu-
p(r) =Uo e-~r
w h e r e t h e p a r a m e t e r s v0 a n d a a r e
ber of seamounts and the rate of
crease with radius, respectively.
power-law cumulative distribution,
b y Pl a n d y, h a s t h e f o r m :
u(r)
=
the total numabundance deSimilarly, the
parameterized
t,'lr "r
Both descriptions predict far fewer large
s e a m o u n t s t h a n s m a l l o n e s . T h e e x p o n e n t i a l distribution predicts a finite number
of small
seamounts while the power-law distribution has an
infinite total number. In order to compare these
two distribution models the differential and
cumulative abundances are shown on both semi-
TABLE 2
Population parameters for height data
A. Exponential model: ~,(h) = u0e-~h
Size range
(m)
fl ~
(kin 1)
% a
Number per 106 km 2 a
( × 1 0 - 3 km-2)
h > 300 m
All data
All data
All data
0-2 Ma
5-10 Ma
30-40 Ma
100-1000
100- 700
300-1000
100- 600
100- 800
100- 800
5,44+-0.27
5,76+-0.25
4,86 +-0.36
12.85 + 2.48
5.51 +-0.52
4.72 + 0.39
12.66 + 0.76
12.83 + 0.76
10.22 + 1.43
27.18+-8.62
10.71 + 1.32
9.83 + 1.11
2473 + 193
2278 __178
2379 + 233
575+-309
2052 + 328
2387 +- 336
55 __14
40 +- 10
79 + 22
0+- 1
43 + 21
88 +- 33
Smith and
Jordan [17], all areas b
Jordan et al. [16] ~
Batiza [13]
400-2500
300-1500
3.47 + 0.21
2.95 +-0.36
5.44+-0.65
3.96+-1.12
1920+- 116
1633+-416
117
169+- 17
207+-62
Region
-
h >1000 m
B. Power-law model: ~,(h) = ulh - ~
a
ul (my-2) "
Number per
h > 300 m
h > 1000 m
0.68 + 0.02
0.56+-0.01
0.97 + 0.03
1.54 +-0.07
1.34-+0.14
2.48+-0.47
0.83+-0.04
0.64 +-0.02
2.08+0.17×10 7
1.36_+0.01×10 7
8.73 + 1.34 × 10- 7
1,80 + 0.75 × 10- 5
4A5 +2.70x 10 5
1,12+2.62x10 3
4.07 + 0.88 x 10- v
1.80 + 0.28 × 10 7
4391+ 375
5744_+ 376
3411 +_ 523
2724 + 1140
1987+1293
792_+1850
3600 + 774
4763 + 726
1944±162
2943_+193
1058 + 162
426 ± 178
396_+258
40+ 93
1326 + 800
2206 + 337
2.37 + 0.00
2.28+0.09x10 -3
3016+
Region
Size range
(m)
Y
All data
All data
All data
All data
0-2 Ma
0-2 Ma
5-10 Ma
30-40 Ma
100-1000
100- 700
200- 800
300-1000
100- 450
150- 500
150 700
150- 700
Smith [18], all areas b
400-2500
a Errors are lo uncertainties from maximum-likelihood estimates.
b Flatness ( r t / r ) = 0.3.
c Flatness = 0.2.
91
10 6
km 2 a
173+-
5
145
TABLE 3
Population parameters for radius data
A. Exponential model: p ( r ) = u0e-~r
Region
All data
0 - 2 Ma
5-10 Ma
30-40 Ma
B. Power-law model:
Region
Number per 106 km 2 a
Size range
(m)
a a
( k m - l)
()<10 -3 km - 2 )
500-6000
500-1750
500-4000
500-4000
1.01 + 0.05
3.02 _+0.50
0.87 + 0.07
1.11 _+0.10
12.76 _+ 0.75
33.32 _+10.37
10.59 _+ 1.19
11.26 _+ 1.28
PO a
r > 5000 m
2799 -+ 208
362 _+200
2858 _+392
2130 _+316
81-+19
0-+ 1
135_+46
43 _+20
u(r) = vlr -r
Size range
3' a
Pl ( m r - 2 ) a
0.71+0.02
1.35_+0.06
0.77_+0.02
2.53+0.44
0.55+0.02
0.81_+0.04
7.54_+ 0.87×10 7
5.37_+ 2.27)<10 5
10.5 _+ 1.5 ×10 -7
81.9 _+235.2 )<10 -3
3.36_+ 0.53)<10 -7
14.23_+ 3.63×10 7
(m)
Alldata
All data
Alldata
0-2Ma
5-10Ma
30-40Ma
r > 1500 m
500-6000
1000-7500
500-7500
750-2000
750-4000
750-4000
Number per 106 km 2 a
r >1500m
r > 5000 m
4253_+ 489
2854_+1205
3812_+ 531
736_+2111
5816_+ 926
3699_+ 943
1814_+208
565_+238
1513_+211
35_+100
2982_+475
1388_+354
a Errors are lo uncertainties from maximum-likelihood estimates.
log plots (Fig. 4, top) and log-log plots (Fig. 4,
bottom). Both abundance diagrams are consistent
with a linear trend suggesting that both decay
functions can describe the observed abundances.
There is some indication on the abundance-radius
diagrams that the semi-log plot is linear to smaller
sizes than the log-log plot, implying that the exponential function has a wider range of validity.
Smith and Jordan [17] found the exponential distribution described observed abundances far better than the power-law distribution over large size
intervals.
Parameters for the decay laws 0'0 and a or ul
and 3') were fit to the measurements using a
maximum-likelihood estimation procedure developed by Smith and Jordan [17]. Parameters for the
exponential function are determined for seamount
sizes from 500 to 6000 m in radius and from 100
to 1000 m in height (Tables 2 and 3). Calculations
were made for a number of other size ranges to
investigate the dependence of the parameters on
the maximum and minimum sizes used.
For all distribution curves (Fig. 4) the large-size
limit of the observed cumulative distribution is set
to the predicted value since the actual number of
seamounts at larger sizes is poorly constrained by
the observations. Therefore, the estimated abundance inherently matches the total observed num-
ber of seamounts exactly [17] and the predicted
and observed cumulative curves will always agree
at the smallest and largest size range in which data
is taken. The parameters are being fit to the
incremental abundances, not the cumulative abundances, so that adjusting the cumulative curve in
this way does not affect the parameter estimates.
Because the counted incremental abundances are
plotted out to larger sizes than were actually fit to
the decay relations, some of the outlying points
appear greater than the cumulative distributions
allow.
The total number of seamounts (e.g. h > 300 m
in Table 2) predicted by both distributions roughly
agrees with wide-beam estimates. For large
seamounts (h > 1000 m in Table 2), however, the
exponential distribution function under-predicts
the wide-beam counts by a factor of 2 - 4 and the
power-law distribution over-predicts the same
counts by nearly an order of magnitude. Observed
abundances at the large sizes are low (Fig. 4), so
that the estimates for h > 1000 m are mostly based
on extrapolation from smaller sizes. Seamount
observations from wide-beam echo-sounders are
plentiful to heights of 1000-2000 m and should
provide more accurate abundance estimates for
these sizes. The large differences between data sets
may suggest that the slopes of the distribution
146
curves ( - / 3 or - "¢) change between the small-size
seamounts sampled in this study and the larger
volcanoes sampled by the wide-beam data set, for
either distribution model. Alternatively, they could
reflect differences in the regions sampled or some
sort of measurement bias. Some differences between wide-beam and Sea Beam seamount counts
were found by Smith and Jordan [17].
From the histograms the resolution limits of
this study can be estimated. At the smallest size
ranges (r < 500 m and h < 100 m), the number of
seamounts is much lower than would be expected
from the rate of abundance increase predicted by
larger seamounts. In part the accuracy of the Sea
Beam instrument (at best + 10 m in depth and +
100 m in location) limits the resolution, but more
often the background "noise" level of topographic
variation interferes with identification of small
seamounts. Features such as linear ridges, abyssal
hills, and relic ridge-crest structures frequently
have heights of 100 m in the Pacific and tend to
obscure identification of volcanic edifices of the
same size or smaller. Existent topography and
structure also significantly affect the morphology
associated with seafloor volcanism so that simple
conical seamounts are less common in more rugged
regions.
5. Variations with crustal age
The main advantage of studying seamount distribution with this data set is that the region
sampled is fairly homogeneous with respect to
tectonic history, although a range of ages are
covered. Therefore, the distribution should be
controlled by relatively few variables. As a first
estimate of heterogeneity in the seamount population, the locations of all features whose centers
were crossed are marked on a map with symbols
scaled to the seamount diameters (Fig. 5, top).
The portion of the East Pacific Rise in the study
area and the major fracture zones crossed by the
ship are shown for reference and indicate possible
controls on population segmentation. The distribution of seamounts appears uneven but other
than a near absence of volcanoes near the equator
where there is a thick sediment layer there are no
obvious large-scale variations parallel to the ridge.
At smaller scales seamounts and seamount-free
areas occur in clumps, and some of these clumps
1~$C°W
O°S
150°~¢
120°W
110°W
100°W
o
10°S
.'..~ ~,]e g
20°S I
I
GjI
seamounts
i
50°S
[
140°'h ,
i
' 30°W
.20o,~i
110°W
100°7,
iI
I
1
C,% l
/ /
i
4
i
j
y,
:argo
seamounts
{ r - ~-O00m},
!
50°S
]
Fig. 5. Map showing locations of seamounts whose centers
were crossed, south of the equator. Top: all the seamounts
observed. Bottom: all seamounts with radii measured larger
than r = 4000 m. The circle size is scaled to the seamount
diameter, exaggerated 5 times relative to map scale. Also
shown are the East Pacific Rise north of Easter Island, and the
major fracture zones.
appear continuous from one line of data to the
next, but it is difficult to be sure that these are not
within the expected range of random fluctuations.
By separating the large seamounts (r > 4000 m)
from the rest some patterns emerge (Fig. 5). Only
one large seamount is found at the ridge-crest
while more than twice as many by area occur on
the lines away from the ridge. By contrast, there
seems to be very little difference in the number of
smaller seamounts near and far from the East
Pacific Rise. Away from the ridge the number of
both large and small seamounts is roughly constant with seafloor age.
A more quantitative evaluation of the changes
in size distribution with crustal age is made by
comparing abundance-size relations for subsets of
the data (Fig. 6). Semi-log abundance histograms
147
All data. h:100
0-2
1 -j _- r.~
~la. t-100-60C, m
10 3
%
%
102
x. x
o
o
0.0
10 ~
o
o'~
o
~ ×
Oo
O..O
O-©
E
c
x
8o
o
"~.x
"0. 0
O O
[0 o
0
ado
~.x
0~
o
0o
0
L'
c
0
10-1
10-1
,
400
5-10
height
(m)
height
,
400
800
~
8 0
(rn)
3 0 - 4 0 Mo. h : 1 0 0 - 8 0 0 m
Ma, h : 1 0 0 - 8 0 0 m
103
'~E
102
10 2
o
o
O0
101
E
c
x. x
x.,x
o
O
~. x
bO
C~x"-.
•"
x'.
x-.
dO
100
o
•
o
Sex.
q
o
10 ~
10 -1
~X
":~"~ "-~ x
( 3 ° o ° Q - Q D "" %~'~-x
"-..o
x~-
;~.~
do..o
dO
Odd-
c
10 °
0
"000
10 ~
6
400
height
800
(rn)
4 0
8 0
height (m)
Fig. 6. Semi-log abundance histograms for the different age regions. Exponential model fits are also shown for the entire data set, for
the line on 0 - 2 M a crust, for the lines on 5 - 1 0 M a crust, and for the lines on 3 0 - 4 0 M a crust• All abundances are normalized by the
area ensonified by Sea Beam in the sample for comparison. Symbols are the same as in Fig. 3.
were generated and maximum-likelihood estimates
of the exponential-decay population parameters
were made for: (1) data just west of the East
Pacific Rise between 9 °S and 22 ° S, with 560 km
of track over 1.5-2 Ma crust and 1000 km over
0.3-0.7 Ma crust, (2) the three parallel lines and
two short segments connecting them between
l 1 5 ° W and 1 2 5 ° W on 5 - 1 0 Ma crust, and (3) the
three lines and two connecting segments between
1 2 8 ° W and 1 3 8 ° W on 30-40 Ma crust. In order
to compare different subsets, abundances in each
histogram were normalized by the seafloor area
sampled. Significant differences between normal-
ized plots should directly correspond to differences in frequency-size relationships, since for
any stationary population the abundances should
scale directly to the area covered (so long as only
seamounts whose centers were crossed are included).
A direct comparison of the normalized distribution histograms for the oldest two age regions
reveals an almost identical size distribution. At the
largest sizes differences exist, but since only one or
two seamounts are in any size range the sampling
may not be significant. In other studies [12-14,17]
it is observed that the number of very large
148
seamounts in the Pacific increases steadily with
crustal age, but the Sea Beam data presented here
show that below some moderate size (4000-6000
m in radius and 800-1000 m in height) there are
no more seamounts on 5-10 Ma crust than on
30-40 Ma crust. Repeated tests with different size
ranges showed that the minimum and maximum
sizes listed in Tables 2 and 3 are approximately
the largest ranges for which estimates were consistent with the observations, and are assumed to
represent the resolution limits of the study.
By contrast, the distribution of sizes for
seamounts on the youngest seafloor falls off significantly faster than for seamounts on older crust
(Fig. 6). The largest size resolved for seamounts on
0.5-2 Ma crust, about 400-500 m in height, is
much smaller than for the other subsets because
only one line was available instead of three. There
is 4-5 times less area covered in this sample than
in each of the older subsets (taking into account
the increase in swath width with water depth) so
the fits to the 0.5-2 Ma counts are somewhat
poor, reflecting the lack of coverage. The differences between the youngest-age subset and the
others is therefore marginally substantial. Twosigma error bars for the abundance estimates in
Fig. 7 would not overlap between the 0.5-2 Ma
sample and the other age samples. The accuracy
of the large relative uncertainty estimates predicted for the young-age sample is difficult to assess,
since the values are critically dependent on the
assumed form of the probability distribution and
the size range used. The inference of a low number
of large seamounts on zero-age crust is also supported by the distribution maps discussed earlier
(Fig. 5).
An added constraint is provided by examining
Sea Beam data directly along the East Pacific
Rise. We examined Sea Beam bathymetry recorded on the PASCUA-2 cruise along 800 km of
ridge-crest between 10°S and 25°S and did not
find any features that could be considered
seamounts on zero-age crust. Therefore, all of the
seamounts observed on the 0 - 2 Ma sample must
have formed away from the ridge, but not more
that 20-50 km distant.
The number of smallest (height < 250 m)
seamounts appears to be 50-100% greater for the
youngest age group than the others (Fig. 6), and is
reflected in a larger value of the zero-value u0
oh seomounts
40,30
35O0
5000
2500
E 2000
o
a 1500
c
1000
Smith ond Jordon: z~
This Study: [ ]
500
.El
'
1'0
2'0
3'0
4'0
5~0
6~0
5'0
6J0
oge (My)
[orge s e o m o u n t s
400
55O
300
250
oE 200
o
A
150
100
+
50
l:m
i Jo
2'0
3'0
4'0
age (My)
Fig. 7. Predicted abundances for all seamounts (h > 300 m)
and for large seamounts (h > 1000 m), using the fits to the
exponential distribution. Points with boxes are from the data
presented in this study, and those with triangles are from the
wide-beam study by Smith and Jordan [17]. Vertical error bars
are 1o and horizontal bars show age ranges. Note change in
vertical scale between the plots.
(Tables 2 and 3). Some possible explanations for
the apparent decrease in the number of small
seamounts with a g e are insufficient near-ridge
sampling, sampling bias due to increased background "noise" with age, and recent changes in
near-ridge seamount production rates. Alternatively, the number of smallest seamounts may
actually decrease with crustal age as clusters of
small volcanoes grow and merge into large single
seamounts. Sedimentary burial may cause a decrease in the number of small seamounts [13], but
149
the observed sediment thickness on all lines was
generally less than 50-100 m.
6. Discussion
The abundance estimates presented here predict that there are over 7000 seamounts with
heights of 100 m or more and over 800 seamounts
with heights greater than 500 m per 10 6 k m 2.
These estimates are similar to those of previous
statistical sampling studies [16,17] but are over an
order of magnitude more than estimated from
map counts (Table 2) [13].
The decrease in the number of seamounts with
increasing size can be described as either an exponential decay (%e -~r) or a power-law decay
(~,lr-V). Seamount counts seem to fit the exponential distribution function better at small sizes than
the power-law, but this is only marginally apparent from our data (Fig. 4). Neither distribution, when fit to the populations counted here,
predicts the abundances of large seamounts determined from wide-beam echo-sounder records
(Table 2) [17]. These simple two-parameter distributions may be inadequate to describe the distribution of seamounts over large size ranges.
The number of seamounts increases dramatically between the ridge-crest and 5-10 Ma crust
but changes little after that (Fig. 7). The maps of
seamount locations (Fig. 5) and the abundance
histograms (Fig. 6) show that the increase in numbers is most rapid for small seamounts which do
not show any significant change in abundance
after 0.5-2 Ma. The number of larger seamounts
increases until 5-10 Ma, and previous studies of
more the extensive sets of wide-beam sonar records [17] indicate that the number of even larger
features may increase on older crust. The numbers
of small seamounts counted at 5-10 Ma and 30-40
Ma are essentially identical to those determined
from wide-beam data in other parts of the Pacific
(Fig. 7, top). The agreement is somewhat surprising since large along-strike regional variations in
abundance have been observed [17].
It is possible that the observed age variations
are due to temporal changes in magma production, and that there are less large seamounts being
produced now than 10 Ma ago. There has been a
reorganization of the Pacific-Nazca spreading system in the last 10 Ma [31] so that changes in
seamount production would not be surprising.
However, observations of similar abundance
changes in other parts of the Pacific (Fig. 7, top)
suggest that the change observed here is related to
processes that depend predominantly on lithospheric age.
We can speculate as to the origin of these
seamounts which (unlike hot-spot volcanoes) are
probably a byproduct of upwelling and accretion
at a normal mid-ocean ridge. Partial melt produced by large-scale upwelling under the ridgecrest is likely to be the dominant source of magma
for the seamounts observed here, because nearly
all the growth in seamount population occurs near
the ridge. Much of the basaltic melt forming at
depth probably reaches the surface at the ridge
crest since only - 0.4% of the total crustal volume
is necessary to produce the volume of seamounts
observed (Table 1). The heterogeneous petrology
of near-ridge seamounts suggests that they do not
tap directly the voluminous and well-mixed magma
bodies just below the ridge crest but rather receive
magma from deeper levels, where there is less
mixing [2]. As melt ascends towards the ridge it is
likely that small quantities of magma (not necessarily well mixed) would not reach the ridge axis
but would be trapped beneath the lithosphere.
Concentrations of trapped melt could and sometimes penetrate the lithosphere to form seamounts.
These small volumes of melt, which would produce small seamounts, could ascend through the
thin lithosphere near the ridge but probably are
unable to penetrate older and thicker lithosphere
because they would solidify before reaching the
surface. The amount of available melt would decrease with time as the lithosphere moves farther
from ascending magma beneath the ridge crest,
and as remaining trapped melt reaches the surface
or crystallizes in place. Production of small
seamounts would stop as the lithosphere becomes
too thick to permit passage of magma and as the
volume of available melt decreases.
Larger volumes of melt would be more likely
than small volumes to pass through thick lithosphere without completely solidifying. Probably
almost all large concentrations of melt formed
near the ridge crest come out as new seafloor so
that there are few sources for large volcanoes on
very young crust (excluding hot-spot sources such
as Iceland). Small melt pockets could coalesce to
150
b e c o m e b o d i e s b i g e n o u g h to f o r m l a r g e r
s e a m o u n t s , b u t t h e m i g r a t i o n w o u l d t a k e time.
T h u s , l a r g e r m a g m a sources m i g h t b e d i s t r i b u t e d
f u r t h e r f r o m the ridge t h a n s m a l l sources. C o n t i n u e d g r o w t h at the s u r f a c e of s m a l l e r s e a m o u n t s
m a y p r o d u c e larger o n e s if a d d i t i o n a l m e l t passes
through lithosphere heated by earlier magma. Also,
isostatic b a l a n c e suggests taller v o l c a n o e s can f o r m
o n o l d e r crust as the m a g m a c o n d u i t s b e c o m e
longer through thicker lithosphere. Continued prod u c t i o n of large s e a m o u n t s is likely d u e to a
c o m b i n a t i o n o f these causes, a n d is f a v o r e d relative to p r o d u c t i o n o f s m a l l e r s e a m o u n t s p r i m a r i l y
b e c a u s e of a g r e a t e r a b i l i t y of l a r g e r m a g m a b o d ies to m i g r a t e t h r o u g h the l i t h o s p h e r e a n d p o s s i b l y
b e c a u s e o f an i n c r e a s i n g a v e r a g e size for m a g m a
sources.
Acknowledgements
G e o f f r e y A b e r s g r a t e f u l l y a c k n o w l e d g e s support from a National Science Foundation graduate
fellowship. T h i s s t u d y is b a s e d o n o b s e r v a t i o n s
m a d e d u r i n g leg R C 2 6 0 8 of R / V " R o b e r t D.
C o n r a d " w h i c h was f u n d e d b y N a t i o n a l S c i e n c e
F o u n d a t i o n g r a n t s O C E - 8 4 1 8 3 7 1 to M I T a n d
OCE-8418119
to C o l u m b i a
University. Tom
J o r d a n a n d D e b b i e S m i t h s p e n t m u c h t i m e disc u s s i n g their w o r k a n d p r o v i d i n g advice. W e w o u l d
like to t h a n k t h e N E C O R Sea B e a m p e r s o n n e l o n
this leg for their efforts: J o y c e Miller, J o h n F r e i tag, a n d N i c k Kallas. J o h n M a d s e n k i n d l y let us
use s o m e p r o g r a m s he h a d w r i t t e n to d i s p l a y Sea
B e a m s w a t h profiles, a n d J e f f F o x p r o v i d e d a t a p e
o f P A S C U A - 2 Sea B e a m r e c o r d s a l o n g the ridgecrest. A d d i t i o n a l s u p p o r t for this analysis c a m e
from Office of Naval Research contract N001486-K-0325.
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