and strong) oceanic lithosphere may in some circumstances attain heights of almost 10 km (measured from
the seafloor to the tallest peak on an island). This is the
case for Mauna Kea, one of five volcanoes that form the
Big Island of Hawaii, which by this definition is the tallest
mountain on Earth. Given that the smallest features considered by most geologists to be seamounts are 50–
100 m tall, the sizes of observed seamounts span almost
three orders of magnitude.
Studies have found that the number of seamounts varies
considerably across the oceans and that they tend to form
both linear and random constellations. Furthermore, their
sizes and distributions provide invaluable information
about their origins. Ranging from single-beam echosounder profiles, via multibeam surveys, to satellite altimetry, studies have found that the distribution of seamounts
can be reasonably well explained by an exponential or
power-law model (e.g., Craig and Sandwell, 1988; Smith
and Jordan, 1987; Wessel, 2001). Such models support
the observation that most seamounts are fairly small.
Extrapolations from the power-law trends obtained for
large seamounts suggest that perhaps over 100,000 seamounts of heights !1 km may be present in the oceans.
Extrapolating further down to the smallest sizes observed
(a few tens of meters) would predict a population over one
million. However, sediments will most likely have buried
the bulk of those seamounts, considering typical 100–
200 m sediment thicknesses in the ocean basins (e.g.,
Ludwig and Houtz, 1979). Consequently, we find that
most of the smallest seamounts reside on young seafloor
where the sediment cover is modest or nonexistent.
We do not know why seamount abundances vary spatially. One factor may be the underlying distribution of
mantle plumes, which seem to be found in higher numbers
beneath plates with the largest seamount abundances.
Another factor may be systematic variations in plate
stresses. We note that smaller plates are possibly in compression, which could prevent the intrusion of magma.
Smaller plates are also less likely to have a directional
regional stress dominating the state of stress, and often
are relatively young and buoyant (e.g., the Cocos plate);
during subduction, such plates would only be associated
with slightly negative buoyancy forces. On the other hand,
the large Pacific plate, and specifically its equatorial
region, appears to be under tension from the slab pull
forces at the distant subduction zones, as evidenced by
widespread extensional volcanism that is neither associated with hotspots nor mid-ocean ridges (e.g., Sandwell
et al., 1995; Wessel and Kroenke, 2007). Finally, plates
that move the fastest over the underlying mantle appear
to have the highest seamount abundances provided they
share at least one spreading plate boundary.
Summary
Oceanic hotspots are the surface expression of rising mantle plumes from the Earth’s interior and are responsible for
much of the intraplate volcanism observed in the ocean
basins. Being relatively stationary with respect to the surrounding mantle, the oceanic hotspots produce linear
chains of islands and seamounts as tectonic plates move
over these sites.
Bibliography
Bercovici, D., and Mahoney, J., 1994. Double flood basalts and
plume head separation at the 660-kilometer discontinuity. Science, 266, 1367–1369.
Burke, K. C., and Wilson, J. T., 1976. Hot spots on the Earth’s surface. Journal of Geophysical Research, 93, 7690–7708.
Cazenave, A., Dominh, K., Rabinowicz, M., and Ceuleneer, G.,
1988. Geoid and depth anomalies over ocean swells and troughs:
evidence of an increasing trend of the geoid to depth ratio
with age of plate. Journal of Geophysical Research, 93(B7),
8064–8077.
Clouard, V., and Bonneville, A., 2001. How many Pacific hotspots
are fed by deep-mantle plumes? Geology, 29, 695–698.
Coffin, M. F., Prince, M. S., and Duncan, R. A., 2002. Kerguelen
hotspot magma output since 130 Ma. Journal of Petrol, 43,
1121–1139.
Craig, C. H., and Sandwell, D. T., 1988. Global distribution of seamounts from Seasat profiles. Journal of Geophysical Research,
93(B9), 10408–10420.
Detrick, R. S., and Crough, S. T., 1978. Island subsidence, hot spots,
and lithospheric thinning. Journal of Geophysical Research, 83,
1236–1244.
Hess, H. H., 1946. Drowned ancient islands of the Pacific Basin.
American Journal of Science, 244, 772–791.
Jordan, T. H., 1979. Mineralogies, densities, and seismic velocities
of garnet lherzolites and their geophysical implications. In
Boyd F. R., and Meyer H. O. A. (eds.), The Mantle Sample:
Inclusions in Kimberlites and Other Volcanics. Washington,
DC: American Geophysical Union, pp. 1–14.
Ludwig, W. J., and Houtz, R. E., 1979. Isopach Map of the Sediments in the Pacific Ocean Basin, Color Map with Text. Tulsa,
OK: American Association of Petroleum Geologists.
McNutt, M. K., 1998. Superswells. Reviews of Geophysics, 36(2),
211–244.
McNutt, M. K., Caress, D. W., Reynolds, J., Jordahl, K. A., and
Duncan, R. A., 1997. Failure of plume theory to explain
midplate volcanism in the southern Austral Islands. Nature,
389(6650), 479–482.
Montelli, R., et al., 2004. Finite-frequency tomography reveals
a variety of plumes in the mantle. Science, 303(5656), 338–343.
Morgan, W. J., 1971. Convection plumes in the lower mantle.
Nature, 230, 43–44.
Nataf, H. -C., 2000. Seismic imaging of mantle plumes. Annual
Review of Earth and Planetary Sciences, 28, 391–417.
Olson, P., 1990. Hot spots, swells and mantle plumes. In Ryan M. P.
(ed.), Magma Transport and Storage. New York: John Wiley,
pp. 33–51.
Phipps Morgan, J., Morgan, W. J., and Price, E., 1995. Hotspot
melting generates both hotspot volcanism and a hotspot swell?
Journal of Geophysical Research, 100(B5), 8045–8062.
Ribe, N. M., and Christensen, U. R., 1999. The dynamical origin of
Hawaiian volcanism. Earth and Planetary Science Letters, 171,
517–531.
Richards, M. A., Duncan, R. A., and Courtillot, V., 1989. Flood
basalts and hot spot tracks: plume heads and tails. Science,
246, 103–107.
Sandwell, D. T., et al., 1995. Evidence for diffuse extension of the
Pacific plate from Pukapuka ridges and cross-grain gravity lineations. Journal of Geophysical Research, 100(B8), 15087–
15099.
OCEANIC HOTSPOTS
739
the seafloor to the tallest peak on an island). This is the
case for Mauna Kea, one of five volcanoes that form the
Big Island of Hawaii, which by this definition is the tallest
mountain on Earth. Given that the smallest features considered by most geologists to be seamounts are 50–
100 m tall, the sizes of observed seamounts span almost
three orders of magnitude.
Studies have found that the number of seamounts varies
considerably across the oceans and that they tend to form
both linear and random constellations. Furthermore, their
sizes and distributions provide invaluable information
about their origins. Ranging from single-beam echosounder profiles, via multibeam surveys, to satellite altimetry, studies have found that the distribution of seamounts
can be reasonably well explained by an exponential or
power-law model (e.g., Craig and Sandwell, 1988; Smith
and Jordan, 1987; Wessel, 2001). Such models support
the observation that most seamounts are fairly small.
Extrapolations from the power-law trends obtained for
large seamounts suggest that perhaps over 100,000 seamounts of heights !1 km may be present in the oceans.
Extrapolating further down to the smallest sizes observed
(a few tens of meters) would predict a population over one
million. However, sediments will most likely have buried
the bulk of those seamounts, considering typical 100–
200 m sediment thicknesses in the ocean basins (e.g.,
Ludwig and Houtz, 1979). Consequently, we find that
most of the smallest seamounts reside on young seafloor
where the sediment cover is modest or nonexistent.
We do not know why seamount abundances vary spatially. One factor may be the underlying distribution of
mantle plumes, which seem to be found in higher numbers
beneath plates with the largest seamount abundances.
Another factor may be systematic variations in plate
stresses. We note that smaller plates are possibly in compression, which could prevent the intrusion of magma.
Smaller plates are also less likely to have a directional
regional stress dominating the state of stress, and often
are relatively young and buoyant (e.g., the Cocos plate);
during subduction, such plates would only be associated
with slightly negative buoyancy forces. On the other hand,
the large Pacific plate, and specifically its equatorial
region, appears to be under tension from the slab pull
forces at the distant subduction zones, as evidenced by
widespread extensional volcanism that is neither associated with hotspots nor mid-ocean ridges (e.g., Sandwell
et al., 1995; Wessel and Kroenke, 2007). Finally, plates
that move the fastest over the underlying mantle appear
to have the highest seamount abundances provided they
share at least one spreading plate boundary.
Summary
Oceanic hotspots are the surface expression of rising mantle plumes from the Earth’s interior and are responsible for
much of the intraplate volcanism observed in the ocean
basins. Being relatively stationary with respect to the surrounding mantle, the oceanic hotspots produce linear
chains of islands and seamounts as tectonic plates move
over these sites.
Bibliography
Bercovici, D., and Mahoney, J., 1994. Double flood basalts and
plume head separation at the 660-kilometer discontinuity. Science, 266, 1367–1369.
Burke, K. C., and Wilson, J. T., 1976. Hot spots on the Earth’s surface. Journal of Geophysical Research, 93, 7690–7708.
Cazenave, A., Dominh, K., Rabinowicz, M., and Ceuleneer, G.,
1988. Geoid and depth anomalies over ocean swells and troughs:
evidence of an increasing trend of the geoid to depth ratio
with age of plate. Journal of Geophysical Research, 93(B7),
8064–8077.
Clouard, V., and Bonneville, A., 2001. How many Pacific hotspots
are fed by deep-mantle plumes? Geology, 29, 695–698.
Coffin, M. F., Prince, M. S., and Duncan, R. A., 2002. Kerguelen
hotspot magma output since 130 Ma. Journal of Petrol, 43,
1121–1139.
Craig, C. H., and Sandwell, D. T., 1988. Global distribution of seamounts from Seasat profiles. Journal of Geophysical Research,
93(B9), 10408–10420.
Detrick, R. S., and Crough, S. T., 1978. Island subsidence, hot spots,
and lithospheric thinning. Journal of Geophysical Research, 83,
1236–1244.
Hess, H. H., 1946. Drowned ancient islands of the Pacific Basin.
American Journal of Science, 244, 772–791.
Jordan, T. H., 1979. Mineralogies, densities, and seismic velocities
of garnet lherzolites and their geophysical implications. In
Boyd F. R., and Meyer H. O. A. (eds.), The Mantle Sample:
Inclusions in Kimberlites and Other Volcanics. Washington,
DC: American Geophysical Union, pp. 1–14.
Ludwig, W. J., and Houtz, R. E., 1979. Isopach Map of the Sediments in the Pacific Ocean Basin, Color Map with Text. Tulsa,
OK: American Association of Petroleum Geologists.
McNutt, M. K., 1998. Superswells. Reviews of Geophysics, 36(2),
211–244.
McNutt, M. K., Caress, D. W., Reynolds, J., Jordahl, K. A., and
Duncan, R. A., 1997. Failure of plume theory to explain
midplate volcanism in the southern Austral Islands. Nature,
389(6650), 479–482.
Montelli, R., et al., 2004. Finite-frequency tomography reveals
a variety of plumes in the mantle. Science, 303(5656), 338–343.
Morgan, W. J., 1971. Convection plumes in the lower mantle.
Nature, 230, 43–44.
Nataf, H. -C., 2000. Seismic imaging of mantle plumes. Annual
Review of Earth and Planetary Sciences, 28, 391–417.
Olson, P., 1990. Hot spots, swells and mantle plumes. In Ryan M. P.
(ed.), Magma Transport and Storage. New York: John Wiley,
pp. 33–51.
Phipps Morgan, J., Morgan, W. J., and Price, E., 1995. Hotspot
melting generates both hotspot volcanism and a hotspot swell?
Journal of Geophysical Research, 100(B5), 8045–8062.
Ribe, N. M., and Christensen, U. R., 1999. The dynamical origin of
Hawaiian volcanism. Earth and Planetary Science Letters, 171,
517–531.
Richards, M. A., Duncan, R. A., and Courtillot, V., 1989. Flood
basalts and hot spot tracks: plume heads and tails. Science,
246, 103–107.
Sandwell, D. T., et al., 1995. Evidence for diffuse extension of the
Pacific plate from Pukapuka ridges and cross-grain gravity lineations. Journal of Geophysical Research, 100(B8), 15087–
15099.
OCEANIC HOTSPOTS
739