Oak Ridge National Laboratory, US Department of Energy,
ORNL/CDIAC-145, NDP-083.
Sandberg, P. A., 1983. An oscillating trend in phanerozoic nonskeletal carbonate mineralogy. Nature, 305,19–22.
Silverman, J., Lazar, B., Cao, L., Caldeira, K., and Erez, J., 2009b.
Coral reefs may start dissolving when atmospheric CO 2 doubles.
Geophysical Research Letters, 36, L05606, doi:10.1029/
2008GL036282.
Silverman, J., Lazar, B., and Erez, J., 2009a. Effect of aragonite saturation, temperature, and nutrients on the community calcification rate of a coral reef. Journal of Geophysical Research, 112,
C05004, doi:10.1029/2006JC003770.
Tribollet, A., Godinot, C., Atkinson, M. J., and Langdon, C., 2009.
Effects of elevated pCO 2 on dissolution of coral carbonates by
microbial euendoliths. Global Biogeochemical Cycles, 23,
GB3008, doi:10.1029/2008GB003286.
Veron, J. E. N., 2008. Mass extinctions and ocean acidification: biological constraints on geological dilemmas. Coral Reefs, 27,
459–472.
Yates, K. K., and Halley, R. B., 2006. CO 3
2À concentration and
pCO 2 thresholds for calcification and dissolution on the Molokai
reef flat, Hawaii. Biogeosciences, 3, 357–369.
Cross-references
Algae, Coralline
Aragonite
Calcite
Carbonate Budgets and Reef Framework Accumulation
Climate Change and Coral Reefs
Corals: Biology, Skeletal Deposition, and Reef-Building
Mass Extinctions, Anoxic Events and Ocean Acidification
Rhodoliths
OCEANIC HOTSPOTS
Paul Wessel
University of Hawaii at Manoa, Honolulu, HI, USA
Definition
Site occupied by a seamount or oceanic island, usually
located away from plate boundaries, where long-lived
(>5 Myr), volcanism occurs or has recently occurred.
Introduction
Volcanism occurs mainly at or near active plate boundaries, in particular at mid-ocean ridges. However, there
are important exceptions where volcanic activity occurs
within plate interiors. In the ocean basins, the prime observational evidence comes from the presence of linear
chains of islands and seamounts, sometimes extending
for thousands of kilometers across the ocean basins. These
island and seamount chains exhibit a general progression
of decreasing elevation along the chain from volcanic
island to fringing reef, to atoll, and finally to a submerged
flat-topped seamount (guyot). An active or dormant volcano usually occupies the young end of an island chain,
with progressively older and extinct volcanoes occurring
along the rest of the chain. Such topographic features were
suggested to have formed as the plate moved over
a relatively stationary “hot spot” in the Earth’s mantle,
where an upwelling mantle plume brings hot magma from
great depths to the surface (Morgan, 1971; Wilson, 1963).
The archetypal example of this process is the HawaiianEmperor seamount chain (Figure 1). The mantle plume
that produced this $6,000 km long chain is presently situated beneath the Big Island of Hawaii, the site of active
volcanism today. Extending 3,500 km to the northwest lies
a linear chain of islands, atolls, and seamounts; there, near
the Diakakuji Seamount the chain exhibits a sharp $60
change in orientation and continues another 2,500 km
north-northwest to the Aleutian trench. The age of the volcanism gets progressively older with increasing distance
from Hawaii, which is seen as a key observation that
validates the hot spot hypothesis. Although predominantly
an oceanic phenomenon, hot spot volcanism is not
restricted to the ocean basins; volcanism of a similar type
occurs within continents, one example being the activity
of Yellowstone National Park in the western North
America.
Origin of oceanic hotspots
The hotspot hypothesis states that seamount chains and
oceanic islands are the surface manifestation of impinging
mantle plumes. These upwelling mantle plumes are
thought to originate either at the core-mantle boundary
(2,900 km depth) or the boundary between the lower and
upper mantle (670 km depth). One theory suggests that
a “plume head” develops, above a “plume stem” (e.g.,
Richards et al., 1989) but other scenarios of plume development with double plume heads (Bercovici and
Mahoney, 1994) and time varying magma output (Coffin
et al., 2002) have also been considered. As originally proposed (Morgan, 1971; Wilson, 1963), these plumes were
considered stationary relative to the lithospheric plates
that move over them, but recent evidence suggests that
some plumes may have experienced considerable drift
over their active lifespan. In particular, deep-sea drilling
of seamounts in the northern Emperor seamount chain
recovered oriented samples the magnetic memories of
which imply that they must have formed at latitudes some
10–15
further north than the present location of the
Hawaii hot spot. These systematic discrepancies have lead
to suggestions that the mantle plume was further north
in the past and consequently has drifted south since
the formation of these seamounts (e.g., Tarduno et al.,
2009).
It is not entirely clear how mantle plumes manage to
reach the surface. Apparently weakened by the impinging
plume, the lithosphere allows magma to migrate through
cracks and fractures and eventually reach the surface.
There, a volcano builds upon the surface of the plate
directly above the plume. The constant motion of the
plate, however, will eventually carry the volcano too
far from the source of magma and the volcano becomes
extinct. As extinct volcanoes no longer have the
OCEANIC HOTSPOTS
737
ORNL/CDIAC-145, NDP-083.
Sandberg, P. A., 1983. An oscillating trend in phanerozoic nonskeletal carbonate mineralogy. Nature, 305,19–22.
Silverman, J., Lazar, B., Cao, L., Caldeira, K., and Erez, J., 2009b.
Coral reefs may start dissolving when atmospheric CO 2 doubles.
Geophysical Research Letters, 36, L05606, doi:10.1029/
2008GL036282.
Silverman, J., Lazar, B., and Erez, J., 2009a. Effect of aragonite saturation, temperature, and nutrients on the community calcification rate of a coral reef. Journal of Geophysical Research, 112,
C05004, doi:10.1029/2006JC003770.
Tribollet, A., Godinot, C., Atkinson, M. J., and Langdon, C., 2009.
Effects of elevated pCO 2 on dissolution of coral carbonates by
microbial euendoliths. Global Biogeochemical Cycles, 23,
GB3008, doi:10.1029/2008GB003286.
Veron, J. E. N., 2008. Mass extinctions and ocean acidification: biological constraints on geological dilemmas. Coral Reefs, 27,
459–472.
Yates, K. K., and Halley, R. B., 2006. CO 3
2À concentration and
pCO 2 thresholds for calcification and dissolution on the Molokai
reef flat, Hawaii. Biogeosciences, 3, 357–369.
Cross-references
Algae, Coralline
Aragonite
Calcite
Carbonate Budgets and Reef Framework Accumulation
Climate Change and Coral Reefs
Corals: Biology, Skeletal Deposition, and Reef-Building
Mass Extinctions, Anoxic Events and Ocean Acidification
Rhodoliths
OCEANIC HOTSPOTS
Paul Wessel
University of Hawaii at Manoa, Honolulu, HI, USA
Definition
Site occupied by a seamount or oceanic island, usually
located away from plate boundaries, where long-lived
(>5 Myr), volcanism occurs or has recently occurred.
Introduction
Volcanism occurs mainly at or near active plate boundaries, in particular at mid-ocean ridges. However, there
are important exceptions where volcanic activity occurs
within plate interiors. In the ocean basins, the prime observational evidence comes from the presence of linear
chains of islands and seamounts, sometimes extending
for thousands of kilometers across the ocean basins. These
island and seamount chains exhibit a general progression
of decreasing elevation along the chain from volcanic
island to fringing reef, to atoll, and finally to a submerged
flat-topped seamount (guyot). An active or dormant volcano usually occupies the young end of an island chain,
with progressively older and extinct volcanoes occurring
along the rest of the chain. Such topographic features were
suggested to have formed as the plate moved over
a relatively stationary “hot spot” in the Earth’s mantle,
where an upwelling mantle plume brings hot magma from
great depths to the surface (Morgan, 1971; Wilson, 1963).
The archetypal example of this process is the HawaiianEmperor seamount chain (Figure 1). The mantle plume
that produced this $6,000 km long chain is presently situated beneath the Big Island of Hawaii, the site of active
volcanism today. Extending 3,500 km to the northwest lies
a linear chain of islands, atolls, and seamounts; there, near
the Diakakuji Seamount the chain exhibits a sharp $60
change in orientation and continues another 2,500 km
north-northwest to the Aleutian trench. The age of the volcanism gets progressively older with increasing distance
from Hawaii, which is seen as a key observation that
validates the hot spot hypothesis. Although predominantly
an oceanic phenomenon, hot spot volcanism is not
restricted to the ocean basins; volcanism of a similar type
occurs within continents, one example being the activity
of Yellowstone National Park in the western North
America.
Origin of oceanic hotspots
The hotspot hypothesis states that seamount chains and
oceanic islands are the surface manifestation of impinging
mantle plumes. These upwelling mantle plumes are
thought to originate either at the core-mantle boundary
(2,900 km depth) or the boundary between the lower and
upper mantle (670 km depth). One theory suggests that
a “plume head” develops, above a “plume stem” (e.g.,
Richards et al., 1989) but other scenarios of plume development with double plume heads (Bercovici and
Mahoney, 1994) and time varying magma output (Coffin
et al., 2002) have also been considered. As originally proposed (Morgan, 1971; Wilson, 1963), these plumes were
considered stationary relative to the lithospheric plates
that move over them, but recent evidence suggests that
some plumes may have experienced considerable drift
over their active lifespan. In particular, deep-sea drilling
of seamounts in the northern Emperor seamount chain
recovered oriented samples the magnetic memories of
which imply that they must have formed at latitudes some
10–15
further north than the present location of the
Hawaii hot spot. These systematic discrepancies have lead
to suggestions that the mantle plume was further north
in the past and consequently has drifted south since
the formation of these seamounts (e.g., Tarduno et al.,
2009).
It is not entirely clear how mantle plumes manage to
reach the surface. Apparently weakened by the impinging
plume, the lithosphere allows magma to migrate through
cracks and fractures and eventually reach the surface.
There, a volcano builds upon the surface of the plate
directly above the plume. The constant motion of the
plate, however, will eventually carry the volcano too
far from the source of magma and the volcano becomes
extinct. As extinct volcanoes no longer have the
OCEANIC HOTSPOTS
737