Geoscience Reference
In-Depth Information
Pacific. Almost every volcanic island, seamount or
seamount chain surmounts a broad topographic
swell. The swells generally occur directly beneath
the volcanic centers and extend along fracture
zones. Small regions of anomalously shallow
depth occur in the northwestern Indian Ocean
south of Pakistan, in the western North Atlantic
near the Caribbean, in the Labrador Sea and in
the southernmost South Pacific. They are not
associated with volcanism but are slow regions
of the upper mantle as determined from seismic
tomography.
Shallow regions probably associated with
plate flexure border the Kurile Trench, the
Aleutian Trench and the Chile Trench. Major
volcanic lineaments without swells include the
northern end of the Emperor Seamount chain,
the Cobb Scamounts off the west coast of North
America and the Easter Island trace on the East
Pacific Rise. Bermuda and Vema, in the south-
east Atlantic, are isolated swells with no associ-
ated volcanic trace. For most of the swells expla-
nations based on sediment or crustal thickness
and plate flexure can be ruled out. They seem
instead to be due to variations in lithospheric
composition or thickness, or abnormal upper
mantle. Dike and sill intrusion, underplating of
the lithosphere by basalt or depleted peridotite,
serpentinization of the lithosphere, delamina-
tion, or reheating and thinning the lithosphere
are mechanisms that can decrease the density
or thickness of the lithosphere and cause uplift
of the seafloor. A higher temperature astheno-
sphere, greater amounts of partial melt, chem-
ical inhomogeneity of the asthenosphere and
upwelling of the asthenosphere are possible sub-
lithospheric mechanisms.
A few places are markedly deep, notably the
seafloor between Australia and Antarctica -- the
Australian--Antarctic Discordance or AAD -- and
the Argentine Basin of the South Atlantic. Other
deep regions occur in the central Atlantic and
the eastern Pacific and others, most notably
south of India, are not so obvious because of
deep sedimentary fill. Most of the negative areas
are less than 400 m below the expected depth,
and they comprise a relatively small fraction of
the seafloor area. They represent cold mantle,
lower melt contents, dense lower crust or an
underlying and sinking piece of subducted slab
or delaminated lower crust.
Dynamic topography
The long-wavelength topography is a dynamic
effect of a convecting mantle. It is difficult to
determine because of other effects such as crustal
thickness. Density and thermal variations in a
convecting mantle deform the surface, and this
is known as the
mantle dynamic topography
.
The
long-wavelength geoid
of the Earth is
controlled by density variations in the deep man-
tle and has been explained by circulation models
involving whole mantle flow. However, the rela-
tionship of long-wavelength topography to man-
tle circulation has been a puzzling problem in
geodynamics. Dynamic topography is mainly due
to density variations in the upper mantle. Lay-
ered mantle convection, with a shallow origin for
surface dynamic topography, is consistent with
the spectrum, small amplitude and pattern of the
topography.
Layered mantle convection,
with a barrier near 1000 km
depth provides a
self-consistent geodynamic model for the amplitude and
pattern of both the long-wavelength geoid and surface
topography
.
The geoid
The centrifugal effect of the Earth's rotation
causes an equatorial bulge, the principal depar-
ture of the Earth's surface from a spherical shape.
If the Earth were covered by oceans then, apart
from winds and internal currents, the surface
would reflect the forces due to rotation and the
gravitational attraction of external bodies, such
as the Sun and the Moon, and effects arising from
the interior. When tidal effects are removed, the
shape of the surface is due to density variations
in the interior. Mean sea level is an equipotential
surface called the geoid or figure of the Earth.
Crustal features, continents, mountain ranges
and midoceanic ridges represent departures of
the actual surface from the geoid, but mass com-
pensation at depth, isostasy, minimizes the influ-
ence of surface features on the geoid. To first
