Geoscience Reference
In-Depth Information
erosion and lower crustal delamination, are asso-
ciated with low relief, less than 1 km, and thin-
ner crusts. Regional changes in the topography
of the continents are generally accompanied by
changes in mean crustal thickness. Continents
stand high because of thick, low-density crust,
compared with oceans. There is a sharp cut-off
in crustal thickness at about 50 km, probably
due to delamination of over-thickened crust at
the gabbro--eclogite phase change boundary. As
the dense root grows, the surface subsides, form-
ing sedimentary basins. Upon delamination, the
surface pops up, forming a swell, often accom-
panied by magmatism. Many continental flood
basalt provinces (CFB) erupt on top of sedimen-
tary basins and the underlying crust is thinner
than average for the continents.
The long-wavelength topography of the ocean
floor exhibits a simple relationship to crustal
age, after averaging and smoothing. The system-
atic increase in the depth of the ocean floor
away from the midocean ridges can be explained
by simple cooling models for the evolution of
the oceanic lithosphere. The mean depth of
ocean ridges is 2.5 km below sealevel although
regional variations off 1 km around the mean
are observed. Thermal subsidence of the seafloor
is well approximated by an empirical relationship
of the form
0 to 70 Myr, topography are described by
d
(
t
)
=
2500
+
350
t
1
/
2
where
t
is crustal age in Myr and
d
(
t
)isthe
depth in meters. Older seafloor does not follow
this simple relationship, being shallower than
predicted, and there is much scatter at all ages.
Slightly different relations hold if the seafloor
is subdivided into tectonic corridors. There are
large portions of the ocean floor where depth
cannot be explained by simple thermal models;
these include oceanic islands, swells, aseismic
ridges and oceanic plateaus as well as other areas
where the effects of surface tectonics and crustal
structure are not readily apparent. Simple cool-
ing models assume that the underlying mantle
is uniform and isothermal and that all of the
variation in bathymetry is due to cooling of a
thermal boundary layer (TBL). The North Atlantic
is generally too shallow for its age, and the
Indian Ocean between Australia and Antarctica
is too deep. Continental insulation, a chemically
heterogenous mantle and accumulated slabs at
depth may explain these anomalies. There is no
evidence that shallow regions are caused by par-
ticularly hot mantle. In fact, there is evidence for
moderate mantle temperature anomalies
associated with hotspot volcanism
.
Residual depth anomalies
, the depar-
ture of the depth of the ocean from the value
expected for its age, in the ocean basins have
dimensions of order 2000 km and amplitudes
greater than 1 km. Part of the residual anoma-
lies are due to regional changes in crustal thick-
ness. This cannot explain all of the anomalies.
Positive (shallow) depth anomalies -- or swells --
are often associated with volcanic regions such as
Bermuda, Hawaii, the Azores and the Cape Verde
Islands. These might be due to thinning of the
plate, chemically buoyant material in the shal-
low mantle, or the presence of abnormally hot
upper mantle. Patches of eclogite in the man-
tle are dense when they are colder than ambi-
ent mantle, but they melt at temperatures some
200
◦
C colder than peridotite and can therefore be
responsible for elevation and melting anomalies.
Shallow areas often exceed 1200 m in height
above the expected depth and occupy almost the
entire North Atlantic and most of the western
At
1
/
2
d
(
t
)
=
d
o
+
where
d
is seafloor depth referred to sea-level
and positive downward,
d
o
is mean depth of mid-
ocean ridges and
t
is crustal age. The value of
A
is around 350 m/(my)
l
/
2
if
d
and
d
o
are expressed
in meters and
t
in my.
Depth anomalies
or
resid-
ual depth anomalies
refer to oceanfloor topog-
raphy minus the expected thermal subsidence.
Although there is a large literature on the inter-
pretation of positive depth anomalies -- swells
-- it should be kept in mind that in a convect-
ing Earth, with normal variations in temperature
and composition, the depth of the seafloor is not
expected to be a simple function of time or age.
Geophysical anomalies, both positive and nega-
tive, are well outside the normal expected varia-
tions for a uniform isothermal mantle.
Data from the western North Atlantic and
central
Pacific
Oceans,
for
seafloor
ages
from
