| The term plume
in the Earth sciences is not always consistently used
or precisely defined. What a geophysicist means by a
plume is not always understood to be the case by a geochemist
or a geologist. Nevertheless, the term has been precisely
defined in classical fluid dynamics, and it is probably
best to provide at least that one description of a plume
in the framework of geophysics as an entrée to
the many contributions that treat them on this website.
Melting anomalies can result from concentrated
hot regions of the shallow mantle – hotspots –
or from upwelling jets – plumes. They can also
result from fertile patches or regions of shallow mantle
with low melting point. Focusing, edge effects (see
also EDGE convection page),
ponding and interactions of surface features with a
partially molten asthenosphere can also create melting
anomalies at the surface. Adiabatic decompression melting
can be caused by passive upwellings, changes in thickness
of the lithosphere, or by recycling of basaltic material
with a low melting temperature. The usual explanation
for melting anomalies is that they result from active
hot upwellings from a deep thermal boundary layer. In
the laboratory, upwellings thought to be analogous to
these are often created by the injection of hot fluids,
not by the free circulation of a fluid (click
for more on this).
Upwelling and downwelling features in a fluid that are
maintained by thermal buoyancy are called plumes. This
standard fluid dynamical definition of a plume does
not exclude types of circulation that might occur in
the Earth’s mantle, or other types of convection
and upwelling. However, the mantle is not the ideal
homogeneous fluid heated entirely from below –
or cooled from above – that is usually envisaged
in textbooks on fluid dynamics. Normal convection in
a fluid with the properties of the mantle occurs on
a very large scale, comparable to the lateral scales
of plates and the thicknesses of mantle layers. In geophysics,
plumes are a special form of small-scale convection
originating in a thin, lower, thermal boundary layer
(TBL) heated from below; in this sense not all upwellings,
even those driven by their own buoyancy, are plumes
(click for more on this).
Narrow downwellings in the Earth are called slabs.
The
dimension of a plume is controlled by the thickness
of the lower boundary layer or the diameter of the hypodermic
needle used in the injection experiment. There is likely
to be a thermal boundary layer at the core-mantle boundary
(CMB), and there is one at the Earth's surface. There
is no reason to believe that these are the only ones,
however. In the Earth, the buoyant products of mantle differentiation tend to collect at the
surface TBL (continents, crust, harzburgite) and the
dense dregs at the lower TBL. Because of these complications,
and the scales of TBLs, it is difficult for seismic
techniques alone to detect the high temperature gradient
that is the signature of a thermal boundary layer. Nevertheless,
Earth scientists are confident that substantial TBLs
exist at the surface and at the CMB. Upwelling plumes
should spread out and pond beneath internal boundary
layers. Large scale low-velocity zones at 650 km or
1000 km, for example, would be good evidence both for
the existence of boundary layers at these depths and
of upwelling plumes.
Convecting systems that are chemically
stratified or involve endothermic phase changes (e.g.
with negative Clapeyron slopes) develop internal thermal
boundary layers. Because of their small density contrasts,
chemical interfaces are predicted to have large relief.
This, plus the low sensitivity of seismic velocity to
temperature at high pressure, further complicates the
seismic detection of deep TBL. On the other hand, the
presence of deep TBLs does not require that they form
narrow upwelling instabilities that rise to the Earth’s
surface. A TBL is a necessary condition for the formation
of a plume – as it is understood in the geophysics
and geochemistry literature – but is not a sufficient
condition. Likewise, the formation of a melting anomaly
at the surface, or a buoyant upwelling, does not require
a deep TBL.
Internal and lower thermal boundary
layers in the mantle need not have the same dimensions
and time constants as the upper one. Plate tectonics
and mantle convection can be maintained by cooling of
plates, sinking of slabs and secular cooling, without
any need for a lower thermal boundary layer, –
particularly one with the same time constants as the
upper one. Buoyant decompression upwellings can also
be generated without a lower thermal boundary layer.
Because of internal heating and the effects of pressure,
the upper and lower thermal boundary layers are neither
symmetric nor equivalent.
Upwellings in the mantle can be triggered
by spreading, by dehydration and melting of slabs, by
phase changes and by displacement by sinking materials.
Cooling of the surface boundary layer creates dense
slabs. These are all plumes in the strict fluid dynamic
sense but in geophysics the term is restricted to narrow
hot upwellings rooted in a deep thermal boundary layer,
and having a much smaller scale than normal mantle convection
and the lateral dimensions of plates. Sometimes geophysical
plumes are considered to be “the way the core
gets rid of its heat”.
If the mantle is homogeneous and convects
as a unit, from top to bottom, it will have a Rayleigh
number of > 107. Convection withhigh Rayleigh
number (> 107) is time-dependent, and
should contain length scales that vary between the thickness
of the boundary layer to many times the depth of the
layer. If pressure did not increase with depth, a physically
impossible circumstance, we would expect to see convective
features throughout the mantle with scales from 50 to
10,000 km. There is good evidence that the mantle is
not homogeneous; some slabs become horizontal at depths
near ~ 650 km and there is a drastic change in the characteristics
of mantle structure at this depth.
If the mantle is chemically stratified
and the effects of pressure on physical properties are
taken into account, the effective Rayleigh number of
the mantle may be orders of magnitude less than 107.
Deep boundary layers must be much thicker and more sluggish
than the surface boundary layer. Lower mantle thermal
features – because of pressure effects –
must be orders of magnitude larger than the thicknesses
of slabs and surface plates, and orders of magnitude
older.
The view that the mantle has high Rayleigh
number and relatively symmetric upper and lower boundary layers
is the conventional wisdom of most geophysicists who
have worked on mantle convection. High Rayleigh numbers imply time-dependent and intermittent convection, small-scale
features, and rapid mixing. A fully self-consistent
thermal-dynamic treatment (non-Boussinesq,
and including deformable boundaries, non-uniform internal
heating and secular cooling) has yet to be done and
much of the fluid dynamic intuition regarding plumes
is based on unrealistic laboratory experiments (low
Prandtl number) involving either heating from below
or the injection of hot fluids. A homogeneous mantle
with constant melting point, well below the solidus,
is the usual starting point in mantle convection simulations.
A plume, in the geophysical sense, requires heating
from below. Upwellings in internally heated fluids,
or secularly cooled fluids, are broad and non-stationary.
Other kinds of upwellings such as at ridges, and island
arcs, or which result from heating and melting of slabs
or the displacement by sinking slabs, are alternatives
to thermal plumes of the type discussed by Morgan
in 1971. Regions of excess magmatism or low seismic
velocity can be confidently attributed to plumes only
if they can be shown to originate in a lower thermal
boundary layer. Cracks and dikes the upper boundary
layer can also generate melting anomalies. Low velocity
zones can be due to composition or the presence of small
amounts of grain boundary fluids; they do not have to
result from deeper upwellings. |
