The outer shells of Earth - New Theory of the Earth - page 55

anomalies at the surface. Adiabatic decompres-
sion melting can be caused by passive upwel-
lings -- such as material displaced upwards by
sinking slabs -- changes in thickness of the litho-
sphere, or by recycling of basaltic material with
a low melting temperature. The usual explana-
tion for melting anomalies is that they result
from active hot upwellings from a deep ther-
mal boundary layer. In the laboratory, upwellings
thought to be analogous to these are often cre-
ated by the injection of hot fluids, not by the free
circulation of a fluid.
In the fluid dynamics literature upwelling
and downwelling features in a fluid that are
maintained by thermal buoyancy are called
plumes . Thermal plumes form at thermal boundary
layers , and rise from boundaries heated from
below. As they rise, they migrate horizontally
along the boundary generating both small-scale
and large-scale flow, including rotation of the
whole fluid layer, a sort of convective wind. 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, or a fluid dynamicist.
The mantle is not the ideal homogenous fluid
heated entirely from below -- or cooled from
above -- that is usually envisaged in textbooks
on fluid dynamics and mantle 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 spe-
cial 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. Narrow downwellings in the Earth are
fluid dynamic plumes but they are called slabs .
The dimension of a plume is controlled by the
thickness of the boundary layer. 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, bound-
ary layers tend to collect the buoyant products
of mantle differentiation at the surface one (con-
tinents, crust, harzburgite) and the dense dregs

at the lower one. They are, therefore, not strictly
thermal; they are thermo-chemical boundary layers .
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 neces-
sary condition for the formation of a plume -- as
it is understood in the geophysics and geochem-
istry 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 dimen-
sions and time constants as the upper one. Plate
tectonics and mantle convection can be main-
tained 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 melting , caused by
upwelling, can also be generated without a lower
thermal boundary layer. Because of internal heat-
ing and the effects of pressure, the upper and
lower thermal boundary layers are neither sym-
metric nor equivalent. The Earth's mantle does
not have the same kind of symmetry regarding
convection that is exhibited by a pot of water on
a stove that is heated from below and cooled from
above.
Upwellings in the mantle can be triggered
by spreading, by hydration, by melting, by phase
changes and by displacement by sinking materi-
als (passive upwelling). In a heterogenous man-
tle with blobs of different melting points and
thermal expansion coefficients, upwellings can
form without thermal boundary layers. Con-
vection in the mantle need not involve active
upwellings. The active, or driving, elements may
be dense downwellings with complementary pas-
sive upwellings. Upper mantle convection is pri-
marily passive; the plates and the slabs are the
active elements.
Cooling of the surface boundary layer creates
dense slabs. Extension of the lithosphere allows
the intrusion of dikes. These are all plumes
in the strict fluid dynamic sense but in geo-
physics the term is restricted to narrow hot
upwellings rooted in a deep thermal boundary
layer, and having a much smaller scale than