DEFINITION OF CONVECTION AND THE EARTH’S MANTLE

The main heat transfer

mechanism in the Earth’s mantle is convection. It is a

thermally driven process in which heating at depth causes

material to expand and become less dense, causing it to rise

while being replaced by complementary cool material that

sinks. This moves heat from depth to the surface in a very

efficient cycle since the material that rises gives off heat as it

rises and cools, and the material that sinks gets heated only

to eventually rise again. Convection is the most important

mechanism by which the Earth is losing heat, with other

mechanisms including conduction, radiation, and advection.

However, many of these mechanisms work together in the

plate tectonic cycle. Mantle convection brings heat from

deep in the mantle to the surface where the heat released

forms magmas that generate the oceanic crust. The midocean

ridge axis is the site of active hydrothermal circulation

and heat loss, forming black smoker chimneys and other

vents. As the crust and lithosphere move away from the midocean

ridges, it cools by conduction, gradually subsiding

(according to the square root of its age) from about 1.5–2.5

miles (2.5–4.0 km) below sea level. Heat loss by mantle convection

is therefore the main driving mechanism of plate tectonics,

and the moving plates can be thought of as the

conductively cooling boundary layer for large-scale mantle

convection systems.

The heat being transferred to the surface by convection

is produced by decay of radioactive heat-producing isotopes

such as U235, Th232, and K40, remnant heat from early heatproducing

isotopes such as I129, remnant heat from accretion

of the Earth, heat released during core formation, and heat

released during impacts of meteorites and asteroids. Very

early in the history of the planet at least part of the mantle

was molten, and the Earth has been cooling by convection

ever since. It is difficult to estimate how much the mantle has

cooled with time, but reasonable estimates suggest that the

mantle may have been up to a couple of hundred degrees hotter

in the earliest Archean.

The rate of mantle convection is dependent on the ability

of the material to flow. The resistance to flow is a quantity

measured as viscosity, defined as the ratio of shear stress to

strain rate. Fluids with high viscosity are more resistant to

flow than materials with low viscosity. The present viscosity

of the mantle is estimated to be 1020–1021 Pascal seconds

(Pa/s) in the upper mantle, and 1021–1023 Pa/s in the lower

mantle, which are sufficient to allow the mantle to convect

and complete an overturn cycle once every 100 million years.

The viscosity of the mantle is temperature dependent, so it is

possible that in early Earth history the mantle may have been

able to flow and convectively overturn much more quickly,

making convection an even more efficient process and speeding

the rate of plate tectonic processes.

There is currently an ongoing debate and research about

the style of mantle convection in the Earth. The upper mantle

is relatively heterogeneous and extends to a depth of 416

miles (670 km), where there is a pronounced increase in seismic

velocities. The lower mantle is more homogeneous and

extends to the D’’ region at 1,678 miles (2,700 km), marking

the transition into the liquid outer core. One school of mantle

convection thought suggests that the entire mantle,

including both the upper and lower parts, is convecting as

one unit. Another school of thought posits that the mantle

convection is divided into two layers, with the lower mantle

convecting separately from the upper mantle. A variety of

these models, presently held by the majority of geophysicists,

is that there is two-layer convection, but that subducting

slabs are able to penetrate the 670-kilometer discontinuity

from above, and that mantle plumes that rise from the D’’

region are able to penetrate the 670-kilometer discontinuity

from below.

The shapes of mantle convection cells include many possible

forms that are reflected to a first order by the distribution

of subduction zones and mid-ocean ridge systems. The

subduction zones mark regions of downwelling, whereas the

ridge system marks broad regions of upwelling. Material is

upwelling in a broad planiform cell beneath the Atlantic and

Indian Oceans, and downwelling in the circum-Pacific subduction

zones. There is thought to be a large plume-like

“superswell” beneath part of the Pacific that feeds the planiform

East Pacific rise. Mantle plumes that come from the

deep mantle punctuate this broad pattern of upper mantle

convection, and their plume tails must be distorted by flow in

the convecting upper mantle.

The pattern of mantle convection deep in geological time

is uncertain. Some periods such as the Cretaceous seem to

have had much more rigorous mantle convection and surface

volcanism. More or different types or rates of mantle convection

may have helped to allow the early Earth to lose heat

more efficiently. Some computer models allow periods of

convection dominated by plumes, and others dominated by

overturning planiform cells similar to the present Earth. Some

models suggest cyclic relationships, with slabs pooling at the

670-kilometer discontinuity, then suddenly all sinking into

the lower mantle, causing a huge mantle overturn event. Further

research is needed on linking the preserved record of

mantle convection in the deformed continents to help interpret

the past history of convection.

See also BLACK SMOKER CHIMNEYS; DIVERGENT OR

EXTENSIONAL BOUNDARIES; PLATE TECTONICS.

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