DEFINITION OF HEAT FLOW

In geology, crustal heat flow is a measure of the

amount of heat energy leaving the Earth, measured in calories

per square centimeters per second. Typical heat flow values

are about 1.5 microcalories per centimeter squared per second,

commonly stated as 1.5 heat flow units. Most crustal

heat flow is due to heat production in the crust by radioactive

decay of uranium, thorium, and potassium. Heat flow shows

a linear relationship with heat production in granitic rocks.

Some crustal heat flow, however, comes from deeper in the

Earth, beneath the crust.

The Earth shows a huge variation in temperature, from

several thousand degrees in the core to essentially zero

degrees Celsius at the surface. The Earth’s heat was acquired

by several mechanisms, including: (1) heat from accretion as

potential energy of falling meteorites was converted to heat

energy; (2) heat released during core formation, with gravitational

potential energy converted to heat as heavy metallic

iron and other elements segregated and sank to form the core

soon after accretion; (3) heat production by decay of radioactive

elements; and (4) heat added by late impacting meteorites

and asteroids, some of which were extremely large in early

Earth history. Heat produced by these various mechanisms

gradually flows to the surface by conduction, convection, or

advection, and accounts for the component of crustal heat

flow that comes from deeper than the crust.

Heat flow by conduction involves thermal energy flowing

from warm to cooler regions, with the heat flux being

proportional to the temperature difference, and a proportionality

constant (k), known as thermal conductivity, related to

the material properties. The thermal conductivity of most

rocks is low, about one-hundreth of that of copper wire.

Advection involves the transfer of heat by the motion of

material, such as transport or heat in a magma, in hot water

through fractures or pore spaces, and more important on a

global scale, by the large-scale rising of heated, relatively lowdensity

buoyant material and the complementary sinking of

cooled, relatively high-density material in the mantle. The

large-scale motion of the mantle, with hot material rising in

some places and colder material sinking in other places, is

known as convection, which is an advective heat transfer

mechanism. For convection to occur in the mantle, the buoyancy

forces of the heated material must be strong enough to

overcome the rock’s resistance to flow, known as viscosity.

Additionally, the buoyancy forces must be able to overcome

the tendency of the rock to lose heat by conduction, since this

would cool the rock and decrease its buoyancy. The balance

between all of these forces is measured by a quantity called

the Raleigh number. Convection in Earth materials occurs

above a critical value of the Raleigh number, but below this

critical value heat transfer will be dominated by conductive

processes. Well-developed convection cells in the mantle are

very efficient at transporting heat from depth to the surface

and are the main driving force for plate tectonics.

Heat transfer in the mantle is dominated by convection

(advective heat transfer), except in the lower mantle near the

boundary with the inner core (the D’’ region), and along the

top of the mantle and in the crust (in the lithosphere), where

conductive and hydrothermal (also advective) processes dominate.

The zones where the heat transfer is dominated by conduction

are known as conductive boundary layers, and the

lithosphere may be thought of as a convecting, conductively

cooling boundary layer.

See also CONVECTION AND THE EARTH’S MANTLE;

GEOTHERMAL ENERGY; PLATE TECTONICS; RADIOACTIVE DECAY.

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