Earth's heat flow The sources of Earth's internal heat are:

heat remaining from the initial accretion of the Earth

gravitational energy released from the formation of the core

tidal heating

radiogenic heating within the mantle and crust.

Although the proportion of each heat source cannot be determined accurately,

radiogenic heat is considered to have been the major component for much of the

Earth's history. There are three main processes by which this internal heat gets

to the Earth's surface, these being conduction, convection and advection.

Heat flow (or heat flux), q, is a measure of the heat energy being transferred

through a material (measured in units of watts per square metre; W m−2). It may

be determined by taking the difference between two or more temperature

readings (ΔT) at different depths down a borehole (d), and then determining the

thermal conductivities (k) of the rocks in between. q can then be calculated

according to the relationship.

Earth scientists are interested in the heat flow measured at the Earth's surface

because it reveals important information concerning the nature of the rocks and

the processes that affect the lithosphere.

The total annual global heat loss from the Earth's surface is estimated as 4.1–4.3

× 1013 W. This yields an average of q ≥ 100 mW m−2 (milliwatts per square

metre), though individual measurements may be much higher than this.

However, values of q decrease to less than 50 mW m-2 for oceanic crust older

than 100 Ma. In continental areas, the younger crust (i.e. mountain belts that are

less than 100 Ma) have relatively high values of q, which are 60–75 mW m−2,

whilst old continental crust and cratons have much lower heat flow values,

averaging q = 38 mW m−2. Thus, variations in heat flow are closely related to

different types of crustal materials and, importantly, different types of tectonic

plate boundary.