Geothermal gradient is the rate of increasing temperature with respect
to increasing depth in the Earth's interior. Away from tectonic plate
boundaries, it is about 25–30 °C/km (72-87 °F/mi) of
depth near the surface in most of the world. Strictly speaking,
geo-thermal necessarily refers to the
Earth but the concept may be
applied to other planets.
The Earth's internal heat comes from a combination of residual heat
from planetary accretion, heat produced through radioactive decay, and
possibly heat from other sources. The major heat-producing isotopes in
Earth are potassium-40, uranium-238, uranium-235, and
thorium-232. At the center of the planet, the temperature may be up
to 7,000 K and the pressure could reach 360
GPa (3.6 million
atm). Because much of the heat is provided by radioactive decay,
scientists believe that early in
Earth history, before isotopes with
short half-lives had been depleted, Earth's heat production would have
been much higher. Heat production was twice that of present-day at
approximately 3 billion years ago, resulting in larger
temperature gradients within the Earth, larger rates of mantle
convection and plate tectonics, allowing the production of igneous
rocks such as komatiites that are no longer formed.
1 Heat sources
2 Heat flow
3 Direct application
5 See also
Earth cutaway from core to exosphere
Geothermal drill machine in Wisconsin, USA
Temperature within the
Earth increases with depth. Highly viscous or
partially molten rock at temperatures between 650 to 1,200 °C
(1,200 to 2,200 °F) are found at the margins of tectonic plates,
increasing the geothermal gradient in the vicinity, but only the outer
core is postulated to exist in a molten or fluid state, and the
temperature at the Earth's inner core/outer core boundary, around
3,500 kilometres (2,200 mi) deep, is estimated to be 5650 ± 600
kelvins. The heat content of the
Earth is 1031 joules.
Much of the heat is created by decay of naturally radioactive
elements. An estimated 45 to 90 percent of the heat escaping from the
Earth originates from radioactive decay of elements mainly located in
Heat of impact and compression released during the original formation
Earth by accretion of in-falling meteorites.
Heat released as abundant heavy metals (iron, nickel, copper)
descended to the Earth's core.
Latent heat released as the liquid outer core crystallizes at the
inner core boundary.
Heat may be generated by tidal force on the
Earth as it rotates; since
rock cannot flow as readily as water it compresses and distorts,
There is no reputable science to suggest that any significant heat may
be created by electromagnetic effects of the magnetic fields involved
in Earth's magnetic field, as suggested by some contemporary folk
The radiogenic heat from the decay of 238U and 232Th are now the major
contributors to the earth's internal heat budget.
In Earth's continental crust, the decay of natural radioactive
isotopes has had significant involvement in the origin of geothermal
heat. The continental crust is abundant in lower density minerals but
also contains significant concentrations of heavier lithophilic
minerals such as uranium. Because of this, it holds the largest global
reservoir of radioactive elements found in the Earth. Especially
in layers closer to Earth's surface, naturally occurring isotopes are
enriched in the granite and basaltic rocks. These high levels of
radioactive elements are largely excluded from the
Earth's mantle due
to their inability to substitute in mantle minerals and consequent
enrichment in partial melts. The mantle is mostly made up of high
density minerals with high contents of atoms that have relatively
small atomic radii such as magnesium (Mg), titanium (Ti), and calcium
Present-day major heat-producing isotopes
Mean mantle concentration
[kg isotope/kg mantle]
9.46 × 10−5
4.47 × 109
30.8 × 10−9
2.91 × 10−12
5.69 × 10−4
7.04 × 108
0.22 × 10−9
1.25 × 10−13
2.64 × 10−5
1.40 × 1010
124 × 10−9
3.27 × 10−12
2.92 × 10−5
1.25 × 109
36.9 × 10−9
1.08 × 10−12
The geothermal gradient is steeper in the lithosphere than in the
mantle because (a) crustal minerals have lower thermal conductivity
than mantle minerals, (b) the crust produces more radiogenic heat than
the mantle and (c) the crust contains numerous fractures.
Main article: Earth's internal heat budget
Heat flows constantly from its sources within the
Earth to the
surface. Total heat loss from the
Earth is estimated at 44.2 TW (4.42
× 1013 watts). Mean heat flow is 65 mW/m2 over continental
crust and 101 mW/m2 over oceanic crust. This is 0.087
watt/square meter on average (0.03 percent of solar power absorbed by
the Earth ), but is much more concentrated in areas where thermal
energy is transported toward the crust by convection such as along
mid-ocean ridges and mantle plumes. The
Earth's crust effectively
acts as a thick insulating blanket which must be pierced by fluid
conduits (of magma, water or other) in order to release the heat
underneath. More of the heat in the
Earth is lost through plate
tectonics, by mantle upwelling associated with mid-ocean ridges. The
final major mode of heat loss is by conduction through the
lithosphere, the majority of which occurs in the oceans due to the
crust there being much thinner and younger than under the
The heat of the
Earth is replenished by radioactive decay at a rate of
30 TW. The global geothermal flow rates are more than twice the
rate of human energy consumption from all primary sources.
Heat from Earth's interior can be used as an energy source, known as
geothermal energy. The geothermal gradient has been used for space
heating and bathing since ancient Roman times, and more recently for
generating electricity. As the human population continues to grow, so
does energy use and the correlating environmental impacts that are
consistent with global primary sources of energy. This has caused a
growing interest in finding sources of energy that are renewable and
have reduced greenhouse gas emissions. In areas of high geothermal
energy density, current technology allows for the generation of
electrical power because of the corresponding high temperatures.
Generating electrical power from geothermal resources requires no fuel
while providing true baseload energy at a reliability rate that
constantly exceeds 90%. In order to extract geothermal energy, it
is necessary to efficiently transfer heat from a geothermal reservoir
to a power plant, where electrical energy is converted from heat.
On a worldwide scale, the heat stored in Earth's interior provides an
energy that is still seen as an exotic source. About 10 GW of
geothermal electric capacity is installed around the world as of 2007,
generating 0.3% of global electricity demand. An additional 28 GW of
direct geothermal heating capacity is installed for district heating,
space heating, spas, industrial processes, desalination and
agricultural applications. Because heat is flowing through every
square meter of land, it can be used for a source of energy for
heating, air conditioning (HVAC) and ventilating systems using ground
source heat pumps. In areas where modest heat flow is present,
geothermal energy can be used for industrial applications that
presently rely on fossil fuels.
The geothermal gradient varies with location and is typically measured
by determining the bottom open-hole temperature after borehole
drilling. To achieve accuracy the drilling fluid needs time to reach
the ambient temperature. This is not always achievable for practical
In stable tectonic areas in the tropics a temperature-depth plot will
converge to the annual average surface temperature. However, in areas
where deep permafrost developed during the
Pleistocene a low
temperature anomaly can be observed that persists down to several
hundred metres. The
Suwałki cold anomaly in
Poland has led to the
recognition that similar thermal disturbances related to
Holocene climatic changes are recorded in boreholes
throughout Poland, as well as in Alaska, northern Canada, and Siberia.
In areas of
Holocene uplift and erosion (Fig. 1) the initial gradient
will be higher than the average until it reaches an inflection point
where it reaches the stabilized heat-flow regime. If the gradient of
the stabilized regime is projected above the inflection point to its
intersect with present-day annual average temperature, the height of
this intersect above present-day surface level gives a measure of the
Holocene uplift and erosion. In areas of
and deposition (Fig. 2) the initial gradient will be lower than the
average until it reaches an inflection point where it joins the
stabilized heat-flow regime.
In deep boreholes, the temperature of the rock below
the inflection point generally increases with depth at rates of the
order of 20 K/km or more. Fourier's law of heat flow
applied to the
Earth gives q = Mg where q is the heat flux at a point
on the Earth's surface, M the thermal conductivity of the rocks there,
and g the measured geothermal gradient. A representative value for the
thermal conductivity of granitic rocks is M = 3.0 W/mK.[citation
needed] Hence, using the global average geothermal conducting gradient
of 0.02 K/m we get that q = 0.06 W/m². This estimate, corroborated by
thousands of observations of heat flow in boreholes all over the
world, gives a global average of 6×10−2 W/m².
Thus, if the geothermal heat flow rising through an acre of granite
terrain could be efficiently captured, it would light four 60 watt
A variation in surface temperature induced by climate changes and the
Milankovitch cycle can penetrate below the Earth's surface and produce
an oscillation in the geothermal gradient with periods varying from
daily to tens of thousands of years and an amplitude which decreases
with depth and having a scale depth of several kilometers.
Melt water from the polar ice caps flowing along ocean bottoms tends
to maintain a constant geothermal gradient throughout the Earth's
If that rate of temperature change were constant, temperatures deep in
Earth would soon reach the point where all known rocks would
eventually melt. We know, however, that the
Earth's mantle is solid
because of the transmission of S-waves. The temperature gradient
dramatically decreases with depth for two reasons. First, radioactive
heat production is concentrated within the crust of the Earth, and
particularly within the upper part of the crust, as concentrations of
uranium, thorium, and potassium are highest there: these three
elements are the main producers of radioactive heat within the Earth.
Second, the mechanism of thermal transport changes from conduction, as
within the rigid tectonic plates, to convection, in the portion of
Earth's mantle that convects. Despite its solidity, most of the
Earth's mantle behaves over long time-scales as a fluid, and heat is
transported by advection, or material transport. Thus, the geothermal
gradient within the bulk of
Earth's mantle is of the order of 0.5
kelvin per kilometer, and is determined by the adiabatic gradient
associated with mantle material (peridotite in the upper mantle).
This heating up can be both beneficial or detrimental in terms of
Geothermal energy can be used as a means for generating
electricity, by using the heat of the surrounding layers of rock
underground to heat water and then routing the steam from this process
through a turbine connected to a generator.
On the other hand, drill bits have to be cooled not only because of
the friction created by the process of drilling itself but also
because of the heat of the surrounding rock at great depth. Very deep
mines, like some gold mines in South Africa, need the air inside to be
cooled and circulated to allow miners to work at such great depth.
Sustainable development portal
Earth's internal heat budget
TauTona Mine The world's deepest mining operation at 3.9 km
(2.4 mi), where the rock face temperature reaches 60 °C
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