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EARTH’S ENERGY SOURCES
The geological processes that shape the Earth’s surface are powered by two major sources of
energy; geothermal heat from the Earth’s interior and external energy from the sun. The
Earth’s internal heat is the ultimate source of energy for virtually all tectonic and mantle
processes, whereas the sun’s energy drives the water cycle, and controls weathering and
erosion. Tectonic activity (mountain building) also generates surface relief which allows the
downslope movement of rock material under the influence of gravity.
Evidence of the Earth’s internal energy is provided by the geothermal heat flow which can be
measured, and in some cases observed, at the surface. Volcanoes, geysers and hot springs, of
course, provide further evidence that the inside of the Earth is much hotter than the surface.
Since the inside of the Earth is hotter than the outside it follows that heat must flow
outwards. Heat flow is measured in Watts per square metre (W/m2). The present rate of heat
flow to the surface is fairly well established and can be measured from the increase in
temperature with depth (known as the geothermal gradient or geotherm) and on how well the
rocks conduct heat (see Figure 45). Heat flow to the Earth’s surface is higher where
geothermal gradients are steep and in rocks which are relatively good conductors. Heat flow
rates within continents average around 0.056 W/m2 and range up to 0.2 W/m2 in areas of
volcanic activity. In the ocean basins the average is about 0.078 W/m2, but increases to rates
of 0.3 W/m2 at volcanic mid-oceanic ridges. These variations in heat flow provide important
clues as to the nature of the Earth’s interior and the tectonic and mantle processes in
operation.
Figure 45
Global Heat Flow
In general, temperature increases with depth at an average rate of 20-30°C/km for the first
200km and thereafter dropping to a rate of about 1°C/km (see Figure 46). However, the rate
at which the temperature rises over the first 200km is very variable. In some parts of the
lithosphere temperature rises very slowly with depth while in other parts temperature rises
very rapidly. In areas of high heat flow the geothermal gradient is steep (e.g. mid-oceanic
ridges), whereas in areas of low heat flow the geothermal gradient is less steep (e.g. oceanic
trenches). Figure 46 shows the relationship between the Earth’s heat flow and the rate at
which temperature increases with depth. Graph A shows the normal situation over most of the
Earth. Here the temperature increases with depth at the average rate of 20-30°C/km and
does not cross the melting point curve (known as the solidus line) of the mantle rocks, so no
melting of rock occurs to form magma. Graph B and C show a situation where the geothermal
gradient is much higher (i.e. temperature increase with depth at a much quicker rate here) and
crosses the solidus line for mantle rocks which causes melting to take place. In Graph D the
geothermal gradient is very similar to the rest of the Earth but melting does occur for reasons
which will be discussed later in the plate tectonic section.
Figure 46
Earth’s Geotherm
The interior of the Earth gives off enormous amounts of heat due the continual decay of
certain radioactive isotopes (radiogenic heat); the residual heat trapped within the earth
since its formation 4.6 billion years ago (primordial heat); and the formation of the core. By
far the most important of these is the decay of long-lived radioisotopes such as those of
uranium (U), thorium (Th) & potassium (K). Long-lived radioisotopes are elements with
particularly long half-lives (U – 4.5 billion years, Th – 14 billion years and K – 1.3 billion years).
About 83% of the heat flow to the Earth’s surface is attributable to this process. The
elements U, Th and K (and their radiogenic isotopes) are particularly concentrated in the
silicate-dominated outer layers of the Earth, and in particular within the continental crust.
They are thought to be virtually absent from the core. As a result the radiogenic heat
produced per unit mass of the continental crust is, on average, over 100 times greater than
that of the underlying mantle. But because the mantle is so much more massive than the crust,
in effect this means that the overall radiogenic heating budget is roughly split equally between
the mantle and the crust despite the much greater mass of mantle material. It is the decay
of these long-lived isotopes that provides sufficient heat energy to keep the Earth geologically
active.
Figure 47
Relationship between the Geotherm, Heat Flow and Plate Tectonics
Residual heat from the formation of the Earth could still be escaping to the Earth’s surface.
The Earth formed by the collision of millions of rocky fragments through a process called
accretion. As these fragments accelerated due to gravitational attraction towards the forming
Earth, their gravitational potential was converted to kinetic energy. When they impacted, much
of it was then transferred to heat energy, causing heating of the Earth. The decay of shortlived radioisotopes (formed in a supernova that may have triggered Solar System formation)
could also have produced a lot of heat soon after the Earth was formed. This heat would have
been produced for the first few million years of the Earth’s existence from this source.
The Earth has differentiated to form a dense core surrounded by a relatively lower density
mantle and even lower density crust. Differentiation is the process by which planets develop
concentric layering, with zones that differ in their chemical and mineralogical compositions,
due to the separating out of materials of different densities. The Earth would have undergone
this process very early on in its formation, when the entire planet was molten. During this early
molten phase the material that now forms the core would have sunk towards the centre under
the influence of gravity because of its relatively high density. The gravitational energy lost by
this inward movement of this material would have been converted first to kinetic energy and
then into thermal energy. It is estimated that the core-forming process would have
contributed significantly to the Earth’s primordial heating.
As we have seen so far, radioactive decay, accretion and core formation are the processes
that have heated the Earth. But how is this internal heat transferred to the surface? There
are four mechanisms of heat transfer: conduction, convection, radiation and advection. In
conduction, heat is transferred from atom to atom or molecule to molecule within a material.
The rate at which it does this depends on the temperature difference between the hot and
cold portions of the material. Different materials, such as rocks of various compositions,
conduct heat at different rates, and the efficiency of heat transfer in this manner is known
as conductivity. Conduction is the most important heat transfer process in the outermost layer
of the Earth (i.e. the lithosphere).
In convection, matter actually moves, driven by density differences causing buoyancy, taking
heat energy with it. Denser (colder) material tends to move downwards, while less dense
(hotter) material moves upwards. During this transfer the material gives up its heat. It is a
particularly efficient method of heat transfer, but the medium through which transfer takes
place must be fluid. However, the term “fluid” describes any substance capable of flowing and
is not restricted to liquids and gases. Under the right conditions, even solid rocks can flow,
albeit at a very slow rate (a few centimetres per year). Over long periods of time, the effect
of such solid-state convection becomes a highly significant way of transferring heat towards
the Earth’s surface. In fact it is the most efficient form of heat transfer within all but the
outermost part of Earth.
Advection is rather like the upward part of convection. In this process heat is transferred
when molten material (magma) moves up through fractures in the lithosphere and remains
there. Advection operates when magma spreads out at the surface as a lava flow or, if it is
injected, cools and crystallises within the lithosphere itself. The effect is the same in both
cases, since heat is transferred by the molten rock from deeper levels where melting is taking
place to shallower levels where it solidifies, losing its heat by conduction to the overlying crust.
Any planetary body that exhibits, or has exhibited, volcanic activity must have lost some of
its internal heat in this manner.
In the case of radiation, photons (electromagnetic radiation) carry away the heat energy from
hotter region (Earth’s surface) to a colder one (atmosphere above). This is the final process
in the transfer of heat from the interior of the Earth to the surface and then into space.
The Earth’s external energy source is from the sun. Solar heating of the Earth’s surface drives
the water cycle and controls weathering and erosional processes. Igneous, sedimentary and
metamorphic rocks are linked through the rock cycle by key rock forming processes
(weathering, erosion, transportation, deposition, lithification, metamorphism, partial melting
and crystallisation). Figure 48 is a schematic diagram of the rock cycle. In Figure 8, boxes
represent earth materials and arrows represent the processes that transform those
materials. The processes are named in bold next to the arrows. The two major sources of
energy for the rock cycle are also shown; the sun provides energy for surface processes such
as weathering, erosion, and transport, and the earth's internal heat provides energy for
processes like subduction, melting, and metamorphism. The complexity of the diagram reflects
a real complexity in the rock cycle. Notice that there are many possibilities at any step along
the way.
Figure 48