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SP E C I A L FE A T U R E : EA R T H
www.iop.org/journals/physed
Why does plate tectonics occur
only on Earth?
Paula Martin, Jeroen van Hunen, Stephen Parman and
Jon Davidson
Department of Earth Sciences, Durham University, Science Laboratories, South Road,
Durham DH1 3LE, UK
E-mail: paula.martin@durham.ac.uk,jeroen.van-hunen@durham.ac.uk,
stephen.parman@durham.ac.uk and j.p.davidson@durham.ac.uk
Abstract
Plate tectonics governs the topography and motions of the surface of Earth,
and the loss of heat from Earth’s interior, but appears to be found uniquely on
Earth in the Solar System. Why does plate tectonics occur only on Earth?
This is one of the major questions in earth and planetary sciences research,
and raises a wide range of related questions: has plate tectonics ever occurred
on other planets in the past? How did plate tectonics start on Earth? Will it
ever end? In the absence of plate tectonics, how do planets lose their heat?
This article provides a brief introduction to the ways in which planets lose
their heat and discusses our current understanding of plate tectonics and the
challenges that lie ahead.
Introduction
Plate tectonics governs the nature and shape of the
surface of Earth, from ocean basins to mountain
ranges. It also governs the motions of the surface
of Earth, providing a range of natural hazards such
as earthquakes and volcanic eruptions. It is a
familiar component of the National Curriculum,
and a major field of on-going scientific research.
This article focuses on the five largest silicate
bodies in the Solar System, namely Mercury,
Venus, Earth, the Moon and Mars, collectively
referred to as ‘terrestrial bodies’ (figure 1).
Terrestrial bodies can be thought of as a series
of approximately spherical layers, defined either
chemically or mechanically. For example, starting
at the centre and working outwards, Earth is
chemically composed of an inner core, outer core,
mantle, and crust; it is mechanically composed
of an inner core, outer core, lower mantle, upper
mantle, asthenosphere and lithosphere (figure 2).
The lithosphere is composed of the crust and the
rigid uppermost part of the mantle, and is the
‘plate’ of plate tectonics. Although it is also
solid, in contrast to the rigid lithosphere, the
underlying asthenosphere is plastic (i.e. it can flow
on geological timescales).
Plate tectonics only occurs on Earth. We
do not know exactly why. We have looked for
plate tectonics on all of the other terrestrial bodies
in the Solar System (i.e. terrestrial planets and
satellites), and found that it is unique to Earth. This
is puzzling. Why should this process be unique
to Earth? How did it get started? Will it ever
stop? Why does it not happen anywhere else?
This article begins with a consideration of how
planets lose their heat, putting plate tectonics into
the larger context, followed by a brief summary of
what we do and do not know about plate tectonics,
and ends with a look at how we hope to find out
more about plate tectonics in the future.
144 PH Y S I C S ED U C A T I O N 43 (2) 0031-9120/08/020144+07$30.00 ©2008 IOP Publishing Ltd
Why does plate tectonics occur only on Earth?
Figure 1. The silicate bodies of the Solar System (Mercury, Venus, Earth, the Moon and Mars). Image courtesy
NASA/JPL-Caltech.
How do planets lose their heat?
Plate tectonics is the primary mechanism through
which Earth loses its heat. This raises the question:
in the absence of plate tectonics, how do the
other terrestrial bodies lose their heat? Terrestrial
bodies are generally thought to have been initially
hot, and gradually cooling, with many planetary
processes (e.g. volcanism and tectonism) being
driven by this cooling. The sources of heat within
planetary bodies can be categorized as either
primordial (i.e. inherited from processes occurring
during formation) or the result of radioactive
decay. Heat is transferred within planetary bodies
and eventually lost to space through a combination
of convection, conduction and radiation. Different
methods of heat loss dominate in the different
layers of planetary bodies, and at the boundaries
between these layers. For example, it is estimated
that every year Earth loses 4.2×1013 W, or
42 TW, of heat: 32 TW conducted through the
lithosphere, and up to 10 TW lost by, for example,
hydrothermal activity at mid-ocean ridges [1].
There are three primary modes of planetary
cooling: magma ocean, stagnant lid, and plate
tectonics. Regardless of the mode of planetary
cooling, all bodies lose heat from their surface
to some degree via radiation. All terrestrial
bodies are thought to undergo a short-lived
magma ocean stage early in their evolution.
The name ‘magma ocean’ refers to the stage
when a body is so hot that the surface is
partially or largely molten, and heat loss from
the surface is primarily through small-scale
convection (figure 3). When a body has cooled
sufficiently, the surface solidifies and the common
mode of heat loss is stagnant lid behaviour, where
heat loss from the surface is primarily through
conduction (although there could probably also
be significant heat loss through widespread large-
scale extrusive volcanism during this stage). It is
possible that other intermediate stages might have
existed between the magma ocean and stagnant
lid stages. These intermediate stages would
probably have been on a significantly smaller scale
than the current plate tectonics regime, and may
have involved, for example, a relatively mushy
lithosphere that could deform and subsequently
form small-scale downwellings or drips in contrast
to the large-scale downwellings (subduction)
associated with plate tectonics. Alternatively
to the stagnant lid regime, if the conditions
are appropriate, a body may begin to lose
heat via plate tectonics. It is theoretically
possible that a body may alternate between a
stagnant lid regime and a plate tectonics regime;
this has never been observed, but the lack of
observation may simply be a reflection of the
long timescales involved. Ultimately, when
they have become sufficiently cool, the fate of
all terrestrial bodies is to continue to cool by
conduction alone; they may then be considered to
be inactive or dead (i.e. lacking in any force to
March 20 08 P H Y S I C S ED U C A T I O N 145
P Martin et al
drive planetary processes such as volcanism and
tectonism).
What are the conditions necessary for plate
tectonics? This question may be thought of as
a Goldilocks problem: everything needs to be
just right. First, the planetary body in question
must have cooled sufficiently so that it is too
cold to sustain a magma ocean. Second, there
needs to be sufficient heat within the interior of
the body to prevent the existence of a stagnant
lid, i.e. sufficient heat to maintain convection
within the upper layers of the body. Third,
the lithosphere needs to be cool enough, dense
enough, strong enough and thin enough to subduct.
Finally, probably the most important ingredient
for successful plate tectonics is liquid water,
which is readily available only on Earth, not
on the other terrestrial bodies. This too is a
Goldilocks problem: the Earth may be at just
the right distance from the Sun to have a surface
temperature between 0 and 100 ◦C, and therefore
be a stable environment for liquid water. So far,
all of the necessary conditions for plate tectonics
have been found together only on Earth. In the
next section we discuss our current understanding
Figure 3. Artist’s conception of a planetary magma
ocean. Image courtesy NASA/JPL-Caltech.
of plate tectonics, based on our only observed
example: Earth.
What do we know about plate tectonics on
Earth?
Plate tectonics is a theory that has been developed
to explain the observed evidence for large-scale
motions of Earth’s lithosphere. The development
of the theory of plate tectonics, including the
146 PH Y S I C S ED U C A T I O N March 2008
Why does plate tectonics occur only on Earth?
combination of concepts such as continental drift
and seafloor spreading, is a very interesting
illustration of how science works. A simple
introduction to the development of the theory of
plate tectonics can be found in a variety of books;
see, for example, [3]. A comprehensive review,
including a wide selection of papers describing
the development of the theory and the personal
stories of the scientists involved, written by the
scientists themselves, is given in Plate Tectonics:
An Insider’s History of the Modern Theory of the
Earth, edited by Oreskes [4].
On Earth, the lithosphere is divided into
rigid plates, separated by linear features that
are identified by their appearance on maps
showing the locations of major tectonic events
(e.g. earthquakes) and topographic features such as
mountain chains, volcanoes and oceanic trenches,
as illustrated in figure 4[5]. There are a
total of seven major tectonic plates, and several
minor tectonic plates on Earth, which all move
in relation to one another at typical rates of a
few centimetres per year. The plate boundaries
may be categorized into one of three types:
convergent or destructive boundaries, divergent
or constructive boundaries, and transform or
conservative boundaries.
Tectonic plates are created at mid-ocean
ridges (where a gap is continuously renewed
when two plates move away from each other)
and destroyed at subduction zones where the
plates sink into the mantle. At the constructive
boundaries (mid-ocean ridges), melting results
in new buoyant, basaltic crust, which today is
typically about 7 km thick. This crust and
underlying mantle material quickly lose their heat
to the surface, and become the lithosphere. This
lithosphere continues to cool, and becomes thicker
and denser. After about 20 million years of
cooling, the lithosphere is already denser than the
underlying mantle, and ‘ready’ to sink down. This
sinking (called subduction), however, has to be
postponed until the plate meets another one at a
subduction zone. The lithosphere does not simply
sink under gravity when it is sufficiently dense
because of a variety of other factors, including
the energy required to bend the plate, and the
fact that it is often attached in some way to
something else (for example, the cold, dense edges
of the oceanic plates in the North Atlantic are
attached to the buoyant continental plates that form
Europe and North America). When two plates
do meet at a subduction zone, one plate bends
down below the other into the mantle, and its
high density (from cooling, and further increased
by the transformation of basaltic crust to much
denser eclogite below 40 km depth) will provide
the gravitational force to sink further down. This
sinking plate (called the ‘slab’) pulls the attached
plate at the surface towards the subduction zone,
and this process is called ‘slab pull’. Slab pull
is the dominant driving force of plate tectonics,
providing 90% of the force required to drive the
plate tectonic process (figure 5) (Stern, 2007).
Water plays a dominant role in the total
process of plate tectonics: for example, by
lubricating the sliding of tectonic plates past each
other at subduction zones; by rapidly cooling
tectonic plates near the ridges by hydrothermal
circulation; by speeding up the transformation
of basalt to eclogite; and by facilitating bending
of plates into the subduction zone by hydrous
weakening and chemical alteration in bending
cracks.
What do we not know about plate
tectonics?
There is still a lot that we do not know about plate
tectonics. For example:
•How and when did plate tectonics start on the
Earth? Did it simply ‘turn on’, or was there a
‘spluttering’ period when it started and
stopped before finally getting going?
•Is plate tectonics a continuous process that
will continue for the foreseeable future, or a
discontinuous process that stops and starts?
If it is a discontinuous process, how many
times in Earth’s history has it actually started
and stopped?
•How does subduction begin (when plate
tectonics first began, and even today)?
•Why do subduction zones have arc shapes?
They are called arcs because of their shape;
several theories have been suggested to
explain the arc shape, but none of these
suggestions can explain all of the
observations.
•Why do subduction zones move around?
They move both towards and away from the
subducting plate, and very little correlation
exists with, for example, plate age or plate
motion.
March 20 08 P H Y S I C S ED U C A T I O N 147
P Martin et al
•Why do only some tectonic plates have a
subducting slab?
•Why do plates without a subducting slab
(e.g. the North and South American plates)
move with significant speed (up to
5 cm yr−1)? This is particularly confusing as
we do know that, in general, plate motion is
primarily driven by slab pull. Does the
underlying mantle play a role (i.e. is it doing
something more than just passively sitting
there)?
•Did plate tectonics look different in the past?
For example, was there always the same
range in sizes of tectonic plates?
•How did plate tectonics influence the
generation of continental crust? This is
particularly interesting, as it is the buoyant
continental crust that rises above sea level
and forms the ‘life rafts’ on which we live.
We are pursuing a number of lines of evidence
in an attempt to answer the above questions. For
example, the critical examination of ophiolites
(which are pieces of oceanic crust that have been
thrust up onto continental crust, for example, as
seen on Cyprus) and transform faults (for example,
the San Andreas fault, USA) will allow us to
148 PH Y S I C S ED U C A T I O N March 2008
Why does plate tectonics occur only on Earth?
develop a better understanding of processes at
plate boundaries.
Is there any evidence for plate tectonics on
other planets?
There is no conclusive evidence for plate tectonics
on any other planets [6]. Both the Moon and
Mercury are significantly smaller than Earth, and
therefore it is likely that they lost all of their
internal heat at a much faster rate, largely because
of their greater surface area to volume ratio.
Now, they both have a single lithospheric plate,
continuing to cool through conduction alone, and
are considered to be geologically inactive. There
is no evidence to suggest that plate tectonics ever
operated on either the Moon or Mercury.
Venus shows no evidence of active plate
tectonics, although the surface does appear to be
relatively young based on the lack of a significant
number of impact craters (we do not yet have any
samples that may be used to date the surface of
Venus by any other methods). The evolution of the
surface of Venus remains a hotly debated issue and
the subject of substantial on-going research. Venus
is similar in size to Earth, and so the question
of why Venus shows no evidence for active plate
tectonics is intriguing. It has been suggested that
the key difference between Venus and Earth may
be the lack of water on Venus, as on Earth water
plays an important role in the evolution of the
surface, particularly in plate tectonics.
In contrast to Venus, Mars is considerably
smaller than Earth, but does have water (mostly
in the form of ice). Some surface features have
been interpreted as indicating the possibility of
plate tectonics operating on Mars in the past.
For example, it has been suggested that magnetic
patterns observed by the Mars Global Surveyor
spacecraft may indicate that a process similar to
plate tectonics may have operated on Mars in the
past. However, other surface features have been
interpreted as indicating that plate tectonics has
not operated on Mars. For example, it has been
suggested that the enormous size of volcanoes
such as Olympus Mons may indicate that the
Martian crust has remained stationary over the
magma source for a protracted period of time,
whereas on Earth the movement of tectonic plates
over magma sources results in linear tracks of
relatively small volcanoes on the surface (for
example, the chain of Hawaiian islands). There
remains no evidence for coherent planet-wide
plate tectonics at any time in the history of Mars.
Discussion and conclusions
We have made substantial progress in understand-
ing plate tectonics since the early development and
acceptance of the theory in the 1960s. We have
solved many puzzles in this field. For example, we
now know that the lithosphere and asthenosphere
behave relatively independently, in contrast to the
original idea that the motion of the tectonic plates
was controlled by motion in the asthenosphere. We
also know that the motion of the tectonic plates has
significant control over motion in the mantle, not
other way around (i.e. the location of downwelling
slabs at subduction zones form and control the lo-
cations of the downwelling zones within the man-
tle). We also know that mid-oceanic ridges spread
passively, and do not provide a significant contri-
bution to driving plate tectonics.
We have also identified new puzzles that we
are only just beginning to address. For example,
Earth is unique in that it has plate tectonics, but
also in that it has continents, and in that it has
life. Are these issues related? There is no clear
consensus on these issues, as we do not yet fully
understand how continental crust is formed. We
do not know whether it would be possible to have
a world with plate tectonics, but no continents, or
conversely a world with continents but no plate
tectonics. The relation between plate tectonics
and life is even more speculative, and this is
currently discussed as a chicken and egg problem:
do we need plate tectonics in order for there to
be life on Earth, or do we need life in order for
there to be plate tectonics on Earth? Of course,
although our attempts to address this puzzle are
more speculative, this puzzle is also very exciting!
There is still a lot that we do not know about
plate tectonics, and there are many tasks that lie
ahead for geophysicists. For example, why do
subduction zones have arc shapes, and why do
they move around? What are the differences
between the various tectonic plates, what causes
those differences, and do those differences control
the process of plate tectonics in any way? How
and when did plate tectonics start on Earth? This
last question is the one that we are most likely to
be able to answer in the near future. If we can
understand how plate tectonics started on Earth it
will help us to figure out why it does not occur
March 20 08 P H Y S I C S ED U C A T I O N 149
P Martin et al
on any of the other terrestrial bodies in the Solar
System. Will we ever find another planet that does
have plate tectonics, or is Earth not just unique
within the Solar System, but also within the wider
universe? If you want to find out the answers to
these kinds of questions, become a geophysicist!
Acknowledgments
The images in figures 1–3in this article are
courtesy of NASA/JPL-Caltech. The NASA
Planetary Photojournal is an excellent resource
bank containing thousands of images, complete
with their original release captions [7].
Received 8 January 2008
doi:10.1088/0031-9120/43/2/002
References
[1] Anderson D L 2007 New Theory of the Earth
(Cambridge: Cambridge University Press)
[2] Stern R J 2007 When and how did plate tectonics
begin? Theoretical and empirical considerations
Chin. Sci. Bull. 52 578–91
[3] van Andel T H 1994 New Views on an Old Planet:
A History of Global Change (Cambridge:
Cambridge University Press)
[4] Oreskes N 2001 Plate Tectonics: An Insider’s
History of the Modern Theory of the Earth
(Oxford: Westview Press)
[5] Davidson J P, Reed W E and Davis P M 2001
Exploring Earth 2nd edn (Englewood Cliffs, NJ:
Prentice-Hall) (ISBN 0-13-018372-5)
[6] Beatty J K, Petersen C and Chaikin A 1999 The
New Solar System 4th edn (Cambridge:
Cambridge University Press)
[7] http://photojournal.jpl.nasa.gov/index.html
Paula Martin is the Science Outreach
Co-ordinator for Durham University. Her
current research is focused on the
geology and geophysics of Venus and
Mars.
Jeroen van Hunen is a lecturer in Earth
Sciences at Durham University. His
current research is focused on dynamic
geophysical models of subduction and the
early Earth.
Steve Parman is a lecturer in Earth
Sciences at Durham University. His
current research is focused on the
chemical evolution of the Earth’s
interior.
Jon Davidson is a professor of Earth
Sciences at Durham University. His
current research is focused on the
generation and evolution of volcanoes at
subduction zones.
150 PH Y S I C S ED U C A T I O N March 2008
