The Living planet: putting our knowledge of plate tectonics to work

105 Pages·2009·5.29 MB·English

The living planet:
putting our knowledge of
plate tectonics to work
Impact N o . 145
3 Comment
5 Plate tectonics: a framework for understanding our living planet
José Achache
21 Heat flow of the earth and geothermal resources
Valiya M. Hamza
33 Continental growth and collision, and mineral prospection, in Southeast Asia
Charles S. Hutchison
45 Sedimentary basins, plate tectonics and oil fields
Bruce Sellwood
55 Volcanoes and m e n
Claude Jaupart
63 Seismicity of Japan: earthquakes and tsunamis
Katsuyuki Abe
75 Earthquake-proof construction and architecture
Ananá S. Ar y a
89 Deep continental drilling on the Kola peninsula and the structure of the Earth's
crust
Olea L. Kuznetsov
97 Readers' forum
99 Erratum—Impact 142
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Comment
For centuries, m a n has observed seismic and volcanic activity and frequently suffered
from it. Early reports of natural disasters m a y , in fact, be considered as being a m o n g the
first scientific geophysical observations. It was not until 50 years ago, however, that the
mechanisms responsible for this activity began to be understood. Until then, it had
seemed as if natural disasters could occur anywhere at any m o m e n t on earth.
But m a n has also lived and produced at the earth's expense. In the nineteenth
century, the development of industry underlined the need for a more systematic
extraction of mineral resources. Prospecting at that time was still largely performed by
trial and error methods, since there were few guidelines on the distribution of ore
deposits and coal measures.
This situation has changed after the emergence during the second half of this
century of the basic principles of plate tectonics. Earth sciences have evolved toward a
global understanding of our planet, from the kinematics and dynamics of surface
phenomena to the thermal, mechanical and chemical couplings between the core, the
mantle, the crust, the ocean and the atmosphere. With this new perspective, the
distribution of active zones and natural resources at the surface of the earth can be
accounted for by simple mechanisms.
As a consequence, the numerous disciplines of geophysics and geology could no
longer work side by side and ignore one another. They had become the tools of every
earth scientist willing to study and understand natural phenomena on earth. For
instance, understanding the structure and, hence, the mechanical behaviour of the
lithosphère, which is thef irsts tep towards earthquake risk assessment, requires the
analysis of m a n y different parameters such as seismic velocities, gravity, topography,
tectonic analyses on various scales, heat flux and magnetics.
In the first article of this issue, we present the basic principles of plate tectonics in the
context of their discovery. W e show that, on a global scale, this theory provides the
appropriate framework for the study of natural phenomena.
V . M . H a m z a pictures the earth as a large heat engine—a nuclear power plant
would be more accurate—which provides the required energy to drive plate tectonics
and all its related surface phenomena. H e , then, gives a global presentation of
geothermal resources which can be diverted for man ' s use. The fundamental
importance of heat on earth is further illustrated by its role in the formation of mineral
resources.
After they have formed, mineral deposits are transported by plate motion. C . S.
Hutchison shows h o w plate tectonics leads to the concentration of these deposits along
belts such as subduction-related volcanic arcs and intracontinental collision zones,
which are particularly developed in Southeast Asia.
3 Impact of science on society, no. 145. 3 4
Comment
B . Sellwood describes the conditions necessary for the formation of hydrocarbon
resources. Although the succession of these conditions is rather stringent, they can be
accounted for by plate tectonics in several environments. Such a theory, therefore,
provides a guideline to assess the global distribution of such reserves.
Volcanic eruptions are the most spectacular evidence of the internal activity of the
earth. In thef iftha rticle C . Jaupart describes the main characteristics of volcanoes on
earth and analyses their impact on society. Surprisingly, he shows that volcanoes are
not solely a source of destruction but m a y sometimes promote the development of
society.
K . A b e gives a detailed account of the seismicity of Japan and the occurrence of
tsunamis generated, in some instances, by underwater earthquakes. H e shows h o w this
activity is distributed with respect to subduction zones, a fundamental feature of plate
tectonics.
O u r current understanding of plate tectonics gives us a clear knowledge of the
global distribution and frequency of earthquakes. It is, however, still not possible to
accurately forecast individual events, particularly in continental areas. A . S. Arya
shows that, while prediction can be of little use in earthquake mitigation, w e n o w have
the capacity to design and build earthquake-proof buildings.
In thef inala rticle, O . L . Kuznetsov reports on the deep drilling project undertaken
by the Soviet Union in the Kola Peninsula. This work illustrates the basic limitation of
direct investigations within the earth. However, it shows that even at rather shallow
depth, in-situ observations m a y sometimes depart from remote sensing and modelling
results universally used in the earth sciences.
José Achache
4
Plate tectonics:
a framework for
understanding our
living planet
José Achache
The development of new concepts in earth sciences has grown rapidly in the course of the
second half of this century and has shaken most of our previous ideas about the earth. It
eventually led to the layout of the theory of plate tectonics. Seismic, volcanic and tectonic
features observed at the surface of the planet are now seen as a consequence of intense
internal activity, and their investigation can no longer be separated from the study of the
internal structure of the earth. Such a global approach provides a powerful framework for
the understanding of catastrophic natural phenomena and the carrying out of prospecting
for natural resources in a more efficient way.
The earth has long been seen as an ageing planet with localized, random signs of
activity, its geological structure inherited from the past andf ixedf or eternity. This static
view of the earth stemmed from the difficulty of prospecting the whole surface of the
planet, in particular ocean basins, and of observing it on a global scale. O n the contrary,
the picture emerging today is that of a slowly but continuously evolving planet. The
first evidence for this maintained internal activity is given by the distribution of
altitudes over the surface of the earth. Indeed, topography results from the combined
action of internal activity, which creates relief, and erosion. If the earth were no longer
active, mountains would tend to be eroded, thusf illingv alleys and ocean basins with
sediments, and in time the average altitude would tend to be zero. The reality is quite
different, since an analysis of the globe shows a bimodal distribution of m e a n altitudes,
with two max i m a at —4500 metres (the m e a n depth of the oceans) and + 100 metres
(the m e a n elevation of continents) (see figure 1). This topography must then be
dynamically maintained and attests to the earth being a living planet.
But observers do not have access to the interior of the earth and all the studies of the
internal processes and structures must rely on indirect observations. This, too, has been
a major obstacle to the understanding of our planet, and has often linked progress in
the earth sciences with technological advances. The discovery of large-scale structures
such as the tectonic plates appeared as a direct consequence of the extensive survey of
the ocean bottom performed after the Second World W a r by modern and well-
equipped océanographie ships. The worldwide distribution of earthquake epicentres
José Achache is Chargé de Recherches at the Instituí de Physique du Globe de Pans. His current work
involves the analysis of satellite measurements of the earth"s magneticf ield,w ith a particular emphasis on the
field of crustal origin and its implications for the determination of the deep structure of the continental crust.
H e m a y be contacted at the following address: Department of Geomagnetism and Paleomagnetism, Institut
de Physique du Globe de Paris. 4, place Jussieu, 75252 Paris, France.
5 Impact of science on society, no. 145. 5 19
José Achachc
lii/urc I. Percentage
of the Earth's
Graph showing surface Sea
percentage of the level
surface of the earth at
various altitudes. Peak 20%
A corresponds to the
mean altitude of the B
continents, Peak B to
the mean depth of the
/ 1
oceans. After Allèque
(1984). 10% / 1
/ 1
' 1
s^ Sy 1
1 X \ ^
'—w
10•0-m -45•0 0m Altitude
was obtained with the development of a new generation of highly sensitive seis-
mometers. M o r e recently, the advent of space techniques has m a d e possible the
observation of the earth on a truly global scale. Space techniques provide a remarkable
insight into the deep interior of the earth, allow the constant monitoring of crustal
motions and constitute a new means of prospecting for natural resources, on a
planetary scale.
Th e stable and stratified body pictured by nineteenth-century scientists in n o w
replaced by a unified system in which surface motions are coupled with internal
processes on large scales both in space and time. O n e cannot understand surface
phenomena on earth without studying the interior of the planet. This intimate
relationship between surface geology and internal geophysics and geochemistry has
emerged as the basic principle of mode rn earth sciences.
F r o m continental drift to plate tectonics
Continental drift
At the beginning of this century, Alfred Wegener, a G e r m a n meteorologist, proposed
that all the continents were once grouped as a single supercontinent that he named
1
Pangaea (see figure 2). S o m e 300 million years ago, Pangaea started to break up.
North and South America drifted away from Africa, opening the Atlantic Ocean. In a
similar way, the Indian Ocean resulted from the separation of Africa, India, Australia
and Antarctica. This suggestion of large horizontal displacements of continents at the
surface of the earth (by several thousands of kilometres) was very m u c h at odds with all
the geological theories prevailing at that time. Indeed, geological processes such as
mountain building were interpreted as the result of small (i.e. of a few kilometres) local
vertical displacements of the crust. T h e debate betweenf ixista nd mobilist views of the
history of the earth was opened.
Wegener's hypothesis wasf irstb ased on the striking similarities in shape of the
coastlines of Africa and South America. But Wegener pursued his idea, accumulating
evidence for the existence of Pangaea from palaeontological observations, sedimen-
tology, mineralogy and m a n y other disciplines. Similar species are observed at the same
6
'<
I
t. Everest
uril Trench
Plate tectonics understanding our planet
20°E
20°E
Figure 2. Wegener's reconstruction of the landmass Pangaea,
approximately 200 million years ago. Panthalassa (meaning 'all
seas') evolved into the Pacific Ocean, and the Mediterranean is
a remnant of the Tethys Sea. Shading represents the polar
glacier thought to have flowed over Southern Gondwanaland
during Permian time, and explaining the glacial deposits found
in South America, Africa, India and Australia.
epoch in the palaeontological record of the continents on both sides of the south
Atlantic. Since m a n y of these species arc strictly continental lifeforms (like the reptile
Mesosaurus), it indicates that land connections must have existed in the past. T h e
Glossopteris flora of the Carboniferous age (about 300 million years ago), which
equally cannot be expected to have crossed an ocean, is nevertheless widespread over
all continents in the Southern Hemisphere. Glacial deposits of the same age define a
continuous polar cap w h e n all southern continents are placed in their position
according to Wegener's reconstruction (figure 2).
Wegener's theory of continental drift was able to account for m a n y more poorly
understood observations, but above all it provided thef irsts atisfactory explanation for
mountain building. However , Wegener w a s not able to determine the forces that
powered the motion of continents over such distances and that had sufficient energy to
build mountains. Led by Sir Harold Jeffreys, a distinguished British geophysicist, the
majority of earth scientists refuted Wegener's theory, and it eventually fell into oblivion
after twenty years of controversy.
Past recordings of the magnetic field
Like the burial, the revival of continental drift theory c a m e from the United K i n g d o m ,
in the late 1950s, with the study of the natural magnetization of rocks.
A strong magneticf ielde xists at the surface of the earth and is generated in the core
of the earth. Between 2900 and 5000 k m depth, the outer core is predominantly m a d e of
7
José Achache
iron and behaves like a liquid. It is therefore a good electric conductor. The magnetic
field of the earth is believed to be produced by a self-sustained d y n a m o process, driven
by convective fluid motion in the electrically conducting outer core. Crustal rocks
containing magnetic minerals are magnetized by the coref ield.I n s o m e instances, this
magnetization can be 'frozen' for millions of years, thus creating a permanent
magnetization in the rocks parallel to the direction of the ambientf ielde xisting at the
time of formation of the rock. This ability of rocks of the crust to retain the m e m o r y of
the earth's magneticf ieldi n the past has been fundamental to the revival of the concept
of continental drift.
The mainf ieldo f the earth (the coref ield)h as two remarkable properties. First, it is
almost dipolar; that is, it is similar to thef ieldt hat would be created by a bar magnet
located at the centre of the earth (figure 3). For this reason, the needle of a magnetic
compass always points to the magnetic north pole. Its second interesting property is
that it undergoes polarity reversals, during which the north and south poles are
switched. Using thef irstp roperty one can determine the position of the north pole by
measuring the direction of the magnetization in crustal rocks anywhere on the surface
of the earth. W h e n performing such measurements on rocks from various continents
2
around the globe, S. K . Runcorn and E . Irving, two British geophysicists, observed
systematic discrepancies between the magnetic pole positions deduced from rocks of
the same age but of different origins. Furthermore, rocks of different ages sampled in a
given location showed a regular migration of the pole position with time. This apparent
wander of the pole implied that either the pole or the continent had actually drifted. All
their observations led them to conclude that the continents had been continously
Geographic Magnetic
North \ /North /
\ Pole \ / Pole /
Figure 3. The Eartrfs magneticf ieldi s much like that which would be
produced if a giant bar magnet were placed at the Earth's centre
and slightly inclined from the axis of rotation.
Plate tectonics—understanding nur planet
drifting. In addition, they were able to show that the movements of the continents thus
predicted brought them in a position close to that proposed by Wegener in his Pangaea
reconstruction.
Sea-floor spreading
But the major obstacle remained. W h a t could be the cause of this motion and what is
the force which drives continental drift? The answer was to come from the study of the
ocean floor.
Several striking features are observed on the topographic m a p of the ocean bottom,
foremost being a network of ridges 2000 to 4000 metres high and about 2000 kilometres
wide which runs continuously through the Atlantic, Indian and Pacific Oceans. These
ridges appear to ber ifteda long their axis and are similar to the rift valley of East Africa.
The second most important topographic features are the deep trenches which girdle the
north and west Pacific Ocean along the Aleutians, Japan, the Marianas and the
Philippines.
Performing geophysical measurements of all kinds, research vessels have been able to
assess m a n y fundamental properties of the oceanic crust. Seismic reflection profiles
showed this crust to be mu c h thinner than under the continents and to be mainly
composed of basaltic rocks rather than granites. The thickness of sediments was also
surprisingly small, given the age of the oceans and the observed rate of sedimentation,
and it was seen to increase away from the ridges. Gravity measurements revealed strong
anomalies above the trenches and to a lesser extent above the ridges. Mid-ocean ridges
were also observed to be regions of anomalously high heat flow, indicating the presence
of volcanic activity. All these observations led to the formulation of the sea-floor
3
spreading hypothesis in a paper referred to by its o w n author as an essay in geopoetry.
According to this hypothesis, the mid-ocean ridges are accreting zones where the ocean
floor is constantly generated from upwelling mantle material. The newly formed sea-
floor then moves away from the volcanic ridges, across the ocean basins. At trenches, it
sinks and returns into the mantle, drawing d o w n the sediments deposited during its
travel through the ocean. The sea-floor is seen as constantly moving at the surface of the
earth and recycling through the mantle in less than 200 million years. This model was
confirmed by the planetary distribution of earthquakes plotted in the early sixties
(figure 4). It shows that the vast majority of earthquakes occur along ridges and
trenches, the ones in the latter being m u c h stronger and located deeper in the mantle
(down to 700 k m ) .
The obstacle to Wegener's theory could then be easily overcome. The continents,
instead of drifting on the underlying mantle, were entrained in the motion of a thicker
surface layer involving the crust of both the oceans and the continents and the upper
4
part of the mantle . This layer, called the lithosphère, is created at mid-ocean ridges
where upwelling mantle material cools and solidifies, then drifts at the earth's surface
5
and eventually returns to the mantle at trenches. In fact, as early as 1931, Holmes had
proposed that continental drift was associated with mantle convection and thus driven
by thermal forces. Indeed, the earth's mantle is being heated by the decay of radioactive
isotopes of uranium, thorium and potassium. Because the temperature increases with
depth in the mantle, the hot rocks at depth are gravitationally unstable with respect to
colder and denser rocks near the surface. This results in a convective motion in which
colder rocks descend into the deep mantle and hotter rocks ascend toward the surface.
9