Introduction to geodynamics

Introduction
• Geodynamics is a subfield of geophysics
dealing with dynamics of the Earth.
• It applies physics, chemistry and mathematics
to the understanding of how mantle
convection leads to plate tectonics and
geologic phenomena such as seafloor
spreading, mountain building, volcanoes,
earthquakes, faulting and so on.
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What is Geodynamics ?
The branch of geophysics concerned with
measuring, modeling, and interpreting the
configuration and motion of the crust, mantle,
and core of the earth and other planets.
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Geodynamics provides the fundamentals necessary for an
understanding of the workings of the solid Earth. The Earth is a
heat engine, with the source of the heat the decay of radioactive
elements and the cooling of the Earth from its initial accretion.
The work output includes earthquakes, volcanic eruptions, and
mountain building.
Geodynamics comprehensively explains these concepts in the
context of the role of mantle convection and plate tectonics.
Observations such as the Earth’s gravity field, surface heat flow,
distribution of earthquakes, surface stresses and strains, and
distribution of elements are discussed.
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Since the l6th century, cartographers have
noticed the jigsaw-puzzle fit of the continental
edges.
Since the 19th century, geologists have known
that some fossil plants and animals are
extraordinarily similar across the globe, and
some sequences of rock formations in distant
continents are also strikingly alike.
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The similarity in shape between the west coast of Africa and the
east coast of South America was noted as early as 1620 by
Francis Bacon.
This “fit” has led many authors to speculate on how these two
continents might have been attached.
A detailed exposition of the hypothesis of continental drift was
put forward by Frank B. Taylor (1910).
The hypothesis was further developed by Alfred Wegener
beginning in 1912 and summarized in his book The Origin of
Continents and Oceans (Wegener, 1946).
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As a meteorologist, Wegener was particularly interested in the
observation that glaciation had occurred in equatorial regions at
the same time that tropical conditions prevailed at high
latitudes.
This observation in itself could be explained by polar wander, a
shift of the rotational axis without other surface deformation.
Wegener argued that for several hundred million years during
the late Paleozoic and Mesozoic eras (200 million to 300 million
years ago), the continents were united into a supercontinent
that he labeled Pangea—all Earth.
10

Alfred Wegener (1880 – 1930)
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He suggested that tidal forces or forces associated with the
rotation of the Earth were responsible for the breakup of this
continent and the subsequent continental drift.
Further and more detailed qualitative arguments favoring
continental drift were presented by Alexander du Toit, particularly
in his book Our Wandering Continents (du Toit, 1937).
Du Toit argued that instead of a single supercontinent, there had
formerly been a northern continent, Laurasia, and a southern
continent, Gondwanaland, separated by the Tethys Ocean.
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At the turn of the 20th century,
Austrian geologist Eduard Suess proposed the theory of
Gondwanaland to account for these similarities:
that a giant supercontinent had once covered much or
all of Earth's surface before breaking apart to form
continents and ocean basins.
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Eduard Suess (1831 –1914)
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Continental drift was not accepted when first
proposed, but in the 1960s it became a cornerstone of
the new global theory of plate tectonics.
The motion of land masses is now explained as a
consequence of moving "plates"—large fragments of
the earth's surface layer in which the continents are
embedded.
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BEFORE CONTINENTAL DRIFT:
VERSIONS OF CONTRACTION THEORY
One of the central scientific questions of 19th-century geology
was the origin of mountains. How were they formed? What
process squeezed and folded rocks like putty? What made the
earth's surface move? Most theories invoked terrestrial
contraction as a causal force.
It was widely believed that Earth had formed as a hot,
incandescent body, and had been steadily cooling since the
beginning of geological time.
Because most materials contract as they cool, it seemed logical
to assume that Earth had been contracting as it cooled, too.
As it did, its surface would have deformed, producing mountains.
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Geologist James Dwight Dana (1813- 1895) had developed a
different version of contraction theory.
Dana suggested that the continents had formed early in earth
history, when low-temperature minerals such as quartz and
feldspar had solidified.
Then the globe continued to cool and contract, until the high-
temperature minerals such as olivine and pyroxene finally
solidified: on the moon, to form the lunar craters; on Earth, to
form the ocean basins.
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James Dwight Dana (1813 – 1895)
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In the early 20th century, contraction theory
was challenged by three independent lines of
evidence. The first came from field mapping.
Nineteenth- century geologists had worked in
great detail to determine the structure of
mountain belts, particularly the Swiss Alps and
the North American Appalachians.
When they mapped the folded sequences of
rocks in these regions, they found the folds to be
so extensive that if one could unfold them the
rock layers would extend for hundreds of miles.
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The second line of evidence came from geodesy—the science of the
shape (or figure) of the earth. While field geologists were unraveling the
structure of the Alps and Appalachians, cartographers with the Great
Trignometrical Survey of India were making geodetic measurements to
produce accurate maps of British colonial holdings.
In the early 1850s, Colonel (later Sir) George Everest, the surveyor-general of
India, discovered a discrepancy in the measured distance between two
stations, Kaliana and Kalianpur, 370 miles (600 kilometers) apart.
When measured on the basis of surveyor's triangulations, the latitude
difference was five seconds greater than when computed on the basis of
astronomical observation.
Everest thought the difference might be due to the gravitational attraction of
the Himalayas on the surveyors' plumb bobs, and enlisted John Pratt (1809-
1871), a Cambridge-trained mathematician and the archdeacon of Calcutta,
to examine the problem.
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Pratt proposed that the observed effects could be
explained if the surface topography of the mountains
were somehow compensated by a deficit of mass
beneath them—
an idea that came to be known as isostasy, or "equal
standing."
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Third, and most fundamental, physicists discovered
radiogenic heat, which contradicted the basic
assumption of contraction theory that the earth was
steadily cooling. With contraction no longer assumed,
earth scientists were motivated to search for other
driving forces of deformation.
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In the United States, the question was addressed by
Harvard geology professor Reginald A. Daly (1871-
1957), North America's strongest defender of
continental drift.
Daly argued that the key to tectonic problems was to
be found in the earth's layered structure.
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Reginald A. Daly (1871-1957)
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• Advances in seismology suggested that the
earth contained three major layers: crust,
substrate (or mantle), and core
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During the 1950s extensive exploration of the seafloor led to an
improved understanding of the worldwide range of mountains
on the seafloor known as mid-ocean ridges.
Harry Hess (1962) hypothesized that the seafloor was created at
the axis of a ridge and moved away from the ridge to form an
ocean in a process now referred to as seafloor spreading.
This process explains the similarity in shape between continental
margins. As a continent breaks apart, a new ocean ridge forms.
The ocean floor created is formed symmetrically at this ocean
ridge, creating a new ocean.
This is how the Atlantic Ocean was formed; the mid-Atlantic
ridge where the ocean formed now bisects the ocean.
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Harry Hammond Hess (1906 - 1969)
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It should be realized, however, that the concept of continental
drift won general acceptance by Earth scientists only in the
period between 1967 and 1970.
Although convincing qualitative, primarily geological, arguments
had been put forward to support continental drift, almost all
Earth scientists and, in particular, almost all geophysicists had
opposed the hypothesis.
Their opposition was mainly based on arguments concerning the
rigidity of the mantle and the lack of an adequate driving
mechanism.
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The propagation of seismic shear waves showed beyond any
doubt that the mantle was a solid.
An essential question was how horizontal displacements of
thousands of kilometers could be accommodated by solid rock.
The fluid like behavior of the Earth’s mantle had been
established in a general way by gravity studies carried out in the
latter part of the nineteenth century.
Measurements showed that mountain ranges had low-density
roots. The lower density of the roots provides a negative relative
mass that nearly equals the positive mass of the mountains. This
behavior could be explained by the principle of hydrostatic
equilibrium if the mantle behaved as a fluid. Mountain ranges
appear to behave similarly to blocks of wood floating on water.
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The fluid behavior of the mantle was established quantitatively
by N. A. Haskell (1935). Studies of the elevation of beach
terraces in Scandinavia showed that the Earth’s surface was still
rebounding from the load of the ice during the last ice age.
In the 1950s theoretical studies had established several
mechanisms for the very slow creep of crystalline materials.
This creep results in a fluid behavior. Robert B. Gordon (1965)
showed that solid-state creep quantitatively explained the
viscosity determined from observations of postglacial rebound.
32

Arthur Holmes (1931) hypothesized that thermal convection was capable of
driving mantle convection and continental drift.
If a fluid is heated from below, or from within, and is cooled from above in the
presence of a gravitational field, it becomes gravitationally unstable, and
thermal convection can occur.
The hot mantle rocks at depth are gravitationally unstable with respect to the
colder, more dense rocks in the lithosphere. The result is thermal convection
in which the colder rocks descend into the mantle and the hotter rocks
ascend toward the surface.
The ascent of mantle material at ocean ridges and the descent of the
lithosphere into the mantle at ocean trenches are parts of this process.
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The Earth’s mantle is being heated by the decay of the
radioactive isotopes uranium 235 (235U), uranium 238 (238U),
thorium 232 (232Th), and potassium 40 (40K).
The volumetric heating from these isotopes and the secular
cooling of the Earth drive mantle convection.
The heat generated by the radioactive isotopes decreases with
time as they decay
34

Arthur Holmes (1890 –1965)
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During the 1960s independent observations supporting continental drift
came from paleomagnetic studies. When magmas solidify and cool, their
iron component is magnetized by the Earth’s magnetic field. This remanent
magnetization provides a fossil record of the orientation of the magnetic
field at that time. Studies of the orientation of this field can be used to
determine the movement of the rock relative to the Earth’s magnetic poles
since the rock’s formation.
Rocks in a single surface plate that have not
been deformed locally show the same position for the Earth’s magnetic poles.
Keith Runcorn (1956) showed that rocks in North America and Europe gave
different positions for the magnetic poles. He concluded that the differences
were the result of continental drift between the two continents.
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By the late 1960s the framework for a comprehensive understanding of
the geological phenomena and processes of continental drift had been built.
The basic hypothesis of plate tectonics was given by Jason Morgan (1968).
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Introduction to geodynamics

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