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    PLATE TECTONICS
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The measurement of seismic waves passing through
the Earth, which has a radius of about 6500 km,
indicates that the Earth is made up of 1) a
partly molten core composed largely of iron 2) a
mantle, largely composed of oxygen, magnesium and
silicon in the ratio of 421, divided into two
shells, an inner shell called the asthenosphere,
and an outer shell called the lithosphere 3) an
outer crust composed of two components, one
represented by the sea floors and the other by
the continents 4) a discontinuous hydrosphere
and polar ice cap and 5) a continuous
atmosphere. These constitute the main material or
chemical RESERVOIRS of the Earth. The boundaries
between the reservoirs are relatively sharp but
the reservoirs themselves may be heterogeneous in
composition. To understand how material and
energy are transferred between these reservoirs
it is necessary to first grasp the concepts of
material creep, thermal convection, and
pressure-release melting.
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• Material Creep - the earth seems to be a very
  solid and elastic body when subjected to short
  term stresses, but when the stresses are imposed
  over long periods of time it behaves more like a
  plastic or viscous material capable of flowing
  like a thick liquid - and the higher the
  temperature, the greater the propensity of the
  material to behave as a plastic material. This
  kind of time-dependent deformation is known as
  creep.

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Thermal convection - in solid material heat is
transferred from regions of high temperature to
regions of low temperature by the process of
thermal conduction. Heat can also be transferred
by the process of radiation, as in the case of
the heat we receive from the Sun, or by
convection, as in the case of rising hot air. All
three processes are involved in the transfer of
heat from the interior parts of the Earth towards
the surface but, surprisingly, the process of
thermal convection is the most important.
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Three ways by which heat is transferred
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    Decompression melting - if materials are
heated to a sufficiently high temperature, they
begin to melt. However the melting temperature is
also a function of the confining pressure acting
on the material. Consequently, it is feasible to
melt material by lowering the pressure rather
than raising the temperature. This is called
decompression melting.
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• The contradictory effect of temperature and
  pressure on the melting of rock material

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    How do these three concepts help us explain
the operation of the Earth? Well, the temperature
of the asthenospheric mantle reservoir is
increased by heat transferred from the molten
core and by increments of heat generated by the
decay of the radioactive elements U, Th, K, Rb,
Sm, etc. At some critical temperature, the mantle
will start to flow buoyantly towards the surface
by the mechanism of deformation creep. Heat is
therefore transferred by the process of
convection, and because the confining pressure
acting on the mantle decreases as it rises, at
some critical depth the mantle will start to
melt. Once a sufficient degree of melting has
been achieved, the melt will separate and rise to
form a body of magma (magma chamber) at the base
of the lithospheric shell, from which it will
find its way to the Earth surface via passageways
created during the process known as sea-floor
spreading. This is the principal way in which the
Earth rids itself of its internal heat.
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    In contrast to the asthenosphere, heat
transfer though the lithosphere is effected by
conduction because the temperature of the
lithosphere is too low to permit convection. As
the average temperature of the Earth decreases,
the lithosphere grows downwards and it becomes
more effective as a thermal insulator. For this
reason the rate at which heat is lost from the
Earth decreases to a self-regulated minimum
value. It is currently estimated that although
the rate of radioactive heat production 3 billion
years ago was twice the rate it is today, the
mean temperature of the mantle at that time was
only 150 degree K higher than its present value.
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    The uprising thermal currents must eventually
turn over and descend back into the
asthenosphere, and the zone of magma formation is
therefore also coincidentally a zone of tensile
stress allowing the magma easy egress to the
surface via fractures created in the lithosphere
by the laterally flowing asthenosphere. These
fractures appear on the surface of the Earth as
topographically elevated linear zones within
ocean basins, and we know them as mid-ocean
ridges. Where the rock magma comes into contact
with sea-water it cools to form distinctively
shaped and aptly named 'pillow lava' units,
whereas within the fractures it cools as tabular
bodies commonly referred to as 'sheeted diabase'.
As the magma within the underlying magma
reservoir cools, minerals crystallizing out of
the melt either sink to the floor of the chamber
to form layers of mineral 'sediments', or are
added to a downward growing roof unit. As the
mass of magma solidifies from top to bottom and
bottom to top, the floor and roof of the chamber
eventually meet and the wholly solid crust is
carried away piggy-back by the laterally flowing
asthenospheric mantle conveyor belt (i.e.
sea-floor spreading). As long as the magma
chamber is continuously fed with new batches of
magma, oceanic crust is thus generated in a
quasi-steady state manner.
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    The formation of an oceanic crustal reservoir
at a Mid-ocean ridge
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• Nature of the Atlantic oceanic crust based on
  seismic experiments

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Drilling of the Pacific sea floor demonstrated
that the age of the oceanic volcanic rocks
increased westwards from the East Pacific Rise
towards the western reaches of the Pacific.
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    It is observable that the ridges are divided
into a large number of segments separated from
one another by fractures which geologists refer
to as transform faults. The presence of
transforms' reflects the fact that the location
of the magma chamber beneath the ridges tends to
jump backwards and forwards along the ridge, and
that the rate of spreading along the length of
the ridge is not uniform. The variation in
seismic activity along the transform provides
remarkable confirmation of the process of sea
floor spreading.
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Earthquakes are only observed in the red segment
of the transform fault, where the crustal
sections on either side of the fault are moving
in opposite directions.
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    If some oceans are getting larger as a result
of sea-floor spreading, then some must be getting
smaller, otherwise the total volume of the earth
would also have to increase commensurate with the
increase in size of the surface area of the
Earth. Since the Atlantic ocean is increasing in
size whereas the Pacific is decreasing in size,
the inference is that Pacific ocean crust is
being consumed back into the asthenosphere at the
margins of the Pacific. This process is called
subduction, and it is intimately linked to the
formation of volcanic island arcs, and eventually
to the construction of continental crust.
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Distribution of island arcs around the Pacific
ocean note the lack of arcs in the Atlantic
ocean other than in the Pacific extrusion zones
represented by the Antilles and Scotia arcs.
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Islands arcs may form on oceanic crust (Marianas)
or on pre-existing continental crust (Japan,
Andes). As you will note on the following two
slides, continental crust becomes progressively
thickened above subduction zones. Some arcs such
as Japan and the Marianas separate from the
continents on which they were initiated, and
float eastwards into the Pacific. Where the
arcs become totally separated, new oceanic
crust forms between the arc and its source area.

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Plate Tectonic Processes

• The formation of ocean crust at mid-ocean ridges
  is balanced by the destruction of oceanic crust
  at subduction zones

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    Based on the distribution of mid-ocean
ridges, subduction zones, and transform faults,
the surface of the Earth can be represented as a
set of moving plates, the relative movement along
whose mutual boundaries may be extensional
(mid-ocean ridges), compressional (subduction
zones), or horizontal (transform faults).
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• The mid-ocean ridges are displaced by -east-west
  trending transform faults

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Continents may aggregate to form
Supercontinents which may in time break up to
form a dispersed set of smaller continental
fragments. The latter may then reassemble to
form a new supercontinent but with the fragments
arranged in a new pattern.
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    Complete consumption of oceanic crust may
lead to the collision of continental masses and
the formation of collisional mountain chains such
as the Himalayas, the Alps, or, closer to home,
the Appalachians. In this way continents
amalgamate to form supercontinents. Where arc
systems participate in continental collision,
they are also amalgamated to the continents and
there is a consequent transfer of new arc
material to the buoyant continental plate. The
rate at which this process has varied over
geological time is a matter of dispute, but it is
this process that is thought to have led to
formation of continents.
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    Continents are also destroyed by erosion and
weathering brought about by the reaction of
silicate minerals with bicarbonate-bearing rain
water (the hydrologic cycle). As a consequence
the oceans become the receptacle of weathered
rock material and an intermediary in the
subsequent transfer of material back to the
continents or to the mantle, thus completing the
material transfer cycle.
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The following two figures illustrate the vastly
different stories represented by the
physiographic (surface) and geologic maps of
North America.
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The complex nature of the geology of Australia is
easily discernable in terms of the variable
magnetic nature of the rocks.
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The geologic history of North America dates
back to almost 4 billion years ago and records a
complicated history of oceanic consumption,
island arc development, and continental collision.
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The line marked Iapetus Suture on the following
geological map of Newfoundland represents the
collisional boundary between North America and
Europe about 400-300 million years ago.
Geologically speaking south-east Newfoundland is
more akin to Europe than it is to North America.
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The yellow area marks the newly developing
Atlantic ocean during the Jurassic the green
line is the line of closure of the earlier
Iapetus ocean separating North America and
Europe.
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    The conversion of thermal energy to chemical
energy by the formation of hydrous minerals.
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    As the mid-ocean ridge basaltic material
derived from the mantle cools, part of its heat
energy is lost by conduction to the overlying sea
water and part is converted to chemical energy by
endothermic reaction of the basalt minerals (Cpx,
Opx, Plagioclase) with sea water to form a new
set of hydrous minerals (amphibole epidote and
haematite (Fe2O3)). These minerals are then
physically transported by the process of
sea-floor spreading' to zones of subduction
where they pass back into the mantle. The water
is supplied via hydrous convection cells which
circulate within the cooling upper part of the
ocean crust. The exit zones of the hydrous fluids
are marked by the oft-publicised black' and
white' smokers located on mid-ocean ridges.
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    The release of chemical energy and water and
the formation of island arcs.
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    At depths of the order of a 100 km, the
hydrous minerals produced by reaction of basaltic
material with sea water at the ocean ridges
undergo an exothermic dehydration reaction to
form a high pressure anhydrous mineral (eclogite)
assemblage (pyroxene (jadeite) garnet) and a
hydrous phase highly charged with metal ions. The
hydrous fluid passes upwards into the overlying
mantle, which as a consequence melts to produce
an oxidized magma which rises to the surface to
form the island arcs found adjacent to subduction
zones.