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PLATE TECTONICS
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 4:2:1, 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 pressurerelease melting.
• 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.
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.
Three ways by which heat is transferred
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.
• The contradictory effect of temperature
and pressure on the melting of rock
material
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.
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.
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.
The formation of an oceanic
crustal reservoir at a Mid-ocean
ridge
• Nature of the Atlantic oceanic crust based
on seismic experiments
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.
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.
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.
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.
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.
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.
Plate Tectonic Processes
• The formation of ocean crust at mid-ocean
ridges is balanced by the destruction of
oceanic crust at subduction zones
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).
• The mid-ocean ridges are displaced by east-west trending transform faults
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.
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.
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.
The following two figures illustrate the vastly
different stories represented by the physiographic
(surface) and geologic maps of North America.
The complex nature of the geology of Australia is
easily discernable in terms of the variable magnetic
nature of the rocks.
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.
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.
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.
The conversion of thermal
energy to chemical energy by
the formation of hydrous
minerals.
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.
The release of chemical
energy and water and the
formation of island arcs.
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.