Transcript Earthsci1

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 a partly molten core composed largely of iron; a
mantle, largely composed of oxygen, magnesium and silicon in
the ratio of 4:2:1, further divided into two shells, an inner shell
called the asthenosphere, and an outer shell called the
lithosphere; an outer crust composed of two components, one
represented by the sea floors and the other by the continents; a
discontinuous hydrosphere and polar ice cap; and 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.
• 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 it behaves
more like a viscous material capable of
flowing like a liquid; and the higher the
temperature, the greater the propensity of
the material to flow. This kind of
deformation is known as creep.
Deformation.Brittle versus
ductile strength of materials.
Modes of Thermal Transfer.
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 convective heat transfer, 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.
Decompression melting - if
materials are heated to a sufficiently
high temperature, they begin to melt.
However the melting temperature is 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.
Pressure v Temperature, Melting.
How do these three concepts help us explain the operation of the Earth?
Well, the temperature of the asthenosopheric 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.
Plate Tectonic Processes
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 selfregulated 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 formation of the oceanic
crustal reservoir
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 whole is carried away piggyback 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.
Formation of Oceanic crust at
Mid-Ocean Ridges.
Seismic model of the Atlantic
oceanic crust.
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.
Transform faults.
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-publicized `black' and `white'
smokers located on mid-ocean ridges.
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.
Creation and consumption of
oceanic crust.
Section through a model island
arc and marginal basin.
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.
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).
Tectonic Plates - Deep Sea
Drilling Project map of the
Pacific Ocean.
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 are
amalgamated 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 boyant 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.
formation of the Himalayas
through the collision of India and
Asia.Formation of the Himalayas.
The location of the ancient
Iapetus Ocean, the closure of
which gave rise to the
Appalachian mountain system.
Continents are also destroyed by erosion
and weathering brought about by the reaction
of silicate mineras 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.