Food Chain and Food Web (Ecosystem) EVS, B. Pharmacy 1st Year, Sem-II
Plate tectonics Theory
1. Mr. Toradmal Agastirishi B.
Assistant Professor
Department of Geography
Dada Patil Mahavidyalaya, Karjat, Dist.- Ahmednagar
2. Oceanic plates: basalt-
Dark (black) and dense rock
type composed of silicates,
iron and magnesium
Continental plates
granite and andesite
Light colored (pink, white
and gray) and low density
rock type composed almost
entirely of silicates.
Plate Types
3.
Plate-
The rigid lithospheric slabs or rigid and solid crustal
layers are tectonically called plates.
‘Plate tectonics-
The whole mechanism of the evolution, nature and motion of
plates and resultant reactions is called 'plate tectonics'.
In other words, the whole process of plate motions is referred to
as plate tectonics. 'Moving over the weak asthenosphere,
individual lithospheric plates glide slowly over the surface of the
globe.
Plate tectonic theory, a great scientific achievement of the
decade of 1960s is based on two major scientific concepts
e.g. (i) the concept of continental drift and (ii) the concept
of sea-floor spreading
4. Lithosphere is internally made of rigid plates .
Six Major and 20 Minor plates have been identified so
far (Eurasian plate, Indian-Australian plate, American
plate, Pacific Plate, African plate and Antarctic plate).
( Scotia plate, Nasca Plate, carribean plate, Arabean
plate, Philippine plate etc.)
It may be mentioned that the term 'Plate' was first used
by Canadian geophysicist J.T. Wilson in 1965
5. Divergent/Constructive Boundaries-
plates move away from each other
Destructive /Convergent Boundaries –
plates move toward one another
Conservative/Transform Boundaries–
plate moves sideways from each other
Plate Boundaries
7.
Now the continental drift and displacement
are considered a reality on the basis of plate
tectonics.
It may be highlighted that tectonically plate
boundaries or plate margins are most
important because all tectonic activities occur
along the plate margins e.g. seismic events,
vulcanicity, mountain building, faulting etc.
Thus, the detailed study of plate margins is
not only desirable but is also necessary. Plate
margins are generally divided into three
groups, as follows :
8. These are also called as 'divergent plate margins' or
'accreting plate margins*. Constructive plate
margins (boundaries) represent zones of divergence
where there is continuous upwelling of molten
material (lava) and thus new oceanic crust is
continuously formed. In fact, oceanic plates split
apart along the mid-oceanic ridges and move in
opposite directions
1.Constructive /divergent Plate
Boundaries-
11.
These are also called as 'consuming plate margins' or
Convergent plate margins' because two plates move
towards each other or two plates converge along a
line and leading edge of one plate overrides the other
plate and the overridden plate is subducted or thrust
into the mantle and thus part of the crust (plate) is lost
in the mantle.
2.Destructive/Convergent
Plate Boundaries-
14. Conservative Plate Margins are also called as shear plate
margins. Here, two plates pass or slide past one another
along transform faults and thus crust is neither created
nor destroyed.
H. Hess postulated the concept of 'plate tec-tonics' in
1960 in support of continental drift. The continents and
oceans move with the movement of these plates. The
present shape and arrangement of the continents and
ocean basins could be attained because of continuous
relative movement of different plates of the second
Pangaea since Carboniferous period, Plate tectonic
theory is based on the evidences of (1) sea-floor
spreading and (ii) Palaeomagnetism.
3. Conservative Plate Margins/
Boundaries-
16.
See the movement of plate
Motion at Plate Boundaries.mp4
plate tectonics.mp4
plate tectonics animation.mp4
Volcanic Activity and Plate
Motions.mp4
17.
1. Sea-Floor Spreading-
The concept of sea floor spreading was first propounded
by professor Harry Hess of the Princeton University in
the year 1960. His concept was based on the research
findings of numerous marine geologists, geochemists
and geophysicists. Mason of the Scripps Institute of
Oceanography obtained significant information about
the magnetism of the rocks of sea-floor of the Pacific
Ocean with the help of magnetometer. Later on he
surveyed a long stretch of the sea-floor of the Pacific
Ocean from Mexico to British Columbia along the
western coast of North America. When the data of
magnetic anomalies obtained during the aforesaid
survey were displayed on a chart, there emerged well
defined patterns of stripes. Based on these information
Harry Hess propounded that the mid-oceanic ridges
were situated on the rising thermal convection currents
coming up from the mantle.
18.
The oceanic crust moves in opposite directions from
mid-oceanic ridges. These molten lavas cool down and
solidify to form new crust along the trailing ends of
divergent plates (oceanic crust). Thus, there is
continuous creation of new crust along the mid-
oceanic ridges and the expanding crusts (plates) are
destroyed along the oceanic trenches. These facts prove
that the continents and ocean basins are in constant
motion.
W.G. Vine and Matthews conducted the magnetic
survey of the central part of Carlsberg Ridge in the
Indian Ocean in 1963 and computed the magnetic
profiles on the basis of general magnetism.
19. Palaeomagnetism refers to the preservation of magnetic
properties in the older rocks of the earth. It may be
mentioned that when any rock, whether sedimentary or
igneous, is formed it gets magnetised depending on the
presence of iron content in the rock and is preserved
(frozen at temperature below Curie point, which is
generally 600°C).
It was the year 1600 A.D. when William Gilbert, the
physician of Queen Elizabeth, postulated that the earth
behaved like a giant magnet and magnetism of the earth
was produced in the inner part of the earth.
2. Palaeomagnetism
20.
The magnetic field of the earth is like a giant bar
magnet of dipoles, located in the centre (core) of the
earth and is aligned approximately along the axis of
rotation of the earth.
When the long axis of dipole bar magnet is extended it
intersects the earth's surface at two centres which are
called north and south magnetic poles. It may be
pointed out that magnetic south pole of the earth is
near its (earth's) geographical north pole and vice-versa
(i.e. magnetic north pole is located near geographical
south pole). If an ordinary small magnet is freely
suspended at the earth's surface then the earth's south
magnetic pole attracts north pole of small magnet and
earth's north magnetic pole attracts south pole of small
magnet.
21.
It may be clarified that as per general rule when two
magnets are brought together, then their similar poles
repel each other but opposite poles attract each other.
A freely suspended magnet on the earth's surface does
not indicate geographical north and south perfectly
because the axis of magnetic north and south poles is
not perfectly alligned along the axis of geographical
north and south poles. This causes angular inclination
between the magnetic and geographical axes. This
angular inclination is called magnetic declination
which, in fact, denotes angular inclination between the
direction of freely suspended magnet at any part of the
earth's surface and the direction of earth's geographical
north-south pole axis.
22.
On the other hand, angular inclination between freely
suspended magnetic needle and horizontal plane of the
earth's surface is called magnetic inclination or
magnetic dip. If a magnetic needle is freely suspended
at the north pole of the earth, the north pole of the
magnet being closer to the south magnetic pole of the
earth (which is, in fact, near geographical north pole)
would be attracted more and magnetic needle becomes
perpendicular Consequently, north pole of the
suspended magnetic needle dips downward vertically.
The situation is reversed in the southern hemisphere.
Thus, magnetic dip becomes 90° on geographical north
and south poles of the earth. Magnetic dip becomes
zero wherever freely suspended magnetic needle
becomes horizontal at the earth's surface.
23.
The imaginary line joining places of zero magnetic
dip angle is called magnetic equator. The magnetic
dip angle increases pole ward. It may be pointed out
that there may be spatial and temporal variation in
the intensity of simple dipole magnetic field.
24.
Source of Geomagnetic Field-
The origin of geomagnetic field is in no case
related to mantle rather it is related to the outer core of
the earth because of the fact that there is gradual
westward migration of geomagnetic field at the rate of
0.18° per year which proves that the rotation of
geomagnetic field is slower than the rotation of the earth.
This indirectly proves that the core of the earth rotates at
slower rate than the overlying mantle. It may be stated
that 'the magnetic field cannot be a permanent property
of the material of the core......must therefore be
continuously produced and maintained' (A. and Doris L.
Holmes, 1978).
25.
If permanent geomagnetic field is not possible then the
continuous production and maintenance of geomagnetic
field may be possible only when there would be
presence of materials of high electrical conductivity in
the core so that electrical currents may be generated. It is
further pointed out that the generation of electrical
currents is possible only in metallic liquid materials and
such situation is found in the outer core of the earth
which functions as self exciting dynamo. Thus, the
energy coming out of the core is transformed into
electrical currents which in association with metallic
liquid substances produce geocentric dipole magnetic
field.
26.
The required energy to maintain geomagnetic field is
believed to come from three possible sources:-
(1)heat energy released from the disintegration of
radioactive elements of the core of the earth. It is argued
that this source of energy for the generation of convective
currents (electrical currents) may not be possible because
if we accept this proposition then difficulty arises in the
process of cooling of the crust of the earth because such
situation (generation of heat energy from radioactive
elements) would also prevail in the mantle and hence the
crust cannot cool because there would be constant
supply of heat energy from below (from the mantle).
27.
(2) The downward transfer of ferromagnesian
materials from mantle into core results in the release
of gravity force in the core which in turn produces
energy.
(3) The movement of materials from inner core to the
outer core results in the heating of outer core
through heat energy released from inner core
The required energy to maintain geomagnetic field is
believed to come from three possible sources:-
28.
Remanent Magnetism-
The geocentric axial dipole magnetic field represents 95
per cent of the earth's total magnetism. The remaining
portion is represented by irregular, scattered and weak
magnetic fields. It may be pointed out that there is no
such giant bar magnet inside the earth but there is more
concentration of magnetism in the rocks of the core of
the earth in the shape of a bar magnet. The hot and
liquid lava and magma with high ferromagnesian
contents, when cooled and solidified to form igneous
rocks, get magnetized, the records of which are
preserved in the rocks. Such magnetism preserved
(frozen) in the rocks are called remanent or
palaeomagnetism.
