1. Mr. Toradmal Agastirishi BMr. Toradmal Agastirishi B..
Assistant ProfessorAssistant Professor
Department of GeographyDepartment of Geography
Dada Patil Mahavidyalaya, Karjat, Dist.- AhmednagarDada 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 nec-essary. 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 (bounda-ries) 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 tectonics' 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) seafloor
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.mp
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 seafloor of the Pacific Ocean
with the help of magnetometer. Later on he surveyed a
long stretch of the seafloor of the Pacific Ocean from
Mexico to British Columbia along the western coast of
North America. When the data of magnetic anomalies ob
tained 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
midoceanic 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 midoceanic
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 viceversa
(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
northsouth 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.
