Introduction: Plate Tectonic
It is known that the earth’s crust must have about 35 km of the dense rock layer, which varies from about 5 km in the oceans to 70-80 km in the mountainous regions of the Alps, And the Himalayas. Plate Tectonic Movements are still frequently proved by earthquakes on the oceans and are rare on continents, with volcanic, rocky, and coastal areas rising and falling at great expense.
When we study the palaeomagnetic, the Earth’s crust (Oceans and Continental–Theory of Continental Drift) meets the upper part of the Mantle, which lowers the asthenosphere (low-velocity zone at a depth of about 100 to 150 km). Lithosphere formation Does, in which renunciation is done. Plates or defects or large blocks with force. This is how the lithosphere is made up of lithospheric plates.
Plate tectonics provides a modern view of the rock-cycle. It involves a world-wide net-work of moving plates of lithosphere.
Fig.1 diagram of Basic Plate Tectonic Theory
Important Features of Plate Tectonics
- The Earth is believed to be made up of 20 lithospheric plates.
- Their thickness ranges from 0-10 km to 100-150 km elsewhere.
- Plates can have continental and oceanic surfaces.
- These plates are in constant motion relative to each other and the axis of rotation of the earth.
- Virtually all seismic, volcanic, and tectonic activity is localized around the margins of the plate and is associated with the difference between adjacent plates.
- These plates are small and large, separated by defects and stiffness, mostly across rocks or similar to continental boundaries (trenches).
- They move at speeds of up to 1-6 cm per year.
- Where two plates are shown, we find extension features, usually similar to the marine coast, the vertical axis.
- Where two plates protrude from each other, we see temporary faults, such as parts joining large sections of sea paths or arcs.
- Where two plates meet and push each other down, we find the island arcs, the vast asymmetric features that are the largest earthquake, explosive volcanism, great topographic relief, and many more because of Plate Tectonic Movements.
a). Plate Boundary:
It is the surface trace of the zone of motion between two plates.
b). Plate Margin: The marginal part of a particular plate.
Types of Plate – Margin
There are three types of plate-margin:
I. Constructive Plate Margin:
In this case, a new entrails are formed when stimulated with the control materials. The lithosphere plates break near the middle-wave box of the ocean, where a new surface emerges. Thus, a ridge is an area where two plates move away from each other, but they do not separate, as new material is constantly added to each. The boundaries at which the net effect of motion is to cause a surface are called “Sources”. In the case of structural plate boundaries, the largest main stress is vertical, and the plate boundary will consist of a series of normal malfunctions that are immersed about 60 ° from the horizontal.
II. Destructive Plate Margin:
It represents the boundary zone of convergent plates, along which two lithosphere plates join together, and one plate is forced to fall into the mantle. In this case, the boundary of the plate will be the opposite error, which is immersed at an angle of 30 ° from the horizontal. This dominance over one plate gives rise to the arches of the trenches and the island. The edges of the plates, in which the net effect of the motion is to destroy the surface, are called “sinks”.
III. Conservative Plate Margin:
When the lithosphere plate can slide side by side, that the plates neither gain nor lose surface area, there is an overflow or transforming fracture that marks the boundaries of the conservation plate.
It is a geometric rule that shows that any movement of the tile from one position on the top of the circle to the other can be thought of as a simple rotation of the tiles around a well-selected row. crossing between sections. All points on the plate travel along small circular paths about the chosen axis in passing from their initial to final position. It follows that any plate boundary which is conservative must be parallel to a small circle, the axis of which is the axis of rotation plate boundaries which are not parallel to such small circles must be either constructive or destructive.
Euler’s formula: Two important mathematical theorems of Leonhard,
F + V = E + 2
F is the number of faces
V the number of vertices
E the number of edges.
Physical Characters Of Lithosphere.
- Thermal Property: Lithosphere changes the pressure and temperature drastically as it transmits them from the atmosphere to the planet. As a newly developed hot, lithospheric plate is removed from the plate, and it continues to flow slowly as the law expands through its maximum temperature.
- Elasticity: A lithospheric plate can be considered as a flexible sheet that floats on water and bends under a load.
- Mechanical Properties: The newly created natural lake near the center of the mountains is warm and very important and will be less fragile than oceans or continents carrying lithospheric plates. Regardless of the size of the slab, the continental plate carrier is lighter than the ocean-carrying plate.
- Lithosphere as a stress-guide: The deep and central earthquake zones are often called Benioff zones, corresponding to the pressure exerted on the lithospheric plate that flows into the atmosphere and not to collide between the atmosphere and the lithospheric plate.
- Kinematics of Relative Movements: According to Wilson, the sign of the creation of the surface creates a variety of elements, while the sign of the destruction of the surface destroys the surface. These processes can suddenly stop against what he calls the error of change, as well as that of the movement being a real strike-slip.
Causes Of Plate Motion
- Oceanic Crust Formation: As the crustal formation at the mid-oceanic ridges is a continuous process, it begins to spread at a rate of 1-6 cm/year and this may cause the motion of the plates.
- Rates Of Motion: Since spreading occurs at ridges, at rates ranging from 1-6 cm/year but crust is consumed at a rate of 5 to 15 cm/year at oceanic trenches, the plates are to move.
- Oceanic-Topography: The mechanism must be consistent with the development of topographic ridges at centers of spreading ridges rises 2 to 4 km above the level of the ocean floor and near the axes slope away more or less symmetrically from the crest.
- Gravity: Ridges are close to isostatic equilibrium but sinks are characterized by topographic trenches which shows the largest negative gravity anomalies. The gravitational differences may cause plate motion.
- Thermal: With increasing distance from ridge crest the scatter of the heat flow values diminish and the mean heat-flow falls until it reaches average level for the oceans. Oceanic trenches have abnormally low heat flow but a short distance away in the adjacent island-arcs, the flow is high. The difference between these heat flow values seem to be responsible for plate motion.
- Convection-Currents: In the mantle zone seems to be responsible for plate motion, as the diverging current drags the lithospheric plates along the direction of their flow.
- Strength of the Lithosphere: Even though lithospheric plates appears to be able to move great distances without undergoing significant internal deformation, in some instances the plate are 20 times as long as they are thick. With such a length to thickness ration neither compressional or tensional stresses could be transmitted from one end of the plate to the other, unless the frictional resistances beneath the plate are very small. As the plates are above the viscous melt, it suffers no friction and can move.
Importance Of Plate Tectonics
The theory of plate tectonics is useful in explaining the phenomena like:
- Mountain Building
- Oceanic Trenches
- Ocean Floor Spreading
- Mid-Oceanic Ridges (Where plates diverge)