Crust is the outer thin layer with a total thickness of around 100Km and forms 0.5% of the earth's volume.Mantle lies between 100 to 2900 Km below the earths surface and forms 16% of the earth's volume.Core lies between 2900 to 6400 Km below the earth's surface and accounts for 83% of the earth's volume.Our earth is undergoing deformations imperceptibly and inexorably. These deformations are caused by the movements generated by various factors:
1. The heat generated by the radioactive elements in earth's interior.
2. Movement of the crustal plates due to tectogenesis.
3. Forces generated by rotation of the earth.
4. Climatic factors
5. Isostacy.
The seafloor points to the movement of the earth's crust, in relation to each other takes place primarily due to three factors - polar wandering( It is the relative movement of the earth's crust and upper mantle with respect to the rotational poles of the earth), continental drift ( It refers to the movement of the continents relative to each other) and sea floor spreading( It describes the movement of the oceanic plates relative to one another).
PERMIAN , 225 million years ago
TRIASSIC , 200 million years ago
JURASSIC , 135 million years ago
CRETACEOUS , 65 million years ago
PRESENT DAY
Continental Drift theory: Wegner,a climatologist wanted to investigate the relative distribution of land and sea and the climatic aberrations of the past.He postulated that originally(225 million years ago) all of the land masses of earth were locked together as a great supercontinent called PANGAEA which was covered by one big ocean called PANTHALASSA. A sea called TETHYS divided the PANGAEA into two huge landmasses _ LAURENTIA to the north and GONDWANALAND to the south of Tethys.The drift started around 200 million years ago and the continents began to break up and drift away from one another. The drift was in two directions - equatorwards due to interaction of forces of gravity and buoyancy, and westwards due to tidal currents because of the earth's motion and the drift is still continuing.From the analysis of the continental drift The following points can be observed.
1. South America and Africa seem to fit in with each other, especially the bulge of Brazil fits into the Gulf of Guinea.
2. Greenland seems to fit in well with Ellesmere and Baffin islands.
3. The east coast of India, Madgascar and Africa seem to have been joined.
4. North and South America on one side and Africa and Europe on the other fit along the mid-Atlantic ridge.
But, the main drawback of the wegner's theory is that he failed to explain why this drift began only in the Mesozoic era and not in the period prior to it.
Sea floor spreading: The authors of this theory recognised the existence of Pangaea , tethys and Panthalassa and tried to explain the apparent movement of sea floor at specific locations.According to this theory, the intense heat generated by radioactive substances in the mantle(100 to 2900 Km below the earth surface) seeks a path to escape, and gives rise to the formation of convection currents in the mantle. Wherever rising limbs of these currents meet, oceanic ridges are formed on the sea floor and wherever the falling limbs meet oceanic trenches are formed.At the ridges, the eruption results in upwelling of the magmatic material. This causes movement of the crust.After upwelling, the hot magma cools down, solidifies and moves apart to make way for the material of successive eruptions. This results in effective sea floor spreading. Thus discrete segments of earth's crust are formed by solidification of this erupted material at mid-oceanic ridges. Based on this, theory of plate tectonics was formulated.
Plate tectonics: According to this theory, the lithosphere(upper 100 Km of the earth's crust) is broken into a number of plates or sections, each of which is capable of independent movement over the asthenosphere.The movement of these crustal plates causes the formation of various landforms and is the principal cause of all earth movements.There are six major plates and many minor ones. The major plates are American, Eurasian, African, Indo-Australian, Pacific and the Antarctic. The minor plates include North American, South American, Nazca, Turkish, Aegean, Arabian, Caribbean, Phillipine, Juan de Fuca and the Iranian plate. The average thickness of these plates is around 100 Km and the entire thickness is involved in crustal spreading and are moving at a speed that has been estimated at 1 to 10 cm per year
Types of Plate Boundaries:
At the boundaries of the plates, various deformations occur as the plates interact.
There are 3 primary types of Tectonic Plate boundaries:Divergent boundaries; Convergent boundaries; and Transform boundaries.
At divergent boundaries new crust is created as two or more plates pull away from each other. Oceans are born and grow wider where plates diverge or pull apart.When a diverging boundary occurs on land a 'rift', or separation will arise and over time that mass of land will break apart into distinct land masses and the surrounding water will fill the space between them.For example, Surtsey island(south of Iceland) was born along mid-Atlantic ridge and even earthquakes also occur at the divergent boundaries.
Here crust is destroyed and recycled back into the interior of the Earth as one plate dives under another.The zone of collision may undergo crumpling and folding and folded mountains may emerge.When one of the plates is an oceanic plate, it gets embedded int he asthenosphere of the continental plate and as a result, trenches are formed at the zone of subduction. The subducted material gets heated up and is thrown out forming volcanic islands.Similarly, When two continents meet head-on, neither is subducted because the continental rocks are relatively light and, like two colliding icebergs, resist downward motion. Instead, the crust tends to buckle and be pushed upward or sideways.The collision of India into Asia 50 million years ago caused the Eurasian Plate to crumple up and override the Indian Plate. After the collision, the slow continuous convergence of the two plates over millions of years pushed up the Himalayas and the Tibetan Plateau to their present heights.
Transform-Fault Boundaries are where two plates are sliding horizontally past one another. These are also known as transform boundaries or more commonly as faults.
When two plates move sideways against each other (at a transform plate boundary), there is a tremendous amount of friction which makes the movement jerky. The plates slip, then stick as the friction and pressure build up to incredible levels. When the pressure is released suddenly, and the plates suddenly jerk apart, this is an earthquake.San Andreas fault along the western coast of USA is an example.
Thus, all landforms are influenced directly or indirectly by tectogenesis which provides energy for formation of landforms. Because of interplay of the factors, the plate margins are areas of intense volcanic and earthquake activity.
Significance of plate tectonics:
If the present trends continue, North and South Americas will separate. A piece of land will separate from western USA, and California will move northwards closer to San Fransisco. A piece of land will separate from the east coast of Africa. Australia will move closer to Asia.Two hundred million years from now a new Pangaea will be formed when all lands again converge. Then, inevitably, another break-up will ensue as our restless planet continues to reform and reshape itself.
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