Earth is dynamic in nature. Endogenetic forces keep on changing the location of the continents and the oceans. There are various theories which explain the evolution of our planet.
1. Continental Drift Theory
Put forward by Alfred Wegner in 1912.
- Also called as ‘displacement hypothesis’
- Wagner believed in three-layer system; outer layer of sial, intermediate layer of sima and lower layer of nife. (sial is restricted to continent only and sialic masses float over)
- According to Wegener, all continents formed a single continental mass (PANGEA) & mega ocean (PANTHALASSA) surrounded the same.
- Around 200 million years ago, Pangaea first broke into two large continental masses as Laurasia (Present day- N. America, Europe and Asia) and Gondwanaland (Present day S. America, Africa, Peninsular India, Australia and Antarctica). Subsequently, Laurasia and Gondwanaland continued to break into various smaller continents that exist today.
- Force for Drifting Wegener suggested that movement responsible for the drifting of the continents was caused by pole-fleeing force and tidal force.
- Polar-fleeing force relates to rotation of the earth.
- Tidal force is due to attraction of moon and sun. Wegener believed that these forces would become effective when applied over many million years. However, most of scholars considered these forces to be inadequate.
Evidence In Support Of Continental Drift Theory
Jig Saw Fit theory: Shorelines of Africa & South America facing each other have a remarkable and unmistakable match and can be refitted together.
- Rocks of Same Age Across the Oceans: The belt of ancient rocks from Brazil coast matches with those from western Africa. Appalachians of N. America are compatible with mountain system of Ireland and North-western Europe.
- Gondwana system of sediments from India is known to have its counterparts in six different landmasses of the Southern Hemisphere.
- Placer Deposits: Brazil has the gold bearing veins but the gold is also found at the coast of Ghana which has absolute absence of source rock.
- Distribution of Fossils: Observations that Lemurs occur in India, Madagascar and Africa led some to consider a contiguous landmass ‘Lemuria’ linking these three landmasses.
- Carboniferous glaciation of Brazil, Falkland, South Africa & peninsular India further prove the unification of landmasses during carboniferous period.
- Seafloor spreading & Plate tectonic theory also proved that lands and seas are not stationary and they keep on drifting.
PALEOMAGNETIC STUDIES & SEA FLOOR SPREADING
When molten hot lava comes up with thermal convection current along M.O.R and gets cooled and solidified, these lavas also get materialised in accordance with the then geomagnetic field and thus alternate bands of magnetic anomalies are formed on either side of M.O.R. These findings validate following things:There is a reversal in main magnetic field of Earth.Normal and reverse magnetic anomalies are found in alternate manner on either side of the M.O.R.There is a complete parallelism in the magnetic anomalies on either side of the M.O.R. There is parallelism in the time sequence of paleomagnetic epochs.
POLAR WANDERING
It is the migration of the magnetic poles over Earth’s surface through geologic time. Polar-wandering curves are the paths of a magnetic pole with respect to a given continent.
2. Sea floor spreading theory
- By Prof Harry Hess.
- Mid oceanic ridges are situated on the rising thermal convection currents coming up from the mantle.
- Constant eruptions at the crest of oceanic ridges cause the rupture of the oceanic crust and the new lava wedges into it, pushing the oceanic crust on either side. The ocean floor, thus spreads.
- Molten lava cools down and solidifies to form new crusts along the trailing ends of divergent oceanic crusts.
- Spreading of one ocean does not cause the shrinking of the other because of the consumption of the Oceanic crust at the oceanic trenches.
EVIDENCE IN SUPPORT OF SEA FLOOR SPREADING
- Rocks equidistant on either side of the crest of mid-oceanic ridges show remarkable similarities in terms of period of formation and chemical composition.
- Rocks on either side of the Mid-Oceanic Ridge have similar magnetic properties in terms of magnetic anomaly and time sequence of magnetic epochs.
- Age of rocks increases as one moves away from crust.
- Ocean crust rocks are much younger than continental rocks. Age of rocks in oceanic crust is nowhere more than 200 million years old. Some of the continental rock formations are as old as 3,200 million years.
- Sediments on ocean floor are unexpectedly very thin.
3. Plate Tectonics Theory
- Suggested by McKenzie and Parker in 1967 and outlined by Morgan in 1968.
- A tectonic plate (also called lithospheric plate) is a massive, irregularly shaped slab of solid rock, generally composed of both continental and oceanic lithosphere.
- Plates move horizontally over the asthenosphere as rigid units.
- Forces responsible for such movements- The heat beneath the earth is generated because of two factors: (i) Radioactive decay (ii) Residual heat.
- The lithosphere includes the crust and top mantle with its thickness range varying between 5 and 100 km in oceanic parts and about 200 km in the continental areas.
A plate may be referred to as the continental plate or oceanic plate depending on which of the two occupy a larger portion of the plate.
THREE TYPES OF PLATE BOUNDARIES
- Divergent: Where new crust is generated (Constructive margins) as plates pull away from each other and new crusts are formed because of solidification of upwelling molten material. These sites are called spreading sites. Ex. Mid-Atlantic Ridge (American Plate(s) being separated from Eurasian and African Plates).
- Convergent: Where the crust is destroyed (Destructive margins) as one plate dives under another. They are also known as consuming plate margins. The location where sinking of a plate occurs is called a subduction zone. There are three ways in which convergence can occur. (i) Ocean-Ocean convergence (ii) Ocean-Continent convergence (iii) Continent-Continent convergence
- Transform: Where the crust is neither produced nor destroyed as the plates slide horizontally past each other. They are generally perpendicular to the mid-oceanic ridges. They are formed due to differential movement. Also, the rotation of the earth has its effect on the separated blocks of the plate portions.
EVIDENCE OF PLATE TECTONICS
Evidence of Continental drift, palaeomagnetism and sea floor spreading theory can be combined. Continental drift proves that there has been a motion i.e., ‘tectonics’. Palaeomagnetism and sea floor spreading theory explains the process behind this movement.
PLATE TECTONICS IMPROVEMENT OVER CONTINENTAL DRIFT THEORY
| Continental Drift Theory | Plate Tectonic Theory |
| Only landmass moves. | Lithospheric slab moves which might be continental or oceanic or both. |
| Forces responsible were incompetent. | Forces were well explained and reasonable. |
| Many evidence were unscientific. Ex. jig-saw fit theory could not be proven because both the coasts of Atlantic Ocean cannot be completely refitted. | Evidence was based on palaeomagnetic studies and other researched theories like convection current theories and sea floor spreading theory. |
| Could not explain satisfactorily the formation of various landforms like volcanic arcs and fold mountains. | Theory explained the formation of fold mountains, plateaus, island arcs, mid oceanic ridges etc. |