Continental Drift and Plate Tectonic Theory
This article deals with ‘Continental Drift and Plate Tectonic Theory.’ This is part of our series on ‘Geography’ which is important pillar of GS-1 syllabus . For more articles , you can click here
Continental Drift Theory
- Given by Alfred Wegener (German Meteorologist) in 1912 . It speaks about rifting and drifting of continents .
- According to Wegener, about 250 million years ago all the
continents formed a single continental mass and mega ocean surrounded the same.
- Super continent was named PANGAEA, which meant all earth.
- Mega-ocean was called PANTHALASSA, meaning all water.
- He argued that, around 200 million years ago, the super continent, Pangaea, began to split.
- Pangaea broke to Laurasia/ Angaraland (forming Northern Continents) and Gondwanaland (Southern Continents) with Tethys Sea between them.
- Subsequently, Laurasia and Gondwanaland continued to break into various smaller continents that exist today. (Note – India was part of Gondwanaland.)
- Interesting theory but was scrapped saying it GEO-POETRY because he wasn’t able to explain forces of movement.
Continental Drift theory was based on following clues
a. Continental fit / Jig Saw Fit
- Continental lands can be joined together like jig saw puzzle.
b. Rocks of same age across oceans
- Belt of ancient rocks of 2,000 million years from Brazil coast matches with western Africa.
- Geological Structure of Appalachian Mountains matches with Morocco and Algeria in North Africa.
|Mesosaurus||Freshwater reptile found in Africa & South America.|
|Glassopteris||Fern found on all southern continents.|
|Lemur||Found in India, Africa and Madagascar.|
d. Placer Deposits
- The occurrence of rich placer deposits of gold in the Ghana coast and the absolute absence of source rock in the region => gold bearing veins are in Brazil => Ghana & Brazil Plateau used to lay side by side.
- Tillite are the sedimentary rock formed out of deposits of glaciers.
- Gondwana system of sediments from India has counter parts in six different landmasses of Southern Hemisphere.
Forces for Drifting
- Wegener suggested that movement responsible for drifting of continents was caused by pole-fleeing force and tidal force.
- Polar-fleeing force relates to the rotation of the earth.
- Tidal force—is due to the attraction of the moon and the sun that develops tides in oceanic waters.
- Wegener believed that these forces would become effective when applied over many million years. However, most of scholars considered these forces to be inadequate
Post Drift Studies
- It is interesting to note that for continental drift, most of the evidences were collected from the continental areas .
- Number of discoveries during the post-war period added new information to geological literature. Particularly, the information collected from the ocean floor mapping provided new dimensions for the study of distribution of oceans and continents.
Convectional Current Theory
- Wegener wasn’t able to explain the force behind Continental Drift.
- Arthur Holmes in 1930s discussed the possibility of convection currents operating in the mantle portion. These currents are generated due to radioactive elements causing thermal differences in the mantle portion. Holmes argued that there exists a system of such currents in the entire mantle portion.
- These convection currents are
nothing but molten rocks
- Rising limb : it will pressurise crust in such a way that crust will break .
- Diverging limbs : take crust away from each other .
- Descending limbs : make two crusts to collide (like Indian & Eurasian plate ).
- According to Holmes , these Convection Currents are the Driving Force . This was an attempt to provide an explanation to the issue of force, on the basis of which contemporary scientists discarded the continental drift theory.
Ocean Seafloor Spreading Theory
Post war studies using which Ocean Map was prepared showed that ocean floor is not just a vast plain but it is full of relief.
Mapping of the ocean floor and palaeo-magnetic studies of rocks from oceanic regions revealed the following facts :
- It was realised that all along the mid-oceanic ridges, volcanic eruptions are common and they bring huge amounts of lava
- Ocean crust rocks are much younger than the continental rocks. The age of rocks in the oceanic crust is nowhere more than 200 million years old. Some of the continental rock formations are as old as 3,200 million years.
- Age of the rocks increases as one moves away from crest.
- Sediments on the ocean floor are unexpectedly very thin => nowhere was sediment column found be older than 200 million years.
- Deep trenches have deep earthquake occurrences while in mid-oceanic ridge areas, earthquake foci have shallow depths
- Concept of zebra strip / Magnetostratigraphy : rocks equidistant on either sides of mid-oceanic ridges show similar magnetic properties
This led Hess (1961) to propose his hypothesis, known as the “sea floor spreading”
- Constant eruptions at crest of oceanic ridges cause rupture of the oceanic crust and new lava wedges into it, pushing the oceanic crust on either side. The ocean floor, thus spreads.
- Ocean floor that gets pushed due to volcanic eruptions at crest, sinks down at the oceanic trenches & gets consumed (Seafloor Spreading Theory).
Plate Tectonic Theory
- Given in 1967 by McKenzie & Parker
- Tectonic plate ( lithospheric plate) is a massive slab of solid rock, generally composed of both continental and oceanic lithosphere. Plates move horizontally over the asthenosphere . Its thickness range varying between 5-100 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
- Pacific plate is largely an oceanic plate
- Eurasian plate may be called a continental plate.
- Theory of plate tectonics proposes that the earth’s lithosphere is divided into seven major and some minor plates.
- Young Fold Mountain ridges, trenches etc are formed due to movement and interaction of these plates
The major plates are
- Antarctic and the surrounding oceanic plate
- North American
- South American
- Pacific plate.
- India-Australia-New Zealand plate .
- Africa with the eastern Atlantic floor plate .
- Eurasia and the adjacent oceanic plate.
Some important minor plates are
- Cocos plate : Between Central America and Pacific plate .
- Nazca plate :Between South America and Pacific plate .
- Arabian plate : Mostly the Saudi Arabian landmass.
- Philippine plate : Between the Asiatic and Pacific Plate .
- Caroline plate : Between the Philippine and Indian plate (North of New Guinea) .
- Fuji plate : North-east of Australia.
- 2017 update – Zealandia is now considered separate Continent/Plate
These plates have been constantly moving over the globe throughout the history of the earth.
- All the plates, without exception, have moved in the geological past, and shall continue to move in the future as well.
- Pangaea of Wegner was also result of convergence of continental masses
There are three types of plate boundaries according to Plate Tectonic Theory (these three types of plate boundaries are discussed in detail below)
Rate of Plate Movement
- Strips of normal and reverse magnetic field that parallel the mid-oceanic ridges help the scientists to determine the rates of plate movement.
- These rates vary considerably.
- Arctic Ridge: slowest rate (less than 2.5 cm/yr),
- East Pacific Rise in the South Pacific : fastest rate (more than 15 cm/yr).
Force for the Plate Movement
- Convectional Currents which were first explained by Hess in his Convectional Current Theory was the main force behind plate movement
Side Topic : Palaeomagnetism & how it prove Plate Tectonics
Palaeomagnetism is the study of the record of the Earth’s magnetic field in rocks and sediments
How it proves Plate Tectonic Theory
- Magnetostratigraphy, with rocks equidistant on either sides of mid-oceanic ridges show similar magnetic properties
- Polar wandering – Magnetic minerals formed at same time but on different continents points have different orientation . So, there were either multiple north poles during the same time period or that the continents moved in relation to a single north pole. Geophysicists concluded that the magnetic poles remained stationary, and the continents moved
- Palaeomagnetism is also used to match once joined landmasses that are now separated. For example, the orientation of magnetic minerals along the eastern coast of South America very closely matches that of similar minerals on the western coast of Africa.
Convergent Plate Boundaries
Convergent plate boundary is the margin where two plates collide with one another.
Convergent Plate Boundaries can be of three types :-
1 . Ocean Ocean Convergent Boundary
- Denser of the two oceanic plates is subducted . It goes to Asthenosphere & generate new Magma .
- Andesitic Magma will from in this case . Andesitic Magma is less mobile and solidifies quickly. As a result, underwater Volcano or Volcanic island arc will form in this case.
- Characterised by Trenches , Underwater Volcanoes , Volcanic Island Arc and Earthquakes .
- Island arc (and not single island) will be formed because they will be formed on whole boundary where Ocean-Ocean plate is converging. All these islands will be volcanic islands .
2. Ocean – Continental Convergence Boundary
- Oceanic Plate is denser than Continental Plate . Hence, Ocean plate will be subducted into Asthenosphere & melt down there. But at the same time, due to the great compressional force between two converging plates, folding will happen on the Continental Plate , resulting in formation of Marginal Fold Mountains . As we know, folding happens along the zones of weakness , hence, when Magma of the subducted Ocean plate will rise , it will come out of the crust through these weak zones resulting in formation of Volcanic Peaks .
- Hence, Volcanic mountains ranges, Trench and earthquakes are common on boundaries
- Examples of such volcanic mountain ranges are
|Andes mountains||South America|
3. Continental – Continental Convergent Boundary
- When continental plate converges into continental plate , crust at both sides is tool light & buoyant to be subducted. Both are compressed against each other and folding happens. Hence, in this case Fold mountain Belt forms.
- No Volcanism but powerful earthquakes are created in this region .
- Himalayas & Urals are formed in this way due to convergence of Indo-Australian and Eurasian plates.
- Pressure between plates is so high that metamorphic rocks form there.
Divergent plate Boundaries
- Divergent plate boundary is the margin where two plates move apart. For instance, African plate and South American plate
- Divergent plate boundary is termed as the constructive plate boundary as it leads to the formation of new lithosphere .
Divergent plate boundaries are of two types :-
1 . Ocean – Ocean Divergent Boundary
- Creation of new crust takes place at submarine mountain ridge . Ocean crust is rifted apart & basaltic magma wells up to fill the opening.
- Basaltic Magma will come out from the Mantle. Since basaltic magma is very mobile, hence it will spread out. This magma hardens & forms igneous rock . Since magma is basaltic, hence ocean crust is basaltic in nature .
- Mid-Atlantic Ridge is an ideal example of a submarine mountain ridge in the Atlantic Ocean. It is the longest mountain ridge in the world. It extends for about 16,000 km, in a ‘S’ shaped path, between Iceland in the north and Bouvet Island in the south
- Water from hydrothermal vents (along the submarine ridges) is rich in dissolved minerals and supports organisms like chemo-autotrophic bacteria.
2 . Continental – Continental Divergent Plate
- Rift valley along with block mountains are formed when two continental plates move apart.
- Initially it leads to the development of a small body of water . But if rifting continues , body of water becomes bigger to juvenile ocean and consequently to Large Ocean.
- Example : The Great Rift of Africa
Side Topic : Great Rift of Africa
- In Great African Rift Valley, Continental Continental Divergence is observed leading to formation of Rift Valley.
- Almost all the lakes in Africa are in the Rift Valley generated by diverging of continental Plates except Lake Victoria.
Transform Plate Boundary
- Where two plates are sliding past each other.
- They are under shear stress.
- The lithosphere is neither destroyed nor created by the transform plate boundary. Hence , it is called Conservative or passive plate boundary.
- Earthquakes are common & Volcanoes are not formed at Transform boundaries. Whenever plate boundary is active, Earthquakes are also experienced in that region.
- Example –San Andreas Fault in USA (Pacific Plate & North American) , Chile etc