Theories of Mountain building

Theories of Mountain building

1.Continental Drift Theory

• This theory was first put forward by Abraham Ortelius in 1596 before fully being developed by Albert Wegener.

• Finally Continental drift theory was proposed by Alfred Wegener in 1912.

Wegener suggested that -

• There was a single landmass called Pangea.

• The mega-ocean is called Panthalssa– which means all water.

• About 200 million years ago(Triassic Period), the mega-landmass Pangea started splitting into two large landmasses namely, Laurasia(The Northern Part) and Gondwanaland(The Southern part)

Lauresia- 

• Comprising of North America

• Greenland.

• Europe and parts of Asia.

Gondwanaland -

It was named after an ancient tribe in Madhya Pradesh

It comprises of-

• South America.

• Antarctica.

• Africa.

• Madagascar.

• India and Australia

In 1937, Alexander Du Toit, Professor of Geology at Johannesburg University and one of Wegener’s staunchest supporters, proposed that Pangaea  first broke into two large continental landmasses, Laurasia in the northern hemisphere and Gondwanaland   in the southern hemisphere

 Laurasia and Gondwanaland then continued to break apart into continents that exist today

Evidences of Continental Drift-

1.Jig-Saw-Fit(The Matching of Continents)

• The coastlines of South America and Africa fronting each other have a remarkable and unique match.

2.Rocks and Fossils similarity

• Wegner found the evidence of 2.2 billion year old igneous rocks in Brazil that closely resembled similarly aged rocks in Africa.

• It is established by radiometric dating methods that the earliest marine deposits along the coastline of south america and Africa are of Jurassic age.

• Fossils of land plants like Glossopteris and Gangamopteris were found in rocks 225 million years ago, widespread in places over Africa, Australia, India, which are now widely separated.

• Lemurs occur in India, Madagascar, and Africa, this information leads to consideration of these three landmasses being connected in the past.

• Same with the case of Mesosauras.

3.Tillite-

• It is the sedimentary rock made from glacier deposits.

• The Gondwana system of sediments from India is recognized as having its counterparts in 6 different landmasses in the Southern Hemisphere.

• Counterparts of this series are found in Madagascar, Africa, Antarctica, Falkland Island, and Australia not to mention India.

• At the base, the system has thick tillite signifying widespread and sustained glaciation.

• Generally, the similarity of the Gondwana type sediments evidently shows that these landmasses had exceptionally similar origins.

4.Paleoclamatic evidences-

• The evidences of similar Tillite represents that there was similar paleoclimate.

• The evidences of glaciations of Carboniferous times  are found equally on the South America, southern Africa, Madagascar, Falkland, peninsular India, Australia and Antarctica

5.Placer deposits-

• Occurrences of rich placer deposits of gold in Ghana coast and the absolute absence of source rock in the region.

• Also, presence of gold bearing veins in Brazil show that obviously gold deposits of Ghana are derived from Brazilian plateau when they were together

6.Sea floor spreading-

• Expansion in the floor area of the ocean is called sea floor spreading.

• The studies of sea floor spreading phenomenon have confirmed that the continents were once united as a supercontinent Pangaea which later got fragmented and drifted

7.Palaeomagnetic evidences-

• These are the evidences which relates to Earths magnetic field

• Magnetic iron-bearing minerals crystallizing from molten lava align themselves with the earth’s magnetic field - thus preserving a record of where the rocks formed relative to the earth’s magnetic poles.

• If we collect rocks from different latitudes, their magnetic direction and inclination will reflect the earth’s magnetic field where they were formed.

• It was discovered that ancient rocks had magnetic directions and inclinations that did not correspond with the present magnetic pole position.

Forces Responsible for Drifting of Continents-

• Pole-fleecing force (equator wards movement )- Force due to rotation of earth.

• Tidal force(westward movement) - This is due to attraction of moon and the sun that develops tides in oceanic waters

• Also According to Alfred Wegener, the continents are dominantly formed of lighter material ‘SiAl’, comprising of silicon (Si) and aluminum (Al) which float over relatively denser rock material ‘SiMa’- comprising of silicon (Si) and magnesium (Mg).

• Due to above forces Sial starts moving towards direction of these forces

Criticism of Continental drift theory

• Wegener did not offer a suitable mechanism that could displace such large masses for a long journey.

• Wegener failed to explain why the drift began only in Mesozoic era and not before.

• Forces like buoyancy, tidal currents and gravity are too weak to be able to move continents

2.Sea Floor Spreading Theory-

• First suggested by Arthur Holmes (1928) based on concepts of continental drift and convection cells within the mantle.

• The term sea floor spreading was proposed in 1961 by Naval Reserve Rear Admiral Robert S. Dietz.

• It was Foformally proposed by Dietz (1961) and Hess (1962).

• Hess believed that sea-floor spreading is the answer to the mystery of continental drift.

• Hess explained the principle of ocean floor spreading with the example of Atlantic Ocean.

• According to him, the drift between the North America and Europe continents would have been accomplished by the gradual growth of Atlantic Ocean.

• The growth of the ocean floor requires formation of new crustal material for which it is believed that volcanic activity was continuously going on under sea along a ridge in middle portion of the ocean and these are called Mid-Oceanic Ridges (MORs)

• During sea-floor spreading, magma rises from the mantle. It oozes out of the rift valley. It cools and forms new crust.

• A small amount of new ocean floor is added to the surface of the earth. It fills the gap at the ridge. As this cycle continues, the newly created crust slowly moves away from the ridge.

Features of Mid oceanic ridges-

• There is high Heat flow in this area.

• There is Shallow Seismicity.

• There is thin or very less sediments.

• There is divergence of plates and this divergence is causing Rifting int his area.

• There is Basaltic eruption

Process-

First Basaltic eruption comes to surface due to divergence of plates than with time time the deposited magma or basalt is moved away from centre of MOR due to opposite movements of the plates and new deposition continues at the centre of MOR and this process continues

Evidences of Sea floor Spreading-

1.Magnetic anomalies

• It is a local variation in the Earth's magnetic field resulting from variations in the chemistry or magnetism of the rocks.

• Studies have shown that the strength of the geomagnetic field is alternately anomalously high and low with increasing distance away from the axis of the mid-ocean ridge system

2.Direct Observations-

• Drilling and dredging have provided evidences for sea floor spreading and it suggested that material near MOR opening is newer than material away from centre of MOR

3.Earthquakes

• Frequent occurrence of earthquakes and lack of sediments at ridge crests explain sea floor spreading

4.Heat Anomaly-

• The Heat near centre of MOR is high as comparison to areas away from the centre of the MOR indicating eruption of volcanic material at the centre of MOR

Forces Responsible for Sea Floor Spreading-

• Arthur Holmes proposed that the chief driving mechanism for the sea floor spreading could be the convection currents generated within the interior of the Earth.

3.Convectional current Theory-

• This theory is based on the heat flow in the mantle.

• Arthur Holmes postulated his thermal convection current theory during 1928-29.

Thermal convection current-

• He suggested that the Thermal convectional current origionating deep within the earth is the main cause of plates movement.

• The main source of Thermal currents is excess heat due to radioactivity within the earth

Mechanism -

• Although the crustal part of the earth have maximum radioactive material but due to heat lost from upper potion f the earth due to conduction or radiation there is no sufficient heat for formation of convection current and it takes place deep inside the earth

• Because of different temperature at pole and equiator there is two different types of currents that are formed I.e downward moving current at pole and upward moving current at equator.

• Under the solid lithosphere, the fluid asthenosphere circulates as convection current.

Action of Divergence Current

• Diverging (moving away) convection current leads to pulling away of crustal plates which leads to breaking of the crustal blocks

Opening and Closing of Tethyas-

• Due to Streching and risng of convection current at equator the North and Southern Blocks started moving away from each other and a Sea is formed which is known as Tethyan sea.

• This phase is called Opening of Tethys.
• Again two sets of convergent or downward moving currents pull Laurasia and Gondowanaland towards each other and thus Tethys was compressed and folded into Alpine mountains including Himalyas.

• This phase is called Closing of Tethys.

• At present also these Divergence currents are responsible for Formation of Rift valley and Block Mountains

Action of Convergence Current-

• The convergent convective currents cause subsidence in the crustal zones giving birth to geosynclines and closing of sea which leads to formation of island arcs, Mountains etc.

Geosynclines are long but narrow and shallow water depressions characterized by sedimentation and subsidence

Due to Convergence of these currents the process of Lithogenesis,Orogenesis and Gliptogenesis takes place

Lithogenesis:This includes Creation sedimentation and subsidence of geosyncline due to converging of convection current

This subsidence is due to increase in density of rocks due to metamorphism

Orogenesis: Further squeezing of Sediments of Geosyncline Leads to initial formation of Folded mountains called as Orogenesis

Gliptogenesis :This phase of mountain building is characterized by a gradual ascent of mountain ranges and the ongoing denudation processes by natural agents.

Criticism of Convectional Current Theory-

Some geographers argued that there is not enough heat to generate such convectional current

The mechanism of these convectional current is still doubted by many geographers

Geosynclinal Orogen Theory of Kober

Geosynclines are long but narrow and shallow water depressions characterized by sedimentation and subsidence

Background-

Geosynclinal theory was proposed by James Hall in 1850s and later modified by J.D. Dana during 1870s

His Theory

Orogen/Geosyncline:Mobile Zone of Water

Kratogen/Foreland : The mobile zones of geocyncline were surrounded by rigid masses which were termed by Kober as 'Kratogen'.

Basis of his Theory-

Kober's Geosynclinal Theory is based on the forces of the contraction

produced by the cooling of the Earth

And there was changes in these orogen and Kratogen with time due to these force of contraction

He identified 8 morphotectonic units-

(i) Africa together with some parts of Atlantic and Indian Oceans

(ii) Indian Australian land mass

(iii) Eurasia

(iv) North Pacific continents

(v) South Pacific continents

(vi) South America and Antarctica

Kober has identified 6 major periods of mountain building.

Three mountain building periods, about which very little is known, are reported to have occurred during pre-Cambrian period.

Palaeozoic era saw two major mountain building periods – the Caledonian orogenesis was completed by the end of Silurian period and the Variscan orogeny was culminated in Permo-Carboniferous period.

The last (6th) orogenic activity known as Alpine orogeny was completed during Tertiary epoch.

He idemtified 3 stages of Mountain building-

1st stage-

Lithogenesis-

This includes Creation ,sedimentation and subsidence of geosyncline between Two Forelands

Orogenesis-

Both the forelands started moving towards each other because of horizontal movement cause by the force of contraction resulting from the cooling of the Earth and Further squeezing of Sediments of Geosyncline Leads to initial formation of Folded mountains called as Orogenesis

Median Mass : middle portion of the geosynclines remains unaffected by folding activity. This unfolded middle portion is called Zwischengebirge or median mass

Gliptogenesis :This phase of mountain building is characterized by a gradual ascent of mountain ranges and starting of denudation process by denudation agents like river,glaciers etc.

Explanation of Formation of Different mountain system by Kober-

According to Kober the Alpine mountain chains were formed because of compressive forces coming from Forelands of two sides (north -Europian and south-African).

Betic Cordillera, Pyrenees, Provence ranges, Alps proper, Carpathians, Balkan mountains and Caucasus mountains were formed due to northward movement of African foreland

 tlas mountain (north-west Africa), Apennines, Dinarides, Hellenides and Taurides were formed due to southward movement of European land mass 

There was formation of Median mass(some flat areas ) in the form of Plateaus like Tibetan plateau between Kunlun and Himalaya ,anatolian plateau between Pantic and Taurus etc. , in the form of Plains like Hungarian plain etc. And in the form of Seas like Mediterranean Sea between African Atlas mountains and European Alpine mountains etc.

Criticism of This Theory-

He explained East-West extending Mountains but he was not able to explain north-south extending mountains like Rockies and Andes

The force of contraction can not be only reason for such a large change on surface of The Earth

Plate Tectonic Theory-

Tectonic plates-

A tectonic plate is a massive, irregularly shaped slab of solid rock, generally composed of both continental and oceanic lithosphere. 

It is also called as Lithospheric plates

They are divided into Two parts-

Oceanic plates(more Dense)

Continental plates (Less denser)

Lithosphere :It consist of Earths crust and upper part of Mantle

Asthenosphere: It is Semi liquid zone below earths lithosphere

The plates of the lithosphere float on the asthenosphere because asthenosphere have more density

Lithospheric plates are thick at continent and known as continental plates and it is thin in oceans known as oceanic plates

Earth lithosphere is divided into seven major and some minor plates.

1. Antarctica and surrounding oceanic plate

2. North America ( western Atlantic floor separated from South America along Caribbean islands) plate

3. South American ( with western Atlantic floor separated from North American plate along Caribbean islands) plate

4. Pacific plate

5. India- Australia-New Zealand plate

6. Africa with eastern Atlantic floor plate.

7. Eurasia and adjacent oceanic plate.

Minor plates:

1. Cocos plate- between central america and pacific plate.

2. Nazca plate – between south america and pacific plate.

3. Arabian plate – Mostly Saudi Arabian landmass.

4. Philippine plate – between Asiatic and Pacific plate.

5. Caroline plate – between the Philippine and Indian plate

6. Fuji plate – North-east of Australia.

Interaction of the plates leads to formation of different geographic structures on the earth

Plate boundaries-

Plate boundary is the zone of separation of two plates

Plate margin is marginal part of a particular plate

Types of Plates boundary-

Convergent boundary: when plates (both oceanic and continental ) collide with each other is called Convergent boundary

Divergent boundary: When plates moves away fro each other

Transform boundary: When plates slide along side each other

1.Convergent boundaries

A.Oceanic continent plates collision

Because the oceanic crust is more dense than continental crust, when these two collide, the continental crust rides up over the oceanic crust and the oceanic crust is bent down and subducted beneath the continental crust. This is called a subduction zone

Deep-sea trenches are created at subduction zone

Volcanoes and island arcs are common around subduction zones as oceanic crust melts into magma and due to low density it rises upwards

Due to this volcanic activity, the Pacific Ocean boundary is named as 'Ring of Fire'.

B.Collision between Continent-Continent plates

In this case both the plates have same density therefore these plates do not undergo subduction instead they collide, deform, get folded and rise upwards to form mountains

Plate tectonics considers that continental part of the plate is made up of lightest material (sial) which cannot subduct (in denser sima) and hence it always float at top parts of the Earth

C.Collision between Ocean-Ocean plates

If Two oceanic plates collide, the older, denser one is subducted downward into the mantle and a chain of volcanic islands can form, called a volcanic arc.

The interaction of the descending oceanic plate causes incredible amounts of stress between the plates. This usually causes frequent earthquakes along the top of the descending plate known as the Benioff Zone

Example: Mariana Islands (Mariana Trench). It is deeper than the earth‘s tallest mountain

Some of the Major features associated with convergent boundaries

1.Formation of Mountains -

• The Himalayas is one such example which was formed due to convergence of two continental plates

2.Volcanism-

• Most volcanoes form above subduction zones because as one slab is subducted beneath the other, the interaction of fluids and geothermal heat form new magma.

• The new magma then rises upward through the overlying plate to create volcanoes at the surface

Example- 

The Andes Mountains are home to many volcanoes that were formed at the convergent boundary of the Nazca and South American Plates.

2.Divergent Boundary-

• When tectonic plates moves away from each other is called as divergent boundary

It is of following types-

A.Divergence of continental plates-

• Due to divergence of earth plates there are formation of Horst and Grabon.

• The uplifted blocks are termed as horsts and the lowered blocks are called graben or rift valley.

Example-

• The East African Rift valley.

• Rift valley of The Narmada in India.

• The Rhine valley and the Vosges mountain in Europe.

B.Divergence of Oceanic plates-

• Sue to divergence of ocean-ocean plates there is eruption of hot magma or lava and different geigraphical structures are formed

Example-

• Mid Oceanic Ridge.

• Shallow focus earthquakes.

• Narrow belt of submarine lava on ridge flanks.

Mid oceanic ridge and Sea floor spreading-

• Oceanic ridge is a continuous submarine mountain chain extending   through all the world’s oceans.

• The oceanic ridges are nearly 5 kms higher than the ocean floor.

• Typical rates of spreading average around 5 centimeters  per year.

• The rock and structures are roughly symmetrical in cross section.

• Earthquakes are common along the crests and in the transform faults that join the offset ridge segments.

• Experimental results indicate that midocean-ridge ) are produced by 10 to 20% partial melting of the upper mantle and majorly consisits of basalt.

• The magnetic reversals are recorded in bands as mirror image of rock on either side of a mid-ocean ridge.

• The rocks at the center of a mid-ocean ridge are the youngest and age increases gradually on moving away from the ridge.

• Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge

Features of MOR-

• Age of rocks at the centre of MOR is lesser as comparison to when we move away from centre.

• There is magnetic reversal on both sides of MOR when we move away from its centre.

• There is zone of subduction at the opposite side from centre of MOR.

3.Transform boundary-

• It was proposed by Wilson in1965.

• There is movement of plates horizontally.

• Crust is neither produced, nor destroyed as the plates slide horizontally past each other.

• Transform faults are the planes of separation generally perpendicular to mid-oceanic ridges.

• The San Andreas Fault is a transform boundary between the Pacific Plate and the North American Plate.

• When tectonic plates slide against each other at transform boundaries, pressure builds up untithe plates suddenly slip.

• This sudden release of pressure causes an earthquake.

• There is very less or no volcanoes formation in transform boundary

Other Mountain building theories

Contraction hypothesis-

• Initially it was given by Warision and was later developed by Jeffreys in 1929.

• He proposed that mountains were formed due to contraction and stretching in the outermost layer of the earth.

• The cooling of earth was major cause for such contraction

Radioactive theory of Joly-