Lecture 9 & 10 : Geography

Topics Covered in this Lecture

  1. Concept of Geomagnetism
  2. Source and Principal of Geomagnetism
  3. Signification of Geomagnetism
  4. Shifting and Reversal of Magnetic Poles
  5. Concept of Paleomagnetism
  6. Polar Wandering Theory
  7. Marine Geological Findings
  8. Seafloor Spreading Theory

Why do continent move?

Idea- Wegener
Force- Arthur Holmes
Evidence- Harry Hess

Convectional current theory

Arthur Holmes gives this theory. According to this theory, the convectional current rises from the heated interior due to the presence of radioactive minerals. Rising limbs of the convectional current give rise to divergent motion of plates, while the descending limb gives rise to convergent motion of plates.

The convectional current theory gives the most satisfactory explanation for the movement of the continents.

Sea Floor Spreading

  • According to Harry Hess, the seafloor or oceanic crust behaves like a conveyor belt.

  • According to Hess, rising conventional currents from the heated interior cause breaking of oceanic crust forming “fissures”.

  • As magma rises, it comes out through FISSURES causing silent fissure eruptions and a parallel mountain system known as Mid Oceanic Ridges (MOR).

  • As the ocean floor expands, new MOR are formed along the divergent plate boundary while the old oceanic ridges become a part of oceanic crust.

  • The oceanic crust is destroyed along the continental margins forming oceanic trenches.

  • Thus, the ocean floor is continuously creating and destroying and behaves like a conveyor belt.

Evidences

  • Presence of mid-oceanic ridge. This MOR is the longest mountain system on the earth, running for more than 70,000 km.

  • Rocks of the same ages across the mid-oceanic ridges.

  • The rocks equidistant from MOR belong to the same origin.

  • Continuous volcanic eruptions along the mid-oceanic ridges.

  • Oceanic crust is much younger as compared to the continental crust. As per the conventional theories, if the Earth was formed around 4.2 billion years ago. Both the oceanic crust and continental crust should belong to the same age. But the creation and destruction of the oceanic crust is evidence of sea-floor spreading and also an answer of the Young age of oceanic crust.

  • Presence of trenches along the continental margins.

  • Paleomagnetism

Paleomagnetism

It is the study of the magnetic inclination of rocks which tells about the geological history of the earth. The minerals in the rocks freeze as per the prevalent magnetic field. But Earth’s magnetic field keeps on changing and it is known to rivers after one lakh years. The study of seafloors have shown that rocks have different magnetic alignment.

Points for Prelims-

  • Atlantic ocean has the best or most well defined MOR.
  • Iceland/Icelandic plateau is a part of MOR.

Practice questions-

  1. What is paleomagnetism? Describe this with the relevance of seafloor spreading theory of Harry Hess.

Topics Covered in this Lecture

  1. Background of Plate Tectonic Theory
  2. Basic Postulates of the Theory
  3. Types and Characteristics of Plates
  4. Plate Boundaries :- Types and Associated Landforms


The lithospheric slabs is broken into major and minor plates. These plates slide or glide over Aesthnosphere, the movement of plates, their study, and resultant effects is known as plate-tectonic.

Some major plates are-

  1. Pacific plate- entirely oceanic
  2. Eurasian plate- largely continental
  3. The indo-Australian plate that consists of peninsular India and Australia.
  4. North and South American plates

Some of the minor plates are-

  1. Nasca plate
  2. Cocos plate
  3. Philippine plate
  4. Burmese plate

The Indonesia region is a trijunction of 3 major plates- Eurasian, Indian, Pacific along with the Burmese plate and Philippine plate. Hence, these are tectonically one of the most unstable regions of the world.

Based on the movement, there are 3 kinds of plate boundary-

  1. Convergent plate boundaries
  2. divergent plate boundaries
  3. transform plate boundaries

Convergent plate boundary (CPB)

In the CPB, there are three types of convergence-

  • continental oceanic convergence which is most common
  • continent-continent convergence
  • oceanic-oceanic convergence

Continent-Oceanic convergence

Its characteristics are- fold mountains, violent volcanic eruptions, trenches, earthquakes.

Continent-Oceanic convergence is the most common type of convergence. In this, convergence happens between 2 plates. One is oceanic and one is continental- moving towards each other and collide due to convectional current. The sediments of the plates are folded due to intense compressive forces giving rise to fold mountains. These are the highest mountains on the surface of the earth.

The denser plate is pulled inside the mantle at a zone known as subduction zone or Benioff zone where trenches are located.

The plate which is subducted goes deep inside the mantle, melts and the magma rises upwards exploding the top of a mountain causing violent volcanic eruptions. The movement of the plates, folding, movement of magma, volcanic eruptions causes seismic activities or earthquakes.

For example- Rockies- North American plate and Pacific plate, Andes- South American and Nasca Plate, Circum pacific belt or Pacific ring of fire.

Pacific ring of fire

It is a tectonically most active region of the world which starts from Antarctica, passes through the mountains of New Zealand, Island arcs of Indonesia, Mountains of Russia, and Japan, Philippines, and the Rockies and Andes mountains of both the Americas, where the most important mountains are Mt. Shasta, Mt. Hood, Mt. Cotopaxi, Mt. Aconcagua, Mt. Ojas Del Saldo, Mt. Fujiyama, Mt. Mayon, Mt. Angung, and Mt. Cook.

Pacific ocean also has the largest no. of trenches. Most trenches are- Mariana Trench (deepest part known as Challenger Deep. Surrounding Mariana is Mindanao Trench. Surrounding Japan is Kurile trench. This region also has HOT SPRINGS and GEYSERS found only in- New Zealand, USA, and Iceland.

For example, the Midcontinental belt- the belt of Atlas and Alps, Mt. Vesuvius, mt. Etna, mt. Stromboli.

Continent-continent convergence

In this continent-continent convergence, both the plates have a similar density. In such a case, the plate which is comparatively denser slides below the comparatively lighter plate. As the plates are not denser it is not pulled into the mantle and therefore it will not destroy. In fact, the thickness of the Earth’s crust is double/greater than other regions and hence magma cannot escape.

Therefore volcanic eruptions are not associated with continent-continent convergence.

Its characteristics are- highest mountains, powerful earthquakes, and depressions.

Formation of Himalayas

  • The Indian plate which consists of present-day peninsular India is one of the most ancient or oldest landmasses in the world. It was a part of Gondwanaland.

  • The Indian plate started drifting northwards. During its mainland journey, the western part of the Indian plate came under the influence of the Reunion hotspot. During Cretaceous period, resulting in the formation of Deccan Lava Plateau or Deccan traps.

  • As peninsular India progressed northward, it started depositing the sediments in the Tethys sea. The floor of Tethys sea was eventually folded to give rise to mountains immediately North of the Himalayas known as Trans Himalayas. Thus, Trans Himalayas are volcanic in origin.

  • Around 50 million years ago, the Indian plate collided with the Eurasian plate to give rise to the great Himalayas.

  • The two arms of a peninsular plateau- Aravalis and Mikir Rengma hills pushed the extremes of the Himalayas giving rise to Hairpins or syntactical belt.

  • The collision continued in 3 phases giving rise to three parallel ranges of the Himalayas.

  • The Indian plate still going below the Eurasian plate resulting in an increase in the height of the Himalayas.

Evidences that Himalayas are still rising and came from Tethys sea-

  • Presence of marine fossils and sediments in the Himalayas. Eg- Sea buckthorn.
  • Studies have shown that there has been a crustal shortening of about 500 km in the Himalayan region.
  • Whenever there is upliftment in the Himalayas there is equivalent seafloor spreading in the Indian Ocean region.
  • Presence of saline lakes in the Himalayas.
  • Frequent earthquakes show that the plate movements are still going.

3. Ocean-Ocean convergence

  • Caribbean Islands is an example of ocean-ocean convergence. The denser plate subducts through a comparatively lighter plate.
  • Due to intense compressive forces, folding occurs continuous volcanic eruptions causes some of the peaks to rise above the sea level forming an arc-shaped group of islands- as Island arch and Festoons.
  • For example- A Caribbean island arc formed due to the collision of a minor Caribbean plate with American plates, Indonesian Island arch is formed due to the collision of the Indian plate, Eurasian plate, and Pacific plate, Philippine island arc is formed due to subduction of Pacific plate below the Philippines plate.

Divergent plate boundary

  • Two plates move away.
  • Also known as a constructive plate boundary as the crust is created.
  • Its major features are- mid-oceanic ridges, fissure eruptions, and earthquakes.

Conservative or Transform plate boundary

The two plates slide or pass each other. Hence, the most important feature of the conservative plate boundary is powerful earthquakes and faults. The best example is- San Andrea fault and East African fault system.

Practice questions-

  1. Elaborate the causes that leads the discovery of plate tectonics theory.
  2. Brief plate tectonics theory and formation of Himalayas.

Topics Covered in this Lecture

  1. Plate Tectonic and Mountain Building.
  2. Plate Tectonic and Volcanism.
  3. Plate Tectonic and Seismicity.
  4. Driving Mechanisms of the Theory.
  5. Evidence in Support of the Theory.
  6. Critical Evaluation of the Theory.
  7. Comparison of Plate Tectonic Theory and Continental Drift Theory.
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