Geography

 

Geography Study material:

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The Earth: Distribution of Ocean and Continents

Alfred Wegener– a German Meteorologist put forth “the Continental Drift theory” in 1912. According to him, all the continents formed a single continental mass and mega ocean surrounded the same. The super continent is called the Pangaea and the mega ocean was called the Panthalassa. Around 200 million years ago pangaea began to split in Laurasia and Gondwanaland. Subsequently the broke into today’s continents.

Evidences in support of Continental Drift:

  • Jig saw fit: The shorelines of Africa and South America have a remarkable match.
  • Rocks of same age across the ocean: Ancient Rocks of 2000 million years ago found in Brazil and Western have similarities in terms of age, composition.
  • Tillite: Tillite is sedimentary rocks formed out of deposits of glaciers. The tillite in gondwana system has its counter parts in six different areas namely in Africa, Falkland island, Madagascar, Antarctica and Australia. This are evidence of paleoclimate(Ice age, not the movie J) and drifting of continents.
  • Placer deposits: Placer deposits of gold are found in Ghana (Africa) and source rocks and gold bearing veins are found in Brazil.
  • Fossils: Distribution of same fossils in two distant continents. Ex: Lemuria, the place of the habitation of Lemurs are found in Africa, India and Madagascar.
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Earlier it was believed that pole fleeing force and tidal force are responsible for the drifting but post drift studies confirmed that the sources of energy are radioactive decay and residual heat from the interior of the earth. They produce convectional current in the mantle portion mainly.

Ocean floor mapping and studies gave a new revelation about the drift studies. It has been found that the age of the rocks in the continent is much older than that of the oceans.

Oceans have three distinct reliefs. They are the Continental margin, the deep sea plain and the mid oceanic ridge.

Continental margin includes continental shelf, continental slope, continental rise and deep oceanic trenches. Most of the economic activities like fishing, tourism are done in continental shelf itself.

Deep sea plains are extensive plains with monotonous plains relief.

Mid oceanic ridges form the interconnected chain of mountain system within the ocean. The rift systen at the crest of ridge is the place of or zone of intense volcanic activity.

The mapping of the ocean floor and the palaeomagnetic studies of the rocks from the oceanic regions revealed the following facts:

  • Volcanic eruptions bring large amount of lava to the surface of the earth.
  • The rocks equidistant on either side of the crest of the mid oceanic ridges show remarkable similarities in   terms of period of formation, chemical compositions and magnetic properties.
  • Oceanic rocks are much younger than the continental rocks.
  • Oceanic floors have recent sediments.

These facts and a detailed analysis of magnetic properties of the rocks on either sides of the mid oceanic ridges led Hess (1961) to propose his hypothesis of “Sea Floor Spreading”.

The spreading of floor is due to volcanic eruption at the ridges and thus new rocks are formed in oceanic floor. It does not create any pressure on the other side as the older rocks of either sides of the ridges are consumed or sunk down at the oceanic trenches and the balance is maintained.

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McKenzie, Parker and Morgan proposed the concept of Plate Tectonics in 1967. A tectonic plate is a massive, irregularly shaped slab of solid rock. Plates move horizontally over mostly liquid or semi-solid Asthenosphere as rigid units. A plate may be Oceanic (Pacific) and continental (Eurasian). The theory proposes that the earth is comprises 7 major and some minor plates. The major plates are:

i.Antarctica  ii. North American iii. South American iv. Pacific v. India- Australia- New Zealand vi. African vii. Eurasia.

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Some minor plates are: Cocos Pate, Nazca Plate, Arabian Plate, Caribbean Pate, Philippine Plate, Coroline Plate Fuji Plate etc. Find out their location in world map.

These plates are moving throughout the history of geological time. The Pangaea is a result of their convergence and the present state is the outcome of the divergence.



There are three kind of Plate boundaries:

  1. Divergent Boundaries: As the plates pull away from each other, new crust is generated. These sites are called spreading sites. The best example is the mid oceanic ridges.
  2. Convergent Boundaries: As one plate collide with the other plate, parts of them are destroyed and the location are called the subduction zone. There are three combination of convergence of plates. Most destructive earthquakes occur in such boundaries.
  3. Oceanic-Continental convergence: Collision of Nazca plate and South American Plate and formation of the Andes.
  4. Oceanic-Oceanic Convergence: Volcanic islands in South East Asia due to collision of Pacific plate and Philippines Plate.
  • Continent-Continent convergence: Formation of Himalaya due to convergence of Eurasian plat and Indian Plate. The movement of Indian plate started 200 million years ago and collided with Eurasian plate 40-45 million years ago and the formation of Himalaya started onwards.
  • Transform boundaries: Two plates slide past each other along a transform fault and no eruption or destruction takes place due to such movement.
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Earth Crust is made up of mainly Oxygen, Silicon, Alluminium, Iron, Magnesium, Calcium, Sodium, Potassium. They constitute 98% of earth’s crust and rest 2% are made up of Hydrozen, Titanium, Sulphur, carbon etc.

Minerals are naturally occurring organic or inorganic substances which accumulate and forms the rocks. Rocks are building block of earth’s crust.

How earth crust is formed

Pictorial representation of earth’s crust

Minerals are originated from Magma which is the molten material stores in the earth’s interior. The upward moving magma which comes out to the earth surface through volcanic eruptions is called the Lava.

There are three types of rocks. They are:

The Igneous rocks: Igneous rocks are also called primary rocks as all other rocks are formed from this kind of rock. They do not have sediments and layer structure. Depending upon cooling, igneous rocks develop their structure.

Plutonic rocks are igneous rocks formed at some depth of the earth’s crust. They cool and solidify slowly they develop the structure of larger grain rocks. Example: granite, diorite, Gabbro. Volcanic rocks come out to the earth surface and solidify rapidly having smaller crystals. Ex: Basalt.

The Sedimentary rocks: When other type of rocks are transformed into some other form of rocks due to denudational agents and deposited further away converting into rocks due to the process of lithification, they are called sedimentary rocks. They have fossils in their layered and stratified structure. Ex. Coal, Chalk (organic), Sandstone, Limestone(inorganic mechanically formed)

The Metamorphic rocks: When rocks are transformed into some other form of rocks due to changes of pressure temperature, and volume, the original rocks go down and undergoes re- crystallization and reorganization of materials and the very change of chemical composition of the rocks gives birth to metamorphic rocks. Ex: Clay-Slate, Limestone-Marble, Sandstone- Quartzite, Granite-Gneiss, Shale-Schist, Coal-Graphite are the example of metamorphic rocks from original rock.

Rocks do not remain in their original form and go into a cycle of transformation from one form to another. It is called rock cycle.

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