Lecture 7 & 8 : Geography

Topics Covered in this Lecture

  1. Introduction to Minerals and Rocks
  2. Classification and Formation of Rocks
  3. Rocks Cycle
  4. Endogenic Force – Causes and Types

Topics Covered in this Lecture

  1. Basic Postulates of Continental Drift Theory
  2. Evidences Supporting the Theory
  3. Stages of Drift Theory
  4. Critical Evaluation
  5. Concretional Current Theory

Rocks are classified into- Igneous, Sedimentary, and metamorphic rocks.

Igneous rocks

  • parent or primary rock.

  • formed due tow cooling and solidification of magma/lava. As they were the first rocks to formed on the earth surface, called as parent or primary rock.

  • As they are formed due to cooling and solidification of molten rock material, they are crystalline rocks.

  • The size of crystals depends up on rate of cooling. In case of slow cooling there will me be larger crystals and in case of faster cooling there will be smaller crystals.

  • Therefore, extrusive igneous rocks have smaller crystals. For example, basalt.

  • Intrusive igneous rocks have large crystals. For example, granite and gabbro.

  • On the basis of depth, Igneous rocks can be classified into extrusive, hypabassal, and plutonic.

  • On the basis of chemical composition they classify as-

    a. Acidic igneous rocks
    b. Basic igneous rocks

  • Acidic igneous rocks has greater amount of silica and are lighter while the basic igneous rocks are darker due to dominance of Iron.

  • Igneous rocks do not contain fossils due to high temperature.

  • They are resistant to weathering and erosion and hence, are formed in massive forms such as Lacoith, Batholith, or volcanic mountain, and plateaus etc.

  • Igneous rocks are generally resistant to weathering but basalt undergoes weathering when expose to water to give rise to REGUR or black soil. AA

Economic importance of igneous rocks

  • They are store house of minerals such as iron, manganese, copper, etc.

  • Due to the resistant nature, they are used for construction purpose. For example, granite.

  • Black soil is very fertile and it is the most preferred soil for cash crop specially in the region of dry zone agriculture because of its water retention capacity.

Sedimentary rocks

The original rock (igneous, sedimentary, metamorphic) in broken into small particles. After breaking they undergo compaction and cementation to form sedimentary rocks. The process of sedimentary rocks. The process of sedimentation + compaction + cementation is known as lithification.

Characteristics

  • Sedimentary rocks are found in layers/strata. Therefore, they are known as stratified rocks or bedding planes.

  • They contain fossils and are found over the largest surface area of the earth but they account for only 5% of earth’s volume.

  • These are weak rocks and hence are susceptible to erosion and weathering.

Types of Sedimentary rocks

  • Physically formed or clastic– These are formed due to disintegration (breaking down) of original rocks due to erosion or weathering. For example, sandstone, siltstone, clay, shale, loess.

  • Chemically formed sedimentary rock– Decomposition of minerals that are dissolved in water forms different kinds of sedimentary rocks. Water carries this particle in solution and deposits them layer after layer causing Lithification and rock formation. Limestone and rocks are rich in calcium are more prone to chemical decomposition. For example, Limestone, chalk, common salt (NaCl), Halite (Rock salt).

  • Organically formed– Rocks formed due to disintegration and or decomposition of organic matter are known as organically formed sedimentary rock. For example, coal, peat, limestone, and chalk.

Metamorphic Rocks

Rocks that are formed due to temperature pressure and volume changes are known as metamorphic rocks.

The parallel arrangement of crystals is the chief characteristic of metamorphic rocks. Hence, they are an example of foliated rocks.

Metamorphic rocks do not contain fossils due to high temperature and pressure. Highly resistant to erosion and weathering.

These rocks like igneous rocks are found in large forms or large structures such as mountains, etc.

Types of Metamorphism

  1. Thermal metamorphism– When subjected to intense heat, due to magma or lava , the rock undergoes change known as thermal metamorphism. Thermal metamorphism is also known as regional metamorphism. For example, carbon (coal)-> diamond= formed due to thermal metamorphism.

  2. Dynamic metamorphism- Due to this process, this is seen in the region of intense folding or the regions of high compressive forces. For example, limestone converted into marble.

Rock Cycle

The rock cycle

It is one of the earliest theory which talks about rifting and drifting of the continents.

Assumptions

  1. SIAL floats on SIMA without any resistance.

  2. Forces responsible for the drift are-


    a- tidal force resulting in the westward movement of the continents.


    b- centrifugal force and the force of buoyancy which resulted in the northward movement of the continents.

Theory

  • Before the Carboniferous period, there was a supercontinent – Pangea surrounded by a superocean-Panthalassa.
  • After the Carboniferous period, In the Mesozoic era, the Pangea broke into two parts, the northern part was known as Lauretia/Angara land while the Southern part was known as Gondwanaland. This phase is known as the “opening of Tethys”.
  • In the later period, Angara land further broke into North America and Eurasia, while the Gondwanaland broke into South America, Africa, Peninsula India, Madagascar, Australia, and Antarctica.
  • The westward movement of both the Americas resulted in the opening up of the S-shaped Atlantic ocean.
  • The northward movement of peninsular India resulted in the formation of the Indian Ocean. The Tethys sea was reduced in size to form the present-day the Mediterranean Sea.
  • According to him, mountains were formed because of fiction between SIAL and SIMA while the trailing margins of the continents gave rise to Island Arc festoons.

Criticism

  1. He could not explain the force that is responsible for drifting.
  2. Also, not unable to explain the trigger mechanism for the breaking of continents after the Carboniferous period.
  3. He cannot explain the sequence of how the continents moved.
  4. His explanation of mountain building and island arc is full of errors.

Evidences

  1. Jig-Saw fit- If we attach all the continents, they fit well.
  2. Rocks of the same age across the coast of Atlantic. In fact, the mountains of Scandinavia, Ireland, and the Appalachians are geologically similar.
  3. Gold deposits in Ghana while the gold-bearing rocks are in South America.
  4. Distribution of fossils, Glossopetrius flora in Brazil, Africa, Peninsular India, and Antarctica. Similarly, the above regions have evidence of glaciation.
  5. The behavior of some animals. For example, Lemmings have a tendency to run towards the west whenever there is an increase in population.

Practice Questions-

  1. Who has given the continental drift theory? Write about its significance.
  2. Elaborate the theory of continental drift with its assumptions and evidences.
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