UPSC (IAS): Ignited Minds Academy.: Geography Lecture 3 UPSC (IAS): Continental Drift theory, Sea Floor Spreading Theory, Plate Tectonics

Geography Lecture No. 3 for UPSC (IAS):

Continental Drift theory, Sea Floor Spreading Theory, Plate Tectonics

Few Facts:

· Only 29% of earth is occupied by continents, remaining is the ocean.

· Both oceans and continents change their position. (mentioned in lecture 2).

Various theories behind the evolution of continents and oceans:

1. Continental drift theory:

(Question asked directly in 2013)

Proposed by Alfred Wegner. He noticed that there is a symmetry on either sides of Atlantic oceans. (i.e. the continents on either side of Atlantic Ocean completely fit together). According to him, (About 200 million years ago) all the continents formed a single continental mass called PANGAEA surrounded by a mega ocean called PANTHALASSA. The continent split into 2 continents called as Laurasia and Gondwanaland forming north and south continents respectively.

Evidences for the Continental Drift Theory:

1. Matching of the continents: Shorelines of Africa and South-America have a remarkable

match.

2. Rocks of same age across the ocean: The belt of the ancient rocks of 2000 million years ago of the Brazil coast match with those of western African coast. (Radioactive dating technique).

3. Tillite: Sedimentary rocks formed out of deposits of glaciers. Gondwana system of sediments from India is known to have its counterparts in six different landmasses. Counter parts are Africa, Falkland Islands, Madagascar, Antarctica and Australia besides India.

Figure: Tillite

4. Placer Deposits: The occurrence of rich placer deposits of gold in Ghana coast and the absolute absence of the source rock in the region. The gold bearing veins in Brazil and it is obvious that gold deposits of Ghana coast are derived from Brazil plateau when two continents were side by side.

5. Distribution of fossils: Identical species of plants and animals are found on either side of marine barriers/ continents.

2. Post Drift Theories:

1. Continental Current Theory:

2. Mapping of Ocean Floor:

Sea Floor Spreading Theory:

Seafloor spreading is a process that occurs at mid oceanic ridges where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. Occurs due to mantle convection. Mantle Convection is a slow churning motion of Earth’s mantle. Usually occurs at divergent plate boundaries. Convection currents carry heat from the lower mantle and core to the lithosphere. As the tectonic (lithospheric) plates move away from each other, heat from the mantle’s convection currents makes the crust more plastic and dense. The less sense materials rises, often forming a mountain or elevated area of the seafloor. Eventually, the crust cracks. Hot magma fueled with mantle convection currents bubbles up to fill these fractures and spills onto the crust. It solidifies and forms igneous rock. This is the new part of earth’s crust.

Figure: Sea Floor Spreading.

Plate Tectonics:

Plate Tectonics is a scientific theory that describes the large scale motion of the earth’s lithosphere. This theoretical model is built on the concept of continental drift theory and the sea floor spreading theory. The lithosphere, which is the rigid outermost shell of our planet (the crust and the upper mantle) is broken up into tectonic plates. Where the plates meet, their relative motion determines the type of the boundary.

Significance of Plate Tectonics:

· earthquake frequency and types are a function of plate setting.

· type of volcanic behavior is function of plate setting.

· there are possible connections between plate tectonic behavior and climate change.

· gives a much better framework for understanding geology overall.

Types of Plate boundaries:

· Convergent Boundary: Also called as destructive plate boundary. In this type the two plates come together. They form either a subduction zone (one plate moving underneath the other) or a continental collision. In this three types of collision takes place i.e. ocean to ocean, continent to continent (Indian Plate, Himalayas are formed due to this), ocean to continent.

· Divergent Plate Boundary (Constructive): These boundaries occur when plates move away from each other. At zones of ocean-to-ocean rifting, divergent boundaries form by seafloor spreading, allowing for the formation of new ocean basin.

· Transform boundaries (Conservative): These boundaries occur where two lithospheric plates slide, or perhaps more accurately, grind past each other along transform faults, where plates are neither created nor destroyed.