Distribution Of Oceans And Continents
Continental Drift
Evidence In Support Of The Continental Drift
Alfred Wegener proposed the theory of continental drift in 1912, suggesting all continents were once part of a supercontinent called Pangaea that later drifted apart.
The Matching Of Continents (Jig-Saw-Fit)
The coastlines of South America and Africa appear to fit together, especially along the eastern coast of South America and the western coast of Africa.
Rocks Of Same Age Across The Oceans
Similar rock formations and mountain structures are found on the coasts of different continents (e.g., Brazil and Africa).
Tillite
Glacial deposits known as tillite found in India match similar deposits in Southern Hemisphere continents, indicating a shared glacial history.
Placer Deposits
Similar placer (mineral) deposits found in different continents (e.g., gold in Ghana and Brazil) suggest they were once joined.
Distribution Of Fossils
Identical fossils (e.g., Mesosaurus, Glossopteris) are found in now-distant continents, implying these landmasses were once connected.
Force For Drifting
Wegener assumed forces like pole-fleeing force and tidal force caused drifting, though these were later found inadequate to explain plate movement.
Post-Drift Studies
Convectional Current Theory
Heat-driven convection currents in the mantle are now considered responsible for the movement of tectonic plates.
Mapping Of The Ocean Floor
Technological advancements after World War II enabled detailed mapping of ocean floors, revealing mid-ocean ridges, trenches, and magnetic striping that supported sea floor spreading.
Ocean Floor Configuration
Continental Margins
Include the continental shelf, slope, and rise. They mark the transition from land to deep sea.
Abyssal Plains
Flat and deep areas of the ocean floor, covered with fine sediments.
Mid-Oceanic Ridges
Underwater mountain chains formed by volcanic activity at divergent plate boundaries. Example: Mid-Atlantic Ridge.
Distribution Of Earthquakes And Volcanoes
Major earthquakes and volcanoes are concentrated along plate boundaries—Pacific Ring of Fire is the most active zone.
Concept Of Sea Floor Spreading
Proposed by Harry Hess, this concept explains the formation of new oceanic crust at mid-ocean ridges and its outward movement, providing evidence for plate tectonics.
Plate Tectonics
Divergent Boundaries
Plates move away from each other, creating mid-ocean ridges and rift valleys.
Convergent Boundaries
Plates move towards each other, leading to subduction, mountain formation, and volcanic activity.
Transform Boundaries
Plates slide past each other, causing earthquakes. Example: San Andreas Fault.
Rates Of Plate Movement
Plates move at rates ranging from 2 to 15 cm per year.
Force For The Plate Movement
Main driving forces include mantle convection, slab pull (gravity-driven subduction), and ridge push (from upwelling magma).
Movement Of The Indian Plate
The Indian Plate was part of Gondwanaland. It moved northwards, collided with the Eurasian Plate around 55 million years ago, forming the Himalayas. The plate continues to push into Asia, making the Himalayas geologically active.