Distribution of Oceans and Continents — NCERT Solutions

Given: Convectional currents are considered the driving force behind plate tectonics.

Answer:
Convectional currents in the mantle are initiated and maintained by radioactive decay within the Earth's interior, which generates enormous amounts of heat.

• The heat produced causes the semi-molten mantle material (magma) to become less dense and rise upward.
• As it rises and reaches the upper mantle/crust, it spreads out and cools.
• The cooled, denser material then sinks back down.
• This continuous cycle of rising hot material and sinking cool material forms convection cells.

These convection cells exert drag on the overlying tectonic plates, causing them to move. Arthur Holmes first proposed this mechanism in the 1930s to explain continental movement.

Answer:

In short, at transform boundaries, plates move laterally (sideways) and there is no creation or destruction of the crust. In contrast, at convergent boundaries crust is destroyed through subduction, and at divergent boundaries new crust is created through volcanic activity.

Answer:
During the formation of the Deccan Traps (approximately 60–65 million years ago, around the Cretaceous–Palaeogene boundary), the Indian landmass was located near the Réunion hotspot, which is presently located in the southern Indian Ocean.

As the Indian plate drifted northward over this hotspot, massive volcanic eruptions occurred, leading to the outpouring of enormous quantities of lava that formed the Deccan Traps — one of the largest volcanic features on Earth, covering a large part of the present-day Deccan Plateau in peninsular India.

Introduction: Alfred Wegener proposed the Continental Drift Theory in 1912, suggesting that all continents were once joined in a supercontinent called Pangaea, which broke apart and drifted to their present positions.

Evidences in support:

1. Jigsaw Fit (Matching Coastlines): The coastlines of South America and Africa fit together remarkably well, like pieces of a jigsaw puzzle. This was the most visually compelling evidence.

2. Rocks of Same Age Across Oceans: Rocks of identical age and composition are found on both sides of the Atlantic Ocean — in Brazil and West Africa — suggesting they were once part of the same landmass.

3. Placer Deposits: Gold deposits are found in Ghana (Africa) and the gold-bearing veins from which they originate are found in Brazil, indicating these regions were once connected.

4. Tillite Deposits (Gondwana System): Glacial deposits called tillites are found in South America, Africa, Madagascar, Arabia, India, Antarctica, and Australia — all of which were once part of the southern supercontinent Gondwanaland. This indicates a common glacial history.

5. Distribution of Fossils: Fossils of the same species are found on continents now separated by vast oceans:

• Mesosaurus (a freshwater reptile) fossils are found in both South America and Africa.
• Glossopteris (a plant fossil) is found across India, South America, Africa, Australia, and Antarctica.

6. Placer Deposits and Coal Fields: Coal fields found in Antarctica and India suggest these regions once had tropical climates, indicating they were located near the equator.

Conclusion: These evidences collectively support the idea that the continents were once joined and have since drifted apart.

Introduction: Both Continental Drift Theory and Plate Tectonics deal with the movement of the Earth's lithosphere, but they differ significantly in their scope, mechanism, and scientific basis.

Differences:

Conclusion: Plate Tectonics is a more comprehensive and scientifically robust theory that incorporates and extends the ideas of Continental Drift, explaining not just the movement of continents but the dynamics of the entire lithosphere.

Introduction: After Wegener's Continental Drift Theory was largely dismissed due to lack of a convincing mechanism, several post-drift discoveries in the mid-20th century revived scientific interest and eventually led to the development of Plate Tectonics.

Major Post-Drift Discoveries:

1. Mapping of the Ocean Floor (Post-World War II): Advances in echo-sounding technology allowed scientists to map the ocean floor in detail. This revealed the presence of a continuous system of mid-oceanic ridges (e.g., the Mid-Atlantic Ridge) — underwater mountain chains running through all major oceans.

2. Sea Floor Spreading (Harry Hess, 1961): Harry Hess proposed that new oceanic crust is continuously being created at mid-oceanic ridges through volcanic activity, and older crust is pushed away and eventually destroyed at deep-sea trenches. This explained how continents could move without ploughing through the ocean floor.

3. Palaeomagnetic Evidence: Studies of the ocean floor rocks revealed alternating stripes of normal and reverse magnetic polarity on either side of the mid-oceanic ridges. This was because as new magma solidified, it recorded the Earth's magnetic field at that time. These symmetrical stripes confirmed that the ocean floor was spreading outward from the ridges.

4. Age of Ocean Floor Rocks: It was discovered that rocks of the ocean floor are much younger than continental rocks, and the youngest rocks are found closest to the mid-oceanic ridges. This supported the idea of continuous creation and destruction of oceanic crust.

5. Deep Sea Drilling: Samples from the ocean floor confirmed the age progression of rocks — youngest near ridges and oldest near trenches — providing direct evidence for sea floor spreading.

Conclusion: These discoveries provided the missing mechanism for continental movement and led to the formulation of the Theory of Plate Tectonics, which is now the cornerstone of modern Earth sciences.