Concept of Continental Drift, Sea-floor Spreading and Theory of Plate Tectonics

1. Continental Drift (Proposed by Alfred Wegener, 1912)

* The **Continental Drift Hypothesis** states that continents were once joined together in a single supercontinent called **Pangaea** (\~300 million years ago), surrounded by a vast ocean called **Panthalassa**.
* Pangaea later broke into two landmasses – **Laurasia (north)** and **Gondwana (south)** – which further drifted to form the present continents.
* **Evidence supporting Continental Drift:**

  * **Fossil evidence** (e.g., Mesosaurus in Africa and South America).
  * **Geological similarities** (rock formations across continents match).
  * **Paleoclimatic evidence** (glacial deposits in present-day tropical regions).
  * **Fit of the continents** (South America fits into Africa like a puzzle).
* **Limitation:** Wegener could not explain the driving mechanism of movement.

---

### **2. Sea-floor Spreading (Proposed by Harry H. Hess, 1960s)**

* Suggests that new oceanic crust is continuously formed at **mid-oceanic ridges** due to upwelling of magma from the mantle.
* The new crust spreads outward, pushing older crust away, and is eventually destroyed at **subduction zones (trenches)**.
* **Key Evidence:**

  * Symmetrical pattern of **magnetic stripes** (paleomagnetism) on either side of mid-ocean ridges.
  * **Age of ocean floor** increases away from ridges.
  * Distribution of earthquakes and volcanic activity along ridges and trenches.

---

### **3. Theory of Plate Tectonics (Unified theory, 1968)**

* Combines **Continental Drift** and **Sea-floor Spreading** into a comprehensive model.
* States that the Earth’s lithosphere is divided into **rigid plates** (7 major + several minor) that float over the **asthenosphere**.
* Plate boundaries are sites of most tectonic activity:

  * **Divergent boundaries** → mid-ocean ridges, rift valleys (e.g., Mid-Atlantic Ridge).
  * **Convergent boundaries** → subduction zones, mountain building (e.g., Himalayas, Andes).
  * **Transform boundaries** → lateral sliding, earthquakes (e.g., San Andreas Fault).

---

### **Geological Significance**

* Explains distribution of **earthquakes, volcanoes, and mountain belts**.
* Provides a unifying theory for **crustal movements, orogeny, and continental evolution**.
* Crucial for understanding **paleogeography, ocean basins, and natural resources**.

---

✅ **In short**:

* *Continental Drift* showed that continents move.
* *Sea-floor Spreading* explained how new oceanic crust forms and spreads.
* *Plate Tectonics* unified both, showing that lithospheric plates move and interact, shaping Earth’s surface.