Divergent Boundary
As already discussed earlier: At divergent boundaries, two lithospheric plates move away from each other.
This movement leads to the formation of new crust — making it a constructive boundary.
🧠 Just as convergent boundaries build fold mountains, divergent boundaries build oceans.
Evolution: From Rift → Sea → Ocean
The journey from a solid continent to a mighty ocean involves four distinct stages:
📌 Stage 1: Upwarping and Fault Zones
👉 It all starts with a plume — a vertical column of hot magma rising from deep within the mantle due to convection currents.
- This plume domes the overlying lithosphere, causing upwarping (uplifting of crust).
- Due to stress, the crust develops faults:
- Normal faults due to stretching
- Occasionally thrust faults due to crustal adjustment
🧠 Analogy: Imagine a thick pizza dough being pushed from below — it bulges, cracks, and stretches.
📌 Stage 2: Formation of Rift Valley
As the stretching continues:
- The lithosphere becomes thinner and eventually fractures.
- This leads to the formation of a Rift Valley — a graben (downdropped block) bounded by horsts (uplifted flanks).
- Along the rift:
- Volcanic activity and earthquakes begin.
- The crust continues to weaken due to horizontal extension.
🌍 Key Example:
- East African Rift System
- Started ~30 million years ago in the Afar Triple Junction
- A mantle plume is doming the Ethiopian Highlands, weakening the crust.
📌 Note: This is an active rift — a continental breakup is in progress!
📌 Stage 3: Formation of Rift Lakes and Linear Seas
As divergence continues:
A. Linear Seas Form
- Continental crust thins so much that oceanic crust starts forming.
- Eventually, seawater invades, creating a narrow sea (linear sea).
- Example: Red Sea (between Africa and Arabian Peninsula)
B. Rift Lakes Form
- If this process happens inland, and the valley is deep enough, rainwater accumulates.
- This gives rise to rift lakes — usually long, narrow, and very deep, bounded by steep escarpments.
🌊 Famous Rift Lakes:
| Lake | Key Fact |
| Lake Baikal (Russia) | Deepest (1642m); Largest by volume; in active rift |
| Lake Tanganyika | Longest, second deepest; in Albertine Rift (East Africa) |
| Lake Superior (USA) | Largest by area; formed on an ancient rift |
| Lake Victoria | Second-largest by area; lies between two rift arms |
📌 Lake Baikal is not only a physical geography marvel but also a hotspot for UPSC prelims questions 😊
📌 Stage 4: Transformation into Ocean
- Continued volcanism causes flood basalts and seafloor spreading.
- Eventually:
- Oceanic crust fully replaces continental crust
- Mid-oceanic ridges form along the spreading zone
- A new ocean basin is born
👉 Example:
- South Atlantic Ocean: Formed ~138 million years ago when South America broke away from Africa.
🧠 Balance Maintained:
- Crust formation at divergent boundaries is offset by crust destruction at convergent boundaries — maintaining Earth’s structural equilibrium.
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