GEOMORPHOLOGY
3. Plate tectonic theory
4. Earthquakes
5. Volcanism
6. Rocks
7. Weathering and erosion
8. Mountain & Buildings
3. PLATE TECTONIC THEORY
Wegener’s Continental Drift (1912): Claimed continents drifted. Believed in Pangaea as original supercontinent.
Contradiction: Later evidence showed continents move as part of plates; Pangaea was formed due to plate convergence.
Need for Plate Tectonic Theory
Earlier theories (Continental Drift, Seafloor Spreading, Convectional Current Theory) explained some features but not:
- Formation of fold mountains
- Causes of earthquakes
- Distribution of volcanoes
Plate Tectonic Theory (PTT) unified these explanations.
Formulation (1950s–1960s)
Evidences:
- Mid-oceanic ridges & seafloor spreading
- Palaeomagnetism
- Age of ocean floors
- Island arcs & submarine trenches
- Distribution of earthquakes & volcanoes
- Identification of hotspots
Main Theory
- Tectonic plate: Large, irregular slab of lithosphere (continental + oceanic parts).
- Movement: Plates float over the asthenosphere, move slowly but continuously.
- Effects: Movement creates stress → folding, faulting, volcanism, earthquakes.
- Driving Force: Mantle convection currents generated by radioactive heat.
Importance
- Geologists: Explains earthquakes, volcanoes, mountain-building, ocean basins.
- Geographers: Explains landforms and deformation of Earth’s surface.
- Mineralogy: Plate boundaries are pathways for mantle rocks, forming mineral deposits.
- Examples: Ring of Fire, Iceland volcanoes (mid-ocean ridge), Japan (fault line).
PLATE MOVEMENTS :
Q :- Explain the formation of mid oceanic ridges with suitable examples.
- The divergence of plates releases pressure in the mantle ( asthenosphere) this leads to decrease in melting point of rocks and the available heat is enough to melt the rocks and create magma.
- This magma rises upward in the form of convection currents , near the lithospheric plate it diverges , this divergence of magma on conventional current is the most important reason for the divergence of plate.
- Same magma erupts on the surface of the ocean flow in the form of Volcanism, since, This magma is less viscous and thin , moreover the faults are already present , and the associated Volcanism is of fissure type.
- The tensional force acting on this boundary leads to earthquakes. The earthquakes are also the result of volcanism.
- Volcanism is also the cause of formation of mid oceanic ridges due to solidification of lava.
- Thus we can see that near the divergent plate boundary the material from The mantle comes out or erupts on the oceanic floor and continuously case its spreading.
- On the sea floor spreading the youngest rocks are found near ridges and this spreading of oceanic floor leads to increase in distance b/w the continents.
- Therefore, due to the formation of crust on sea floor also cause constructive plate boundary.
- Eg : Mariana trench.
A. CONVERGENT PLATE BOUNDARY { DESTRUCTIVE }:
2.1 - Oceanic Continental : Q: Explain the formation of fold mountains and also discuss the associated Volcanism.
- when two plates moves towards each other it is convergent plate boundary.
- On this boundary it is a higher density plate which undergoes subduction.
- This subducted plate, when it reaches a particular depth in the mantle , undergoes melting. Here the magma generated has higher concentration of silica.
- Thus the magma is thick and more viscous, and comes out of the earth in the form of explosive volcanism.
- The compressional force along this boundary causes a folding of sediments present along the continental margins in the form of fold mountains.
- Eg:- Himalayas, Rockies, Andes, Alps.
- The huge compressional force and the volcanic activity causes High earthquakes in these regions of fold mountains.
- Apart from this if the subducted plate is oceanic plate then deep trenches formed in this region.
- Because of loss Of crust at this boundary it is called destructive plate boundary.
- Example: Juan de Fuca Plate under North America (Washington–Oregon).
2.2 - Oceanic oceanic : Q: write down the formation of the Indonesian group of islands.
- Older plate subducts → trench + volcanic island arc.
- Example: Mariana Trench
2.3 - Continental ContinentaL
Q : The regions of young fold mountains are associated with volcanism but surprisingly the Himalayan region is devoid of such thing, explain.
- Indian Plate is not subducted deep into the mantle, Hence no magma generated and no volcanism.
Q : The Continental shelf is narrow where there is a fold mountain chain on the continental margins. Explain.
- Due to the collision of plate a compressional force acts on the sediments of Continental plates which results in folding of sediments and foundation of steep slope of the Continental shelf.
- In India the continental shelf is broad towards ARABIAN SEA while it is broad towards the BAY OF BENGAL , because western Ghats are the Block mountains.
C. TRANSFORM PLATE BOUNDARY :
Plates slide past each other → no crust created/destroyed.
Examples:
- San Andreas Fault (California)
- 1906 San Francisco earthquake
