(a) What are lithospheric plates? Discuss the theory of plate tectonics. Which plates were involved when a major earthquake caused tsunami in Indian Ocean in 2004? (20 marks)
(b) Discuss the geomorphic processes. Cite any four aggradational and four

Question

(a) What are lithospheric plates? Discuss the theory of plate tectonics. Which plates were involved when a major earthquake caused tsunami in Indian Ocean in 2004? (20 marks)
(b) Discuss the geomorphic processes. Cite any four aggradational and four degradational land forms associated with fluvial processes. (15 marks)
(c) What does attitude of a bed mean? Define strike, dip direction and dip amount of a bed and explain how these are represented on a map for vertical, horizontal and inclined strata. (15 marks)

UPSC Answer Check · Accepted Answer

Lithospheric plates and plate tectonics
Lithospheric plates are rigid, brittle blocks of crust and uppermost mantle, generally 50–150 km thick, that move over the weaker asthenosphere. Plate tectonics integrates continental drift, seafloor spreading, and subduction into one framework: mantle convection, plate boundaries, and recycling of lithosphere. At divergent boundaries, upwelling mantle creates new oceanic crust; at convergent boundaries, dense oceanic lithosphere subducts beneath continental or oceanic plates, producing trenches, volcanic arcs, and earthquakes; at transform boundaries, plates slide past one another. Triple junctions, where three plates meet, show how spreading, subduction, and transform motion are linked. The theory also explains mountain building, such as the Himalayan orogeny from the ongoing collision of the Indian Plate with Asia, and the Wilson cycle of ocean opening and closing. A cross-section diagram would show a spreading ridge, a subduction zone, and a transform fault. The 2004 Sumatra–Andaman earthquake (Mw 9.1–9.3) occurred at the Sunda Trench, where the Indian/Indo-Australian plate was thrust beneath the Burma/Sunda plate. The plates involved were the Indian Plate, the Indo-Australian Plate, and the Burma microplate; the thrust-fault mechanism generated a megathrust slip that displaced seawater and caused a destructive Indian Ocean tsunami.

Geomorphic processes
Geomorphic processes are endogenic and exogenic. Endogenic processes are internal and constructive or destructive: tectonism, volcanism, and diastrophism. Exogenic processes are external and driven by climate, water, wind, and gravity: weathering, erosion, transportation, and deposition. Fluvial processes produce both aggradational and degradational landforms. Aggradation dominates lower courses and floodplains, while degradation is strongest in upper courses and tectonically uplifted regions. Aggradational landforms are built by deposition: alluvial fans, such as the Kosi fan; point bars on the inside of river bends; natural levees along floodplains; and deltas, especially the Ganga–Brahmaputra delta. These show sediment accumulation where river velocity decreases. Degradational landforms are cut by erosion: V-shaped river valleys, gorges, canyons, and waterfalls, such as Jog Falls; the Chambal badlands illustrate intense fluvial degradation, while the Grand Canyon shows long-term downcutting. A profile diagram would show a river cutting a valley and depositing a fan or delta downstream.

Attitude of a bed
The attitude of a bed is its three-dimensional orientation in space, expressed by strike, dip direction, and dip amount. Strike is the compass direction of the line formed where a bedding plane intersects a horizontal plane; it is measured in degrees from north. Dip direction is the direction in which the bed slopes downward, always perpendicular to strike. Dip amount is the angle of inclination measured from the horizontal plane, ranging from 0° to 90°. On a geological map, horizontal strata are shown by a strike line marked H or with tick marks indicating no dip, because the bed is parallel to the horizontal plane. Vertical strata are shown by a straight line with V or arrows, indicating 90° dip; the strike is recorded and the symbol shows the vertical plane. Inclined strata are shown by a strike line with a short perpendicular tick or arrow pointing down-dip and the dip angle, for example 30°, written near the symbol. GSI (Geological Survey of India) maps use these symbols to represent bedding planes, faults, and structural-field measurements. A map sketch would show horizontal beds as H, vertical beds as V, and inclined beds as a strike line with a 30° down-dip arrow.

Conclusion
Together, plate tectonics and geomorphic processes explain Earth’s dynamic surface: plates create uplift, subsidence, and seismic hazards, while fluvial processes build and erode landforms. Understanding plate boundaries and bed attitudes helps map hazards, reconstruct geological history, and manage natural resources.

(a) What are lithospheric plates? Discuss the theory of plate tectonics. Which plates were involved when a major earthquake…

Model answer

What "Discuss" is asking you to do

Structure that answers it

Where marks are lost

How this answer will be evaluated

Approach

Key points expected

Evaluation rubric

Must cover

Loses marks

Earns more

Extra mark

Must cover

Loses marks

Earns more

Extra mark

Must cover

Loses marks

Earns more

Extra mark

Practice this exact question

More from Geology 2021 Paper I