Classification and Orogenesis of Mountains
Introduction & Conceptual Foundation
The Earth's surface is a dynamic mosaic shaped by continuous endogenetic (internal tectonic) and exogenetic (external denudation) forces. Geomorphologists classify the resultant relief features into three distinct orders of landforms based on their scale, mode of origin, and temporal sequence:
- Primary Landforms: These are the largest relief features of the Earth's crust, formed primarily due to the initial differentiation of the lithosphere into continental blocks and oceanic basins. They provide the fundamental structural framework upon which all other landforms develop.
- Secondary Landforms: These features are constructed upon the primary landforms through endogenetic processes such as diastrophism (orogenesis and epeirogenesis) and volcanism. Examples include mountains, plateaus, and plains.
- Tertiary Landforms: These are micro-level relief features sculpted on secondary landforms by exogenetic agents of erosion and deposition, such as rivers (fluvial), wind (aeolian), glaciers, groundwater (karst), and waves. Examples include river deltas, gorges, sand dunes, and beaches.
Orogenesis (derived from the Greek oros meaning mountain and genesis meaning creation) refers to the structural processes of mountain building, which typically involve folding, faulting, thrusting, and igneous intrusions driven by tectonic plate movements.
UPSC Prelims Perspective
For the Prelims exam, understanding the classification of mountains, key tectonic terminologies, and their global distribution is essential.
1. Folded Mountains
These are formed by the compressional forces generated when tectonic plates converge. The process involves the deformation of thick sedimentary layers accumulated in geosynclines.
- Old Folded Mountains: Formed before the breakup of Pangea (pre-Mesozoic eras, such as the Caledonian and Hercynian orogenies). They are highly denuded and possess low relief.
- New Folded Mountains: Formed during recent geological eras (mainly Cenozoic alpine orogeny). They are characterized by high peaks, deep valleys, and active tectonic instability.
| Parameter | Old Folded Mountains | New Folded Mountains |
|---|---|---|
| Geological Age | Formed during Paleozoic era (250–500 million years ago) | Formed during Cenozoic era (last 65 million years ago) |
| Topography | Low peaks, rounded summits, mature relief due to prolonged erosion | High peaks, rugged relief, steep slopes, youth stage |
| Examples | Aravallis (India), Urals (Russia), Appalachians (USA), Pennines (UK) | Himalayas (Asia), Rockies (North America), Andes (South America), Alps (Europe) |
2. Block Mountains
Block mountains are created when large blocks of the Earth's crust are uplifted or subsided along faults due to tensional or compressional tectonic stresses.
- Horst: An uplifted block of crust between two faults. It forms the prominent mountain mass.
- Graben: The depressed block of crust between faults, forming a valley floor.
- Rift Valley: An elongated valley bounded by parallel fault lines.
Key examples include:
- Rhine Rift Valley: Located between the Vosges Range (France) and the Black Forest (Germany), which act as block mountains flanking the graben.
- Narmada Rift Valley: Located between the Vindhyan Range and the Satpura Range in Central India.
- Tapi Rift Valley: Located between the Satpura Range and the Ajanta Range.
3. Volcanic Mountains
Formed by the accumulation and solidification of molten rock material (magma/lava) erupted from the Earth’s interior.
- Central Eruptions: Eruptions through a central vent forming steep cones (e.g., Mount Stromboli, Mount Vesuvius, Mount Fuji).
- Fissure Eruptions: Lava flows out through elongated cracks, constructing expansive volcanic plateaus or underwater mountain ranges like the Mid-Atlantic Ridge (the longest mountain range in the world, stretching over 65,000 km).
4. Relic (Residual) Mountains
These are remnants of pre-existing mountains that have been severely dissected and lowered by weathering and erosion over geological epochs. Examples include the Aravallis, Eastern Ghats, and the Urals.
UPSC Mains Perspective
Multi-Dimensional Analysis of Fold Mountain Orogeny
The Plate Tectonic Theory provides the most scientific explanation for the origin of folded mountains. The process unfolds in three key phases:
- Geosynclinal Deposition: Rivers and winds transport eroded materials from adjacent landmasses into long, narrow, shallow, subsiding marine basins called geosynclines (e.g., the Tethys Ocean). Over millions of years, these sediments accumulate to form thick sedimentary layers. Under the pressure of overlying sediments, compaction and cementation turn these layers into sedimentary rocks.
- Lithospheric Plate Convergence: As convective currents in the asthenosphere drive tectonic plates toward one another, compressional forces are generated. In the case of the Himalayan orogeny, the Indo-Australian plate moved northward to collide with the Eurasian plate.
- Folding and Upliftment: The thick sedimentary rocks within the geosynclinal basin are squeezed, buckled, and folded upwards. The crust undergoes thickening, crustal shortening, and vertical upliftment, giving rise to high mountain ranges.
Structural and Socio-Economic Significance of Mountain Ecosystems
- Hydrological Lifelines: Mountains act as water towers. Glaciers feed major perennial rivers (e.g., Ganga, Brahmaputra, Indus) that support billions of people downstream.
- Climatic Barriers: The Himalayas act as a barrier to the cold Siberian winds, keeping the Indian subcontinent warm, while also trapping the moisture-laden South-West Monsoon winds to cause rainfall across Northern India.
- Resource Reservoirs: Mountainous regions are rich in minerals, forest resources, and possess massive potential for run-of-the-river hydroelectric projects.
- Ecological Vulnerability: Mountains are highly susceptible to natural hazards such as earthquakes, landslides, cloudbursts, and glacial lake outburst floods (GLOFs), which are exacerbated by climate change and unplanned infrastructure projects.
Practice Questions
Prelims Practice Question
Q1. Consider the following pairs of mountain ranges and their associated rift valleys or geological classifications:
- Black Forest & Vosges : Graben structure of the Rhine Rift Valley
- Satpura & Ajanta Ranges : Tapi Rift Valley
- Ural Mountains : New Folded Mountain system
- Aravalli Range : Relic Mountain system
Which of the pairs given above are correct?
(a) 1, 2 and 3 only
(b) 1, 2 and 4 only
(c) 2, 3 and 4 only
(d) 1, 3 and 4 only
Correct Answer: (b)
Explanation:
- Statement 1 is correct: The Rhine Rift Valley is a classic graben flanked by the Vosges (horst/block mountain in France) and the Black Forest (horst/block mountain in Germany).
- Statement 2 is correct: The Tapi River flows through a rift valley located between the Satpura Range to the north and the Ajanta Range to the south.
- Statement 3 is incorrect: The Ural Mountains in Russia are classified as Old Folded Mountains, formed during the Uralian orogeny in the late Paleozoic era, and are highly eroded.
- Statement 4 is correct: The Aravalli Range is one of the oldest fold mountain systems in the world, now classified as a relic or residual mountain system due to millions of years of exogenetic denudation.
Mains Practice Question
Q1. Explain the process of fold mountain formation with reference to Plate Tectonic Theory. Discuss how the mountains of India influence the socio-economic and climatic conditions of the Indian subcontinent. (15 Marks, 250 Words)
Answer Framework / Approach:
- Introduction (30-40 words): Define orogenesis and folded mountains. State that plate tectonics is the dominant modern theory explaining their formation through crustal compression and plate convergence.
- Body Section 1: Process of Formation (80-90 words):
- Detail the geosynclinal deposition phase (sediment accumulation and subsidence).
- Explain the tectonic plates' convergence (compressive forces, crustal shortening).
- Mention folding, faulting, and vertical upliftment. Use the Himalayan orogeny (Indo-Australian colliding with Eurasian plate squeezing the Tethys sediments) as a prime example.
- Body Section 2: Climatic and Socio-Economic Influences (100-110 words):
- Climatic: Barrier effect against dry Siberian winds; interception of South-West monsoon resulting in orographic rainfall.
- Socio-Economic: Perennial river source supporting agriculture, domestic water, and industrial needs; hydroelectric power generation potential; forest wealth and rich biodiversity; tourism and cultural significance.
- Geological hazards: Seismically active zones (Zone V in Himalayas) leading to landslides and earthquakes.
- Conclusion (30-40 words): Summarize the significance of mountains. Emphasize the need for sustainable development policies in mountainous regions to balance economic growth with environmental conservation.