"The Himalaya is still rising." Expand this statement and describe the processes involved in it with suitable sketches and examples.

The Ongoing Uplift of the Himalayas

The assertion that "The Himalaya is still rising" is supported by substantial geological and geodetic evidence. This continuous uplift is a direct consequence of the active continent-continent collision between the Indian and Eurasian plates, a process that commenced in the Cenozoic Era.

Processes Involved in Himalayan Uplift

The formation and ongoing uplift of the Himalayas are primarily governed by the principles of plate tectonics, specifically continental convergence. The key processes involved are:

1. Continental Collision and Subduction of Oceanic Crust

• Initial Stage: Around 200 million years ago, the Indian subcontinent was part of the supercontinent Gondwana. It began drifting northward after breaking away, separated from Eurasia by the ancient Tethys Ocean.
• Oceanic Subduction: As the Indian Plate moved northward at a rapid pace (around 9-16 cm per year), the oceanic crust of the Tethys Sea subducted beneath the Eurasian Plate. This process led to the formation of volcanic island arcs along the Eurasian margin, similar to the Andes today.
• Closure of Tethys: Over millions of years, the Tethys Ocean gradually narrowed as its oceanic crust was consumed through subduction. Sediments accumulated in the Tethys geosyncline were compressed and uplifted.

2. Continent-Continent Convergence

• Collision Onset: Approximately 50 million years ago (though some estimates vary from 40 to 55 million years), the leading edge of the Indian continental plate finally collided with the Eurasian continental plate. Unlike oceanic crust, continental crust is less dense and highly buoyant, preventing significant subduction of one beneath the other.
• Crustal Shortening and Thickening: The immense compressional forces resulting from this collision led to intense folding, faulting, and thrusting of the continental crust. Both the Indian and Eurasian plates experienced significant shortening, with the crust thickening substantially (up to 75 km beneath the Himalayas and Tibetan Plateau, which is about twice the average continental thickness).
• Formation of Thrust Faults: The compression created a series of imbricate (overlapping) thrust faults. Key thrusts, younging from north to south, include:
  • Main Central Thrust (MCT): Separates the Higher Himalayas from the Lesser Himalayas.
  • Main Boundary Thrust (MBT): Divides the Lesser Himalayas from the Siwalik Range.
  • Main Frontal Thrust (MFT): Marks the southern boundary of the Siwalik Range and the northern limit of the Indo-Gangetic Plain.
  These thrusts act as planes along which blocks of crust are pushed upward and over adjacent blocks, contributing significantly to the vertical growth of the mountains.

3. Isostatic Adjustment and Continuous Uplift

• Isostasy: The thickened continental crust of the Himalayas exerts a greater downward force due to its weight. According to the principle of isostasy, this mass must be compensated by a buoyant force from the underlying mantle, causing the crust to "float" higher. This ongoing adjustment contributes to the uplift.
• Active Indian Plate Movement: The Indian Plate continues to move northward into the Eurasian Plate at a rate of approximately 4-5 cm per year. While the overall convergence rate is absorbed by various processes, about 20 mm per year of this convergence is absorbed by thrusting along the Himalayan southern front, directly contributing to the mountains rising by approximately 5 mm annually.

Evidence of Ongoing Uplift

• High Seismicity: The Himalayan region is one of the most seismically active zones globally. Frequent earthquakes, some of high magnitude (e.g., the 2015 Nepal earthquake), are direct evidence of the ongoing tectonic stress accumulation and release due to plate convergence. These earthquakes are concentrated along the active thrust faults.
• Geodetic Measurements: GPS and satellite-based geodetic data consistently show that the Himalayan peaks are rising. Studies have estimated uplift rates of up to 7-10 mm per year in certain areas like Nanga Parbat.
• Presence of Marine Fossils: The discovery of marine fossils (e.g., ammonites, trilobites) at high altitudes in the Greater Himalayas, including Mount Everest, provides compelling evidence that these rocks were once part of the Tethys Sea floor and have been uplifted thousands of meters.
• Deep Gorges and Youthful Topography: The presence of deep, V-shaped gorges carved by rivers (e.g., Indus, Brahmaputra) through the mountain ranges indicates rapid uplift exceeding the rate of erosion. The jagged peaks, steep slopes, and active landslides also point to a geologically young and active mountain system.
• Fossil Formations: Similar fossil formations found in the Siwalik hills and the Tibetan Plateau further demonstrate the continuity of the geological processes that lifted these regions.

Sketches and Examples

Sketch 1: Plate Tectonic Setting of Himalaya Formation

[Imagine a cross-sectional sketch showing two continental plates: the Indian Plate (south) and the Eurasian Plate (north). The Indian Plate is shown moving northward, colliding with the Eurasian Plate. In between, remnants of the subducted Tethys oceanic crust are depicted. The collision zone shows intense folding, faulting, and stacking of crustal material, leading to the elevated Himalayan mountain range and the Tibetan Plateau. Arrows indicate the direction of plate movement and compressional forces.]

• Indian Plate: Moving North.
• Eurasian Plate: Resisting movement.
• Tethys Sediments: Folded and uplifted between the plates.
• Thrust Faults: Illustrate major thrust faults (MFT, MBT, MCT) showing crustal shortening and stacking.

Sketch 2: Crustal Thickening and Uplift Mechanism

[Imagine a simplified cross-section showing the thickened continental crust beneath the Himalayas. Below this, the Indian lithosphere is depicted underthrusting the Eurasian Plate. The concept of isostatic rebound is represented by the upward buoyant force compensating for the weight of the thickened crust. Arrows show ongoing compression, leading to vertical uplift.]

• Thickened Crust: Label the 70-75 km thick crust beneath the Himalayas.
• Underthrusting Indian Plate: Show the Indian Plate moving beneath the Eurasian Plate.
• Isostatic Rebound: Indicate upward forces.

Implications of Ongoing Uplift

The continuous rise of the Himalayas has several significant implications:

• Enhanced Seismic Hazard: The region remains highly prone to earthquakes, posing significant risks to human settlements and infrastructure. Recent studies indicate the possibility of major earthquakes (e.g., magnitude 8.8) due to accumulated stress.
• River Systems and Erosion: The uplift drives the vigorous erosion by Himalayan rivers, forming deep valleys and transporting vast amounts of sediment, which contribute to the formation of the Indo-Gangetic Plains.
• Climate and Monsoons: The rising mountains act as a significant barrier, influencing atmospheric circulation patterns and strengthening the Indian monsoon system, thus affecting rainfall distribution across South Asia.
• Glacier Dynamics: Tectonic activity can influence glacier stability, contributing to glacial lake outburst floods (GLOFs) and other cryospheric changes.