Geography Optional (Landforms and Processes) by Anurag Pathak

Model Answer

Question #1. Explain the concept of isostasy and its significance in understanding mountain building. 10 marks (150 words)

Hint

While isostasy provides a foundational model for understanding vertical equilibrium in the Earth's crust, it's essential to consider it as part of a broader suite of geological processes and phenomena, especially when delving into the complexities of mountain building.

The concept of isostasy pertains to the equilibrium that exists within Earth's lithosphere. It's analogous to floating objects in water. Just as an iceberg floats with a portion of it submerged in water based on its density and mass, Earth's crust "floats" on the denser, semi-fluid asthenosphere beneath it. If weight is added to or removed from the crust (e.g., through sediment deposition or erosion), the crust will adjust by rising or sinking until it reaches a new equilibrium.

Significance in Mountain Building:

• Compensatory Mechanism: As mountains form, they add a significant weight to the crust. This could potentially cause the crust to sink. However, the crust adjusts (or "floats") by having a corresponding root or thickness that extends into the asthenosphere. The taller the mountain, the deeper its root. This relationship ensures that the weight of the mountain is balanced.
• Erosion and Rebound: Over time, erosion wears down mountains, removing material from the crust. As this weight is removed, isostatic rebound can occur where the crust rises in response to the reduced load, similar to how an iceberg might rise in water if portions of it were removed.
• Glacial Isostatic Adjustment: This principle can also be seen in places where large ice sheets used to exist. When these ice sheets melted at the end of ice ages, the previously compressed crust began to rise – a process that's still ongoing in some parts of the world.

While isostasy provides a model for understanding vertical adjustments of the Earth's crust, it's important to integrate this with other geological processes for a holistic understanding of mountain building.

• Tectonic Forces: Mountain building isn't just about vertical adjustments. Horizontal tectonic forces, especially at convergent plate boundaries, play a crucial role. For instance, the Himalayas are still rising due to the collision of the Indian and Eurasian plates.
• Time Scale: Isostatic adjustments can occur over different time scales. While tectonic forces might push up mountains relatively quickly (geologically speaking), isostatic rebound from glacial melting can take thousands of years.
• Complex Interplay: Mountains aren't just eroding and rising in isolation. The sediment eroded from them is often deposited elsewhere, potentially leading to isostatic adjustments in those deposition areas.
• Limits of the Model: The simplistic model of isostasy might not account for all vertical movements in the crust. Factors like mantle convection and lithospheric rigidity can also influence these movements.

Question #2. Discuss the processes of chemical and mechanical weathering. How do they impact various types of rocks differently? 15 marks (250 words)

Hint

• Interplay Between Processes: Often, mechanical weathering can enhance chemical weathering. By breaking rocks down into smaller pieces, mechanical weathering increases the surface area exposed to agents of chemical weathering.
• Environmental Factors: The rate and type of weathering are highly dependent on environmental factors. For example, freeze-thaw weathering is more prevalent in alpine and polar regions, while chemical weathering dominates in warm, humid climates.
• Feedback Loops: As weathering breaks down rocks, it can lead to the formation of soil, which can then host vegetation. This vegetation can further influence weathering rates, especially through biological activity and the production of organic acids that can boost chemical weathering.
• Cultural Impact: Human-made structures, monuments, and artifacts, often made from rocks or metals, are subjected to these weathering processes. The understanding of these processes is crucial for the preservation of our cultural heritage.
• Economic Implications: Weathering processes play a role in soil formation, influencing agriculture. They also impact the lifespan and maintenance needs of infrastructure

Chemical and Mechanical Weathering:

1. Chemical Weathering:

This involves the breakdown of rocks and minerals due to chemical reactions. Some common types include:

• Hydrolysis: Water reacts with minerals in rocks, leading to the formation of new minerals. For example, feldspar reacts with water to form clay minerals.
• Oxidation: This occurs when minerals, especially those containing iron, react with oxygen in the air or water, forming oxidized compounds. For example, iron can form iron oxide, leading to a reddish coloration.
• Carbonation: Carbon dioxide in the air or water reacts with certain minerals in rocks, such as calcium carbonate in limestone, causing it to dissolve.

2. Mechanical (or Physical) Weathering:

This pertains to the physical breakdown of rocks into smaller pieces without changing their chemical composition. Examples include:

• Freeze-thaw: Water enters cracks in rocks, freezes, expands, and then causes the rock to fracture when it thaws.
• Thermal Expansion: Daily temperature changes can cause rocks to expand and contract, leading to weakening and eventual breakdown.
• Exfoliation: As overlying rocks are removed (by erosion, for instance), underlying rocks can expand and form layers or sheets that peel off.
• Biological Activity: Plant roots can grow into cracks in rocks and break them apart.

Impact on Different Rocks:

Different rock types have different susceptibilities to chemical and mechanical weathering due to their mineralogical composition, structure, and the conditions to which they're exposed.

• Granite: Being coarse-grained and composed mainly of quartz and feldspar, granite is resistant to chemical weathering but can undergo mechanical weathering, especially freeze-thaw and exfoliation.
• Limestone: Composed mainly of calcium carbonate, limestone is particularly susceptible to carbonation.
• Basalt: This rock can be weathered chemically due to its iron-rich minerals undergoing oxidation, leading to a reddish-brown colour.