Defence Exam Notes: Weathering and Mass Movement

The process of wearing away the Earth causes a general lowering and levelling out of the surface. It's known as denudation and is carried out in four phases-

Weathering
Erosion
Transportation
Deposition

WEATHERING

Factors influencing Weathering Processes

Types and rates of weathering vary, at both the regional and local spatial scales In addition to climate, the type of rock (lithology) and the nature and amount of fractures or other weaknesses in it are major influences on the effectiveness of the various rock weathering processes.

Climate

Different climatic conditions are associated with different weathering processes. For example; chemical weathering is particularly effective and rapid in humid climates. Chemical weathering is much more restricted in arid climates. Arid regions typically receive sufficient moisture for physical weathering by salt crystal growth and the hydration of salts. Abundant salts, high humidity and contact with seawater make salt weathering processes very effective in marine coastal locations.

Rock Type

Weathering is greatly influenced by the character of the bedrock: hard or soft, soluble or insoluble, broken or unbroken. Areas of diverse rock types undergo differential weathering and erosion; easily eroded rocks exhibit more extensive effects of weathering and erosion than the resistant rocks.

Slope orientation

The geographic orientation of a slope-whether it faces north, south, east or west-controls the slope's exposure to sun, wind and precipitation.

Vegetation

Although vegetative cover can protect rocks by shielding them from raindrop impact and providing roots to stabilize soil, it also produces organic acids from the partial decay of organic matter, which contribute to chemical weathering Plant roots can enter crevices and break up a rock, exerting enough pressure to force rock segment apart, thereby exposing greater surface area to other weathering processes.

In the complexity of nature, physical and chemical weathering processes usually operate together. Of course, in all of this time, time is the crucial factor for these processes requires a long period of time to operate.

Types of weathering

1 Chemical weathering

Air and water contain chemical elements, which even in small quantities, are sufficient enough to set up chemical reactions in the surface layers of exposed rocks. Such reactions may weaken or entirely dissolve certain constituents of the rock, thus loosening the other crystals and weakening the whole surface.

With the presence of all over rocks. Existing chemical weathering of the underlying rocks does not cease; on the contrary, it is usually enhanced. This is because the soil absorbs rainwater and keeps the underlying rocks in contact with this moisture. The rainwater absorbs organic acids from the soil and thus becomes a stronger weathering agent than pure rainwater acting on bare rock.

There are three major chemical weathering processes

(a) Solution: Many minerals are dissolved by water, especially when, as with rainwater, it contains enough carbon dioxide to make it a weak acid. The rate at which solution takes place in affected not only by the mineral composition of the rock but also by its structure, sedimentary rocks often have pores spaces between the grains in which air and water can lodge and thus attack the rock. Rates of weathering are also affected by climate. Warm wet climates promote rapid chemical weathering while dry climate inhibits chemical weathering. Dry climates, however, provide good conditions for physical or mechanical weathering.

(b) Oxidation: Oxidation is the reaction of oxygen in air or water with minerals in the rock. For example, most rocks contain a certain amount of iron, which when it comes in contact with air is changed to iron oxide, familiar brownish crust or rust.

(c) Decomposition by organic acids: Within the soil which covers most rocks are bacteria which thrive on decaying plant or animal material. These bacteria produce acids which, when dissolved in water help to speed up the weathering of the underlying rocks.

2. Physical or mechanical weathering.

Mechanical weathering is the physical disintegration of a rock by the actual prising apart of separate particles. It takes place in a number of ways:

(a) Repeated temperature changes: in deserts, rocks exposed to the blazing sun during the day are intensely heated. The outer layers exposed much faster than the cooler interiors of the rock and tend to pull away from the rest.

At nightfall, the temperature drops rapidly and the outer layers contract more rapidly than the interior, setting up internal stresses. Such stresses, repeat for a long time causes the rock to crack and split.

(b) Frost action: If the water enters cracks of the rocks and freezes, the cracks widen due to expansion of water.

3.Biotic Weathering

Biotic weathering involves faunal weathering by animals or floral weathering by plants. But recently 'economic and technological man' has become one of the biggest agents of weathering due to anthropogenic activities which involve agriculture, construction, transport, industrialization, urbanization etc.

4.MASS WASTING / MASS MOVEMENT

Mass movement is the movement of weathered material down a slope due to gravitational forces.

Soil Creep: This is a slow, gradual, but more or less continuous movement of soil down the hill slopes.

Soil Flow (Solifluction): When the soil is completely saturated with water, the individual particles are almost suspended in the water and move easily over one another and over the underlying rock. The soils act as liquid and soil-flow or mudflow occur. In temperature and tundra region soil flows occur when the surface layers froze ground to thaw in spring. In Ireland, such flows are known as 'bog-bursts'.