Topics Covered in this Lecture
Weathering is an in-situ process and involves no transportation.
Disintegration or decomposition of the rocks due to either change in temperature and action of water as well as biotic factors is known as weathering.
In weathering, no large scale transportation is involved except under the influence of gravity (mass movement) but weathering is not a prerequisite condition for mass movement (earthquakes and volcanoes).
Physical weathering is breaking or disintegration of rock.
Chemical weathering is decomposition of rocks.
Biotic weathering is disintegration and decomposition due to flora and fauna.
In the regions of steep slope, the weathered material moves down the slope exposing the new layer of rocks for weathering. On the other hand, in the areas of gentle or no slope the weathered material is not displaced and thus protects the lower rocks from weathering. For example, Himalayas versus great plains.
Burrowing animals, vegetation as well as human beings are responsible for weathering. For example, trees exert or put tremendous pressure on the rocks through their root system causing the breaking of the rocks.
Disintegration is due to freeze and thaw action of water due to temperature changes.
(i) Block disintegration- It occurs in well-jointed rocks due to temperature changes or freezes and thaw actions. In the temperate region, especially during the winter season, there is freeze and thaw action of the water. During the night time due to freezing, there is an expansion in volume causing enlargement of cracks or joints. But during the day time, snow melts, daily expansion, and contraction cause weakening of the joints till the time rocks break in the form of large blocks known as block integration.
(ii) Granular/crystalline integration– It occurs in crystalline rocks. The darker crystals absorb more insulation as compared to lighter crystals. As a result there is a differential expansion and contractions of crystals causing granular disintegration.
(iii) Exfoliation/onion peeling– It occurs in the compact rocks, the upper layers of rocks which absorb more insulation break into concentric layers due to differentiated expansion and contraction. Continued expansion and contraction will break/weather the rock layer by layer similar to the peeling of onion and therefore it is also known as onion weathering. Exfoliation is common in a dry climate as well as in monsoon regions. In the hilly regions, exfoliation may result in the formation of rounded hills known as exfoliation domes.
(iv) shattering– Due to sudden expansion and contraction the rocks may break into angular sharp fragments and such weathering is known as shatter. It is a common type of weathering during pre-monsoon rainfall.
(v) unloading and expansion- When the top load on the rocks superincumbent load is removed the rocks expand and develop cracks parallel to the ground surface. Such weathering is unloading and expansion. Unloading and expansion can occur due to tectonic activities, due to anthropogenic activities such as mining, changing of land use pattern and may trigger landslides.
Decomposition of rocks is in-situ.
(i) Solution– When the minerals in the rocks dissolve in water in solution. Such minerals are deposited to form concentrated residues. It makes mineral exploration profitable.
(ii) Carbonation- It occurs in a calcium rich rocks like limestone, chalk.
Atmospheric Co2 reacts with rain water to form a weak acid (carbonic acid) which will react with calcium rich rocks known as carbonation.
(iii) Oxidation– Iron rich rocks react with oxygen to form oxides or hydroxides. This process is known as oxidation. Oxidation causes rusting of the rocks and hence, especially in the tropical regions, rocks have reddish-orange covering on tops.
(iv) Hydration– Minerals in the rocks take up water and expand which causes positive change in their volume (expansion) and they develop cracks.
It is caused due to flora and fauna.
Trees and vegetation are both the factors and and protectors of weathering. Dead and decayed organic matter will add humic acids to the soil causing chemical weathering. The microorganism such as lichens and fungus break down the rocks by the process of humification and mineralization to form a finely weathered soil, which may help in ecological succession.
Humans beings have accelerated the rate of weathering by destroying weathering, mining activities, plowing, and tilling of the land as well as by changing the land use patterns.
The weathered rock material or debris if it moves down the slope under the influence of gravity alone. Then it is known as a mass movement.
Though mass movement can occur even in dry climatic conditions but water play an important role in the mass movement.
For weathering and mass movement the nature of a slope plays an important role.
Weathering of rocks is not a pre requisite condition for mass movement.
The very slow movement of weathered rocks down the slope is known as creep. It is so slow that it cannot be noticed by human eyes. Water plays a minimal role in soil. Soil creep is responsible for bending electric poles in the mountainous region. Soil creep occurs when individual particles jump upon each other use to grazing, animals, movements of humans, vehicles, etc. as well as due to the gentle nature of slope. Rock creep when the sediments of larger size move down in a very slow manner it is known as rock creep.
It is a rapid mass movement caused when the debris becomes fully saturated with water [the 100% capacity]. The mud starts behaving like water and starts flowing down the slope under the influence of gravity.
Mudflow occurs in the region of very heavy rainfall, regions having clayey soli, and in the regions of the absence of vegetation. Mudflow is classified as a natural disaster. Bog burst is a name of mudflow in Ireland. Lahar is the mudflow that occurs in the volcanic regions. It occurs when lava and soil saturated with water flow together.
Instant movement of weathered rocks particles down the slope is known as slides. In the landslides water, it acts as a lubricating agent. But does not play the role of saturating agent.
Its causes are-