crustal deformation and landslide

Crustal Deformation

In the boundaries of the earth lithospheric plates, enormous stress is imposed and where the convection currents in the mantle tug and tear the crust above. When such stress is imposed on the rock material, the rock probably changes its shape and also its volume.Thus, this change in shape as a result of imposing a stress is called strain.

There are three basic kinds of stress in which compression occurs when rock masses are pushed together like that, which occurs when the plate collide. The rocks tend to shorten laterally and thicken vertically when exposed to the compressional stress. The tensional stress pulls the crust apart, like that which occurs along diverging the plate boundaries. The tension extends the crust causing it to thin and lengthen. The shearing stress occurs when the plates slide past one another in opposite directions along transform plate boundaries which cut the crust into the parallel blocks displacing them horizontally relative to one another. The rocks behave in the different way when stress is imposed on them. The major thing is that the reaction of the rock material to an imposed stress depends on the temperature and the pressure conditions. As the stress is imposed on the rock, it starts to deform up to its yield point. Before it gets to the yield point, the rock will undergo elastic deformation like the rubber band, and if the stress is released before its yield point, the rock material will return to its original shape.

However, under low temperature and pressure, once the rock reaches its yield point, it will break and it is called brittle failure. The rock after reaching the yield point under high temperature and pressure may undergo plastic deformation, where the rock would not return to its original shape after the stress is released. During the plastic deformation, mineral bonds adjust to the stress by breaking, moving about, and then reforming and when the rock reaches to the rupture it will break.

Landslide

The downward movement of the mass of the land or the bedrock under the influence of the gravity is called a landslide. Usually, the landslide occurs on the steep slopes where the hard and heavy rocks overlie softer or more the lubricated materials such as clay, gypsum in which the bedding materials or the joint planes dip downward towards the valley.

The landslide can is categorized as:

1. Slump

It involves the movement of the curved slip surfaces, the rock mass moves down it rotates on the horizontal axis and therefore the upper surface of the block becomes tilted towards the cliff that remains. In such cases of the mass of rockfalls along two surfaces, one is the common surface of shear failure usually on the curved surfaces, usually the curved outline and the other surface is the surface separating one block from the other. Normally, the three slumping takes place as the several small independent units which give rise to the number of the steps like features.

2. Debris slide

It is the rapid downslope movement of the mass consisting of the regolith of the varied texture. They mostly occur on the steep slopes covered with less vegetation . The amount of the water is usually small, so it can be distinguished from the debris avalanches. It involves a sliding and the rolling motion but not the type of the backward rotation as in the case of slumping. In this case, the materials fall from the vertical or overhanging cliff.

3. Rock Slide

It consists of the slipping down of the rock mass from the slopes which are usually the planes of weakness like bedding joint or the fault planes.

The rock fall is the free falling or the rolling of the single masses of rock from a steep cliff and thus involves the disintegration of the rock mass.

Cause and the effect of the landslide

As the landslide gives the meaning as the downward and outward movement of the slope forming material along the surface of separation by falling, standing and flowing of the mass of land or soil at a faster rate. Its causes includes the following factors which impose the following factors:

1. Natural factor

The natural factor involves the following :

• Step slope : The steep slopes when gets heavy rainfall and if there is no presence of the vegetation and the trees they may fall as the landslide.
• Undulating of the banks of the rivers by the abrasive action of the river itself: The abrasive action of the river causes the loosening the soil masses that may fall at any time as a landslide.

• Types of rocks present: If there is a presence of weaker rocks like chalk slate, phyllite they can be easily weathered which may cause the fall of the mass after the certain time like the landslide of Church, Sindhupalchok in 2015.

• Triggering : Cloudburst rainfall of about 200- 1000mm per day may cause the uncontrolled flow of water on slope surface which cause its mass flow.

• Earthquake vibration : The earthquake vibration loosens the soil surface that when meets with heavy rain fall down as the landslide or sometimes the dry land mass may also fall due to the pressure to the vibrated soil mass.

2. Anthropogenic cause

• Deforestation : The deforestation is the root cause of the landslide that reduces the soil holding capacity of root which influences the erosion of the soil and also the masses of soil to fall.

• Improper land use pattern: Improper land use pattern involves

The agricultural practice on the steep slope, irrigation on the steep and vulnerable slope, overgrazing by livestock, grazing for the construction of the material without considering the condition of the train are the anthropogenic improper land use pattern that suggests the erosion of the soil and also the land mass.

3.Construction and the Developmental Activities

• Construction of road
• Channels in the hillside
• Vibration caused by blasting
• Slope cutting action

Effects of landslides

The landslide causes the loss of life and property of the settlement area and the physical structure of the roads, bridges, channels etc. Not only this, the landslide carries away the fertile soil and also affects the local ecosystem of the area. Sometimes the landslide may block the river causing the damming of the water for temporary and gets burst that may cause the flooding to the downstream causing the great losses of lives and properties and also inviting the diseases. The ecosystem when gets hampered it reduces the number of living species and also may lead to its extinction.