Tectonic movement

Tectonic Movement

The tectonic movement takes place inside the globe (Earth) which is governed by the forces inherent in the earth and belongs to the class of endogenous process. It can also be defined as the process involving the breaking and the bending of the earth’s crust under the internal earth’s forces is known as the tectonic movement. The science that deals with the study of the structures of the upper layer of the earth i.e the crust and the upper mantle, as well as the movements producing these structures, are called the Geotectonics.

It has been proved and established that the rocks do not always behave in the same way, as they seem at the surface. The rock fall types are elastic i.e compression, elongation, and shearing. The earth’s crust is compiled with the rocks. So the rocks within the earth’s crust experience greater confining pressure as the result of the load of the overlying rocks, the depth of the burial, high temperature due to the general geothermal gradient and also the presence of the gases and the solution arising from the magmas. The rocks do not behave as strictly elastic material but the elastic viscous material according to E. W. Spencer. According to him “The rocks are the substances exhibiting the characteristics of elastic solids under some conditions and those of the fluid under other conditions”. Besides, from the gravitational stress the rock mass within the earth’s crust is acted upon by the number of processes like an igneous intrusion, flow movement in the mantle, the tidal stress contraction and expansion of some parts of the earth’s crust etc. These forces cause the rock to crumple into folds, thrust over one another or to break up so that the landforms on the surface undergoes changes, mountains are formed and deep basins are developed. Such crustal deformations mostly occur as a result of the different stress acting upon the rocks and the whole crustal deformations are known as diastrophism.

The crustal disturbances are most preserved in the sedimentary rocks as these rocks remain undisturbed and exhibit almost horizontal deposition over the vast area of the earth’s crust and even a slight bending, breaking or some sort of the distortion of the strata can be easily detected on them. Thus it can be said that the sedimentary rocks have remained as the index of the movements which have occurred in nature.

At the origin of the earth, the sedimentary rocks were horizontal but moving with the time along the movement of the earth’s crust they are often tilted out of their original position. Sometimes the tilting of the beds even takes the horizontal beds to the vertical position, but due to the unequal uplift or the subsidence of the earth crust moves into an inclined position. The altitude of the inclined bed is defined by two elements namely dip and strike.

The dip is the angle of inclination of a bed with respect to the horizontal plane. The dip has got two components as direction as well as magnitude in which the angle of dip varies between 0 (for horizontal beds) to 90 (for vertical beds). The direction of the dip is the geographical direction, along which a bed has the maximum slope. The strike is the line of intersection of a bed with the horizontal plane.

As a direct result of the diastrophism, irrespective of the size or scale, four major groups of structures are produced which are as follows:

1. Folding
2. Faults
3. Joints
4. Unconformities

Folding

The rocks can be bend or fold by plastic deformation. The process of folding occurs when the rock is compressed, as it is along the colliding plate boundaries. The upturned worlds are known as anticlines whereas the downturned folds are known as synclines. The anticlines and the synclines are the geologic structures folded in the rock material. They give the expression to the surfaces as linear ridges (anticlines) and troughs (synclines). The sides of the fold are called limbs. Each of the fold has an axial plane, an imaginary plane that runs down its length and divides the fold into half.

The symmetrical or open folds with their near vertical axial planes and gently dipping limbs of about the same angle are the product of the gentle compression and are found near the margins of the mountain system where the tectonic activity is relatively quiet. If the compression is more from one direction then an overturned fold may occur. The extreme pressure may lay the fold over with its axial plane nearly horizontal with the surface producing the recumbent fold. Sometimes the length of the folds is tilted creating the plugging folds. The Appalachian mountain in North America, Himalaya mountains of Asia,

The Alps in Europe, The Andes in South America are the examples of the folded mountain.

Elements of the folds

Crest and troughs

The convex and the concave portions of the wavy undulation formed in the alternate series is known as crest and trough respectively.

Core

The core is the inner part of the fold.

Limb

The limb is the stretch of the rock beds lying between and its vertical troughs i.e side of the fold also known as flank.

Axial plane

Axial plane is the plane dividing the fold symmetrically.

Axis

Axis is the line of intersection of the axial plane and the ground surface.

Hinge

The hinge is the line running through the points of maximum curvature of any of the beds forming the fold also known as fold bend or flexure.

Plunge

The plunge of the fold is the angle which the fold axis makes with the horizontal.

Pitch

The pitch is the angle between the horizontal line and the axis measured on the axial plane. The plunge and the pitch coincide when the axial plane is upright.

Angle of the fold

It is the interlimb angle formed by the lines continuing the limbs of the fold up to the plane of the intersection.

Height of the fold

It is the vertical distance between the hinges of the adjacent anticline and the synclines and is also known as the amplitude of the fold.

Width of the fold

It is the distance between the axial lines of the two adjacent anticlines or the synclines.

Nose or the curve of the fold

It is the part of the fold adjacent to the hinge line also known as hinge area.

References:

Keller, E.A. Environmental Geology. Columbus, Ohio: Charles E. Miller Publishing Company, Bell, and Howell Company, 1985.

Mahapatra, G.B. Textbook of Physical Geology. Shahadra,Delhi-110032: CBS Publishers and Distributers Pvt.Ltd., 1992.