Forces Affecting the Earth’s Surface
The forces acting on the earth’s surface leads to the creation, destruction, recreation and maintenance of geo-materials on the earth’s crust. The forces which affect the crust of the earth are divided as:
The forces coming from within the earth are called as endogenetic forces which cause two types of movements in the earth, viz, (i) Horizontal movements, and (ii) Vertical movements.
These movements motored by the endogenetic forces introduce various types of vertical irregularities which give birth to numerous varieties of relief features on the earth’s surface, e.g. , mountains, plateaus, plains, lakes, faults, folds, etc.
The endogenetic forces and movements are divided, on the basis of intensity, into two major categories:
a) Sudden forces
b) Diastrophic forces
Sudden forces are the result of long period preparation deep within the earth. Only their cumulative effects on the earth’s surface are quick and sudden. Geologically, these sudden forces are termed as ‘constructive forces’ because these create certain relief features on the earth’s surface.
Diastrophic forces include both vertical and horizontal movements which are caused due to forces deep within the earth. These diastrophic forces operate very slowly and their effects become discernable after thousands and millions of years. These forces also termed as constructive forces, affect larger areas of the globe and Produce meso-level reliefs, for example, mountains, plateau, plains, lakes, big faults, etc.
They include: (i) Orogenic processes involving mountain building through severe folding and affecting long and narrow belts of the earth’s crust; (ii) epeirogenic processes involving uplift or warping of large parts of the earth’s crust (iii) earthquakes involving local relatively minor movements; (iv) plate tectonics involving horizontal movements of crustal plates.
(A) Epeirogenetic Movements:
Epeirogenetic word consists of two words, viz: ‘epiros’ (meaning thereby con-tinent) and ‘genesis’ (meaning thereby original). Epeirogenetic movement causes upliftment and subsidences of continental masses through upward movements are infact, vertical movements. These forces and resultant movements affect larger parts of the continents. These are further divided into two types: upward movement and downward movement.
(B) Orogenetic Movement:
The word orogenetic has been derived from two Greek words, ‘pros’ (meaning thereby mountain) and ‘genesis’ (meaning thereby origin or formation). Orogenetic movement is caused due to endogenetic forces working in horizontal movements. Horizontal forces and movements are also called as tangential forces.
Orogenetic or horizontal forces work in two ways, namely, (i) in opposite direction, and (ii) towards each other. When it operates in opposite directions it is called as ‘tensional force’. Such type of force and movement are also called as divergent forces. Thus, tensional forces create rupture, cracks, fracture and faults in the crustal parts of the earth.
The force that operates face to face is called compression force or convergent force. Compressional force causes crustal bending leading to the formation of fields or crustal warping leading to local rise or subsidence of crustal parts.
A layered rock that exhibits bends is said to be folded. The layered rock was at one time uniformly straight but was stressed to develop a series of arches and troughs. A compressive stress compact horizontal rock layers and forces them to bend vertically, forming fold patterns.
Anticlines and Synclines
An anticline is a fold that is arched upward to form a ridge; a syncline is a fold that arches downward to form a trough. Anticlines and synclines are usually made up of many rock units that are folded in the same pattern. Two anticlines are always separated by a syncline, and two synclines are al- ways separated by an anticline. One side of the fold is called the limb; a side-by-side syncline and anticline share a limb.
A variety of kinds of folds generally reflects increasing amounts of tectonic stress.
Open folds are those in which the angle between the two limbs of the fold is more than 900 but less than 1800.
Closed folds are those in which the angle between the two limbs of a fold is acute angle.
Symmetrical folds are simple folds in which both the limbs incline uniformly.
Monoclinal folds are those in which one limb inclines moderately with regular slope while the other limb inclines steeply at right angle and the slope is almost vertical.
Isoclinal folds are those in which the compressive forces are so strong that both the limbs of the fold be- come parallel but not horizontal.
Recumbent folds are formed when the compressive forces are so strong that both the limbs of the fold become parallel as well as horizontal.
Overturned folds are those folds in which one limb of the fold is thrust upon another fold due to in- tense compressive forces. Limbs are seldom horizontal.
A fault is a fracture or zone of fractures between two blocks of rock. Faults allow the blocks to move relative to each other. This movement may occur rapidly, in the form of an earthquake – or may occur slowly, in the form of creep. Faults may range in length from a few millimeters to thousands of kilometers. Most faults produce repeated displacements over geologic time. During an earthquake, the rock on one side of the fault suddenly slips with respect to the other. The fault surface can be horizontal or vertical or some arbitrary angle in between.
Landforms formed by faulting
1. Block Mountains : Block mountains form when the layers of the Earth’s crust are forced upward near fault lines. Fault-block mountains have a steep, sharp front side and a gentler, sloping back. Examples of block mountains are the Teton
Range of Wyoming, the Alps in Europe and the Urals.
2. Rift Valley : When a depression appears between two block mountains, that depression is called a rift valley, which can be thousands of miles long. In the United States, one example of these flat-bottomed valleys is Death Valley in California. Some of the largest rift valleys include the East African Rift Valley. Russia’s Baikal Valley, Germany’s Rhine Valley and the Red Sea. Ocean rift valleys occur where tectonic plates on the seafloor spread apart. The largest lakes in the world are all found in rift valleys.
Exogenetic forces also called as denudational processes. These forces are engaged in destruction of the relief features by the process of weathering, erosion etc.
It is the breakdown and decay of rocks in-situ related to elements of the weather (e.g. temperature, rainfall, frost etc.
Weathering is the first stage in the denudation of the landscape. Rocks are weakened and loosened by weathering processes. This weakened material is then removed by agents of erosion (e.g. ice, water, etc.)
Weathering can be classified as:
a) Physical Weathering : Physical weathering is also known as mechanical weathering and it involves the physical breakdown of rock – it does not involve chemical change.
This is the breakdown of rocks due to the expansion of water during freezing, a process common in upland Britain where evening temperatures often fluctuate around 00C. Freeze thaw is most effective in jointed rock (e.g. granite). During freezing, water expands by 9% in volume. Water freezing in cracks in rocks, exerts pressure. Alternating freeze-thaw cycles gradually force the rock to split or cause rock fragments to break off. Where this process occurs on steep slopes, rock fragments collect at the base of the slope due to gravity in the form of a scree slope.
Pressure Release or Dilation
Rocks such as granite, formed as igneous intrusions are formed under pressure: When weathering and erosion removes overlying rocks, the pressure is released and the underlying rock expands. This expansion results in the fracturing of the rock, which weakens it by making it susceptible to other weathering agents. If cracks develop parallel to the surface, sheeting of rock layers may occur.
This process results from large diurnal temperature ranges which result in heating and cooling of the rock. When heated, expansion of the rock occurs, whilst during cooling the rock contracts. This expansion and contraction during cycles of temperature change results in stresses in the rock layers. Outer layers of rock heat and cool quicker than inner layers and over time the upper layers flake / peel off (exfoliation). It should be noted that the effectiveness of this process is heavily debated and some believe that it is only really effective when water is also present.
Water passing through crevases and joints in rocks, may be saline (carrying salts in solution). As the water evaporates, the dissolved salts precipitate and crystalise froming salt crystals. This may also take place where in rocks such as chalk, the rock is decomposed by solution to form salt solutions such as sodium carbonate which will then crystalise upon evaporation of the moisture. The salts may expand up to 3x their original size, and therefore the crystals put stresses upon the rock as they grow, resulting in granular disintegration (gradually breaking off individual grains of rock).
b) Chemical Weathering : Chemical weathering is where rocks are decomposed by chemical reaction between elements of the weather and rock minerals, resulting in either the alteration of a rock’s internal mineral structure or the formation of new minerals (e.g. feldspar forming Kaolin in the process of hydrolysis). Weakened rock or the consequent deposits are then more easily removed by erosion processes. Water plays a key role in most chemical reactions and also provides a transport mechanism for other elements that carry out weathering. Chemical weathering is most dominant in hot and humid areas such as equatorial zones and least effective where there is little rain such as in desert or polar regions (where most water is held as ice). The susceptibility of rocks to chemical weathering is determined by the types of minerals they contain and their mineral structure. There are a number of different types of chemical weathering.
The exposure of rocks to oxygen in air or water can result in a reaction between the oxygen and iron-based minerals in the rocks. Iron readily oxidises and during oxidation blue grey ferrous iron (Fe2+) is transformed to red ferric iron (Fe3+). This causes a weakening of the rock structure enabling them to crumble easily and making them more susceptible to other weathering processes.
Rainwater contains dissolved CO2 which forms a weak carbonic acid
(H2O + CO2 = H2CO3). Carbonic acid is able to react with calcium carbonate (common in rocks such as limestone and chalk) to form calcium bicarbonate which is then easily removed in solution in water. Limestone is gradually dissoved in this way as the calcium carbonate is converted to calcium bicarbonate and carried away in solution by running water.
Water can act as a solvent by breaking down chemical bonds in minerals causing them to dissolve in a process known as solution – carbonation is therefore a form of solution although it is mineral specific in relation to calcium carbonate. Solution rates tend to increase with an in- creased acidity of water.
This is where acidic water reacts with rock forming minerals such as feldspar. This is a common process in the weathering of granite. Hydrogen ions in the water displace potassium ions in the feldspar. This causes the feldspar to break down into a secondary mineral, Kaolin (China Clay). Whilst the feldspar in granite decomposes, the quarz and mica remain relatively unaffected but the structure weakened.
This occurs as the addition of water causes minerals in rock to swell (by about 0.5%) due to a chemical reaction as the mineral absorbs water (‘hydrates’), thus involving both chemical and physical (mechanical weathering). The formation of gypsum when water combines with anhydrite (CaS04) (anhydrite) + 2H2O (water) = CaS04.2H20 (gypsum)). Gypsum is fairly soluble and can then be fairly easily removed by solution.
c) Biological Weathering: It usually consists of a combination of physical (growth of roots into joints in rocks) and chemical (e.g. impact of organic acids) processes.
As roots of plants and trees grow downwards, they often enter and exploit cracks joints in rock. As they grow they are able to gradually wedge the joints further apart, eventually resulting in detatchment of rock fragments (simillar to freeze-thaw).
As roots as well as surface litter decays, organic acides are released into the ground. Percolating rainwater moves these acids further down and the organic acids may react with minerals in the rock through a process called chelation. The combination of rainwater and organic acids combines with aluminium and iron which are washed out of the soil.
Respiration of bacteria and tree roots also releases C0, which when becomes dissolved in water forms a weak carbonic acid which can increase the chemical weathering process, carbonation.
Burrowing animals help to open up joints in rock and also help to bring rock fragments to the surface, where they are exposed to further weathering. At the coast, animals such as limpets increase the rate of chemical weathering through the acids, secreted as they cling to rock surfaces.