Changing Face of the Earth

Geomorphic Processes

(The endogenic forces are mainly land building forces and the exogenic processes are mainly land wearing forces)

One must astonish that why is the surface of the earth so uneven? Our earth’s surface is subject to constant change i.e. dynamic, must be the answer. Of course, earth's crust moved a bit faster in the past than the rate at which it is moving now. The differences in the internal forces operating from within the earth which built up the crust have been responsible for the variations in the outer surface of the crust. The earth’s surface is being continuously subjected to external forces induced basically by energy i.e. sunlight. Another truth, the internal forces are still active though with different intensities. That means, the earth’s surface is being continuously subjected to by external forces originating within the earth’s atmosphere and by internal forces from within the earth. These changes occur beneath the earth’s crust due to folding or faulting of rock strata by earthquake or volcanic activity. On the other hand changes are also brought about very slowly by processes like weathering, erosion or human activities. The external forces are termed as exogenic forces and the internal forces are known as endogenic forces. 

Internal Forces

Internal Forces

The lithosphere is broken into several sections or plates. Each plate carrying oceanic and continental crusts alike moves independently over the asthenosphere. The energy required to move these plates is produced by the internal heat of the earth. At places, these plates move away from each other creating wide rifts on the earth’s surface for example San Andreas fault whereas at some places, these plates come closer and collide. 

• When an oceanic plate collides with a continental plate, the denser oceanic plate is forced below the continental plate. As a result of the pressure from above, the rocks heat up and melt. The molten rocks (magma) rise again forming volcanic mountains along the continental edge. Alternatively a trench may be formed between two plates. 
• When two continental plates converge, neither plate can be forced under the other. Instead, folds may be created. Great mountain ranges like the Himalayas have been formed in this way.

Movements of the crustal plates cause compression and tension on rocks. Thus the areas of collision are marked with deep trenches on the ocean floor, rows of volcanic islands and mountain ranges. Along the converging boundaries of the lithospheric plates, tectonic activity is that of compression happened. As a result, the sedimentary layers of the oceanic floor get compressed to form folds. The wavelike undulations have alternating upfolds (anticlines) and downfolds (synclines). In areas of rifting of continental plates, faulting takes place. Tension is responsible for the formation of faults. A fault is a fracture along which displacement of rock layers takes place. These landforms may be formed on land as well as under oceans.

Volcano

A volcano is an opening in the earth’s crust that allows magma to reach the earth’s surface. The magma that reaches the earth’s surface is called lava, and the passage through which it travels is called a vent. In course of time, lava and other materials thrown out of a volcano accumulate around the opening and form a conical hill or mountain. The top of this cone is usually marked by a funnel shaped depression called crater. Sometimes, magma rises slowly to the surface and spreads over a vast area. This is known as lava flow. Some plateaus and plains have been formed in this way, e.g. Deccan plateau in India and the Colombian plateau in North America. The viscocity of Lava is determined by the amount of silica and water in magma. High viscocity lava is rich in silica and has little water. Low viscocity lava has little silica but a lot of water. It moves rapidly forming smooth flows. If the magma rises quickly to the surface, lava is thrown high into the atmosphere. Besides lava, ash, steam, gasses and pieces of rocks are also thrown out. The shape of a volcano depends on the type of lava and the force of the eruption. On the basis of shape, there are three types of volcanoes. They are shield, cinder-cone and composite volcanoes.

• A shield volcano is formed by quiet eruption of lava with low silica content. Such a volcano has wide base and a cone with gentle slopes. Volcanos of the Hawaii islands are of this type.
• Silica rich magma traps gases inside the volcano until enough pressure is built to push the magma out of the earth’s crust. When this type of volcano erupts, it shoots gases, ash etc. such volcanoes have steep slopes and are made of cinder and ash. They are known as cinder-cone volcanoes. Many volcanoes of Mexico and Central America belong to this group.
• A series of successive eruptions of different types of lava gives rise to a composite volcano. Such volcanoes are made of alternative layers of lava, cinder and ash. They have undulating slopes. Mt. Fujiyama of japan is an example of composite volcano.

(Note: There is a circular belt of volcanoes round the Pacific Ocean. It is commonly known as the ring of fire)

Earthquake

Earthquake is a vibratory motion of the ground surface. The point where these vibrations originate is called the focus of the earthquake. The point of the earth’s surface directly above the focus is called the epicentre of the earthquake. From the focus, the earthquake vibrations travel in different directions in the form of seismic waves. The earthquake waves are recorded by an instrument known as Seismograph. The magnitude of an earthquake is measured by the Richter scale. The number on this scale range from 0 to 9, though there is no upward limit except for nature’s own limit of energy release. The region around the Pacific Ocean, which is also the area of most prominent tectonic activities, experiences frequent earthquakes. In India the Himalayan region and the Ganga-Brahmaputra valley are prone to earthquakes. The Deccan plateau is supposed to be comparatively free from the dangers of the earthquake.

External Processes

External Forces

The mediums of the hydrosphere and the atmosphere sculpture the landforms created by tectonic and volcanic activities. They are powered by solar energy, weathering, and mass wasting as well as erosion, transportation, and deposition of earth materials by agents of gradation produce a variety of landforms. When rocks are exposed to the environment at or near the surface of the earth, they are gradually disintegrated into small pieces at the same place (in situ). The process is called weathering. Breaking of rocks takes place due to change in temperature and pressure as well as effects of air and water and the mechanised actions of animals, plants and humans.

Weathering & Erosion

Weathering is often described as the first phase in the gradation of the landscape because it prepares rock materials. It is an important pre-requisite to the formation of soils. Once the rock is broken by weathering, the small pieces can be moved by water, moving ice, wind or gravity. In this process broken rocks are carried from one place and get deposited at other place. This process is called erosion. Weathering and erosion take place almost everywhere and all the time. Sometimes one process is more prominent than the other. Their rate of action may vary due to climatic change or change in the environment. The rate of weathering and erosion depends on the following factors :- (a) temperature and rainfall of a place (b) vegetation cover (c) change in land use (d) type of soil (e) slope of the land. Weathering and erosion that spread over millions of years change the land surface features like mountains, hills, plateaus, and plains on the earth. For very long time these two forces are acting together. The processes of weathering and erosion are called denudation. Generally there are three types of weathering - Physical, chemical and Biological.

Physical Weathering

Mechanical disintegration of rocks without any change in their chemical composition is physical weathering. It takes place in many ways.

• Change in temperature causes rock forming minerals to expand when heated and contract when cooled. In areas of extreme range of temperature, rock surface are exposed daily to intense heating during the day and intense cooling during the night. As a result of the daily cycles of expansion and contraction rocks are weakened and they break apart.
• Change in the temperature also affects the water collected in cracks or pore spaces of rocks. Due to fall in temperature at night, water freezes and expands. Consequently the cracks also enlarge under pressure. Gradually widening of cracks leads to disintegration of rocks. This is called frost action.
• In the process of expansion and contraction due to the difference in day and night temperature, the outer layers of rocks get peeled off. This is called exfoliation which is very common in granitic rocks.

Chemical Weathering

In chemical weathering, the mineral composition of rocks changes due to chemical reaction of water or air. When air contacts with any minerals then the oxidation starts. Similarly when rain falls co2 reacts with several minerals. Limestone and marble react to carbonic acid and produce interesting features in humid regions, for example, rock cavities, sinkholes, and caverns.

Biological Weathering

When seeds sprout in a rock there is crack and begin to grow roots, they exert pressure on rock walls and expand the cracks. This leads to breaking of rocks. Humans and animals also contribute to this process by burrowing.

Mass Wasting 

The various kinds of downhill movement occurring under the pull of gravity are collectively known as mass wasting or mass movement. When the rock materials such as clay or silt saturated with water move down a gentle slope, it is called an earth-flow. The mud flow, are the movement of water-saturated rock debris down channels on steep hill slides. Quite often dry soil and rock pieces suddenly move down a steep slope. This is known as landslide.