The term lithosphere refers to the solid crust of the earth. The materials constituting the crust of the earth are called rocks. The rocks can be defined as aggregate of minerals.

The earth is divided into three major layers, and two transitional zones. The layers of the earth are The Crust, The Mantle and The Core.

The Crust is the topmost and the thinnest layer of the earth. A transitional zone called Mohorovicic Discontinuity or Moho, marks the lower limit of the crust. The rocks forming the crust of the earth are rich in lighter minerals like silica and aluminium. Hence this layer is called sial (silica and aluminium).

Crust is the most important part of the earth from human point of view. The continents and oceans are situated on this part only. The most important constituents of crust are oxygen (46.6%), silicon (27.7%), aluminium (8.1%), iron (5%), calcium (3.6%), sodium (2.8%), potassium (2.6%) and magnesium (2.1%).

The Mantle is the intermediate layer of the earth. Due to the presence of minerals like silica and magnesium in the rocks forming this layer of the earth, the mantle layer is also known as sima (silicon and magnesium).

The transitional zone separating the mantle from the core is called the Gutenberg Discontinuity.

The core is the innermost layer of the earth and occupies its center. Core is the densest part of the earth and the rock material forming this zone of earth is rich in metallic minerals like iron and nickel. It is due to the large concentration of such minerals in the core of the earth, that this zone is known as nife (nickel and iron).

The material of which the crust is formed is called the rocks. A rock can be defined as an aggregate of minerals. For being called a proper rock, the matter should be in a consolidated form.

The process of rock formation, consolidation of mineral particles, is called lithification. Rocks can be soft, such as clay and sandstone, and hard, such as granite and marble.

A rock containing large proportion of a single mineral so that said mineral can be economically extracted from it is called an ore of that mineral.

Rocks are divided into the categories of igneous, sedimentary, metamorphic and organic rocks. Since these categories of rocks are identified on the basis of the mode of their formation, this classification is called a genetic classification.

Igneous rocks are also called as the primary rocks. Igneous rocks are formed by the cooling and solidification of lava. These rocks are generally hard, massive and crystalline and contains no fossils.

Igneous rocks are of two types, they are Intrusive Igneous rocks and Extrusive Igneous rocks.

Intrusive Igneous rocks are formed due to solidification of lava under the surface of the earth. Intrusive rocks are called as acid lava rocks. These rocks have high content of silica and are poor in metallic minerals. Granite is a typical example of acid lava rocks.

Rocks that are formed at greater depths below the surface are called Plutonic rocks. Granite is an example of both Plutonic as well as acid lava rocks.

Extrusive Igneous rocks are also called as Basic Lava rocks. Basic lava rocks are generally formed on the surface of the earth. Basalt is a typical example of basic or extrusive igneous rocks.

Some of the important igneous rocks are Granite, Rhyolite, Pegmatite, Andesite, Gabbro, Basalt, Dolerite, and Periodotite.

Sedimentary rocks are also called as Secondary rocks or Stratified rocks or argillaceous rocks. Sedimentary rocks are made up of sediments derived from other rocks and deposited by agents of erosion such as rivers, glaciers, and winds on the beds of lakes and seas, etc.

Sedimentary rocks have a layered structure and they are generally softer than igneous rocks and can be scratched. Fossils are commonly found in the sedimentary rocks.

Some of the important sedimentary rocks are Limestone, Gypsum, Sandstone, and Dolomite.

Metamorphic rocks are also called as altered rocks. When exposed to excessive heat or pressure or both, igneous or sedimentary rocks become hard and compact and the joints, if any, get thoroughly compressed.

Some important metamorphic rocks are slate, marble, quartzite, schist, and gneiss.

The process of transformation of rocks is called metamorphism. Metamorphism is of two types, they are thermal or contact metamorphism and regional metamorphism.

Organic rocks are formed from remains of plants and animals. Peat and Coal are examples of organic rocks formed from the remains of plants. Coral is an example of organic rocks formed from the remains of animals. Chalk is also an organic rock made up of microscopic remains of lime secreting organisms.

Oceans and continents form the first order in the hierarchy of landforms.

The tetrahedral theory was proposed by L.Green. It is a theory based on the concept of cooling of the earth and its resultant distortion of shape.

The Continental Drift Theory was put forward by Wegner. The idea of horizontal displacement of continents and oceans is the basis of this theory.

Wegner applied the term Pangea to the super-continent out of which all continents have been formed. Though it has been considered as a single landmass, the Pangea consisted of two parts, a northern block called Lauratia and a southern block called Gondwanaland. A shallow sea was situated between the Lauratia and Gondwanaland. The Pangea was surrounded by an extensive ocean that has named Panthalasa.

Plate Tectonics Theory is the latest and most accepted theory explaining the distribution of oceans and continents.

According to the Plate Tectonics Theory, the lithosphere (the solid crust of the earth) is divided into a number of relatively rigid plates. These plates rest over the molten asthenosphere like floating rafts. A plate according to this theory may include continental or oceanic or both oceanic and continental lithosphere.

Due to convection currents originating in the interior of the earth the plates move in various directions. This movement of plates has been responsible for the present distribution of oceans and continents.

Major landform units found on the continents and ocean beds are called the landforms of second order. The major landforms of second order on the continents are the plains, plateaus, mountains and lakes. The major landforms of second order on the ocean beds are the continental shelf, continental slope and deep sea plain etc.

A plain is defined as an area with gently sloping land and very low local relief (difference between the highest and the lowest point within an area).

Plains are of different types and they can be categorized in various ways. On the basis of their formation they can be grouped as peneplains, flood plains, delta plains, alluvial plains, coastal plains, lacustrine plains, karst plains and glacial plains etc.

Peneplains are formed by the wearing down of land due to weathering and erosion. Peneplains can be defined as a featureless plain formed through fluvial erosion. Remnants in peneplains are called monadnocks.

Flood plains, delta plains and alluvial plains are all formed through the deposition of eroded material transported by rivers. Ganga Yamuna Doab plain is an example of a flood plain; Sunderbans is an example of a delta plain.

Coastal plains are those parts of the continental shelf which have been uplifted and have emerged out of water. Example: Parts of eastern coastal plain of India are of this type.

Lacustrine plains are old lake beds and are made up of sediments deposited mainly by the agency of underground water.

Glacial plains are formed through glacial erosion and deposition. Such plains are found in high latitude areas where glaciers are the chief agents of erosion.

An extensive level or near-level area of elevated land is termed a plateau. It is marked by the absence of a peak or summit and rises almost abruptly from the neighbouring low lands. Plateaus are also called as tablelands.

Plateaus can be classified as inter-montane, piedmont, and continental plateaus.

Inter-montane plateaus are the ones that are either surrounded by mountains on all sides or are situated between two mountain ranges. Tibetan plateau is an example of inter-montane plateaus.

Piedmont plateaus lie between mountains on one side and the sea or plains on the other. The Patagonian plateau in South American is an example of this.

Continental plateaus rise abruptly from the seas or lowlands and are extensive. Indian plateau (Peninsular India), Arabian plateau and Greenland are examples.

Mountains are masses of land considerably higher than the surrounding areas and much of their area is under steep slopes. They have sharp conical peaks and they are areas of high local relief.

On the basis of the mode of their formation, mountains are classified as fold, block, volcanic and residual mountains.

Fold Mountains are formed due to crumpling of the earth's crust due to compression caused by earth movements. This process is called folding. The tallest mountains of the world including the Himalayas, the Andes and the Rockies are all fold mountains.

Due to the forces of compression the rocks are folded upwards and downwards. The up-folded parts or the crests of the folds are called anticlines and the down-folded parts or the troughs in the folds are called synclines. A mountain range is thus an extensive anticline and the mountain valleys represent the extensive syncline situated between two anticlines.

Sometimes, the crust of the earth gets fractured due to stress upon the rocks, usually as a result of forces causing tension. This phenomenon is known as faulting.

Because of faulting some blocks are pushed up while the others are pushed down. The blocks this uplifted are called Block Mountains. The Basin Ranges of USA, and the Black Forest Mountains and the Vosges of Europe are examples of Block Mountains.

The steep sided depressions resulting through faulting are called rift valleys. The Red Sea, Dead Sea and Narmada Valley in India are examples of rift valleys. Areas uplifted due to faulting are called horsts.

Volcanic mountains are formed through the deposition of lava due to volcanic eruptions. In contrast to the fold and block mountains that can be considered structural mountains, the volcanic mountains can be called mountains of accumulation. The Fujiyama and the Hawaiian Islands are examples of volcanic mountains.

The residual mountains are formed due to erosion of raised areas. The residual mountains are also called relict mountains. The Highlands of Scotland and the Sierras of Central Spain are examples of residual mountains.

The forces originating in the interior of the earth are called the internal or the endogenetic forces. Forces causing phenomena like earthquakes and volcanoes are internal forces. Internal forces are also called Constructive Forces.

The forces affecting the surface of the earth from outside are called external or exogenetic forces. Weathering and erosion are a result of the action of external forces.

Volcanic activity or vulcanicity is defined as the ejection of the material from the interior of the earth to the surface. Such material is called volcanic material. The volcanic material may include sold boulders or ashes, molten magma or the gaseous substances. The molten rock material is the most common form of volcanic material ejected during volcanic activity. This material is called magma. Upon reaching the surface of the earth it is called lava.

Lava that is rich in silica and poor in metallic minerals is called acid lava.

The magma that is rich in metallic minerals is called Basic or Basaltic magma.

Typically a volcano is a hole or crack-like opening in the crust through which the volcanic material is ejected to the surface. This opening is called crater.

The magma that rises thorough a nearly circular opening it is called central type of volcanic activity. Mount Etna in Sicily is an example of such volcanoes.

The magma that rises along a long fissure instead of a single nearly circular opening is called a fissure type of volcano. The volcanic activity responsible for the formation of the Deccan Trap of India has been of this type.

Like the volcanic forces the diastrophic forces also originate in the interior of the earth but they do not involve ejection of magma or lava. They rather cause vertical or horizontal displacement of rock masses. The diastrophic forces can thus be defined as the forces responsible for earth movements.

Depending upon the nature of the diastrophic forces the earth movements can be of two types – the vertical movements and the horizontal movements. The vertical earth movements are called the epirogenic or continent building movements and the horizontal movements are called the orogenic movements or mountain building movements.

Folding is a result of the horizontal compressional earth movements so that the rocks are bent or flexed to form arches and troughs. Folding is possible only in the case of Sedimentary rocks.

Warping is also a result of the compressional deformation of the earth's crust. However, warping may involve minor uplift or downward movements and the rocks are not folded. Warping results in formation of domes, shields and depressions. The upward or downward movement in this case is limited.

Faulting implies fracture and vertical or horizontal displacement of fractured rock strata under the influence of tensional forces.

Tremors or shaking of the earth's crust is called an earthquake. Intensity of earthquakes is measured on Richter scale and the instrument providing a continuous record of earthquake waves is called a seismograph.

Richter scale is based on the magnitude of energy released during an earthquake. An increase in the magnitude of an earthquake by one on this scale implies ten times more amplitude or intensity of the earthquake and 32 times more energy.

Prior to the development and adoption of the Richter scale the measurement of earthquakes was done on the basis of either of the two existing scales – Mercalli Scale and the Rossi Forril Scale. Both these older scales were closed end scales and they were based on the degree of damage to structures due to earthquakes.

Most of the earthquakes originate at a depth of about 60 kms below the surface of the earth. The point of origin of an earthquake in the interior of the earth is called the seismic focus.

From the seismic focus, the tremors are carried by the earthquake waves and the point immediately above the focus where the earthquake is felt first on the surface of the earth is called the epicentre.

Lines joining such places that experience a given earthquake at the same time are called coseismal lines.

Lines which join places experiencing the same intensity of an earthquake are called the isoseismal lines.

The energy released during an earthquake moves from the seismic focus in different directions in the form of seismic waves.

There are three types of earthquake waves. They are called the P,S and L waves.

The "P" waves or the push waves are also called as Primary waves or the Longitudinal waves. They move like the sound waves and in case of these waves the direction of wave propagation and the displacement of particles is the same. These waves get deflected while passing through molten rock material.

The "S" waves or the secondary waves are also called the transverse waves. They are like light waves and the direction of displacement of particles in this case is at right angles to the direction of wave movement. These waves cannot pass through the molten rock material.

The "L" waves or the surface waves originate only on the surface of the earth.

If the seismic record of a distant earthquake is analysed, it is observed that the area diametrically opposite (antipodal) the epicentre does not experience the earthquake. This zone around the antipode of the epicentre does not experience the earthquake and the zone is called the shadow zone.

Isostacy is defined as the state of equilibrium that exists between the landforms like mountains, plateaus and plains etc.

External or extrusive forces affect the surface of the earth from above. The external forces have a tendency of removing the surface irregularities and they are called destructive forces. The process of removal of surface irregularities through external processes is called planation or gradation.

The term weathering refers to disintegration and decay of rocks under the influence of the elements of weather, such as temperature and humidity, etc. It involves no transportation of the broken material. Thus weathering involves simply the breaking or crumbling down of rocks in situ.

Depending upon the changes brought about in the rocks due to the influence of the elements of weather, the weathering may be called physical/mechanical, chemical and biological.

Under Physical/Mechanical weathering the rocks are broken down into progressively smaller fragments and the chemical composition of the rocks remains unchanged. Changes in temperature are the primary cause for Physical/Mechanical weathering.

Chemical weathering involves not only the breaking down of rocks into smaller fragments but also changes in their chemical composition. Under chemical weathering water in combination with atmospheric gases, etc., carries out chemical changes such as oxidation (formation of oxides), carbonation (formation of carbonates and bi-carbonates), desilication (removal of silica), and hydrolysis (action of water on minerals).

The total contribution of plants and animals to weathering is called Biological Weathering.

Erosion means wearing down of the earth's surface and it involves not only breaking down of rocks but also the removal of rock material from higher areas. As it involves transportation of rocks, erosion is performed by mobile agents such as streams, glaciers, wind, waves and the underground water.

The cycle of erosion is one of the most important concepts in geomorphology. Put forth by W.M.Davis, this concept deals with the development of landforms under the influence of the agents of erosion.

Rivers are among the most important agents of erosion on account of the total area of the world over which they are active. Landforms made by rivers are often called fluvial landforms.

The chief mechanism involved in erosion by rivers includes corrasion (physical process of removal of rock particles), corrosion (chemical effect of water on rock minerals) and attrition (reduction in size of particles in the eroded material being transported due to mutual friction of the particles.)