MAJOR FEATURES OF EARTH SURFACE & INTERNAL STRUCTURE OF THE EARTH - ENGINEERING GEOLOGY (StudyCivilEngg.com)

MAJOR FEATURES OF EARTH SURFACE &
INTERNAL STRUCTURE OF THE EARTH

SUBJECT : ENGINEERING GEOLOGY

MAJOR FEATURES OF EARTH SURFACE

• The earth's surface is made of diversified features of limitless  sizes and shapes which are classified into several orders on  the basis of their scales. Features of the highest order are  seen only with the help of satellite images or may be  identified on the world map. These cover an area of several  million square kilometres and are the products of internal  earth processes, for example, continents, oceans, major  mountain chains, rift valleys etc. 
• Features of intermediate order cover an area of several  hundred square kilometres. They are formed due to earth's  external processes controlled by climatic conditions and also  originated due to earth's internal processes, for example, San  Andreas fault, river terraces, tectonic landforms etc. 
• Features of the lower order can be easily recognised on aerial  photographs or on topo sheets having a scale 1: 25,000, for  example, a meandering steam, soil horizons etc. The features  of different orders are summarised in below table

• From the above table, it can be said that the features are  prominently formed due to the earth's internal and external  processes. The external processes can be visualised by  considering weathering erosion and deposition caused by  the work of natural agencies including running water, wind,  glaciers and sea. The internal processes may be explained  with the concept of plate tectonics, the most important  discovery in the 20th century.
• The major features of the earth show that about 71% of the  earth's surface is covered by oceans and their average depth  is 3800 metres. The depth is extremely variable and  maximum depth, 11035 metres, is recorded at Mariana  Trench, in the Western Pacific. (From below figure).
• The remaining 29% of the earth's surface is occupied by land.  It has an average elevation of 800 metres above mean sea  level. The surface of continents is constantly modified by  weathering and erosion. Thus, the surface morphology is  dissected in a very complicated manner. The maximum  elevation, 8848 metres, is recorded at Mount Everest, Nepal.

Hypsographic Curve

• Modern coastal lines do not coincide exactly with the  boundaries between continental and oceanic crust. The  boundaries are covered by oceanic water and the present  day boundaries are actually all the continents. Thus, each  continent is surrounded by a gently dipping continental  shelf. With the changes in slope, continental shelf passes into  continental slope, which may be considered as a bottom of  continental surface. If we take this as a boundary of the  continents, then it is estimated that about 60% of the earth's  surface is occupied by the oceanic basins while the remaining  40% is covered by continents. Thus, 10% of the continental  crust is occupied by oceanic water.
• The continental rise lies at the base of continental slope. It is  a region of gently changing slope where the floor of oceanic time, which cannot be studied in the laboratory. One must  also understand limitations of the study. The surface  exposures can be studied in greater details with satisfactory  results. The same can be said of shallow sub-surfaces studies.
• However, deeper sub-surface studies are to be carried out  with the help of geophysical methods. In this, the  information received is in the form of magnetic, and electric  properties of rocks, velocity of sound waves etc. In other  words, interpretation of this information varies from person  to person. Hence, the same information may be interpreted  in different ways by different persons producing  heterogeneity in the results.

INTERNAL STRUCTURE OF EARTH

• Planet earth contains three compositional layers. At  the centre is the dense most of these three layers, the core.  The core is a spherical mass, composed mainly of metallic  iron, with minor amounts of nickel and other elements.
• The thick shell that surrounds the core is called the mantle. It  is less dense than the core, but denser than the outermost  layer. Above the mantle lies the thinnest and outermost layer,  the crust
• It is seen from below figure that the core and mantle have almost  uniform thickness. However, the crust differs in thickness  from place to place. The crust beneath oceans known as  oceanic crust, has an average thickness of 8 kilometres, while  continental crust has an average thickness of 45 kilometres. It  shows variation in thickness from 30 to 70 kilometres
• In addition to compositional layering, other changes  including changes in physical properties also occur within the  earth. These are largely controlled by temperature and  pressure, rather than rock composition. The places where  changes in physical properties do not coincide with the  compositional boundaries, are shown in below figure

• Within the core exists an inner region where iron is  interpreted to be in solid state despite of very high  temperature. This solid centre of the earth is called inner  core. It is believed that the solid state of the iron is due to  presence of very high pressure. Surrounding inner core is a  zone of outer core where temperature and pressure are  balanced in such a way that iron is molten and behaves like a  liquid.
• Similarly, difference in temperature and pressure divides the  mantle and crust into three distinct regions: the mesosphere,  the asthenosphere and the lithosphere. (Figure (a)).
• In the lower part of the mantle, the rock has a considerable  strength despite of very high temperature (from 2883 km to  350 km). This solid region of high strength and temperature  is called the mesosphere (intermediate or middle sphere).
• Within the upper mantle i.e. from 350 km to about 100 km, is  a region called the asthenosphere (weak sphere). Here the  rocks are weak and easily deformed like butter.
• Above the asthenosphere, i.e. the uppermost region of 100  km, is called the lithosphere (rock sphere) where rocks are  cooler, stronger and rigid than plastic asthenosphere. The  lithosphere is not a homogeneous continuous layer. Instead  it is broken into a number of larger or smaller plates. 

FAQs COVERED IN THIS POST

Features of Earth Surface

Earth Surface

Internal Structure of Earth

Hypsographic Curve

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