See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/310021377 The Earth's Lithosphere-Documentary Presentation · November 2011 CITATIONS READS 0 1,973 1 author: A. Balasubramanian University of Mysore 348 PUBLICATIONS 315 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: Indian Social Sceince Congress-Trends in Earth Science Research View project Numerical Modelling for Prediction and Control of Saltwater Encroachment in the Coastal Aquifers of Tuticorin, Tamil Nadu View project All content following this page was uploaded by A. Balasubramanian on 13 November 2016. The user has requested enhancement of the downloaded file. THE EARTH’S LITHOSPHERE- Documentary By Prof. A. Balasubramanian University of Mysore 19-11-2011 Introduction Earth’s environmental segments include Atmosphere, Hydrosphere, lithosphere, and biosphere. Lithosphere is the basic solid sphere of the planet earth. It is the sphere of hard rock masses. The land we live in is on this lithosphere only. All other spheres are attached to this lithosphere due to earth’s gravity. Lithosphere is a massive and hard solid substratum holding the semisolid, liquid, biotic and gaseous molecules and masses surrounding it. All geomorphic processes happen on this sphere. It is the sphere where all natural resources are existing. It links the cyclic processes of atmosphere, hydrosphere, and biosphere. Lithosphere also acts as the basic route for all biogeochemical activities. For all geographic studies, a basic understanding of the lithosphere is needed. In this lesson, the following aspects are included: 1. The Earth’s Interior. 2. The Lithospheric Plates . 3. The Plate tectonics. 4. The Earth’s Internal processes. 5. The Magma and its emplacements. 1. THE EARTH’S INTERIOR The Earth’s lithosphere is composed of three major solid layers as outermost Crust, middle Mantle and the central Core. The Crust is the outermost layer of the earth on which all living organisms survive. This is a very thin layer. It is ranging from 5 km under the oceans to 100 km under the mountainous areas of continents. Usually, it’s about 40 km thick under the flat continents. The crust is made of many types of rocks and thousands of minerals. These rocks and minerals are made from just 8 elements. They are Oxygen (46.6%), Silicon (27.72%), Aluminum (8.13%), Iron (5.00%), Calcium(3.63%), Sodium (2.83%), Potassium (2.70%) and Magnesium (2.09%). The rocks present in the earth’s crust are solid, rigid and brittle in nature. They are also highly variable, including rocks of molten origin, rocks of sedimentary origin, and rocks that have undergone all sorts of structural and chemical alterations through metamorphism. The crust itself can be divided into two sub-layers. One kind of layer is found everywhere, under the oceans and also below the continents. This is called as the oceanic crust. This layer is dominated by relatively heavy, dark, dense rocks of “mafic” composition. Most of these mafic rocks are of volcanic in origin and are called “basalts.” This dense, heavy mafic layer is sometimes called the “SiMa” denoting its chemistry as silica and magnesium. It tends to be relatively thin, usually from about 5-12 km in thickness. A second layer is normally found in the continents. It is made up of light colored rocks. These rocks are primarily composed of silicates enriched in lighter elements, such as aluminum (Al), potassium (K), and sodium (Na). This layer is called as “SiAl” as it is dominated by silicate rocks with lighter elements mixed with aluminum. These rocks are granitic masses and hence, this layer is called as the granitic layer. This is considerably thicker, around 40 km, than the basaltic lower layer, 1 Below this crust, the earth’s solid constituents have shown a transition. The density of the mass is very high and very rigid. Seismic soundings have identified a discontinuity between the crust and this layer. This is the mantle layer of the earth. There is a sharp increase in the velocity of seismic waves as they pass into this layer of differing density and rigidity. The Mohorovicic discontinuity (often called, simply, “the Moho”) marks the transition from the bottom of the crust to the top of this mantle layer. Andrija Mohorovicic first noticed this effect in the year 1909. He found that some of the earthquake waves near the surface, moved slower than the earthquake waves that passed through the interior of the Earth. He also noticed that the P (primary, first and strongest) waves that passed through the interior of the Earth, did move in a straight line. These waves were bent or deflected by something. He decided that the outside layer of Crust was made of less dense material (Rock) and the mantle. The Mantle was much denser. Waves of all other kinds move faster and straighter through this denser, more solid layer. Based on this observation, the nature of the mantle was identified. The Earth’s Mantle The middle, mantle, layer makes up the largest volume of the Earth’s interior. It is almost 2900 kilometers thick and comprises of about 83% of the Earth’s volume. It is divided into two layers as upper mantle and lower mantle. The upper mantle is about 670 kilometers in depth. It is brittle and less dense. It is made up of peridotites. These are rocks made up of olivine and pyroxene minerals. These are largely silicate minerals and the rocks are basic in character. These rocks are highly enriched with iron and magnesium, and hence they are called as “ultramafic” rocks. These ultramafic rocks are dark in color due to the presence of iron and magnesium. These rocks are extremely heavy and dense compared with the typical surface rocks. The rocks in the upper mantle are more rigid and brittle because of cooler temperatures and lower pressures. The Lower Mantle is much thicker and denser. It is 670 to 2900 kilomteres below the Earth’s surface. This layer is hot and plastic. The higher pressure existing in this layer causes the formation of minerals that are different from those of the upper mantle. The mantle varies in its state of matter. It is soft and in nearly liquid condition near its inner boundary with the liquid outer core and again near the top, a few kilometers under the earth’s crust. In other areas, it may show nearly brittle condition and solidity. The Earth’s Core: The Earth’s central Core contains two different layers as Outer Core and Inner Core. The Outer Core is a hot liquid layer and the Inner Core is a hot and solid layer. Beno Gutenberg discovered the boundary as a discontinuity between the mantle and the outer core. This boundary was named after him, as Gutenberg discontinuity. The outer core is at 2890-5150 km below the earth’s surface. 2 The temperature in the outer core is about 4000-50000C. The molten, liquid iron in the outer core is important because it helps to create the Earth’s magnetic field. The outer core is about 2250 km thick. The outer core is known to exist in a liquid state because of the behavior of earthquake waves, particularly shear body waves or secondary waves. Liquid cannot respond to shear forces, so it can’t transmit shear waves. As a result, there is a seismic shadow on the side of the earth antipodal to an earthquake’s epicenter. Solid Inner Core The inner core is 5150-6370 km below the earth’s surface. It mainly consists of iron, nickel and some lighter elements , probably sulphur, carbon, oxygen, silicon and potassium. The temperature in the inner core is about 5000-60000C. Because of the high pressure, the inner core is solid. The solidity of the inner core is due to the presence of iron and nickel. The core is incredibly hot in the centre and the pressure is so great that the melting point of iron and nickel is elevated far beyond those high temperatures (6,500 K), leaving the nickel-iron as solid. 2. THE LITHOSPHERIC PLATES The lithosphere is broken up into tectonic plates. There are currently seven or eight major and many minor plates. The lithospheric plates ride on the asthenosphere. These plates move in relation to one another at one of three types of plate boundaries as convergent( or collisional )boundaries, divergent boundaries(also called as spreading centers); and conservative transform boundaries. Plate size can vary greatly, from a few hundred to thousands of kilometers across; the Pacific and Antarctic Plates are among the largest. Plate thickness also varies greatly, ranging from less than 15 km for young oceanic lithosphere to about 200 km or more for ancient continental lithosphere. Tectonic plates probably got developed in the earlier period of the Earth's 4.6-billion-year old history. They have been drifting on the surface ever since-like slow-moving bumper cars repeatedly clustering together and then separating each other. The movement of plates has caused the formation and break-up of continents over time, including occasional formation of a supercontinent that contains most or all of the continents. The supercontinent Columbia or Nuna was formed during a period of 2.0–1.8 billion years. It got broken up about 1.5–1.3 billion years ago. The supercontinent Rodinia is thought to have formed about 1 billion years ago and to have embodied most or all of Earth's continents. It also got broken up into eight continents around 600 million years ago. The eight continents later re-assembled into another as a supercontinent called as Pangaea.