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Geomorphology
1.0 INTRODUCTION
Geomorphology is the science that analyses and describes the origin, evolution, form, classification, and spatial distribution of landforms. The term arose in the Geological Survey in the USA in the 1880s, possibly coined by J. W. Powell and W.J. McGee. Geomorphology is that part of geology which enabled the geomorphologists to construct earth history by looking at the evidence for past erosion. In recent years there has been a tendency for geomorphologists to become more deeply involved with understanding the processes of erosion, weathering, transport and deposition, with measuring the rates at which such processes operate, and with quantitative analysis of the forms of the ground surface (morphometry) and of the materials of which they are composed. Geomorphology now has many component branches, e.g. Anthropogeomorphology, Applied Geomorphology etc.
2.0 Earth's Four Spheres
Four systems interface and interact on earth. The three abiotic (nonliving) systems are overlapping to form the realm of biotic (living) system. The abiotic spheres are the atmosphere, hydrosphere and lithosphere. The biotic sphere is called the biosphere. Because these four spheres are not independent units in nature, their boundaries must be understood as transitional rather than sharp delimitations.
Atmosphere: The atmosphere is a thin, gaseous envelop surrounding the Earth, held by the force of gravity. Formed by gases arising from within the Earth's crust and interior, and the exhalations of all life over time, the lower atmosphere is unique in the solar system. It is a combination of nitrogen, oxygen, argon, carbon dioxide, water vapour, and small amount of trace gases.
Hydrosphere: Earth's water exists in the atmosphere, on the surface and in the crust near the surface, in liquid, solid, and gaseous states. Water occurs in two forms, fresh and saline (salty), and exhibits important heat properties as well as playing its extraordinary roles as a solvent. Among the planets in the solar system, only the Earth possesses sufficient water in quantity.
Lithosphere: Earth's crust and a portion of the upper mantle directly below the crust form the lithosphere. The crust is quite brittle compared to the layers beneath it, which are in motion in response to an uneven distribution of heat and pressure. In broad sense, the term lithosphere sometimes refers to entire solid planet.
Biosphere: The intricate, interconnected web that links all organisms with their physical environment is the biosphere. Sometimes called the ecosphere, the biosphere extends from the seafloor to about 8 km (5 miles) into the atmosphere. Biosphere is that area where the atmosphere, lithosphere, and hydrosphere function together to form the context within which life exists; an intricate web that connects all organisms with their physical environment. Life is sustainable within these natural limits. In turn, life processes have powerfully shaped the other three spheres through various interactive processes. The biosphere has evolved, reorganised itself at times, faced extinction, gained new vitality, and managed to flourish overall. Earth's biosphere is the only one known in the solar system; thus, life as we know is unique to the Earth (in the solar system).
Today, over seven thousand million humans, approximately one million animal species and 3,55,000 known plant species depend on the air, water and land of the planet Earth.
3.0 The Interior of the Earth
The interior of the Earth and its constitution has always been a matter of great controversy among geologists and geomorphologists. In the absence of direct evidence, the interior of the Earth was estimated with the help of change in temperature, pressure and density with depth. A reliable picture of the physical constitution of the Earth was however, ascertained with the help of seismic waves.
3.1 Chemical Differentiation
Crust: The crust of the earth was formed roughly 4.6 billon years ago. The two types of crust are oceanic and continental and they form the first order relief of the earth’s crust. The depth of the crust varies from 70 km under mountains to 5 km under oceans. Thin oceanic crust is composed of dense iron, magnesium silicate, rocks like basalt. The thick continental crust is less dense, composed of sodium, potassium, aluminium silicate, rocks like granite. The boundary between crust and mantle is called Mohorovicic discontinuity. Named after Andrij Mohorivicic, the velocity of seismic waves increases rapidly at this boundary.
Mantle: The mantle is the thickest layer of the Earth’s crust and is divided into the upper mantle, the intermediate mantle and the inner mantle. The motion of tectonic plates is due to convection in the mantle. The intermediate mantle consists of minerals of a greater density than the upper mantle. The average density in the intermediate mantle is approximately 7 gm/cm3 compared to 4.5 gm/cm3 in the upper mantle.
Outer core and the Inner core: Convection in the outer core gives rise to earth's magnetic field. The mechanism of the magnetic field is explained by the Dynamo Theory, which was proposed by Joseph Larmor in 1919. This magnetic field keeps our planet safe from burning away due to Sun’s continuous dangerous radiations (which it deflects and stops). The outer core is liquid in composition and has a density of 11-12 gms/cm3. The discontinuity between the core and the mantle is called Gutenberg’s discontinuity.
The Inner core is believed to consist of an iron-nickel alloy and is the hottest part of the earth. temperature may reach that of Sun's surface i.e 5700 K. Solid in composition, compressional waves can pass through it but not shear waves. Inner core is younger than the age of the earth. It is 2-4 billion years compared to the earth which is 4.5 billion years old. Inner core is cooling slowly (about 100° C per billion years). The inner core is too hot to hold a permanent magnetic field. It has been speculated that the inner core may rotate slightly faster than the rest of the earth (about 0.3 to 0.5 degrees per year).
3.2 Mechanical Differentiation
Lithosphere: Includes the crust and uppermost parts of the mantle and constitutes the hard and rigid outer layer of the Earth. Lithosphere is broken down into tectonic plates, is rigid and deforms through brittle failure, causing faults. Lithosphere is thought to float or move around on the Asthenosphere, creating plate tectonics.
The inner mantle is the densest part of the mantle with an average density of 9 gm/cm3. The discontinuity between the intermediate and the inner mantle is called the Birch discontinuity. Though the inner mantle is solid, because of radioactive heat parts of it can melt. Magma sourced from such deep mantle may result in volcanic eruptions.
Asthenosphere: It lies below the lithosphere and constitutes the weaker, hotter and deeper part of the upper mantle. It is involved in plate movements. It deforms viscously and accommodates strain through plastic deformation. Due to high temperature, the rock becomes ductile, leading to convection currents. The boundary between Lithosphere and Asthenosphere is defined by a change in seismic velocity. In asthenosphere, seismic waves pass relatively slowly and hence it is called a low-velocity zone.
The mesosphere (not to be confused with a layer of the Earth's atmosphere) refers to the mantle in the region under the lithosphere and the asthenosphere, but above the outer core. It is located from the core-mantle boundary to a depth of about 350 km. The pressure in the mesosphere is so great that even though the rock is hot, it is solid and considerably more rigid than the rock on top of it.
The base of the mesosphere includes the “D'' zone which lies just above the mantle-core boundary at approximately 2,700 to 2,890 km (1,678 to 1,796 mi). The base of the lower mantle is at about 2700 km.
The mesosphere makes up most of the volume of the mantle and is entirely solid. The temperature and pressure of the rock in the mesosphere keep it from breaking; therefore, no earthquakes originate from the mesosphere.
The upper mesosphere is a transition zone in which the rock rapidly becomes denser with depth in response to the increasing lithostatic pressure.
The lower mesosphere starts at a depth of 660 km from earth's surface. At that depth there is an abrupt increase in density. This increase is caused by changes in the crystal structures of the most abundant minerals in the rock. These minerals change from less dense crystal structures above the boundary to more dense crystal structures below the boundary. The lower mesosphere undergoes little density change from its top boundary at 660 km to its base at 2900 km where it meets the outer core.
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4.0 Earthquake waves and interior of the Earth
The behaviour of the earthquake waves in the different layers of the Earth provides the most authentic evidence about the composition and structure of the Earth. For example, when an earthquake or underground nuclear test sends shock waves through the Earth, the cooler areas, which generally are rigid, transmit these waves at a higher velocity than the hotter areas.
The different types of waves generated during the occurrence of an earthquake are generally divided into three broad categories:
- Primary waves,
- Secondary waves, and
- Surface waves.
The behaviour of different types of earthquake waves in the different mediums (solid, liquid and gaseous) has been described ahead.
Primary Waves: These are also called the longitudinal or compressional waves. In this type of wave motion, particles of the medium vibrate along the direction of propagation of the wave. These are the high frequency, short wavelength, longitudinal waves. These waves travel not only through the solid part of the earth but also through the liquid part of the core. A primary wave travels with fastest speed through solid and more dense materials, and under certain circumstances, it changes into a secondary wave on refraction, or vice versa. In the liquid materials, their speed is slowed down. These waves are analogous to sound waves wherein particles move to-and-fro in the line of propagation of the ray.
Secondary Waves: These are also called as transverse or distortional waves. Secondary waves are like water ripples or light waves, wherein the particles move at right angles to the rays. A secondary wave cannot pass through liquid materials. These are the high frequency, short wavelength waves which are propagated in all directions from the focus and travel at varying velocities (proportional to density) through the solid part of the Earth's crust, mantle and core. The shallow zone of 'S' waves extends almost halfway around the globe from the earthquake's focus. This can be explained if the outer core of the Earth is liquid. Since S waves cannot travel through liquid, they do not pass through the core.
Surface Waves: Surface waves are called long period waves. These waves generally affect the surface of the Earth only and die out at smaller depth. The surface waves are characterised with low frequency, long wavelength, and transverse vibration which develop in the immediate neighbourhood of the epicenter. These waves are responsible for most the destructive force of earthquakes. They cover the longest distance of all the seismic waves and are recorded in the end on the seismograph.
4.1 Seismic waves as probes of Earth's interior
Seismic waves passing through the earth are refracted in ways that show distinct discontinuities within the Earth's interior and provide the basis for the belief that the Earth has:
- a solid inner core,
- a liquid outer core,
- a soft asthenosphere, and
- a rigid lithosphere.
If the Earth were a homogeneous solid, seismic waves would travel through it at a constant speed. A seismic ray (a line perpendicular to the wave front) would then be a straight line as shown in the figure. Early investigations, however, found that seismic waves arrive correspondingly sooner than was expected at stations progressively farther from an earthquake's course. The rays arriving at a distant station travel deeper through the Earth than those reaching stations closer to the epicenter. Obviously, then, if travel times of long-distance waves are progressively shortened as they go deeper into the Earth, they must travel more rapidly at depth then they do near the surface. The significant conclusion drawn from these facts is that the Earth is not a homogeneous, uniform mass, but has physical properties that change with depth. As a result seismic rays are believed to follow curved paths through the Earth.
Seismic discontinuities: Seismologists have located two major layers which separate zones within the Earth having markedly different properties. The outer one - Mohorovicic Discontinuity (Moho) - separates the crust from the mantle, its average depth being about 35 km. The second discontinuity lies between the mantle and the outer core known as Gutenberg Discontinuity which is about 2900 km.
Thus, the Earth is a differentiated planet. Its constituent materials are separated and segregated into layers according to density. The denser materials are concentrated near the centre, the less dense near the surface. The internal layers are recognised on the basis of composition, and physical properties. The chemical compositional layers are:
- crust
- mantle
- core
Whereas the layers based on physical properties are:
- lithosphere
- asthenosphere
- mesosphere
- outer core, and
- inner core.
5.0 EARTH’S MAGNETIC FIELD
Although the Earth's magnetic field resembles that of a bar magnet scientists have strived to find another explanation for the field's origin. Permanent magnets cannot exist at the temperatures found in the Earth's core. We also know that the Earth has had a magnetic field for hundreds of millions of years. We cannot, however, simply attribute the existence of the present geomagnetic field to some event in the distant past. Magnetic fields decay, and we can show that the existing geomagnetic field would disappear in about 15,000 years unless there were a mechanism to continually regenerate it.
Many mechanisms have been postulated to explain how the magnetic field is generated, but the only one that is now considered plausible is analogous to a dynamo, or generator - a devise for converting mechanical energy to electrical energy. To understand how a dynamo would work in the context of the Earth, we need to understand the physical conditions in the Earth's interior.
The Earth is composed of layers: a thin outer crust, a silicate mantle, an outer core and an inner core. Both temperature and pressure increase with depth within the Earth. The temperature at the core-mantle boundary is roughly 4800° C, hot enough for the outer core to exist in a liquid state. The inner core, however, is solid because of increased pressure. The core is composed primarily of iron, with a small percentage of lighter elements. The outer core is in constant motion, due both to the Earth's rotation and due to convection. The convection is driven by the upward motion of the light elements as the heavier elements freeze onto the inner core.
The actual process by which the magnetic field is produced in this environment is extremely complex, and many of the parameters required for a complete solution of the mathematical equations describing the problem are poorly known. However, the basic concepts are not difficult. For magnetic field generation to occur these conditions must be met:
- there must be a conducting fluid;
- there must be enough energy to cause the fluid to move with sufficient speed and with the appropriate flow pattern; &
- there must be a "seed" magnetic field.
All these conditions are met in the outer core. Molten iron is a good conductor. There is sufficient energy to drive convection, and the convective motion, coupled with the Earth's rotation, produce the appropriate flow pattern. Even before the Earth's magnetic field was first formed, magnetic fields were present in the form of the sun's magnetic field. Once the process is going, the existing field acts as the seed field. As a stream of molten iron passes through the existing magnetic field, an electric current is generated through a process called magnetic induction. The newly created electric field will in turn create a magnetic field. Given the right relationship between the magnetic field and the fluid flow, the generated magnetic field can reinforce the initial magnetic field. As long as there is sufficient fluid motion in the outer core, the process will continue.
6.0 Rocks
Rocks are the solid constituents of earth's surface containing minerals in various proportions. They can be formed from a number of processes. Some rocks come readymade from under the surface of earth and are brought out through volcanoes and others are formed on the spot during eruptions. Some also get their shape and nature from the processes occurring on the surface of earth. The place of origin, nature of elements and processes determine the final product.
Chemical and mineral compositions: They are the rock DNA. They play a large part in determining their place of origin. For example, rocks from deep below the surface contain a larger proportion of higher metals as compared to others.
Texture and structure: Texture refers to the size, shape and orientation of the grains. In lay terms, texture is the feeling you get when you rub your fingers over a piece of rock. On large scale, texture determines the structure and in turn physical properties like permeability, brittleness, etc.
Mode of occurrence: The process which led to the formation of a rock also determines the final look and feel of the rock. For example, lava which cools down faster results in rocks which are more crystalline than the one which cools down gradually.
Rocks have only three main classifications, namely Igneous, Sedimentary and Metamorphic. The complex part is that each one has sub-categories!
6.1 Igneous rocks
Igneous rocks are formed by the cooling and solidification of magma (layer of molten mass of minerals below earth's crust) from beneath the Earth's surface. They reach the surface of the Earth through volcanic fissures. The process is called crystallization because most of the igneous rocks are crystalline in nature. Igneous rocks have three sub-classifications:
Acidic & Basic: Those rocks which contain high proportion of silica are called Acidic and those high in basic oxides like Iron, Magnesium, Aluminium, etc. are called Basic.
Plutonic & Volcanic: The molten rocks which solidify before coming to the surface are called Plutonic rocks. In contrast to this, Volcanic rocks are the ones which solidify after molten lava reaches the Earth's surface.
Intrusive - Extrusive: These are the other names for Plutonic and Volcanic, respectively.
6.2 Sedimentary rocks
These types of rocks are formed by the accumulation of sediments (broken down pieces) over a long period of time usually by the action of water and wind. They are also called stratified rocks because they form in layers. They often contain various types of fossils (remains of organic matter). They are sub-classified on the basis of their origin mechanism.
Mechanically formed sediments: These rocks are formed by the cementing of material derived from other rocks. They are generally used for building materials as Sandstone, Clay, Sand and Gravel. Quartz is also formed by this process.
Organically formed sediments: They are formed by the remains of living organisms such as shell fishes and corals. The Great Barrier Reef of Australia is a great example of it. Limestone, Coal and Chalk are also examples of these type of rocks. They are sub-classified as:
Calcareous: Formed by the remains of living organisms. Some examples are Limestone and Chalk.
Carbonaceous: Formed by the remains of Vegetative matter in swamps and forests. Its examples are Peat and Lignite.
Chemically Formed Sediments: These types of rocks are chemicals which have precipitated from solutions of some form. Gypsum is one such example formed by the evaporation of salt lakes which have a high level of salinity. Similarly, chemical rocks like Potashes, Rock Salts and Nitrates are formed.
6.3 Metamorphic rocks
It is easy to define Metamorphic rocks. But its definition opens a Pandora's Box of convertibility of rocks. This process of conversion of one type of rocks into another is a continuous one and is called Rock Cycle.
The Rock Cycle is a way to depict the changes in rocks from one form to another in a recurring sequence. It was first suggested by James Hutton, the founder of modern geology. Let us examine the Rock Cycle.
Weathering and Erosion: This refers to the action of wind and water. It leads to the segmentations and layering of other types of rocks.
Deposition and Diagenesis: After weathering and erosion the rocks remain buried. They undergo chemical, mechanical and biological change called diagenesis to form sedimentary rocks.
Pressure and Heat: This refers to the conditions of underground rocks which undergo high pressure and temperatures and are turned into metamorphic rocks.
Conversion of Rocks back to Magma: This happens around seismic zones. When two tectonic plates strike each other, chances are that one of them will slide underneath. This layer then turns into magma and gets recycled on some other part of the earth.
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