🌏INTERIOR OF THE EARTH
The configuration of the surface of the earth is largely a product of the processes operating in the interior of the earth. Exogenic as well as endogenic processes are constantly shaping the landscape. Human life is largely influenced by the physiography of the region. Therefore, it is necessary that one gets acquainted with the forces that influence landscape development.
Sources of Information about the Earth’s Interior
There are two sources for information about interior of the earth:
1. Direct Sources : Mining, drilling and volcanic eruption are examples of direct sources. Direct sources are not very reliable because mining and drilling can be done up to some depth only. During the process of mining and drilling rocks and minerals are extracted which gives information that there are layer system in the crust. Crust is made of many kinds of rocks and minerals. Volcanic eruption suggests that there is some zone inside the earth which is very hot and in liquid condition.
2. Indirect Sources : Indirect Sources: Seismic waves, gravitational field, magnetic field, falling meteors etc. are examples of indirect sources. They are very important to understand about earth’s interior. Seismic activity is one of the most important sources of information about the interior of the earth. Movement of seismic waves suggests that there are three layers in the earth and each layer has different density. Density increases toward the center of the earth.
The gravitational force (g) is not the same at different latitudes on the surface. It is greater near the poles and less at the equator. Magnetic surveys also provide information about the distribution of magnetic materials in the crustal portion, and thus, provide information about the distribution of materials in this part.
We know through the mining activity that temperature and pressure increase with the increasing distance from the surface towards the interior in deeper depths. Moreover, it is also known that the density of the material also increases with depth, knowing the total thickness of the earth, scientists have estimated the values of temperature, pressure and the density of materials at different depths.
Another source of information are the meteors that at times reach the earth. However, it may be noted that the material that becomes available for analysis from meteors, not from the interior of the earth but the material and the structure observed in the meteors are similar to that of the earth. They are solid bodies developed out of materials same as, or similar to, our planet.
Earthquake
An earthquake in simple words is shaking of the earth. It is caused due to release of energy, which generates waves that travel in all directions.
Why Does The Earth Shake?
The release of energy occurs along the fault line. Rocks along the fault tend to move in opposite directions as the overlying strata press them the friction locks them together. However, the tendency of movement overcome the friction. As a result, blocks get deformed so they slide over another, as a result energy releases.
Energy waves travel in all directions. The point where energy released is called focus of an earthquake or hypocenter. Above the focus point on the surface it is called epicenter
Earthquake Waves
All natural earthquakes take place in the lithosphere (depth up to 200 km from the surface of the earth). An instrument called ‘seismograph’ records the waves reaching the surface. Earthquake waves are basically of two types:
i). Body waves : Body waves are generated due to the release of energy at the focus and move in all directions travelling through the body of the earth.
ii). Surface waves : The body waves interact with the surface rocks and generate a new set of waves called surface waves. These waves move along the surface.
Body Waves : Body Waves are of two types : they are P-Waves and S-Waves.
a). P-Waves: These waves move faster and are the first to arrive at the surface and are also called ‘primary waves’. These are similar to sound waves and travel through gaseous, liquid and solid materials as sound P-waves vibrate parallel to the direction of the wave.
b). S-Waves: These waves arrive at the surface with some time lag and are called secondary waves. These waves can travel only through solid materials which has helped scientists to understand the structure of the interior of the earth. Reflection causes waves to rebound whereas refraction makes waves move in different direction. These waves are more destructive, as they cause displacement of rocks, and hence, the collapse structures occur. The direction of vibrations of S-waves is perpendicular to the wave direction in the vertical plane. Hence, they create troughs and crests in the material through which they pass.
Emergence Of Shadow Zone
i). Earthquake waves get recorded in seismo-graphs located at far off locations. However, there exist some specific areas where the waves are not reported. Such a zone is called the ‘shadow zone’.
ii). A zone between 105° and 145° from epicenter was identified as the shadow zone for both the types of waves.
iii). The entire zone beyond 105° does not receive S-waves. The shadow zone of S-wave is much larger than that of the P-waves.
iv). The shadow zone of P-waves appears as a band around the earth between 105° and 145° away from the epicenter.
v). The shadow zone of S-waves is not only larger in extent but it is also a little over 40 percent of the earth surface.
Types of Earthquakes
I). Tectonic Earthquake : The tectonic earthquakes are generated due to sliding of rocks along a fault plane.
II). Volcanic Earthquakes : A special class of tectonic earthquakes. These are confined to areas of active volcanoes.
III). Collapse Earthquakes : In the areas of intense mining activity, sometimes the roofs of underground mines collapse causing minor tremors.
IV). Explosion Earthquakes: Ground shaking may also occur due to the explosion of chemical or nuclear devices.
V). Reservoir induced Earthquakes: The earthquakes that occur in the areas of large reservoirs.
Measuring Earthquakes
i) Richter Scale: It is a scale which measures the magnitude of an earthquake. Generally, it is from 0 to 10. An earthquake measuring 6 on the Richter Scale is 10 times more stronger than 5 and 100 times more stronger than 4.
ii) Mercalli Scale: It was developed by an Italian seismologist. It measures the destruction caused by an earthquake. It ranges from 1 to 12.
Effects Of Earthquake
i) Earthquakes are a natural hazard. The following are the immediate hazardous effects of earthquake:
ii) Ground Shaking, Differential ground settlement, Land and mudslides, Soil liquefaction, Ground lurching and Avalanches Have some bearings upon landforms.
iii). While others like Ground displacement, Floods from dam and levee failures, Fires, Structural collapse, Falling objects and Tsunami may be considered the effects causing immediate concern to the life and properties of people in the region. (The effect of a tsunami would occur only if the epicentre of the tremor is below oceanic waters and the magnitude is sufficiently high).
iv). Frequency of Earthquake Occurrences: If a tremor of high magnitude takes place, it can cause heavy damage to the life and property of people. However, not all the parts of the globe necessarily experience major shocks. The quakes of high magnitude, i.e. 8+ are quite rare, they occur once in 1-2 years whereas those of ‘tiny’ types occur almost every minute.
Structure Of The Earth
The Crust ( sub heading) - It is the outermost solid part of the earth, It is brittle in nature. The thickness of the crust varies under the oceanic and continental areas. Oceanic crust is thin as compared to the continental crust. The continental crust is thicker in the areas of major mountain systems. The heavier rocks having a density of 3 g/cm3, This type of rock found in the oceanic crust is basalt. The mean density of material in the oceanic crust is 2.7 g/cm3.
The Mantle - The portion of the interior beyond the crust is called the mantle. The mantle extends from Moho’s discontinuity to a depth of 2,900 km. The upper portion of the mantle is called the asthenosphere ( weak). It is considered to be extending up to 400 km. It is the main source of magma that finds its way to the surface during volcanic eruptions. It has a density higher than the crust (3.4 g/cm3). The crust and the uppermost part of the mantle are called lithosphere. Its thickness ranges from 10-200 km. The lower mantle extends beyond the asthenosphere. It is in solid state.
The Core - The core-mantle boundary is located at the depth of 2,900 km. The outer core is in liquid state while the inner core is in solid state. The density of material at the mantle core boundary is around 5 g/cm33. The core is made up of very heavy material mostly constituted by nickel and iron. It is sometimes referred to as the nife layer.
Volcanoes And Volcanic Eruption
A volcano is a place where gases, ashes and/or molten rock material – lava – escape to the ground. A volcano is called an active volcano, if the materials mentioned are being released or have been released out in the recent past.
Volcanoes are classified on the basis of: i). Nature of Eruption. ii). Form developed at the surface.
The molten rock materials find their way to the surface from the Asthenosphere. The material in the upper mantle portion is called magma. Once it starts moving towards the crust or it reaches the surface, it is referred to as lava.
Types Of Volcanoes
Shield Volcanoes : Barring the basalt flows, the shield volcanoes are the largest of all the volcanoes on the earth. The Hawaiian volcanoes are the most famous examples. These volcanoes are mostly made up of basalt, a type of lava that is very fluid when erupted. For this reason, these volcanoes are not steep. They become explosive if somehow water gets into the vent; otherwise, they are characterized by low-explosivity. The upcoming lava moves in the form of a fountain and throws out the cone at the top of the vent and develops into a cinder cone.
Composite Volcanoes : These volcanoes are characterized by eruptions of cooler and more viscous lavas than basalt. These volcanoesoen result in explosive eruptions. Along with lava, large quantities of pyroclastic material and ashes find their way to the ground. This material accumulates in the vicinity of the vent openings leading to formation of layers, and this makes the mounts appear as composite volcanoes.
Caldera : These are the most explosive of the earth’s volcanoes because when they erupt they tend to collapse on themselves rather than building any tall structure. The collapsed depressions are called calderas. Their explosiveness indicates that the magma chamber supplying the lava is not only huge but is also in close vicinity.
Flood Basalt Provinces : These volcanoes outpour highly fluid lava that flows for long distances. Some parts of the world are covered by thousands of sq. km of thick basalt lava flows. There can be series of flows attaining thickness of more than 50 m. Individual flows may extend for hundreds of km. The Deccan Traps from India, presently covering most of the Maharashtra plateau, are a much larger flood basalt province.
Mid-Oceanic Ridge Volcanoes : These volcanoes occur in the oceanic areas. There is a system of mid-ocean ridges more than 70,000 km long that stretches through all the ocean basins. The central portion of this ridge experiences frequent eruptions.
Volcanic Landforms
Intrusive Forms :
The lava that is released during volcanic eruptions on cooling develops into igneous rocks. The cooling may take place either on reaching the surface or also while the lava is still in the crustal portion. Depending on the location of the cooling of the lava, igneous rocks are classified as volcanic rocks (cooling at the surface) and plutonic rocks (cooling in the crust). The lava that cools within the crustal portions assumes different forms and these forms are called intrusive forms. Some of the intrinsic forms are:
Batholiths : Batholiths are the cooled portion of magma chambers. They appear on the surface only after the denudational processes remove the overlying material. They cover large areas, and at times, assume depth that may be several km. These are granitic bodies.
Laccoliths : These are large dome-shaped intrusive bodies with a level base and connected by a pipe-like conduit from below. It can be regarded as the localized source of lava that finds its way to the surface.
Lopolith, Photolith and Sills : When the lava moves upwards, a portion of the same may tend to move in a horizontal direction wherever it finds a weak plane. It may get rested in different forms. In case it develops into a saucer shape, concave to the sky body, it is called lopolith. A wavy mass of intrusive rocks, at times, is found at the base of synclines or at the top of anticline in folded igneous country. Such wavy materials have a definite conduit to source beneath in the form of magma chambers (subsequently developed as batholiths). These are called the photoliths. The near horizontal bodies of the intrusive igneous rocks are called sill or sheet, depending on the thickness of the material. The thinner ones are called sheets while the thick horizontal deposits are called sills.
Dykes : When the lava makes its way through cracks and the fissures dev eloped in the land, it solidifies almost perpendicular to the ground. It gets cooled in the same position to develop a wall-like structure. Such structures are called dykes. These are the most commonly found intrusive forms in the western Maharashtra area (Deccan Traps).
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