Interior of the Earth Class 11 Geography

There are two sources for information about the interior of the earth:
a) Direct Sources
b) Indirect Sources

Direct Sources

• Surface rock or the rocks we get from mining areas. Gold mines in South Africa reach depths of 3-4 km.
• Scientists are working on major projects such as the "Deep Ocean Drilling Project" and the "Integrated Ocean Drilling Project."
• The deepest drill at Kola in the Arctic Ocean has reached 12 km.
• Volcanic eruptions provide direct information as magma is available for laboratory analysis.
• These projects and natural events have provided a large volume of information through the analysis of materials collected at different depth

• Mining reveals that temperature, pressure, and material density increase with depth.
• Scientists estimate temperature, pressure, and density at various depths using the total thickness of the Earth.
• Meteors provide indirect information, as their materials and structures are similar to Earth's.
• Indirect sources include gravitation, magnetic field, and seismic activity.
• Gravitation varies with latitude, being greater at the poles and less at the equator due to differences in distance from the center.
• Gravity values differ according to the mass of the material and other factors, resulting in gravity anomalies that reveal mass distribution in the Earth's crust.
• Magnetic surveys provide information about the distribution of magnetic materials in the crust.
• Seismic activity offers crucial insights into Earth's interior.

Why does the Earth Shake?

 P-waves: Move faster and are the first to arrive at the surface. These are also called ‘primary waves’.

• The P-waves are similar to sound waves.
• They travel through gaseous, liquid, and solid materials (as sound).
• P-waves vibrate parallel to the direction of the wave (P for parallel).
• This exerts pressure on the material in the direction of the propagation.
• As a result, it creates density differences in the material leading to stretching and squeezing of the material

S-waves: Arrive at the surface with some time lag. These are called secondary waves.

• An important fact about S-waves is that they can travel only through solid materials. (important)
• It 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 directions.
• These waves are more destructive. They cause the displacement of rocks, and hence, the collapse of structures occurs.
• 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.

Surface Waves: 

• The surface waves are the last to report on seismograph. These waves are more destructive. 
• They cause displacement of rocks, and hence, the collapse of structures occurs

• 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’.
• A zone between 105^° and 145^° from epicenter was identified as the shadow zone for both types of waves.
• 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.
• The shadow zone of P-waves appears as a band around the earth between 105°and 145° away from the epicenter

Types of Earthquakes

• The most common ones are tectonic earthquakes. These are generated due to sliding of rocks along a fault plane.
• A special class of tectonic earthquakes is sometimes recognized as volcanic earthquakes. However, these are confined to areas of active volcanoes.
• In the areas of intense mining activity, sometimes the roofs of underground mines collapse causing minor tremors. These are called collapse earthquakes.
• Ground shaking may also occur due to the explosion of chemical or nuclear devices. Such tremors are called explosion earthquakes.
• The earthquakes that occur in the areas of large reservoirs are referred to as reservoir induced earthquakes

Measuring Earthquakes

• The earthquake events are scaled either according to the magnitude or intensity of the shock.
• The magnitude scale is known as the Richter scale.
• The magnitude relates to the energy released during the quake.
• The magnitude is expressed in absolute numbers, 0-10.
• The intensity scale is named after Mercalli, an Italian seismologist.
• The intensity scale takes into account the visible damage caused by the event.
• The range of intensity scale is from 1-12.

Effects of Earthquake

Earthquake is a natural hazard. The following are the immediate hazardous effects of earthquake:

• Ground Shaking
• Differential ground settlement
• Land and mud slides
• Soil liquefaction
• Ground lurching
• Avalanches
• Ground displacement
• Floods from dam and levee failures
• Fires
• Structural collapse
• Falling objects
• Tsunami

The first six listed above have some bearings upon landforms, while others may be considered the effects causing immediate concern to the life and properties of people in the region.

The effect of tsunami would occur only if the epicenter of the tremor is below oceanic waters and the magnitude is sufficiently high. Tsunamis are waves generated by tremors and not an earthquake in itself.

Though the actual quake activity lasts for a few seconds, its effects are devastating provided the magnitude of the quake is more than 5 on the Richter scale. 

Structure of the Earth

• 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 thinner as compared to the continental crust.
• The mean thickness of the oceanic crust is 5 km whereas that of the continental is around 30 km.
• It is as much as 70 km thick in the Himalayan region.

• 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  asthenosphere.
• The word astheno means 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’s (3.4 g/cm³).
• The crust and the uppermost part of the mantle are called the 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 a liquid state while the inner core is in solid-state.
• 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 Landforms

• These volcanoes are characterized by eruptions of cooler and more viscous lavas than basalt.
• These volcanoes often 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 the formation of layers, and this makes the mounts appear as composite volcanoes. 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.

Caldera

• These are the most explosive of the earth’s volcanoes.
• They are usually so explosive that 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 a series of flows with some flows attaining a thickness.

Mid-Ocean Ridge Volcanoes

• These volcanoes occur in 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

• 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).

Batholiths

• A large body of magmatic material that cools in the deeper depth of the crust develops in the form of large domes.
• They appear on the surface only after the denudational processes remove the overlying materials.
• They cover large areas, and at times, assume depth that may be several km. These are granitic bodies.
• Batholiths are the cooled portion of magma chambers.

Lacoliths

• These are large dome-shaped intrusive bodies with a level base and connected by a pipe-like conduit from below.
• It resembles the surface volcanic domes of the composite volcanoes, only these are located at deeper depths.
• It can be regarded as the localized source of lava that finds its way to the surface.
• The Karnataka plateau is spotted with domal hills of granite rocks.
• Most of these, now exfoliated, are examples of laccoliths or batholiths.

Lopolith, Phacolith, and Sills

• As and 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 the 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 phacoliths.
• 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 developed 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.
• These are considered the feeders for the eruptions that led to the development of the Deccan traps.