Latest Geography NCERT Notes, Solutions and Extra Q & A (Class 8th to 12th)
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Class 11th Chapters
Fundamentals of Physical Geography
1. Geography As A Discipline	2. The Origin And Evolution Of The Earth	3. Interior Of The Earth
4. Distribution Of Oceans And Continents	5. Geomorphic Processes	6. Landforms And Their Evolution
7. Composition And Structure Of Atmosphere	8. Solar Radiation, Heat Balance And Temperature	9. Atmospheric Circulation And Weather Systems
10. Water In The Atmosphere	11. World Climate And Climate Change	12. Water (Oceans)
13. Movements Of Ocean Water	14. Biodiversity And Conservation
Indian Physical Environment
1. India — Location	2. Structure And Physiography	3. Drainage System
4. Climate	5. Natural Vegetation	6. Natural Hazards And Disasters: Causes, - Consequences And Management
Practical Work In Geography
1. Introduction To Maps	2. Map Scale	3. Latitude, Longitude And Time
4. Map Projections	5. Topographical Maps	6. Introduction To Remote Sensing

Chapter 3 Interior Of The Earth

When we think about the Earth, do we picture it as a solid ball, or perhaps something with a hollow inside beneath a rocky crust? Visuals of volcanic eruptions, showing molten lava, ash, smoke, and fire emerging from deep within, give us a hint that the Earth's interior is anything but uniform and static.

Directly reaching the Earth's core, with a radius of about 6,378 km, is impossible. Therefore, our understanding of what lies beneath the surface relies entirely on **indirect evidence**.

The shape and features of the Earth's surface (its physiography) are significantly influenced by powerful processes originating from within the planet (endogenic processes), as well as those acting on the surface (exogenic processes). To fully grasp regional landscapes and phenomena like earthquakes or tsunamis, we need to understand the forces and materials within the Earth.

As discussed in the previous chapter, Earth materials are arranged in distinct layers from the crust to the core. This chapter explains how scientists have uncovered information about these layers and their characteristics.

Sources Of Information About The Interior

Given the impossibility of direct exploration into the deep Earth, scientists primarily rely on various sources, both direct and indirect, to gather information about its composition and structure.

Direct Sources

Direct methods involve obtaining and analyzing actual material samples from the Earth's crust.

The most accessible direct source is **surface rocks** and materials obtained from **mining**. However, mining depth is severely limited by rapidly increasing temperature. For example, gold mines in South Africa reach depths of only 3-4 km before the heat becomes prohibitive for human activity.

To overcome these limitations, scientists globally have undertaken ambitious drilling projects. Notable examples include the "Deep Ocean Drilling Project" and the "Integrated Ocean Drilling Project". The deepest such borehole, located at Kola in the Arctic Ocean, penetrated to a depth of approximately **12 km**. Analysis of the materials collected from these depths provides valuable insights into the upper crust.

**Volcanic eruptions** offer another direct source. Molten rock material (called **magma** beneath the surface and **lava** once it erupts onto the surface) is brought up from the interior during eruptions. Collecting and analyzing this erupted material provides information about the composition of the source region. However, pinpointing the exact depth from which this magma originates can be challenging.

Indirect Sources

Indirect sources involve inferring information about the interior by studying the properties of matter or phenomena influenced by internal conditions.

Observations from mining activities reveal that both **temperature** and **pressure** increase significantly with depth inside the Earth. Additionally, the **density** of Earth materials also generally increases deeper down. Scientists can estimate the rate at which these characteristics change with depth.

Knowing the Earth's total radius (approx. 6,378 km) and the relationships between depth, temperature, pressure, and density allows scientists to estimate the conditions and material properties at various depths, even far beyond the reach of drilling or mining.

**Meteors** that fall to Earth provide another indirect source. While their material doesn't come from Earth's interior, meteors are rocky or metallic bodies formed in the early solar system from material similar to that which formed Earth. Studying their composition and structure offers clues about the potential composition of Earth's deep interior, particularly the core.

Other significant indirect methods include studies of **gravitational force**, Earth's **magnetic field**, and most importantly, **seismic activity** (earthquakes and the waves they generate).

The force of gravity (g) varies slightly across the Earth's surface, being higher near the poles than at the equator due to the Earth's shape (bulging at the equator). Local variations in gravity values, known as **gravity anomalies**, indicate uneven distribution of mass within the Earth's crust. Measuring these anomalies helps geophysicists understand subsurface density variations.

**Magnetic surveys** involve mapping the variations in Earth's magnetic field across the surface. These variations can reveal information about the distribution of magnetic minerals within the crustal rocks.

However, the most crucial indirect source for understanding the Earth's layered structure is the study of **seismic waves** generated by earthquakes. This method provides the most comprehensive picture of the deep interior.

Earthquake

An earthquake is essentially a sudden shaking or trembling of the ground. It is a natural event resulting from the release of enormous amounts of energy accumulated within the Earth's lithosphere. This energy travels outward in all directions in the form of waves.

Earthquakes occur when stress builds up along a **fault**, which is a fracture or zone of weakness in the Earth's crustal rocks. Rocks on either side of a fault attempt to move past each other, but friction often locks them in place. As tectonic forces continue to apply stress, the rocks deform. Eventually, the stress exceeds the frictional resistance, causing the rocks to suddenly slip and slide past each other. This abrupt movement releases stored energy as seismic waves.

The point within the Earth's crust where the energy is released and the rupture originates is called the **focus** or **hypocentre** of the earthquake.

The point on the Earth's surface located directly above the focus is called the **epicentre**. This is typically the area where the earthquake's effects are felt most strongly and where the seismic waves arrive first.

Earthquake Waves

Natural earthquakes primarily occur within the **lithosphere**, which is the rigid outer shell of the Earth, extending roughly up to a depth of 200 km. The energy released at the focus generates different types of seismic waves.

These waves are recorded by an instrument called a **seismograph**. The recording produced is called a seismogram, which shows the different wave patterns arriving over time. (This refers to Figure 3.1).

Diagram showing a seismograph recording with distinct wave patterns arriving at different times, representing P-waves, S-waves, and Surface waves.

Earthquake waves are categorized into two main types:

Body Waves: These waves are generated at the earthquake's focus and travel through the interior (the "body") of the Earth in all directions.
Surface Waves: These waves are generated when body waves interact with the Earth's surface. They travel along the surface and are typically the slowest but cause the most significant ground shaking and damage.

Body waves are further divided into two types based on their motion and speed:

P-waves (Primary Waves): These are the fastest seismic waves and are the first to arrive at a seismograph. P-waves are **compressional** waves, meaning they cause the particles of the material they pass through to vibrate back and forth parallel to the direction the wave is traveling, similar to sound waves. A key characteristic is that **P-waves can travel through solid, liquid, and gaseous materials**.
S-waves (Secondary Waves): These waves are slower than P-waves and arrive second at a seismograph. S-waves are **shear** waves, causing particles to vibrate perpendicular to the direction of wave propagation, creating a shearing or side-to-side motion. A crucial property of S-waves is that **they can only travel through solid materials**, not liquids or gases.

This inability of S-waves to pass through liquids has been fundamental in determining that a significant part of Earth's interior is liquid.

As seismic waves travel through materials of varying densities, their **velocity changes**. Generally, wave speed increases with increasing material density. Also, when waves encounter boundaries between materials with different densities, they can be **reflected** (bouncing back) or **refracted** (bending as they pass through). Seismograph recordings and the analysis of these changes in velocity and direction provide indirect information about the layers within the Earth.

Propagation Of Earthquake Waves

The way earthquake waves travel (propagate) causes different kinds of vibrations in the rocks they pass through.

P-waves vibrate parallel to their direction of movement. This creates alternating areas of compression (squeezing) and rarefaction (stretching) in the material, causing density variations. The velocity of P-waves in a material depends on its bulk modulus ($\kappa$), shear modulus ($\mu$), and density ($\rho$) according to the formula $v_p = \sqrt{(\kappa + \frac{4}{3}\mu)/\rho}$.

S-waves vibrate perpendicular to their direction of movement. In a vertical plane, this creates up-and-down motion, forming crests and troughs. The velocity of S-waves depends only on the material's shear modulus and density: $v_s = \sqrt{\mu/\rho}$. Since liquids and gases have a shear modulus of zero ($\mu=0$), S-waves cannot propagate through them.

Surface waves cause complex ground motion (rolling and side-to-side) and are generally the slowest waves but are the most damaging because they cause the greatest displacement of the ground, leading to structural collapse.

Emergence Of Shadow Zone

Seismic waves from an earthquake are recorded by seismographs across the globe. However, scientists observed that certain areas on the Earth's surface do not receive specific types of seismic waves. These areas are called **shadow zones**.

The existence and characteristics of these shadow zones have been crucial in revealing the internal structure of the Earth, particularly the presence of a core with different properties from the mantle.

For P-waves and S-waves, specific shadow zones are observed (This refers to Figure 3.2 (a) and (b)).

$Diagram showing the Earth's interior layers (crust, mantle, outer core, inner core) and the paths of P and S waves originating from an epicentre. The diagram illustrates how S waves are blocked by the liquid outer core, creating a large shadow zone, while P waves are refracted, creating a smaller shadow zone band.$

Seismographs located within 105° from the earthquake's epicentre typically record both P and S waves.

Beyond 145° from the epicentre, seismographs record P-waves, but no S-waves are detected. This indicates that there is a large zone from 105° onwards that S-waves cannot penetrate. This is because S-waves cannot travel through the liquid **outer core** of the Earth, which lies beneath the mantle.

For P-waves, a specific zone exists between 105° and 145° from the epicentre where they are not recorded. This P-wave shadow zone is a result of P-waves being significantly **refracted** (bent) as they enter and exit the Earth's core due to the sudden change in material properties (density, rigidity) between the mantle and the outer core.

Thus, the area between 105° and 145° from the epicentre is a shadow zone for both P and S waves. The entire region beyond 105° does not receive S-waves, making the S-wave shadow zone much larger (covering over 40% of the Earth's surface) than the P-wave shadow zone, which appears as a band between 105° and 145°.

Types Of Earthquakes

Earthquakes can be caused by various phenomena, leading to different classifications:

Tectonic Earthquakes: These are the most frequent type. They are caused by the movement and sliding of large blocks of the Earth's crust (tectonic plates) along fault lines.
Volcanic Earthquakes: These are seismic events associated with volcanic activity. They are caused by the movement of magma beneath the surface or explosions within the volcano. They are confined to areas near active volcanoes.
Collapse Earthquakes: Minor tremors can occur in areas of intense mining when the roofs of underground mines collapse.
Explosion Earthquakes: These are caused by artificial explosions, such as the testing of chemical or nuclear devices.
Reservoir-Induced Earthquakes: Earthquakes can sometimes be triggered in areas where large reservoirs are created behind dams. The immense weight of the water can increase stress on the underlying rock, leading to seismic activity.

Measuring Earthquakes

Earthquakes are quantified using scales that measure either their energy release or the observed effects:

Magnitude Scale (Richter Scale): Developed by Charles Richter, this scale measures the **magnitude** of an earthquake, which is related to the amount of energy released at the focus. It is a logarithmic scale, typically expressed in numbers from 0 to 10 (though theoretically higher values are possible). Each whole number increase on the Richter scale represents a tenfold increase in wave amplitude and roughly 32 times more energy released.
Intensity Scale (Mercalli Scale): Named after Italian seismologist Giuseppe Mercalli, this scale measures the **intensity** of shaking and the visible damage caused by an earthquake at a specific location. It is based on observed effects on people, buildings, and the natural environment. The scale ranges from I (not felt) to XII (catastrophic destruction).

Effects Of Earthquake

Earthquakes are significant natural hazards with a wide range of immediate and potentially devastating effects:

Immediate hazardous effects include:

Ground Shaking: The primary and most widespread effect, causing vibrations that damage and collapse structures.
Differential Ground Settlement: Uneven sinking of the ground, which can severely damage buildings and infrastructure.
Land and Mud Slides: Shaking can trigger landslides and mudslides on unstable slopes.
Soil Liquefaction: In saturated sandy soils, shaking can cause the soil to lose its strength and behave like a liquid, leading to building collapse.
Ground Lurching: Wavelike motion of the ground surface.
Avalanches: Earthquakes can trigger avalanches in snowy or icy mountainous regions.
Ground Displacement: Visible offsets or cracks in the ground surface along fault lines.
Floods from Dam and Levee Failures: Damage to dams or protective levees can lead to severe flooding.
Fires: Earthquakes can rupture gas lines and electrical wires, leading to widespread fires.
Structural Collapse: Buildings, bridges, and other structures can fail due to intense shaking.
Falling Objects: Non-structural elements, furniture, and other objects can fall and cause injury or damage.
Tsunami: Large ocean waves generated by underwater earthquakes, volcanic eruptions, or landslides.

Some of these effects (like ground shaking, settlement, slides, liquefaction, lurching, avalanches, displacement) directly involve changes to the landforms, while others (like floods, fires, structural collapse, falling objects, tsunami) pose immediate threats to human life and property.

A **tsunami** is specifically a series of large waves in a water body (usually the ocean) caused by the displacement of a large volume of water. While often triggered by underwater earthquakes, it is the wave itself, generated by the tremor's displacement of the seafloor, not the earthquake shaking felt on land, that constitutes the tsunami hazard. Tsunamis occur when an earthquake's epicentre is beneath the ocean and its magnitude is sufficiently high to cause significant seafloor movement.

Although the actual shaking of an earthquake might only last seconds, its consequences can be devastating, particularly for quakes with magnitudes above 5 on the Richter scale.

Frequency Of Earthquake Occurrences

Earthquakes are a continuous process, but their intensity and frequency vary greatly across the globe. While major, highly destructive earthquakes (magnitude 8+) are relatively rare, occurring perhaps once or twice a year globally, smaller tremors happen almost constantly, sometimes multiple times a minute.

The distribution of earthquake activity is not uniform and is largely concentrated along specific zones related to tectonic plate boundaries, which will be discussed in detail in the next chapter.

Structure Of The Earth

Based largely on the analysis of seismic waves and other indirect evidence, scientists have determined that the Earth is composed of several concentric layers. (This refers to Figure 3.3)

Cross-section diagram of the Earth showing the different layers: Crust, Mantle (Upper and Lower), Outer Core, and Inner Core. Also labels Asthenosphere and Lithosphere.

The Crust

The crust is the **outermost solid shell** of the Earth. It is relatively thin and **brittle** compared to the layers below. Its thickness is not uniform; it varies significantly between oceanic and continental areas.

The **oceanic crust** is typically thinner, with an average thickness of around **5 km**. It is primarily composed of denser basaltic rocks.

The **continental crust** is considerably thicker, averaging about **30 km**. It is made of less dense granitic rocks. Beneath major mountain ranges, the continental crust can be exceptionally thick, reaching depths of up to **70 km**, such as under the Himalayas.

The Mantle

Located directly beneath the crust is the **mantle**, which extends from the base of the crust (marked by the Mohorovičić discontinuity, or Moho) down to a depth of approximately **2,900 km**.

The **upper portion of the mantle**, extending roughly up to 400 km depth, is called the **asthenosphere**. The word "astheno" means weak. This layer is partially molten or highly viscous, behaving plastically. It is considered the primary source of **magma** that fuels volcanic eruptions.

The rigid outer part of the Earth, which includes the **crust** and the **uppermost solid part of the mantle**, is collectively called the **lithosphere**. Its thickness varies from about 10 km in oceanic areas to up to 200 km or more beneath continents.

Below the asthenosphere lies the **lower mantle**, which extends from about 400 km down to 2,900 km. Despite high temperatures, the immense pressure at these depths keeps the lower mantle in a **solid state**.

The Core

The Earth's innermost layer is the **core**, situated below the mantle at a depth of 2,900 km and extending to the center of the Earth (approx. 6,371 km). The existence and properties of the core were deduced from the behavior of seismic waves, particularly the S-wave shadow zone.

The core is divided into two parts:

The **outer core**, from 2,900 km to about 5,100 km depth, is in a **liquid state**. This is confirmed by the fact that S-waves cannot travel through it. Convective currents within the liquid outer core are believed to generate Earth's magnetic field.
The **inner core**, from about 5,100 km to the center, is in a **solid state**. Although temperatures are extremely high, the even greater pressure at the very center keeps the material solid.

The core is thought to be composed of very dense, heavy materials, primarily **Nickel (Ni)** and **Iron (Fe)**. For this reason, it is sometimes referred to as the **NIFE** layer.

Volcanoes And Volcanic Landforms

A **volcano** is a geological feature where molten rock, ash, gases, and other materials erupt from the Earth's interior onto the surface. A volcano is considered **active** if it is currently erupting or has erupted in the recent past.

The source of the material erupted by volcanoes is the hot, partially molten rock found in the upper mantle, specifically the **asthenosphere**. This material is called **magma** while it is beneath the surface. When magma rises towards or reaches the surface, it is then called **lava**. The substances ejected during an eruption can include lava flows, fragmented rocky material (pyroclastic debris), volcanic bombs, ash, dust, and various gases such as compounds of nitrogen and sulphur, as well as chlorine, hydrogen, and argon.

Volcanoes

Volcanoes are classified based on the nature of their eruptions (e.g., effusive or explosive) and the resulting shape or form of the volcanic structure on the surface.

Shield Volcanoes

Excluding extensive basalt flows, **shield volcanoes** are the largest type of volcanoes on Earth. They are typically built up by repeated eruptions of highly fluid basaltic lava. Because the lava is very fluid, it flows long distances before solidifying, creating broad, gently sloping structures that resemble a warrior's shield lying on the ground. Hawaiian volcanoes are classic examples.

Shield volcanoes are generally characterized by **low explosivity**, although they can become explosive if water interacts with the magma chamber. Lava often erupts in fountains, which can build small cones of fragmented lava (cinder cones) around the vent. (Related to Figure 3.3 example).

Composite Volcanoes

Also known as stratovolcanoes, **composite volcanoes** are characterized by eruptions of lava that is cooler and more viscous (thicker) than basaltic lava. This higher viscosity prevents the lava from flowing far, resulting in steeper slopes. The eruptions are often **explosive**, ejecting significant amounts of pyroclastic material (ash, bombs, pumice) along with lava.

Composite cones are built up by alternating layers of hardened lava flows and pyroclastic deposits, giving them a layered or "composite" structure. These volcanoes typically have a conical shape with a distinct crater at the summit. (Related to Figure 3.3 example).

Caldera

**Calderas** are among the most explosive volcanic features. Instead of building tall cones, their eruptions are so powerful that the volcano's summit or even the entire structure collapses inward into the emptied magma chamber, forming a large, bowl-shaped depression called a caldera. Their extreme explosiveness suggests a very large magma reservoir located relatively close to the surface.

Flood Basalt Provinces

These features result from massive eruptions of extremely fluid basaltic lava from fissures or cracks in the Earth's crust, rather than from a central vent. The lava flows are so extensive and voluminous that they can cover thousands of square kilometers, forming thick layers of basalt. Individual flows can spread hundreds of kilometers, and successive flows build up vast, flat-topped plateaus. The **Deccan Traps** in India, covering much of the Maharashtra plateau, are a prime example of a large flood basalt province.

Mid-Ocean Ridge Volcanoes

These volcanoes are found along the extensive underwater mountain ranges called **mid-ocean ridges** that crisscross the global ocean basins, stretching for over 70,000 km. Volcanic activity and frequent eruptions occur along the central rift valleys of these ridges as new oceanic crust is created. These eruptions are typically effusive (non-explosive) and contribute to seafloor spreading.

Volcanic Landforms

When molten rock cools and solidifies, it forms **igneous rocks**. This cooling can happen on the Earth's surface after eruption (forming extrusive or volcanic rocks) or within the Earth's crust before reaching the surface (forming intrusive or plutonic rocks).

Intrusive Forms

The magma that cools and solidifies **within** the Earth's crust creates various geological structures called **intrusive forms**. These forms are exposed at the surface only after the overlying rock layers are removed by erosion. (This refers to Figure 3.4).

Diagram illustrating various intrusive volcanic landforms: Batholith, Laccolith, Sill, Dyke, Lapolith, Phacolith.

Batholiths

**Batholiths** are very large, irregularly shaped masses of intrusive igneous rock that cool deep within the crust. They are the cooled remnants of large magma chambers. Batholiths are typically composed of granitic rock. They are revealed at the surface only after significant erosion removes the overlying rocks. They can cover vast areas and extend several kilometers deep.

Lacoliths

A **laccolith** is a mushroom-shaped intrusive body. It is a large, dome-shaped mass with a relatively flat base, connected to a deeper magma source by a narrow conduit or pipe. Magma intrudes between rock layers and pushes the overlying strata upwards into a dome. Laccoliths can resemble small composite volcanoes on the surface if the overlying rock is eroded away, but they solidified underground. The domal granite hills seen on the Karnataka plateau are often cited as examples of laccoliths (or batholiths) where the covering rock has been stripped away by exfoliation.

Lapolith, Phacolith And Sills

When magma moves upwards, it may sometimes spread out horizontally along existing planes of weakness within the rock layers.

If the intrusive body cools into a saucer-like shape, concave upwards, it is called a **lapolith**.
A **phacolith** is a lens-shaped or wavy intrusive body found in folded rock layers. It typically occupies the crest of an anticline (upward fold) or the trough of a syncline (downward fold). These bodies also connect to deeper magma sources.
**Sills** and **sheets** are horizontal or near-horizontal intrusive bodies formed when magma intrudes between existing rock layers and solidifies as a layer. A sill is generally thicker than a sheet.

Dykes

A **dyke** (or dike) is a vertical or near-vertical sheet of intrusive igneous rock. They are formed when magma rises through cracks or fissures in the host rock and solidifies in place. Dykes cut across the existing rock layers. They are very common intrusive forms, for instance, in the western Maharashtra region, where they are believed to have acted as conduits or "feeders" for the massive flood basalt eruptions that created the Deccan Traps.