Chapter 3 Interior Of The Earth

Interior Of The Earth

Understanding the Earth's interior is fundamental to comprehending many surface phenomena, such as volcanic eruptions and earthquakes. The configuration of the Earth's surface, its varied landscapes, is significantly influenced by processes originating deep inside the planet (endogenic processes).

Although direct observation is impossible due to the extreme depth and conditions, scientists have developed methods to infer the Earth's internal structure and composition. The Earth's radius is approximately 6,370 kilometers ($6,370 \, km$), making it impossible for humans to physically reach the center.

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Sources Of Information About The Interior

Information about the Earth's deep interior comes from both direct and indirect sources.

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Direct Sources

Direct sources provide actual material or direct observations from within the Earth's crust:

• Surface Rocks and Mining: The most accessible material is found at the Earth's surface or obtained through mining operations. Mines, such as gold mines in South Africa, can reach depths of 3-4 km. However, increasing temperature with depth limits further direct exploration this way.
• Deep Drilling Projects: Scientists undertake projects to drill deep boreholes into the Earth's crust. Notable examples include the "Deep Ocean Drilling Project" and "Integrated Ocean Drilling Project". The Kola Superdeep Borehole in the Arctic reached a depth of about 12 km. Analysis of rock samples retrieved from these depths provides valuable data about the crust's composition and conditions.
• Volcanic Eruptions: During volcanic eruptions, molten material (magma) from the interior rises to the surface as lava. This lava, ash, and other ejected materials can be collected and analyzed in laboratories. While this provides direct samples of material from within the Earth, determining the exact depth of the magma's origin is challenging.

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Indirect Sources

Indirect sources involve analyzing physical properties and phenomena on the surface to infer conditions and composition deep inside the Earth:

• Temperature, Pressure, and Density Increase with Depth: Observations from mining and drilling show that temperature and pressure consistently increase as one goes deeper into the Earth. Similarly, the density of Earth materials also increases with depth. By estimating the rate of change for these properties and knowing the Earth's overall size, scientists can infer their values at different depths.
• Meteors: Meteorites, which are fragments of asteroids or comets that fall to Earth, are considered potential sources of information. Scientists believe that the material composition and structure of some meteorites are similar to the primitive material from which Earth formed and possibly resemble the Earth's core. Studying meteorites provides clues about the composition of planetary interiors.
• Gravitation: The Earth's gravitational force (g) varies slightly across its surface. This variation is due to factors like latitude (Earth's shape causes poles to be closer to the center than the equator) and, more importantly, the uneven distribution of mass within the Earth. Differences between observed gravity values and expected values are called gravity anomalies. Positive or negative gravity anomalies can indicate concentrations or deficiencies of mass within the crust, providing insights into subsurface structures and material distribution.
• Magnetic Field: Magnetic surveys measure the Earth's magnetic field at different locations. Variations in the magnetic field can be related to the distribution of magnetic materials within the Earth's crust and upper mantle, giving clues about the composition of these layers.
• Seismic Activity (Earthquake Waves): This is one of the most crucial indirect sources. Earthquakes generate seismic waves that travel through the Earth's interior. By studying how these waves behave (their speed, direction changes, reflection, and refraction) as they pass through different layers, scientists can determine the physical properties (density, phase - solid, liquid, or gas) of the materials within the Earth.

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Earthquake

An earthquake is essentially the shaking or trembling of the Earth's surface. It is a natural phenomenon caused by the sudden release of energy within the Earth's crust, which generates seismic waves that propagate in all directions.

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Why Does The Earth Shake?

Earthquakes originate along geological faults. A fault is a fracture or zone of fractures between two blocks of rock in the Earth's crust. The rocks on either side of a fault are constantly under stress due to tectonic forces, causing them to attempt to move relative to each other.

However, friction along the fault surface often locks the rocks in place. As tectonic forces continue, stress builds up, causing the rocks to deform elastically. Eventually, the stress overcomes the frictional resistance, and the rocks on opposite sides of the fault suddenly slip past each other. This abrupt movement releases the stored energy as seismic waves, causing the ground to shake. The point within the Earth where this energy release originates is called the focus or hypocenter. The point on the Earth's surface directly above the focus is the epicenter; this is typically where the shaking is strongest and where surface waves first arrive.

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Earthquake Waves

The energy released during an earthquake travels through the Earth in the form of seismic waves. These waves are recorded by sensitive instruments called seismographs, which produce seismograms.

There are two main categories of earthquake waves:

• Body Waves: These waves are generated at the earthquake's focus and travel through the body (interior) of the Earth in all directions. There are two types of body waves:
  • P-waves (Primary Waves): These are the fastest seismic waves and are the first to arrive at a seismograph. P-waves are compressional waves, similar to sound waves, meaning they cause particles of the material they pass through to vibrate parallel to the direction the wave is traveling. P-waves can travel through solids, liquids, and gases.
  • S-waves (Secondary Waves): These waves arrive after P-waves. S-waves are shear waves, meaning they cause particles to vibrate perpendicular to the direction of wave propagation, creating crests and troughs in solids. A critical characteristic of S-waves is that they can travel only through solid materials; they cannot pass through liquids or gases. This property has been instrumental in determining that parts of the Earth's interior are liquid.
• Surface Waves: These waves are generated when body waves reach the Earth's surface and interact with the surface rocks. They travel along the Earth's surface. Surface waves are generally slower than body waves but are often the most destructive because they cause the most significant ground displacement.

The velocity and direction of seismic waves change depending on the density and rigidity of the material they pass through. Waves speed up in denser, more rigid materials. When waves encounter a boundary between different materials or layers, they can be reflected (bounce back) or refracted (bend and change direction). Studying these changes in wave speed and path allows seismologists to map the Earth's internal structure.

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Propagation Of Earthquake Waves

As seismic waves travel through rock, they cause the rock particles to vibrate. The nature of this vibration depends on the type of wave:

• P-waves: They vibrate the material back and forth along the path of the wave. This alternately compresses and expands the material, creating density variations (like sound waves).
• S-waves: They vibrate the material side-to-side or up-and-down, perpendicular to the wave's path. This creates shear stresses and distortions in the material.

Surface waves cause complex motions, often rolling or side-to-side, resulting in the greatest displacement of the ground and structures.

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Emergence Of Shadow Zone

Seismic waves from an earthquake are recorded by seismographs worldwide. However, there are specific areas on the Earth's surface where direct seismic waves are not detected. These regions are called shadow zones.

The existence and patterns of these shadow zones provide key evidence for the structure and state (solid or liquid) of the Earth's interior layers.

• S-wave Shadow Zone: Seismographs located anywhere beyond 105° from the earthquake's epicenter do not receive S-waves. This is because S-waves cannot travel through liquid. The large S-wave shadow zone, extending across the entire Earth opposite the epicenter, indicates the presence of a large liquid layer within the Earth – the outer core. The S-wave shadow zone covers slightly more than 40% of the Earth's surface.
• P-wave Shadow Zone: P-waves are received up to 105° from the epicenter. Beyond 145°, P-waves are received again, but not in the zone between 105° and 145°. This band-like shadow zone for P-waves is explained by the refraction (bending) of P-waves as they enter and exit the Earth's core. The core, being denser than the mantle, causes the P-waves to bend downwards, creating the shadow zone. While P-waves can travel through the liquid outer core, their path is significantly altered.

The distinct shadow zones for P and S waves provided crucial evidence for the existence of the Earth's core and its liquid outer portion.

Image depicting the shadow zones for P and S waves, showing areas where seismic waves are not detected on Earth's surface due to the refraction and absorption properties of the Earth's interior layers.

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Types Of Earthquakes

Earthquakes can be classified based on their cause:

• Tectonic Earthquakes: These are the most common type, caused by the movement and sliding of rock blocks along faults in the Earth's crust.
• Volcanic Earthquakes: These occur in active volcanic areas and are caused by the movement of magma beneath the surface or eruptions.
• Collapse Earthquakes: Small tremors caused by the collapse of mine roofs or underground cavities, often in areas of intense mining.
• Explosion Earthquakes: Ground shaking caused by the detonation of chemical or nuclear explosive devices.
• Reservoir-Induced Earthquakes: Earthquakes that occur in areas of large artificial reservoirs, believed to be caused by the immense weight of the water pressing down on the crust and increasing pore pressure in rocks, which can lubricate faults and trigger slips.

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Measuring Earthquakes

The size or strength of an earthquake is measured using scales:

• Magnitude Scale (Richter Scale): This scale, developed by Charles Richter, measures the energy released at the earthquake's focus. It is a logarithmic scale, meaning each whole number increase represents a tenfold increase in amplitude and about 32 times more energy released. Magnitudes are typically expressed in numbers from 0 to 10 (though theoretically limitless).
• Intensity Scale (Mercalli Scale): Named after Giuseppe Mercalli, this scale measures the visible effects and damage caused by an earthquake at a specific location. It is based on observed impacts on people, buildings, and the environment. The scale ranges from I (not felt) to XII (catastrophic destruction).

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Effects Of Earthquake

Earthquakes can cause a wide range of immediate and hazardous effects:

• Ground Shaking: The primary and most direct effect, causing buildings and structures to vibrate and potentially collapse.
• Differential Ground Settlement: Uneven sinking or rising of the ground surface.
• Landslides and Mudslides: Shaking can trigger mass movements on slopes.
• Soil Liquefaction: In saturated, loose soils, shaking can cause the soil to lose its strength and behave like a liquid, leading to buildings sinking or collapsing.
• Ground Lurching: Rolling or wave-like motion of the ground surface.
• Avalanches: On snow-covered slopes, shaking can trigger snow avalanches.
• Ground Displacement: Visible offsets or ruptures along fault lines on the surface.
• Floods: Caused by the failure of dams, levees, or dikes due to shaking.
• Fires: Often ignited by damaged gas lines or electrical wiring.
• Structural Collapse: The failure of buildings, bridges, and other constructions.
• Falling Objects: Non-structural elements like ceilings, light fixtures, and building facades falling.
• Tsunami: Giant ocean waves generated by large earthquakes occurring beneath the seafloor.

While ground shaking, settlement, landslides, liquefaction, lurching, and avalanches directly affect the landforms, effects like structural collapse, fires, and tsunamis are immediate threats to life and property. A tsunami occurs only when a significant earthquake ($>$ magnitude 5) happens under the ocean, displacing a large volume of water. Earthquakes themselves last only seconds or minutes, but their consequences can be devastating, especially for magnitudes above 5 on the Richter scale.

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Frequency Of Earthquake Occurrences

Earthquakes are frequent events globally, though high-magnitude quakes are relatively rare. Minor tremors occur almost constantly, while very large earthquakes (magnitude 8 or higher) happen only once every few years. The distribution of earthquakes across the globe is not uniform; they tend to concentrate along specific zones related to tectonic plate boundaries.

Structure Of The Earth

Based on extensive studies, particularly using seismic waves, scientists have determined that the Earth has a layered structure, with distinct shells differing in composition and physical properties. From the outside moving inwards, these layers are the crust, mantle, and core.

Cross-section diagram illustrating the different layers composing the Earth's interior: the thin crust, the vast mantle, and the central core (divided into outer and inner core).

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The Crust

The crust is the Earth's outermost solid shell. It is relatively thin and brittle compared to the layers beneath it.

• Its thickness varies significantly:
  • Oceanic Crust: Thinner, with an average thickness of about 5 km. It is primarily composed of basaltic rocks.
  • Continental Crust: Thicker, averaging around 30 km. It is mainly composed of lighter, granitic rocks. Under major mountain ranges, the continental crust can be exceptionally thick, reaching up to 70 km (e.g., under the Himalayas).

The boundary between the crust and the mantle is known as the Mohorovičić discontinuity, or Moho.

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The Mantle

Located directly beneath the crust, the mantle extends from the Moho discontinuity down to a depth of approximately 2,900 km.

• It constitutes the largest volume of the Earth's interior.
• The upper portion of the mantle, extending roughly down to 400 km depth, is called the asthenosphere. The term 'astheno' means 'weak'. This layer is depicted as being in a ductile or plastic state, capable of flowing very slowly under stress. It is believed to be the primary source of magma that erupts onto the Earth's surface during volcanic activity.
• The crust and the rigid, uppermost part of the mantle together form the lithosphere. The lithosphere is broken into large plates that move over the ductile asthenosphere. Its thickness varies from about 10 km in oceanic areas to 200 km or more in continental regions.
• Below the asthenosphere, the lower mantle extends from 400 km down to 2,900 km depth. Despite very high temperatures, the immense pressure at these depths keeps the lower mantle material in a solid, rigid state.

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The Core

The Earth's core is the innermost layer, located below the mantle at a depth of 2,900 km down to the Earth's center (6,370 km radius).

• The core-mantle boundary is sharply defined.
• Based on seismic wave analysis (specifically the absence of S-waves and the bending of P-waves), the core is understood to have two parts:
  • Outer Core: Extends from 2,900 km to about 5,100 km depth. It is in a liquid state. Convection currents within the liquid outer core are believed to generate the Earth's magnetic field.
  • Inner Core: Extends from 5,100 km to the Earth's center. Despite being hotter than the outer core, the extreme pressure at this depth compresses the material into a solid state.
• The core is thought to be primarily composed of very dense materials, mainly iron (Fe) and nickel (Ni). This is why it is sometimes referred to as the NiFe layer.

Volcanoes And Volcanic Landforms

A volcano is a vent or opening in the Earth's crust through which molten rock (magma), hot gases, ash, and other materials erupt from the interior onto the surface. A volcano is considered active if it is currently erupting or has erupted in the recent past.

The source of volcanic material is the hot, molten rock within the Earth. This material is called magma when it is beneath the surface, typically originating from the asthenosphere (the weak, upper part of the mantle). Once magma reaches the surface through a volcanic vent, it is referred to as lava.

Materials ejected during volcanic eruptions include lava flows, fragments of rock and volcanic glass called pyroclastic debris, volcanic bombs (large molten rock fragments), ash, dust, and various gases such as compounds of nitrogen and sulfur, along with minor amounts of chlorine, hydrogen, and argon.

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Volcanoes

Volcanoes can be classified based on the type of material erupted and the resulting shape of the volcanic structure.

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Shield Volcanoes

These are among the largest types of volcanoes on Earth, formed mainly by the eruption of highly fluid basaltic lava (except for large flood basalts). Famous examples include the volcanoes in Hawaii.

• Shield volcanoes have gently sloping sides and a broad, dome-like shape, resembling a warrior's shield lying on the ground.
• The lava is very fluid, meaning it flows easily and spreads out over large areas before solidifying, preventing the formation of steep cones.
• Eruptions are typically effusive (lava flows) and non-explosive, unless water enters the vent, which can cause steam explosions.
• If lava is ejected from the vent in a fountain, it can cool and accumulate around the vent to form a small, steep cone called a cinder cone on top of the shield.

Diagram illustrating the broad, gently sloping shape of a Shield Volcano formed by fluid lava flows.

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Composite Volcanoes

Also known as stratovolcanoes, these volcanoes are characterized by eruptions of cooler, more viscous (thicker) lavas, such as andesite or rhyolite. This viscosity leads to different eruption styles and cone shapes.

• Composite volcanoes have steeper slopes and are conical in shape.
• Eruptions are often explosive and violent, alternating between flows of viscous lava and explosive emissions of pyroclastic material (ash, cinders, bombs) and gases.
• The erupted materials (lava and pyroclastics) accumulate in layers around the central vent, building up the tall, stratified structure that gives them the name "composite".

Diagram illustrating the conical shape and layered structure of a Composite Volcano, formed by alternating lava flows and ash/pyroclastic layers.

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Caldera

Calderas are not a type of volcano structure built up by eruptions, but rather large volcanic depressions formed by the collapse of a volcano summit following an extremely powerful explosive eruption.

• These represent the most explosive volcanic events.
• After the eruption empties a large volume of magma from the subsurface chamber, the unsupported ground above collapses inward, creating a large basin-like structure called a caldera.
• Their explosive nature suggests a large, shallow magma chamber below.

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Flood Basalt Provinces

These are massive outpourings of highly fluid basaltic lava that cover vast areas, rather than building a distinct cone. They are not characterized by a central volcano but by fissure eruptions over a large region.

• Highly fluid lava flows can travel hundreds of kilometers.
• Successive flows accumulate over time, building up extremely thick layers of basalt over thousands of square kilometers.
• A famous example is the Deccan Traps in India, which cover much of the Maharashtra plateau and are believed to have originally covered a much larger area. Individual flows within flood basalts can be over 50 meters thick.

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Mid-Ocean Ridge Volcanoes

These volcanoes are located along the extensive underwater mountain ranges in the world's oceans, known as mid-ocean ridges. This is the most volcanically active region on Earth.

• Mid-ocean ridges form a continuous system stretching over 70,000 km through all ocean basins.
• Volcanic eruptions, primarily basaltic lava flows, occur frequently along the rift valleys that run along the crest of these ridges. These eruptions are mostly submarine.

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Volcanic Landforms

When molten rock cools and solidifies, it forms igneous rocks. This cooling can happen on the surface (forming extrusive or volcanic rocks like basalt and rhyolite) or within the Earth's crust (forming intrusive or plutonic rocks like granite and gabbro). Intrusive igneous rocks create distinctive landforms within the crust when they cool before reaching the surface. These are called intrusive forms.

Diagram illustrating different shapes formed by magma solidifying within the Earth's crust: Batholith, Lacolith, Sill, and Dyke.

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Intrusive Forms

Different shapes are formed by magma solidifying beneath the Earth's surface:

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Batholiths

These are very large bodies of solidified magma that cool at great depths within the crust. They are typically granitic in composition.

• They often form the core of mountain ranges.
• Batholiths are the cooled and solidified remnants of large magma chambers.
• They are only exposed at the surface after the overlying rock layers are eroded away over geological time. They can cover vast areas and extend several kilometers deep.

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Lacoliths

A laccolith is a mushroom-shaped or dome-shaped intrusive body. Magma pushes upwards but is prevented from reaching the surface. Instead, it spreads horizontally between rock layers, pushing the overlying strata upwards into a dome, while the base remains relatively flat.

• They are connected to a magma source below by a pipe-like conduit.
• They resemble surface volcanic domes but are located underground.
• Areas like the Karnataka plateau in India show examples of domal granite hills formed by the exposure of laccoliths or batholiths through erosion.

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Lapolith, Phacolith And Sills

These are other forms created by magma injecting itself horizontally or conforming to folded rock structures:

• Lapolith: If the intrusive body takes on a saucer shape, concave upwards (like a bowl), it is called a lapolith.
• Phacolith: These are wavy, lens-shaped intrusive bodies found at the crests (anticlines) or troughs (synclines) of folded rock layers. They follow the shape of the folds.
• Sills (or Sheets): When magma injects itself horizontally between existing layers of rock and solidifies as a relatively thin, flat body, it forms a sill (if thicker) or a sheet (if thinner). These are essentially horizontal intrusions.

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Dykes

Dykes are vertical or near-vertical intrusive bodies. They form when magma rises through cracks or fissures that cut across existing rock layers and then solidifies within these fractures.

• Dykes appear as wall-like structures, often more resistant to erosion than the surrounding rock.
• They are very common intrusive forms, for example, found extensively in the western Maharashtra area of India.
• Dykes are often considered to be the conduits or 'feeders' through which magma traveled from deeper sources towards the surface, potentially leading to eruptions or feeding other intrusive forms. The dykes in Maharashtra are thought to be related to the eruptions that formed the Deccan Traps.

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Exercises

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