Fundamentals of Physical Geography: Interior of the Earth

Understanding the Interior of the Earth

• The nature of the earth interior is a subject of scientific inference because it is impossible to reach it directly. It is often conceptualized either as a solid ball (similar to a cricket ball) or as a hollow ball with a thick rock covering known as the lithosphere.

• The configuration of the earth's surface is primarily a product of processes operating in the interior. Landscape development is constantly shaped by both exogenic (external) and endogenic (internal) processes.

• A comprehensive understanding of a region's physiographic character requires the inclusion of endogenic effects, as these forces influence human life and generate natural phenomena like earthquakes and tsunamis.

• The materials forming the earth are distributed in layers, extending from the outer crust to the inner core.

Sources of Information About the Interior

• The radius of the earth is approximately $6,378\,km$. Due to this extreme distance, observations and material collections from the center are currently impossible.

• Most knowledge of the interior is based on estimates and inferences, though some information is derived from direct observations.

Direct Sources

• Surface Rocks and Mining: Surface rocks and materials from mining are the most accessible direct sources. Gold mines in South Africa reach depths of $3 - 4\,km$. Increasing heat at greater depths prevents further penetration.

• Deep Drilling Projects: Scientists globally have initiated projects to explore deeper crustal portions, including:

  • Deep Ocean Drilling Project

  • Integrated Ocean Drilling Project

  • Kola Drill: Located in the Arctic Ocean, this is the deepest drill recorded, reaching a depth of $12\,km$.

• Volcanic Eruptions: Molten material (magma) ejected during eruptions provides material for laboratory analysis. However, determine the exact depth of the magma's source remains a challenge.

Indirect Sources

• Physical Properties: Mining activities reveal that temperature, pressure, and material density increase progressively with distance from the surface towards the interior.

• Meteors: Although not from the earth's interior, meteors provide a source of information because they are solid bodies composed of materials similar to those that formed the earth.

• Gravitation ($g$): The gravitational force is not uniform across the surface; it is greater at the poles and less at the equator because the distance to the center is greater at the equator.

• Gravity Anomaly: Readings of gravity at different locations often differ from expected values based on the uneven distribution of mass within the crust. This difference is termed a gravity anomaly.

• Magnetic Surveys: These provide data on the distribution of magnetic materials within the crustal portion of the earth.

• Seismic Activity: The study of seismic waves is considered the most important source for creating a complete picture of the layered interior.

Earthquake Characteristics and Waves

• An earthquake is a natural event defined as the shaking of the earth caused by a release of energy that generates waves traveling in all directions.

• Faults: Energy release typically occurs along a fault, which is a sharp break in crustal rocks. Friction usually locks rocks together, but when the tendency to move overcomes friction, blocks slide past one another abruptly, releasing energy.

• Focus (Hypocentre): The specific point where the energy is released.

• Epicentre: The point on the surface directly above the focus; it is the first location to experience the waves.

Classification of Earthquake Waves

• Natural earthquakes occur in the lithosphere, which refers to the portion of the earth up to a depth of $200\,km$.

• Waves are recorded by a seismograph, which displays distinct sections representing different wave patterns.

• Body Waves: Generated at the focus, these move through the body of the earth in all directions.

  • P-waves (Primary Waves): These are the fastest waves and the first to reach the surface. They are similar to sound waves and can travel through gaseous, liquid, and solid materials. They vibrate parallel to the direction of propagation, causing stretching and squeezing of materials.

  • S-waves (Secondary Waves): These arrive with a time lag. They can only travel through solid materials, a characteristic that helps scientists define the interior structure. They vibrate perpendicular to the direction of propagation in the vertical plane, creating troughs and crests.

• Surface Waves: Created when body waves interact with surface rocks. These move along the surface and are the last to be recorded on a seismograph. They are the most destructive waves, causing rock displacement and structural collapse.

Earthquake Shadow Zones

• Shadow zones are specific areas where seismic waves are not reported by seismographs.

• Observations relative to Epicentre:

  • Seismographs within $105$ from the epicentre record both P and S-waves.

  • Seismographs beyond $145$ from the epicentre record P-waves but not S-waves.

  • The zone between $105$ and $145$ is a shadow zone for both types of waves.

  • The entire zone beyond $105$ serves as the shadow zone for S-waves.

  • The S-wave shadow zone is significantly larger than the P-wave shadow zone, covering over $40\%$ of the earth's surface.

Types and Measurement of Earthquakes

Types of Earthquakes

• Tectonic: The most common type, generated by sliding rocks along a fault plane.

• Volcanic: Confined to regions of active volcanoes.

• Collapse: Minor tremors caused by the collapse of roofs in underground mines.

• Explosion: Shaking caused by the detonation of chemical or nuclear devices.

• Reservoir-Induced: Occurs in areas surrounding large reservoirs.

Measurement Scales

• Richter Scale: A magnitude scale relating to the energy released during the event. It is expressed in numbers from $0-10$.

• Mercalli Scale: An intensity scale named after the Italian seismologist Mercalli. It measures visible damage and ranges from $1-12$.

Effects of Earthquakes

• Earthquakes are natural hazards with immediate effects including:

  • Ground shaking and displacement.

  • Land and mudslides, avalanches, and soil liquefaction.

  • Differential ground settlement and ground lurching.

  • Floods resulting from dam and levee failures.

  • Fires, structural collapse, and falling objects.

  • Tsunami: Waves generated by tremors rather than the earthquake itself; these occur only if the epicentre is under oceanic water and the magnitude is sufficiently high.

• Quake activity usually lasts for seconds, but effects can be devastating if the magnitude exceeds $5$ on the Richter scale.

• Frequency: High-magnitude quakes ($8+$) are rare, occurring roughly once every $1 - 2$ years, whereas tiny tremors occur almost every minute.

Structure of the Earth

• The Crust: The outermost, brittle solid part.

  • Oceanic crust mean thickness: $5\,km$.

  • Continental crust mean thickness: $30\,km$ (reaching up to $70\,km$ in the Himalayas).

• The Mantle: Extends from Moho’s discontinuity to a depth of $2,900\,km$.

  • Asthenosphere: The upper portion of the mantle, extending up to $400\,km$. "Astheno" means weak. It is the primary source of magma.

  • Lithosphere: Comprises the crust and the uppermost part of the mantle; thickness ranges from $10 - 200\,km$.

  • Lower Mantle: Extends beyond the asthenosphere and remains in a solid state.

• The Core: Located at the core-mantle boundary at $2,900\,km$.

  • Outer Core: In a liquid state.

  • Inner Core: In a solid state.

  • Composition: Primarily heavy materials like nickel and iron, often called the nife layer.

Volcanoes and Volcanic Landforms

• A volcano is a site where molten rock (lava), gases, and ashes escape to the ground.

• Active Volcanoes: Those currently releasing materials or those that have done so in the recent past.

• Ejected Materials: Lava flows, pyroclastic debris, volcanic bombs, ash, dust, and gases including nitrogen compounds, sulphur compounds, and trace amounts of chlorine, hydrogen, and argon.

Major Types of Volcanoes

• Shield Volcanoes: Largest on earth (excluding basalt flows), primarily composed of fluid basalt lava. They are not steep unless water enters the vent, creating explosivity. Examples include Hawaiian volcanoes.

• Composite Volcanoes: Characterized by cooler, more viscous lava and explosive eruptions. They produce large quantities of pyroclastic material and ash, forming layered mounts.

• Caldera: The most explosive volcanoes. They often collapse on themselves upon eruption, forming large depressions called calderas. This indicates a massive, nearby magma chamber.

• Flood Basalt Provinces: Outpour highly fluid lava over vast distances. The Deccan Traps in India (Maharashtra plateau) are a prominent example. Individual flows can exceed $50\,m$ in thickness.

• Mid-Ocean Ridge Volcanoes: Occur in oceanic areas along a ridge system over $70,000\,km$ long. The central portion experiences frequent eruptions.

Intrusive Volcanic Landforms

• Lava cooling within the crust forms intrusive forms (plutonic rocks), while lava cooling on the surface forms volcanic rocks.

• Batholiths: Large granitic bodies of magmatic material that cool at great depth into large domes. They are the cooled portions of magma chambers.

• Lacoliths: Large, dome-shaped intrusive bodies with a level base, connected by a pipe-like conduit from below. The Karnataka plateau features such domal hills.

• Lapolith: Lava that develops into a saucer-shaped body, concave toward the sky.

• Phacolith: A wavy mass of intrusive rocks found at the base of synclines or the top of anticlines in folded igneous regions.

• Sills and Sheets: Horizontal bodies of intrusive igneous rock. Thinner deposits are called sheets; thicker ones are called sills.

• Dykes: Wall-like structures formed when lava solidifies almost perpendicularly to the ground within cracks and fissures. Frequently found in western Maharashtra, these were feeders for the Deccan Traps.

Questions & Discussion

• Question: Which one of the following earthquake waves is more destructive?

  • Answer: Surface waves.

• Question: Which one of the following is a direct source of information about the interior of the earth?

  • Answer: Volcanoes.

• Question: Which type of volcanic eruptions have caused Deccan Trap formations?

  • Answer: Flood.

• Question: Which one of the following describes the lithosphere?

  • Answer: Crust and upper mantle.

• Question: What are body waves?

  • Answer: Body waves are generated due to the release of energy at the focus and move in all directions through the interior body of the earth. They are divided into P-waves and S-waves.

• Question: Name the direct sources of information about the interior of the earth.

  • Answer: Surface rocks, materials from mining (such as South African gold mines), deep ocean drilling projects (like the Kola drill), and volcanic eruptions.

• Question: Why do earthquake waves develop shadow zones?

  • Answer: Shadow zones occur because earthquake waves (specifically P and S waves) change direction through refraction or are blocked (in the case of S-waves) as they encounter materials of different densities and states (solid vs. liquid) in the earth's interior.

• Question: Briefly explain the indirect sources of information of the interior of the earth other than those of seismic activity.

  • Answer: Indirect sources include the analysis of temperature, pressure, and density changes with depth; the study of meteors; variations in gravitational force (gravity anomalies); and magnetic surveys that show the distribution of materials in the crust.

• Question: What are the effects of propagation of earthquake waves on the rock mass through which they travel?

  • Answer: As waves propagate, they cause vibration. P-waves cause stretching and squeezing (parallel vibration), while S-waves create troughs and crests (perpendicular vibration). Surface waves cause the most destructive displacements and structural collapses.

• Question: What do you understand by intrusive forms? Briefly describe various intrusive forms.

  • Answer: Intrusive forms are igneous rock structures created when lava cools and solidifies within the earth's crust rather than on the surface. Key forms include batholiths (large deep domes), lacoliths (dome-shaped with a flat base), lapoliths (saucer-shaped), phacoliths (wavy masses in folds), sills/sheets (horizontal layers), and dykes (vertical wall-like structures).