Geography: Class 11 (Part 2) — Chapter 3: Interior of the Earth

• Surface Rocks: Easily available solid materials from the surface or mining areas (e.g., gold mines in South Africa reaching depths of 3 to 4 km).
• Deep Drilling Projects: Projects like the "Deep Ocean Drilling Project" and "Integrated Ocean Drilling Project". The deepest drill at Kola in the Arctic Ocean has reached a depth of 12 km.
• Volcanic Eruptions: Molten material (magma) thrown onto the surface provides direct samples for laboratory analysis, though determining its exact depth of origin remains difficult.

• Temperature, Pressure, and Density: These physical properties increase progressively with depth. Knowing the earth's total thickness allows scientists to estimate these values at various depths.
• Meteors: The material and structure of meteors are similar to those of the earth, offering clues about the materials from which the planet developed.
• Gravitation: The gravitational force ($g$) varies across latitudes, being greater at the poles and lesser at the equator. Deviations from expected values are termed gravity anomaly, which reveal the uneven distribution of mass within the earth's crust.
  $$\text{Gravity Anomaly} = \text{Observed Gravity} - \text{Expected Gravity}$$
• Magnetic Surveys: Provide information on the distribution of magnetic materials in the crust.
• Seismic Activity: The study of earthquake waves, which is the most crucial source of information about the earth's interior.

• Fault: A sharp break in the crustal rocks where rocks tend to slide past one another.
• Focus / Hypocentre: The point within the earth where the energy is initially released.
• Epicentre: The point on the surface directly above the focus; it is the first to experience the seismic waves.

Seismic waves are recorded by an instrument called a seismograph. All natural earthquakes occur within the lithosphere (up to a depth of 200 km). Waves are broadly classified into two categories:

Seismographs at certain distances from the epicentre do not record specific waves. These regions are called shadow zones:

• P-wave Shadow Zone: Appears as a band around the earth between $105^\circ$ and $145^\circ$ away from the epicentre.
• S-wave Shadow Zone: Much larger, extending entirely beyond $105^\circ$ from the epicentre. It covers over 40% of the earth's surface, proving that the outer core is liquid (since S-waves cannot travel through liquids).

• Tectonic Earthquakes: Caused by sliding of rocks along a fault plane (most common).
• Volcanic Earthquakes: Confined to areas of active volcanoes.
• Collapse Earthquakes: Occur in intense mining areas due to the collapse of underground roofs.
• Explosion Earthquakes: Triggered by chemical or nuclear detonations.
• Reservoir-induced Earthquakes: Occur in areas of large reservoirs.

Earthquakes are measured using two distinct scales:

1. Richter Scale: Measures the magnitude (energy released) on a scale of $0 - 10$.
2. Mercalli Scale: Measures the intensity (visible damage) on a scale of $1 - 12$.

The outermost, brittle solid layer of the earth.

• Thickness: Varies between oceanic crust (mean thickness of 5 km) and continental crust (mean thickness of 30 km, reaching up to 70 km in the Himalayan region).
• Nature: Thinner and denser under oceans compared to continents.

The layer extending beyond the crust down to a depth of 2,900 km.

• Moho's Discontinuity: The boundary separating the crust from the mantle.
• Asthenosphere: The upper portion of the mantle (extending up to 400 km). The word astheno means weak. It is the primary source of magma that reaches the surface during volcanic eruptions.
• Lithosphere: Comprises the crust and the uppermost solid mantle, with a thickness ranging from 10 to 200 km.

The innermost layer of the earth, starting at a depth of 2,900 km.

• Outer Core: Exists in a liquid state.
• Inner Core: Exists in a solid state.
• Composition: Composed of very heavy materials, primarily nickel and iron, and is hence referred to as the nife layer.

• Shield Volcanoes: Largest of all volcanoes on Earth (excluding basalt flows), built of highly fluid basalt lava. They are not steep and have low explosivity unless water enters the vent (e.g., Hawaiian volcanoes).
• Composite Volcanoes: Characterized by eruptions of cooler, viscous lavas. They erupt explosively, depositing layers of pyroclastic material and ashes around the vent.
• Caldera: The most explosive volcanoes. When they erupt, they tend to collapse in on themselves rather than building a tall structure, forming large depressions called calderas.
• Flood Basalt Provinces: Outpour highly fluid lava over vast distances. The Deccan Traps of India (Maharashtra plateau) are a prime example of a flood basalt province.
• Mid-Ocean Ridge Volcanoes: Occur along oceanic ridge systems stretching over 70,000 km across ocean basins, experiencing frequent eruptions.

Lava that cools within the crustal portions forms intrusive igneous rock structures:

• Batholiths: Large granitic bodies of magmatic material that cool in the deep crust, forming massive domes exposed only after extensive erosion.
• Lacoliths: Large dome-shaped intrusive bodies with a level base, connected by a pipe-like conduit from below.
• Lopolith: Saucer-shaped intrusive bodies, concave to the sky.
• Phacolith: A wavy mass of intrusive rocks found at the base of synclines or top of anticlines.
• Sills / Sheets: Near-horizontal bodies of intrusive igneous rocks. Thinner deposits are called sheets, while thicker ones are called sills.
• Dykes: Wall-like structures formed when lava forces its way through cracks and solidifies almost perpendicular to the ground. Commonly found in western Maharashtra.