Interior of the Earth

Correct Answer: (c) Surface waves

Surface waves (also called L-waves or Long waves) travel along the surface of the Earth. They have the largest amplitude and the slowest velocity among all earthquake waves. Because they travel along the Earth's surface and cause the ground to move both vertically and horizontally, they produce the maximum damage to structures and are therefore the most destructive of all earthquake waves.

Correct Answer: (b) Volcanoes

Volcanoes bring material from deep within the Earth directly to the surface in the form of lava, gases, and pyroclastic material. This volcanic material gives us direct physical evidence about the composition and nature of the Earth's interior. Earthquake waves, gravitational force, and Earth magnetism are all indirect sources of information.

Correct Answer: (b) Flood

The Deccan Traps were formed by flood (fissure) volcanic eruptions. In this type of eruption, highly fluid basaltic lava pours out quietly from long fissures or cracks in the Earth's surface and spreads over vast areas, forming extensive horizontal lava plateaus. The dykes found in western Maharashtra are considered the feeders for these eruptions that led to the development of the Deccan Traps.

Correct Answer: (b) crust and upper mantle

The lithosphere is the solid outermost shell of the Earth. It comprises the crust (both continental and oceanic) and the uppermost part of the mantle. It extends to a depth of about 100 km. The lithosphere is broken into several tectonic plates that float over the semi-fluid asthenosphere below.

Given/Asked: Definition and nature of body waves.

Answer:

Body waves are seismic waves generated by an earthquake that travel through the interior (body) of the Earth. They originate from the focus (hypocentre) of the earthquake and radiate outward in all directions through the Earth's interior.

There are two types of body waves:
1. P-waves (Primary waves): These are longitudinal/compressional waves. They travel through solid, liquid, and gaseous materials and are the fastest seismic waves.
2. S-waves (Secondary waves): These are transverse/shear waves. They can travel only through solid materials and are slower than P-waves.

Body waves interact with surface rocks to generate surface waves upon reaching the Earth's surface.

Given/Asked: Direct sources of information about the Earth's interior.

Answer:

The direct sources of information about the interior of the Earth are:

1. Volcanic eruptions: Lava, gases, and pyroclastic material brought to the surface from deep within the Earth provide direct evidence of interior composition.
2. Mining and drilling: Deep mines and bore wells (e.g., the Kola Superdeep Borehole in Russia, drilled up to about 12 km) provide rock samples and direct measurements of temperature and pressure from depth.
3. Meteorites: Since meteorites are believed to have a similar composition to the Earth's interior, they serve as a direct analogue for studying the Earth's core and mantle materials.

These sources give us actual physical material or direct measurements from depth.

Given/Asked: Reason for the development of shadow zones of earthquake waves.

Answer:

A shadow zone is an area on the Earth's surface where no direct earthquake waves are recorded after a seismic event.

Reason:
- P-waves are refracted (bent) when they pass through the liquid outer core because the outer core has different physical properties (density, state). This refraction causes P-waves to bend away, creating a shadow zone between 105° and 145° from the epicentre.
- S-waves cannot travel through the liquid outer core at all (they are transverse waves and require a solid medium). Therefore, S-waves do not appear beyond 105° from the epicentre, creating a larger shadow zone.

Thus, the change in the nature and density of materials inside the Earth causes refraction and absorption of seismic waves, leading to the formation of shadow zones.

Given/Asked: Indirect sources of information about the Earth's interior (excluding seismic activity).

Answer:

The important indirect sources of information about the Earth's interior (other than seismic activity) are:

1. Temperature: Temperature increases with depth inside the Earth (geothermal gradient — about 1°C per 32 metres). This tells us about the thermal state of the interior.

2. Pressure: Pressure also increases with depth due to the weight of overlying rocks. This helps us understand the density and state of interior materials.

3. Density: The mean density of the Earth is about 5.5 g/cm³, while surface rocks have a density of only about 2.7–3 g/cm³. This difference indicates that the interior must contain much denser materials (like iron and nickel).

4. Gravitational force: Variations in gravitational force at different locations indicate differences in the distribution of mass and density within the Earth.

5. Earth's magnetic field: The presence of a strong magnetic field suggests the existence of a metallic (iron-nickel) liquid outer core that generates the geodynamo effect.

6. Meteorites: The composition of meteorites (stony and iron meteorites) provides clues about the likely composition of the Earth's mantle and core.

Given/Asked: Effects of earthquake wave propagation on rock masses.

Concept: Earthquake waves are of three types — P-waves, S-waves, and Surface waves — and each interacts differently with rock material.

Effects:

1. Effect of P-waves (Primary/Compressional waves):
- P-waves are longitudinal waves that cause the rock particles to vibrate parallel to the direction of wave propagation.
- They alternately compress and dilate (expand) the rock material as they pass through it.
- They can travel through solid, liquid, and gaseous materials.
- They cause changes in the volume of the rock but not in its shape.

2. Effect of S-waves (Secondary/Shear waves):
- S-waves are transverse waves that cause rock particles to vibrate perpendicular to the direction of wave propagation.
- They cause the rock to move in an up-and-down or side-to-side motion (shearing motion).
- They can travel only through solid materials because liquids and gases cannot resist shear stress.
- They cause changes in the shape of the rock material.

3. Effect of Surface waves (L-waves):
- Surface waves travel along the Earth's surface and have the largest amplitude.
- They cause the most severe shaking of the ground.
- They move the ground both vertically and horizontally, causing rolling and swaying motions.
- They are responsible for the maximum destruction to buildings, roads, and other surface structures.

General effects on rock mass:
- When waves pass from one medium to another (e.g., from mantle to core), they undergo refraction (bending) and reflection.
- Sudden changes in wave velocity indicate discontinuities (e.g., Mohorovičić discontinuity, Gutenberg discontinuity) between layers of different composition and density.
- The waves can cause fracturing, faulting, and displacement of rock masses near the surface.

Conclusion: The study of how earthquake waves are affected as they travel through different rock masses has helped scientists understand the layered internal structure of the Earth.

Given/Asked: Meaning of intrusive forms and description of various intrusive forms.

What are Intrusive Forms?
When magma, while moving upward, does not reach the Earth's surface but instead cools and solidifies within the Earth's crust itself, the resulting igneous rock bodies are called intrusive igneous rocks and the landforms they create are called intrusive forms (or plutonic forms). They are exposed on the surface only after the overlying material is removed by denudation over millions of years.

Various Intrusive Forms:

1. Batholith:
- A large body of magmatic material that cools at great depth in the crust.
- It forms a large dome-shaped structure covering large areas, sometimes several kilometres deep.
- It is composed of granitic rocks.
- Batholiths are essentially the cooled and solidified magma chambers.
- They appear on the surface only after overlying rocks are removed by erosion.

2. Lacolith:
- These are large dome-shaped intrusive bodies with a flat/level base, connected to the magma source below by a pipe-like conduit.
- They resemble surface volcanic domes but are located at greater depths.
- They can be regarded as localised sources of lava.
- Example: The domal hills of granite on the Karnataka plateau are examples of lacoliths or batholiths.

3. Lapolith:
- When lava moves horizontally and settles in a saucer-shaped (concave upward) body, it is called a lapolith.
- It is concave to the sky.

4. Phacolith:
- A wavy mass of intrusive igneous rock found at the base of synclines or at the top of anticlines in folded igneous terrain.
- It has a definite conduit connecting it to the magma chamber (batholith) below.

5. Sill and Sheet:
- When lava moves horizontally along weak planes and solidifies, it forms near-horizontal bodies.
- Sheets are the thinner horizontal deposits.
- Sills are the thicker horizontal deposits.

6. Dyke:
- When lava makes its way through cracks and fissures in the rock and solidifies almost perpendicular to the ground, it forms a wall-like structure called a dyke.
- Dykes are the most commonly found intrusive forms in western Maharashtra.
- They are considered the feeders for the eruptions that led to the formation of the Deccan Traps.

Conclusion: Intrusive forms provide valuable information about the movement of magma within the Earth's crust and the geological history of a region.