Static Quiz 25 November 2025 (Geography)

Q1. Consider the following statements:

1. The average density of the Earth is about 5.5 g/cm³ whereas surface rocks are only 2.7–3.0 g/cm³.
2. The Gutenberg discontinuity separates the core from the mantle.
3. The Mohorovičić discontinuity is the boundary between crust and mantle.

How many are correct?
(a) 1 only
(b) 2 only
(c) All three
(d) None

Correct Answer: (c) All three

Explanation:

• Statement 1 – Correct
  • Earth’s mean density ≈ 5.51 g/cm³ (higher due to metallic core).
  • Surface rocks (granite, basalt) ≈ 2.7–3.0 g/cm³ → indicates dense interior.
• Statement 2 – Correct
  • Gutenberg discontinuity (at ~2900 km depth) separates the lower mantle (solid) from the outer core (liquid).
  • Marked by sudden disappearance of S-waves.
• Statement 3 – Correct
  • Moho marks the crust–mantle boundary.
  • Identified by sudden increase in seismic velocity.

---

Q2. Which of the following are correct about the Earth’s core?

1. Outer core is liquid.
2. Inner core is solid.
3. Core is mainly composed of iron and nickel.

How many are correct?
(a) 1 only
(b) 2 only
(c) All three
(d) None

Correct Answer: (c) All three

Explanation:

• 1 – Correct
  • Outer core (2900–5150 km) is liquid → explains absence of S-waves and generation of Earth’s magnetic field (geodynamo).
• 2 – Correct
  • Inner core (5150–6371 km) is solid due to immense pressure despite high temperature.
• 3 – Correct
  • Core composition: Fe-Ni alloy with some lighter elements (S, Si, O).
  • Density ≈ 10–13 g/cm³ supports metal-rich nature.

---

Q3. S-waves can pass through:

1. Solid mantle
2. Liquid outer core
3. Solid inner core

How many are correct?
(a) Only one
(b) Only two
(c) All three
(d) None

Correct Answer: (a) Only one

Explanation:

• S-waves travel only through solids.
• 1 – Correct → Mantle is solid, so S-waves propagate.
• 2 – Incorrect → S-waves cannot pass through liquid outer core → creates S-wave shadow zone (103°–142°).
• 3 – Incorrect → Although inner core is solid, direct S-wave paths do not reach it because S-waves are blocked at the liquid outer core boundary. Only PKJKP-type rare conversions occur, not counted in standard propagation.

---

Q4. The temperature at the inner core–outer core boundary is estimated to be around:

(a) 3000°C
(b) 4000°C
(c) 5700°C
(d) 6000°C

Correct Answer: (c) 5700°C

Explanation:

• Experimental data + seismic models: ~5700°C, comparable to the surface of the Sun (~5778 K).
• Corresponds to pressure of ~330 GPa.

---

Q5. Shadow zones for seismic waves are caused by:

1. Refraction at discontinuities
2. Liquid outer core
3. Sudden increase in density

How many are correct?
(a) 1 only
(b) 1 and 2 only
(c) 2 and 3 only
(d) All three

Correct Answer: (d) All three

Explanation:

• 1 – Correct
  • Seismic rays refract sharply at Moho, Gutenberg, Lehmann discontinuities → creates shadow regions.
• 2 – Correct
  • Liquid outer core blocks S-waves → creates S-wave shadow zone (103°–142°).
  • Strong refraction of P-waves at liquid boundary → causes P-wave shadow zone (~105°–142°).
• 3 – Correct
  • Density jumps at discontinuities cause major changes in wave velocity → responsible for angular shadow zones.

---