Which of the following variations in magnetic declination are correctly paired with their causes? (1) Diurnal — Temperature variation (2) Secular — Earthquakes (3) Irregular — Magnetic storms
Correct Answer: D. Only 1 and 3
📚 Detailed Explanation: Correct Pairings Are 1 (Diurnal → Temperature) and 3 (Irregular → Magnetic Storms)
Why D (Only 1 and 3) is correct: Pairing 2 is wrong — secular variation is caused by long-term changes in Earth's molten iron core, not earthquakes. Pairings 1 and 3 correctly describe the causes of diurnal and irregular variations.
Types of Magnetic Declination Variation
| Variation Type | Timescale | Correct Cause | Pairing 2 Claim | Verdict |
|---|---|---|---|---|
| Diurnal variation | 24-hour cycle | Solar radiation & daily temperature changes that shift ionospheric currents and induce daily magnetic fluctuations | Temperature variation (Pairing 1) | ✓ CORRECT |
| Secular variation | Hundreds of years | Gradual changes in Earth's liquid iron outer core and mantle; ~250-year cycle | Earthquakes (Pairing 2) | ✗ WRONG — earthquakes cause IRREGULAR, not secular variations |
| Irregular variation | Unpredictable | Magnetic storms, solar flares, volcanic eruptions, earthquakes | Magnetic storms (Pairing 3) | ✓ CORRECT |
Common exam trap: Earthquakes cause irregular variation (unpredictable, sudden), NOT secular variation (slow, cyclical, spanning centuries).
- Pairing 1 (Diurnal ← Temperature/solar): CORRECT.
- Pairing 2 (Secular ← Earthquakes): WRONG — secular is caused by Earth's core changes, not earthquakes.
- Pairing 3 (Irregular ← Magnetic storms): CORRECT.
- Answer: Only 1 and 3 are correctly paired → Option D.