UPSC Prelims 2001 Q55: Geography – Physical Geography & Geomorphology
Consider the following statements:
I. Most magmas are a combination of liquid, solid and gas.
II. Water vapour and carbon dioxide are the principal gases dissolved in a magma.
III. Basaltic magma is hotter than the silicic magma.
IV. The magma solidified between sedimentary rocks in a horizontal position is known as dike.
Which of these statements are correct?
- (a) I, II and III
- (b) II, III and IV
- (c) I and IV
- (d) I, II and IV
Answer: (a) I, II and III
Explanation
Statements about magma
| Statement | Verdict |
|---|---|
| I. Most magmas are a combination of liquid, solid and gas | Correct — melt with crystals and dissolved gases |
| II. Water vapour and carbon dioxide are the principal gases dissolved in a magma | Correct |
| III. Basaltic magma is hotter than the silicic magma | Correct — basaltic about 1,000-1,200 °C; silicic about 650-800 °C |
| IV. Magma solidified between sedimentary rocks in a horizontal position is a dike | Wrong — a horizontal sheet is a sill; a dike is vertical and cuts across layers |
Statements I, II and III are correct. Magma is usually molten rock with crystals and gases, mainly water vapour and carbon dioxide, and basaltic magma is hotter than silicic magma. Statement IV describes a sill, not a dike, so it is wrong. (Our bank had lost statement IV and the original options; both have been restored from published copies of the paper.)
Answer: (a) I, II and III.
Why each option is right or wrong
- (a) CORRECT. I, II and III are correct; IV describes a sill, not a dike.
- (b) Incorrect. Statement IV is wrong, and I is left out.
- (c) Incorrect. Statement IV is wrong, and II and III are left out.
- (d) Incorrect. Statement IV is wrong, and III is left out.
Key point
Intrusive forms: sill (horizontal), dike (vertical), laccolith (dome), lopolith (saucer), batholith (large deep mass).
Remember
Basaltic magma is hotter; H2O and CO2 are the main gases; horizontal intrusion = sill.
Common mistakes
- Mixing up sill (horizontal) and dike (vertical).
- Guessing that silicic magma is hotter because it is more explosive.
- Naming sulphur dioxide as the main dissolved gas.
Exam tip
Sill lies flat like a window sill; a dike stands up like a wall.
Sources
- NCERT Class XI — Fundamentals of Physical Geography, chapter 'Interior of the Earth' (volcanoes and intrusive forms)
- Arthur Holmes — Principles of Physical Geology, chapter on igneous activity