Earth, landforms & geomorphology

Answer: (c) Only four

The Andes run along South America's western side through seven countries: Venezuela, Colombia, Ecuador, Peru, Bolivia, Chile and Argentina. From the list, Bolivia, Colombia, Ecuador and Venezuela lie on the chain; Brazil and Paraguay lie wholly to the east.

• ✓ I Bolivia is one of the countries with large portions of the Tropical Andes.
• ✗ II Brazil is not one of the Andean countries; it lies east of the chain, in the Amazon basin and the Brazilian Highlands.
• ✓ III Colombia includes large portions of the Tropical Andes; the northern Andes split into several ranges (cordilleras) there.
• ✓ IV Ecuador includes large portions of the Tropical Andes; its Andean cordilleras carry volcanoes such as Chimborazo and Cotopaxi.
• ✗ V Paraguay is a landlocked lowland country of the Paraná–Paraguay basin and the Gran Chaco, with no Andean ranges.
• ✓ VI The Andes hotspot extends from western Venezuela, where the northernmost Andes (the Venezuelan or Caribbean cordillera) lie.

Remember · Andes countries (7), north to south: Venezuela, Colombia, Ecuador, Peru, Bolivia, Chile, Argentina. Not Brazil, Paraguay, Uruguay or the Guianas.

Question and answer: UPSC's official GS Paper I (2025, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink · Report an error

Answer: (c) I, II and III

All three are classic evidences Wegener's supporters used to show the southern continents were once joined: matching ancient rock belts on either side of the South Atlantic, placer gold in Ghana whose source veins lie in Brazil, and the same glacial Gondwana sediments in India and five other southern landmasses.

• ✓ I Rocks about 2,000 million years old on the Brazilian coast match those of western Africa, and the oldest marine deposits along both coasts are only Jurassic, so the ocean between them is young.
• ✓ II Ghana has rich placer gold but no source rock; the gold-bearing veins are in Brazil, so the deposits must have come from the Brazil plateau when the continents touched.
• ✓ III The Gondwana sediments of India, with thick glacial tillite at the base, are matched in Africa, the Falkland Islands, Madagascar, Antarctica and Australia.

Remember · Continental drift evidence: jig-saw fit, matching rocks across the Atlantic (Brazil–West Africa), Ghana's placer gold from Brazil, Gondwana tillite in six southern landmasses, fossils like Mesosaurus and Glossopteris.

📘 Read it in NCERT: Class 11 Fundamentals of Physical Geography, Ch 4 (practise this chapter) · Class 11 Fundamentals of Physical Geography, Ch 4 (practise this chapter) · Class 11 Fundamentals of Physical Geography, Ch 4 (practise this chapter)

Question and answer: UPSC's official GS Paper I (2025, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink · Report an error

Answer: (c) Only one of the Statements II and III is correct and that explains Statement I

Porosity is how much empty space a rock holds; permeability is how easily water passes through it. Chalk is a soft limestone, and limestone is porous and gives up its pore water freely, which explains Statement I. Clay is actually highly porous (USGS puts it at about 50%), but its pores are so tiny that water barely moves, so Statement III ('not at all porous') is wrong.

• ✓ Statement I Chalk is a variety of limestone (Britannica), and USGS shows limestone yielding most of its pore water freely, while groundwater moves very slowly through relatively impermeable materials such as clay and shale.
• ✓ Statement II Chalk, as a limestone, is porous: USGS lists limestone at 20% porosity with 18% specific yield, so it takes up and releases water readily. That is why chalk is permeable, so II explains I.
• ✗ Statement III Clay is not free of pores: USGS lists its porosity at 50%, but only 2% of that drains freely because the pores are tiny. Britannica notes that permeability falls as the average pore size becomes smaller.

Remember · Porosity = pore space; permeability = ease of water flow. Chalk (limestone): porous and permeable. Clay: highly porous but nearly impermeable because its pores are minute.

📘 Read it in NCERT: Class 11 Fundamentals of Physical Geography, Ch 5 (practise this chapter)

Question and answer: UPSC's official GS Paper I (2025, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 1 Oct 2026 (how we verify). Permalink · Report an error

Answer: (b) Both Statement II and Statement III are correct but only one of them explains Statement I

Both Statement II and Statement III are true, but only Statement III explains Statement I. Melting polar ice moves mass from the poles towards the equatorial ocean, and such shifts of mass change Earth's rotation and axis. A solar storm heats the upper atmosphere but does not shift the planet's mass, so it cannot explain a change in rotation or axis.

• ✓ I NASA-funded studies show Earth's spin axis drifts (polar motion) and its rotation slows, as mass is redistributed on and within the planet.
• ✓ II Solar flares and coronal mass ejections do hit Earth's upper atmosphere with charged particles and energy and can heat it. This is a space-weather effect and does not move Earth's mass, so it does not explain Statement I.
• ✓ III Faster melting of ice sheets has shifted mass from the poles towards the equatorial ocean. This redistribution of mass is the cause of the change in rotation and axis, so it explains Statement I.

Remember · Ice melt shifts mass from the poles to the equatorial oceans, changing Earth's spin axis and lengthening the day; solar storms only affect the upper atmosphere.

Question and answer: UPSC's official GS Paper I (2025, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink · Report an error