Consider the following statements : 1. The Earth's magnetic field has reversed every few hundred thousand years. 2. When the Earth was created more than 4000 million years ago, there was 54% oxygen and no carbon dioxide. 3. When living organisms originated, they modified the early atmosphere of the Earth. Which of the statements given above is/are correct ?
- A.1 only
- B.2 and 3 only
- C.1 and 3 only
- D.1, 2 and 3
Correct answer
Explanation
The correct answer is C, 1 and 3 only. The magnetic field has flipped many times and life changed the early air, but the young Earth had almost no free oxygen.
Rocks on the ocean floor record the Earth's magnetic field as they cool, and their striped pattern shows that the north and south magnetic poles have swapped many times, on average every few hundred thousand years; the last full reversal was about 7,80,000 years ago. The early atmosphere, formed largely by gases escaping from volcanoes, was rich in water vapour, carbon dioxide and nitrogen and had almost no free oxygen. Oxygen built up only after cyanobacteria began photosynthesis; the Great Oxidation Event about 2.4 billion years ago is the clearest sign that living organisms reshaped the air.
A is wrong because it leaves out the true statement 3. B is wrong because statement 2 is false. D is wrong because it includes statement 2.
Exam tip: early atmosphere: carbon dioxide-rich, oxygen-poor; oxygen came from photosynthesis.
Practice Questions
View allConsider the following statements : 1. Areas where rainfall exceeds 250 cm 2. Annual temperature 25°C - 27°C 3. Average humidity exceeds 75% 4. Trees do not shed the leaves To which one of the following types of vegetation does the above represent ?
- A.Tropical wet evergreen
- B.Tropical semi evergreen
- C.Tropical moist deciduous
- D.Tropical dry evergreen
Show answer
Very heavy rainfall, high and steady temperature, humidity above 75 per cent and trees that stay green all year are the marks of tropical wet evergreen forests.
Read the full explanation →In shifting cultivation, the ash of the burnt vegetation serves mainly as
- A.A pesticide
- B.A manure that supplies nutrients to the soil
- C.A weedicide that kills the forest roots
- D.A means of levelling the slope
Show answer
In slash-and-burn farming the felled trees and undergrowth are dried and burnt on the spot, and the ash left behind is rich in potash and other mineral nutrients, which the cultivator depends on because no chemical fertiliser or farmyard manure is used.
Read the full explanation →Mediterranean agriculture is particularly famous for which activity?
- A.Viticulture
- B.Dairy farming
- C.Shifting cultivation
- D.Commercial grain farming
Show answer
The Mediterranean climate, with its hot dry summer and mild wet winter, suits the vine better than almost any other region, and the lands around the Mediterranean sea, together with California, central Chile, the Cape region of South Africa and parts of southern Australia, are the world's great wine-producing areas…
Read the full explanation →The 'kolkhoz' of the former Soviet Union was an example of
- A.Cooperative farming
- B.Collective farming
- C.Plantation agriculture
- D.Market gardening
Show answer
The kolkhoz was the collective farm of the former Soviet Union, in which land, implements, livestock and labour were pooled and the farm worked under state direction with production targets fixed by the plan; the state farm, worked wholly by wage earners, was called the sovkhoz.
Read the full explanation →Intensive subsistence agriculture, marked by very small holdings, heavy use of manure and a high yield per hectare, is characteristic of
- A.The prairies of North America
- B.The monsoon lands of Asia
- C.The steppes of Eurasia
- D.The pampas of South America
Show answer
Where population is dense and holdings have been divided by inheritance over generations, as in India, China, Bangladesh, Vietnam and Indonesia, the farmer puts in abundant labour, manure and irrigation water on a tiny field and takes more than one crop a year.
Read the full explanation →