What is the average thickness of the continental crust of earth?
- A.300 km
- B.5 km
- C.30 km
- D.2.5 km
Correct answer
Explanation
The correct answer is C, 30 km. The crust is the outermost and thinnest layer of the earth and is of two kinds. The continental crust, made largely of lighter granitic rock rich in silica and aluminium, averages about thirty kilometres in thickness and reaches about seventy kilometres under young fold mountains such as the Himalaya. The oceanic crust, made of heavier basaltic rock, is only about five kilometres thick, which is option B and the commonest distractor here. Because continental crust is thick and light it is not easily subducted, and that is why a collision of two continental plates raises mountains instead of consuming crust. Option A, three hundred kilometres, is far greater than the crust and lies within the mantle. Option D, two and a half kilometres, is too small for either kind of crust.
Read the full article: Continental Drift and Plate Tectonics: Exam Notes
Practice Questions
View allThe theory of continental drift was propounded by:
- A.Alfred Wegener
- B.Harry Hess
- C.Arthur Holmes
- D.Charles Lyell
Show answer
Wegener, a German meteorologist and climatologist, put forward the continental drift theory in 1912 and developed it in his book The Origin of Continents and Oceans; he argued that all the continents once formed a single landmass, Pangaea, which broke up about two hundred million years ago.
Read the full explanation →In Wegener's theory, the single great ocean that surrounded the supercontinent Pangaea was called:
- A.Tethys
- B.Panthalassa
- C.Laurasia
- D.Gondwana
Show answer
Wegener held that about two hundred and fifty million years ago all the land of the earth lay together as one mass, Pangaea, and that one enormous ocean, Panthalassa, surrounded it.
Read the full explanation →Which pair of forces did Wegener suggest as the cause of continental drift?
- A.Ridge push and slab pull
- B.Pole-fleeing force and tidal force
- C.Isostasy and gravity
- D.Convection currents and radioactivity
Show answer
Wegener attributed the horizontal movement of the continents to the pole-fleeing force, which arises from the rotation of the earth and the bulge of the equator, and to the tidal force, which arises from the attraction of the moon and the sun.
Read the full explanation →The Tethys Sea separated which two landmasses after the break-up of Pangaea?
- A.Laurasia and Gondwanaland
- B.Africa and South America
- C.India and Australia
- D.Antarctica and Australia
Show answer
When Pangaea began to break about two hundred million years ago, it parted into a northern mass called Laurasia or Angaraland and a southern mass called Gondwanaland, and the long narrow Tethys Sea lay between the two.
Read the full explanation →The concept of sea floor spreading was given by:
- A.Alfred Wegener
- B.Harry Hess
- C.Alfred Lothar Nazca
- D.Edward Suess
Show answer
After the detailed mapping of the ocean floor during and after the Second World War revealed mid-oceanic ridges, deep trenches and a surprisingly thin cover of sediment, Hess proposed in 1961 that molten material rises along the ridge, forms new ocean floor and pushes the older floor away on both…
Read the full explanation →