Discussion
Which of the following statements about seafloor hydrothermal vents are correct ?
(i) They are most common near passive continental margin.
(ii) They are so hot and deepseated, the environment prevents organisms from living there.
(iii) They form when dissolved minerals in the hot fluid form solid precipitates when exposed to cold seawater.
(iv) They form when seawater percolates into the oceanic crust and gets superheated by magma chamber.
Answer: C
Statements iii and iv are correct, so the answer is (iii) and (iv).
Hydrothermal vents form when seawater percolates down through fractures in young oceanic crust, is heated by an underlying magma chamber and leaches metals (iv). When the hot, metal-laden fluid jets back into near-freezing seawater, sulphides and other minerals precipitate, building chimneys such as black smokers (iii).
Statement i is wrong: vents cluster at mid-ocean spreading ridges and back-arc basins, where magma is shallow, not at passive continental margins. Statement ii is wrong: vents support rich chemosynthetic communities of tube worms, clams and bacteria.
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