Question:medium

Passage:

The theory of plate tectonics describes the motions of the lithosphere, the comparatively rigid outer layer of the Earth that includes all the crust and part of the underlying mantle. The lithosphere is divided into a few dozen plates of various sizes and shapes; in general the plates are in motion with respect to one another. A mid-ocean ridge is a boundary between plates where new lithospheric material is injected from below. As the plates diverge from a mid-ocean ridge they slide on a more yielding layer at the base of the lithosphere.

Since the size of the Earth is essentially constant, new lithosphere can be created at the mid-ocean ridges only if an equal amount of lithospheric material is consumed elsewhere. The site of this destruction is another kind of plate boundary: a subduction zone. There one plate dives under the edge of another and is reincorporated into the mantle. Both kinds of plate boundary are associated with fault systems, earthquakes and volcanism, but the kinds of geologic activity observed at the two boundaries are quite different.

The idea of sea-floor spreading actually preceded the theory of plate tectonics. In its original version, in the early 1960s, it described the creation and destruction of the ocean floor, but it did not specify rigid lithospheric plates. The hypothesis was substantiated soon afterward by the discovery that periodic reversals of the Earth's magnetic field are recorded in the oceanic crust. As magma rises under the mid-ocean ridge, ferromagnetic minerals in the magma become magnetized in the direction of the geomagnetic field. When the magma cooks and solidifies, the direction and the polarity of the field are preserved in the magnetized volcanic rock. Reversals of the field give rise to a series of magnetic stripes running parallel to the axis of the rift. The oceanic crust thus serves as a magnetic tape recording of the history of the geomagnetic field that can be dated independently. The width of the stripes indicates the rate of the sea-floor spreading.

Question:
What does the author imply about the periodic reversal of the Earth's magnetic field?

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Trace what "the hypothesis" refers to in the sentence right before the discovery of magnetic reversals is mentioned.
Updated On: Jul 20, 2026
  • It analyses the association of plate boundaries
  • It indicates the amount of magma present in earth's crust
  • It clarifies formation of new lithosphere
  • It supports the hypothesis of sea-floor spreading
  • It explains the creation of mid-ocean ridges
Show Solution

The Correct Option is D

Solution and Explanation

Work backward from the paragraph structure to see what job the magnetic reversal discovery is doing.
  • The paragraph opens by introducing sea-floor spreading as a hypothesis that came before plate tectonics.
  • It then says this hypothesis "was substantiated soon afterward" by the magnetic reversal discovery, and "substantiated" means backed up with evidence.
  • So structurally, the reversals function as proof for sea-floor spreading, not as an explanation of plate-boundary relationships, magma volume, lithosphere formation, or ridge origin, each of which is covered by other parts of the passage.
That confirms option (d): the reversals support the sea-floor spreading hypothesis.
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The result is not just a lofty declaration of principles but a blueprint for a practical programme of action. The policy delineates a series of concrete measures to accord women a decision-making role in the political domain and greater control over their economic status. Of especially far-reaching impact are the devolution of control of economic infrastructure to women, notably at the gram panchayat level, and the amendments proposed in the Hindu Succession Act of 1956 to give women coparcenary rights.
 An enlightened aspect of the policy is its recognition that actual change in the status of women cannot be brought about by the mere enactment of socially progressive legislation. Accordingly, it focuses on reorienting development programmes and sensitizing administrations to address specific situations as, for instance, the growing number of households headed by women, which is a consequence of rural-urban migration. The proposal to create an equal-opportunity police force and give women greater control of police stations is an acknowledgement of the biases and callousness displayed by the generally all-male lawenforcement authorities in cases of dowry and domestic violence. While the mere enunciation of such a policy has the salutary effect of sensitizing the administration as a whole, it does not make the task of its implementation any easier. 
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  • IBSAT
  • Reading Comprehension
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  • IBSAT
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So wrote Thomas Hardy in his poem, Heredity, describing direct descent of life from one generation to the next. Indeed, the poem reflects the DNA in our genome. Dr.Drew Endy of MIT quoted this when he described how people at the J. Craig Venter Institute (JCVI) bypass nature’s constraint of direct descent. Scientists there have used chemistry and biochemistry to produce the first synthetic genome in the laboratory. They chemically synthesized many fragments of the DNA, encoding the 582,970-units-long genome of a bacterium called Mycoplasma genitalium. Next, they assembled these fragments in perfect order to generate the genome of the bacterium.
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This surely raises ethical questions, a matter that JCVI is keenly aware of and is already engaged in with ethicists. Even their present work on M.genitalium was done with prior approval of ethical experts. But then, today it is M.genitalium, tomorrow it could be a more advanced, multi-cellular organism, and that could flummox even the ethicist. Assisted reproduction, which is the other side of the coin and truly a recently initiated technology, has become ethically and morally acceptable. Cloning of Dolly, the sheep, has not raised any outrage, but cloning a human certainly does.
  • IBSAT
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