gmatrant wrote:
First identified in 1969, komatiites are Earth’s oldestknown volcanic rocks and contain three times as muchmagnesium as do most volcanic rocks. This chemicalcomposition suggests that komatiites formed from thehottest lava known ever to have erupted: a highconcentration of magnesium changes the physicalproperties of lava so that unusually high temperatureswould be required for the lava to exist as a liquid.
Komatiites’ discovery was surprising in light of then-current geological theories about magmas, moltenrock that forms in the Earth’s mantle (the layerbeneath the crust) and composes volcanic lavaeruptions. Prior to 1960, geologists Bowen and Hessdisagreed over whether or not the very hightemperatures needed to produce magmas rich inmagnesium could have existed on Earth. Hesssuggested that the presence of water, probablyreleased from minerals decomposing in the Earth’smantle, might have meant that a high-magnesiummagma could have existed at a lower temperature. ButBowen showed experimentally that the hightemperatures were indeed necessary. By 1960, it wasgenerally accepted that volcanic rocks with such highlevels of magnesium could not exist, and thus thediscovery of komatiites changed geologists’ assumptions about the characteristics of the Earth’smantle around the time of the formation of komatiites,between 2.5 and 4 billion years ago.
Which of the following most accurately describes theorganization of the passage?
A. Two divergent views of a scientific phenomenon arereconciled.
B. A phenomenon is described and its scientificsignificance is discussed.
C. The discovery of a scientific phenomenon is tracedand its implications for further research are suggested.
D. A long-standing scientific theory is examined andrecently discovered evidence is shown to support it.
E. The ways in which a particular geologicalphenomenon is exceptional are detailed and classified
OA is B. Can you please tell me why D is not correct?
Hii GMATRANT.
The passage discusses the discovery of Komatites or whatever, which is rich in Mg.
Then the passage goes on to tell that two scientists Hess and Bowen examined the formation of molten rocks with high Mg, at certain temperatures.
Hess suggested in favour i.e. rocks rich in Mg can be formed at lower temperature whereas Bowen countered by saying that high temperature is necessary.
Later it was proved that such volcanic rocks with high Mg could not exist.
D suggests "A long-standing scientific theory is examined and recently discovered evidence is shown to support it". We can't say whether a theory if persists for 9 years can be proclaimed as long standing theory. Moreover The discovery of Komatites did not support the view but rather changed the previous perception and proved that such rocks, which have high Mg content can exist.
B is correct since it is the best among the choices. However, I am not very convinced about the first part of B; second part looks perfect.
Please mention the source of the question while posting the questions.
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