Question

to investigate the history of plate subduction - when one of earths tectonic plates slides beneath another - sarah m. aarons and colleagues compared ancient rocks from the acasta gneiss complex in canada to modern rocks. using isotope analysis, the researchers found that acasta rocks dating to about 4.02 billion years ago (bya) most strongly resemble modern rocks formed in a plume setting (an area in which hot rocks from earths mantle flow upward into the crust). by contrast, they found that acasta rocks dating to about 3.75 bya and 3.6 bya have an isotope composition that is similar to that of modern rocks formed in a subduction setting. aaronss team therefore concluded that ______. which choice most logically completes the text? choose 1 answer: a the majority of the rocks in the acasta gneiss complex formed through subduction. b subduction - like processes began occurring in some locations no later than 3.75 bya. c the rocks in the acasta gneiss complex are of a more recent origin than scientists previously thought. d subduction replaced mantle plume formation as the most common geological process by about 4.02 bya.

Explanation:

The text states that Acasta rocks dating to about 3.75 bya and 3.6 bya have an isotope composition similar to modern subduction - setting rocks. This implies that subduction - like processes were occurring at those times. Option A is incorrect as only some (not the majority) of the rocks are mentioned in relation to subduction. Option C is not supported as there's no indication of a change in the rocks' origin time perception. Option D is wrong because 4.02 bya rocks resemble plume - setting rocks, not subduction replacing plume formation.

Answer:

B. subduction - like processes began occurring in some locations no later than 3.75 bya.