Early Earth is thought to have been characterized by a stagnant lid tectonic regime, in which the upper lithosphere (the outer rocky layer) was essentially immobile and there was no interaction between the lithosphere and the underlying mantle. Researchers investigated the timing of the transition from a stagnant lid regime to a tectonic plate regime, in which the lithosphere is fractured into dynamic plates that in turn allow lithospheric and mantle material to mix. Examining chemical data from lithospheric and mantle-derived rocks ranging from 285 million to 3.8 billion years old, the researchers dated the transition to 3.2 billion years ago.
Which finding, if true, would most directly support the researchers' conclusion?
Explanation
Context: This passage describes research into early Earth's tectonics, which is thought to have started as a stagnant lid regime where the lithosphere was immobile and did not interact with the mantle below. Researchers examined chemical data from lithospheric and mantle-derived rocks ranging from 285 million to 3.8 billion years old to find when this changed to a plate tectonic regime, in which lithospheric and mantle material mix, and dated that transition to 3.2 billion years ago. The question asks which finding would most directly support the researchers' conclusion about the timing of this transition.
Answer: C) Mantle-derived rocks younger than 3.2 billion years contain some material that is not found in older mantle-derived rocks but is found in older and contemporaneous lithospheric rocks.
Why? After plate tectonics began (post-3.2 bya), lithospheric and mantle materials started mixing. Finding lithospheric material in young mantle rocks — absent from older mantle rocks — marks when this mixing began and directly confirms the 3.2-billion-year transition date.
Traps:
- A) "mantle-derived rocks older than 3.2 billion years show significantly more chemical variability" → about old-rock variability, not about when mixing began
- B) "there is a positive correlation between the age of lithospheric rocks and their thickness" → about lithospheric thickness over time, irrelevant to the mixing transition
- D) "among rocks known to be older than 3.2 billion years, significantly more are lithospheric than mantle-derived" → about the proportion of rock types in the ancient record, not about the transition timing