Question 81·Hard·Inferences
Paleoclimatologist Dr. Reyes analyzed ice cores from two glaciers located roughly 800 kilometers apart. In layers dated to about 12,800 years ago, she detected identical spikes in volcanic ash particles. Because simultaneous eruptions at distant sites are uncommon, Reyes proposed that the ash originated from a single, exceptionally large eruption. She next began examining marine sediment cores collected from locations between the two glaciers, looking for the same ash signature. This decision suggests that Reyes assumed that if the ash really came from one eruption, ______
Which choice most logically completes the text?
For inference/assumption questions about scientific scenarios, restate the observation and hypothesis, then focus on the researcher’s next action and ask what must be true for that action to test the hypothesis. Prefer the option that links the hypothesis to the new evidence being sought, and eliminate choices that shift the timeframe, introduce a condition that would prevent a meaningful comparison, or narrow the expected evidence in a way that doesn’t fit the researcher’s plan.
Hints
Focus on the scientist’s next step
Look closely at the last sentence: after forming her hypothesis about one big eruption, what does Reyes decide to do next with the marine sediment cores?
Think about what makes that next step reasonable
For her to search marine sediments between the glaciers, what would she have to believe about whether the same eruption’s ash could be found there in a comparable way?
Eliminate choices that break the link between hypothesis and test
Cross out any option that would mean she couldn’t reasonably match the ash in marine sediments to what she found in the ice cores, or that shifts the ash to a different time frame.
Step-by-step Explanation
Restate what the scientist observed and proposed
Reyes examined ice cores from two glaciers that are about 800 kilometers apart. In layers dated to around 12,800 years ago, she saw identical spikes in volcanic ash particles in both cores. Because it's rare for distant volcanoes to erupt at exactly the same time, she proposed that these ash layers came from a single, very large eruption that spread ash over a wide region.
Understand what her new action is
After forming her hypothesis, she next began examining marine sediment cores from locations between the two glaciers, specifically looking for the same ash signature. She is using additional evidence from the in-between region to test whether the ash points to one widespread event.
Identify the assumption behind checking sediments in between
Her choice to look between the glaciers suggests she thinks that, if one eruption caused both ice-core spikes, then ash from that eruption should also have reached the area between the glaciers and been preserved there in a way she can match to the ice-core ash signature.
Match each option to that assumption
Compare the choices:
- Choice 1 says the ash would be present but its signature would differ because seawater altered it. That would undermine the idea of finding the same signature.
- Choice 2 says the ash would show up only in layers much younger than 12,800 years because it was redeposited later. That doesn’t match the logic of checking the same time period as the ice-core spikes.
- Choice 3 limits the matching signature to areas near the glaciers and not throughout the region between them; this does not align with the inference that the in-between sediments from that same period should also record the eruption.
- Choice 4 states that sediment layers between the glaciers from the same period would contain the same distinctive ash particles, which directly fits the assumption behind her decision.
Therefore, the best answer is sediment layers laid down between the two glaciers from the same period would contain the same distinctive ash particles.