In the central Amazon basin, sediment deposits more than sixteen million years old consist of clays formed from rocks found...

GMAT Critical Reasoning : (CR) Questions

Source: Mock

Critical Reasoning

Strengthen

MEDIUM

...

Notes

Post a Query

In the central Amazon basin, sediment deposits more than sixteen million years old consist of clays formed from rocks found in the basin itself. But the sediment deposits sixteen million years old or younger also include dark sand resembling rocks found in the Andes mountain range. These observations have led geologists to conclude that around sixteen million years ago, rivers in the Andes first began to drain into the central Amazon basin.

Which of the following, if true, would most strengthen the geologists' reasoning?

Sediment deposits more than sixteen million years old in both the Andes and the central Amazon basin contain similar types of clays.

Rivers in the Andes and elsewhere form sediment deposits today similar to deposits formed sixteen million years ago in the same regions.

The sediment deposits with dark sand contain fossilized pollen from plant species that grew only in the Andes, but the older deposits do not.

Rocks resembling the dark sand in the younger sediment deposits are common between the Andes and the central Amazon basin.

Sediment deposits less than sixteen million years old in the eastern Amazon, far from the Andes, also include the dark sand.

Solution

Passage Analysis:

Text from Passage	Analysis
In the central Amazon basin, sediment deposits more than sixteen million years old consist of clays formed from rocks found in the basin itself.	What it says: Old sediment layers (16+ million years) only contain local clay materials What it does: Sets up baseline - what sediments looked like before something changed What it is: Scientific observation Visualization: Timeline: 16+ million years ago → Only local Amazon clay in sediment layers
But the sediment deposits sixteen million years old or younger also include dark sand resembling rocks found in the Andes mountain range.	What it says: Newer sediments (16 million years or less) contain both local clays AND Andes-like sand What it does: Shows a clear shift - contrasts with the previous baseline to highlight when something new appeared What it is: Scientific observation Visualization: Timeline: 16 million years ago and after → Local Amazon clay + Andes-like dark sand in sediment layers
These observations have led geologists to conclude that around sixteen million years ago, rivers in the Andes first began to drain into the central Amazon basin.	What it says: Scientists think Andes rivers started flowing into Amazon basin 16 million years ago What it does: Draws conclusion from the sediment evidence - explains why Andes material suddenly appears What it is: Geologists' conclusion

Argument Flow:

The argument moves from older evidence to newer evidence, then draws a conclusion. We start with what sediments looked like before 16 million years ago (only local stuff), then see what changed after that point (Andes material appears), and finally get the scientists' explanation for this change.

Main Conclusion:

Around sixteen million years ago, rivers in the Andes first began to drain into the central Amazon basin.

Logical Structure:

The geologists use a before-and-after comparison in sediment composition as their evidence. Since Andes-like material only shows up in sediments from 16 million years ago onward, they conclude that's when Andes rivers first connected to the Amazon basin and started carrying that material there.

Prethinking:

Question type:

Strengthen - We need to find information that would make the geologists' conclusion more believable. The conclusion is that Andes rivers first started draining into the Amazon basin around 16 million years ago.

Precision of Claims

The key claims involve timing (16 million years ago), geographical sources (Andes vs local Amazon basin), and material composition (dark sand vs clays). We need to be precise about when this drainage started and why Andes material suddenly appears in sediments.

Strategy

To strengthen this argument, we need new information that supports the idea that Andes rivers began flowing into the Amazon exactly when the sediments changed. We should look for evidence that rules out alternative explanations for why Andes-like material appears, confirms the timing, or provides additional support for the river drainage theory.

Answer Choices Explained

Sediment deposits more than sixteen million years old in both the Andes and the central Amazon basin contain similar types of clays.

This tells us that both regions had similar clays in older deposits, but this doesn't help the geologists' argument. If anything, it makes it harder to distinguish between local Amazon material and Andes material. We need evidence that the dark sand specifically came from the Andes via rivers, not evidence that both regions had similar materials before the drainage began.

Rivers in the Andes and elsewhere form sediment deposits today similar to deposits formed sixteen million years ago in the same regions.

This talks about what's happening today and draws parallels to 16 million years ago, but it doesn't provide any new evidence about whether Andes rivers actually started draining into the Amazon basin at that specific time. It's more about general sediment formation processes rather than the specific geographical connection we're trying to prove.

The sediment deposits with dark sand contain fossilized pollen from plant species that grew only in the Andes, but the older deposits do not.

This is perfect! It gives us biological evidence that confirms the dark sand really did come from the Andes - the fossilized pollen from Andes-only plants proves it. Plus, the timing matches perfectly: older deposits (before 16 million years) have no Andes pollen, while newer deposits (after 16 million years) do contain it. This provides independent confirmation beyond just the physical rock similarities, making the river drainage theory much stronger.

Rocks resembling the dark sand in the younger sediment deposits are common between the Andes and the central Amazon basin.

This actually weakens the argument rather than strengthening it. If rocks similar to the dark sand are common between the Andes and Amazon basin, then the dark sand might not have come specifically from Andes rivers - it could have come from these intermediate locations. This gives us alternative explanations for how Andes-like material got into the Amazon sediments.

Sediment deposits less than sixteen million years old in the eastern Amazon, far from the Andes, also include the dark sand.

This tells us the dark sand appears in eastern Amazon deposits too, but this doesn't strengthen the argument about when Andes rivers first began draining into the central Amazon basin. If anything, it raises questions about whether there were other sources or processes involved, potentially complicating the simple river drainage explanation.

Rate this Solution

Tell us what you think about this solution

...

Forum Discussions

Start a new discussion

Post

Previous Attempts

Loading attempts...