Difficulty: Medium

Average Score: 50%

Which of the following statements best explains the presence of seashell fossils in the Andes Mountains in South America?

Rationale

The Andes Mountains were at one time under the Pacific Ocean and have since been pushed upward.

The presence of seashell fossils in the Andes Mountains indicates that these mountains were once submerged beneath the ocean, where marine life thrived. Geological uplift caused by tectonic activity has since raised these fossilized remains to their current elevation.

A (While flying over the Andes Mountains, seagulls excreted the undigested remains of the shelled animals they had eaten.) YOUR ANSWER

This statement suggests an implausible scenario for the presence of seashell fossils. Seagulls' excrement would not account for the widespread and ancient deposits of seashells found in sedimentary rock, which indicate a much larger and historical marine environment rather than casual feeding by birds.

B (The Andes Mountains are the highest in South America.) YOUR ANSWER

While it is true that the Andes are the highest mountain range in South America, this fact alone does not explain the presence of seashell fossils. The elevation of the mountains does not provide a geological rationale for the fossils, which are indicative of a former underwater habitat rather than a consequence of their current height.

C (The Andes Mountains were at one time under the Pacific Ocean and have since been pushed upward.) CORRECT

This statement accurately describes the geological history of the Andes Mountains, where tectonic processes have uplifted land that was once covered by ocean. The seashell fossils are remnants of marine organisms that existed when this area was submerged, supporting the explanation for their presence.

D (Populations of marine invertebrates migrated out of the Pacific Ocean and into the Andes Mountains.) YOUR ANSWER

This choice misrepresents the geological history of the region. Marine invertebrates cannot migrate to the Andes Mountains as it is not a marine environment. The fossils indicate that these organisms were once part of an oceanic ecosystem, not the result of migration to an area that has since been uplifted.

Conclusion

The presence of seashell fossils in the Andes Mountains is best explained by the historical context that these mountains were once part of the Pacific Ocean floor before tectonic activity lifted them above sea level. This explanation aligns with geological evidence, while the other choices fail to provide a reasonable account for the fossil findings. Understanding this process is crucial for comprehending the geological history of mountain ranges and their formation.

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