Final answer:
Animal migration would have the least impact on already formed fossils as it does not disrupt the geological layers where fossils are found. Geophysical processes like earthquakes and extreme weather can significantly affect the integrity of fossils.
Step-by-step explanation:
The question asks which factor would have minimal or the least impact on already formed fossils. Among the options provided, animal migration would likely have the least direct impact on the integrity of existing fossils. Natural events such as earthquakes, extreme weather, uplift of the Earth, and other geological phenomena like volcanic eruptions and continental drift can significantly alter the landscape and may potentially damage or bury fossils, making them harder to recover. Animal migration, on the other hand, typically does not disrupt the geological layers where fossils are found and would have minimal effect on the fossils themselves. This is an important aspect of biogeography and biodiversity, which also involves understanding how species and ecosystems are affected by environmental changes.