Final answer:
The presence of similar igneous rocks indicates past volcanic activity, suggesting that the main answer is that a nearby volcano had once been active.
Step-by-step explanation:
The observation that many igneous rocks are similar in a particular location suggests a common origin for these rocks. In the context of volcanic activity, the presence of similar igneous rocks typically indicates past volcanic activity, rather than a prediction about future eruptions or the current size of a volcano. Therefore, the main answer to the student's question would be A nearby volcano had once been active.Igneous rocks are categorized as either plutonic or volcanic based on their formation. Plutonic, or intrusive, rocks form when magma cools slowly underground, while volcanic, or extrusive, rocks form when lava cools rapidly on the surface after an eruption. Identifying these rocks helps geologists understand the geological history of an area. Considering that the igneous rocks in question are very similar, they likely result from a common source or event, such as past eruptions from a once-active volcano.Furthermore, the nature of these igneous rocks and their location can give clues to the tectonic setting of the past. For instance, with reference to Figure 2.24 from the Peruvian Andes, we see both intrusive and extrusive rocks associated with past Neogene volcanic activity. If these rocks are similar to the extrusive volcanic rocks of the Neogene shown in that figure, it would further support the conclusion that the volcano in question was active in the past, during that geologic time period.