Question

the main, or central, idea of the passage is specific conditions in the cave of crystals led to the development of the enormous crystal structures found there. how does the detail in bold develop the main idea?
while excavating a tunnel under mexicos naica mountain, silver miners were stunned to discover a cavern full of luminous, elongated underground crystals. composed of the mineral gypsum, some of the crystals extended as far as thirty - six feet into the air. what accounted for these amazing mineral structures? millions of years ago, volcanic eruptions released a large amount of mineral - rich water into the cave, now known as the cave of crystals. one of those minerals was anhydrite, which slowly changes into gypsum when exposed to water. originally, the cave was too hot for this change to take place. eventually, however, the temperature cooled to about 136°f, and gypsum crystals began to form. this process continued as the temperature and other conditions in the cave remained constant for thousands of years.
it challenges the influence of volcanic eruptions on the formation of gypsum crystals.
it identifies a condition that led to the formation of gypsum crystals.

Explanation:

The bolded detail "Millions of years ago, volcanic eruptions released a large amount of mineral - rich water into the cave" provides a specific condition (the release of mineral - rich water from volcanic eruptions) that was part of the process leading to the formation of gypsum crystals (as anhydrite in the mineral - rich water can change to gypsum under certain conditions). It does not challenge the influence of volcanic eruptions but rather supports the idea that specific conditions (including volcanic activity) in the cave led to crystal formation.

Answer:

It identifies a condition that led to the formation of gypsum crystals.