Question

read the passage.
crystal caves of mexico
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.
what is the main, or central, idea of the passage?
gigantic crystals in mexicos cave of crystals were discovered by silver miners who were excavating a tunnel.
specific conditions in the cave of crystals led to the development of the enormous crystal structures found there.

Explanation:

The passage first mentions the discovery of crystals, then focuses on the process of their formation. It explains that volcanic eruptions released mineral - rich water (with anhydrite), the cave was initially too hot for anhydrite to change to gypsum. But when the temperature cooled to about \(136^{\circ}F\), gypsum crystals began to form and this process continued as conditions remained constant. So the main idea is about the conditions (temperature, mineral - rich water, constant conditions over time) that led to the crystal formation.

Answer:

Specific conditions in the Cave of Crystals led to the development of the enormous crystal structures found there.