The Naica caves had quite a unique and rare process in how they formed. It is a known as a thermal aquifer. An aquifer is a body of saturated rock through which water can easily move. Aquifers must be both permeable and porous and include such rock types as sandstone, conglomerate, fractured limestone and unconsolidated sand and gravel. Fractured volcanic rocks known as columnar basalts also make good aquifers. The rubble zones between volcanic flows are generally both porous and permeable and make excellent aquifers. In order for a well to be productive, it must be drilled into an aquifer. Rocks such as granite and schist are generally poor aquifers because they have a very low porosity. However, if these rocks are highly fractured, they make good aquifers. A well is a hole drilled into the ground to penetrate an aquifer. Normally such water must be pumped to the surface. If water is pumped from a well faster than it is replenished, the water table is lowered and the well may go dry. When water is pumped from a well, the water table is generally lowered into a cone of depression at the well. Groundwater normally flows down the slope of the water table towards the well.
Every aquifer is unique. Volcanic thermal activity heats the water to extremely hot temperatures. The boundaries of an aquifer are usually gradational (taking place by different degrees) into other aquifers, so that an aquifer can be part of an aquifer system. The top of an unconfined aquifer is what is known as the water table. A confined aquifer has at least one aquitard at its top and, if it is stacked with others, an aquitard at its base. Continental cooling and thermal expansion make this possible.
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figure 1. Click on image for larger view.
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http://www.telegraph.co.uk/travel/picturegalleries/8660364/Amazing-caves-of-giant-crystals-inside-the-Naica-Mine-in-Chihuahua-Mexico.html
http://www.rtbot.net/crystal_caverns
http://geology.geoscienceworld.org/content/35/4/327.abstract
