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Volcanoes. One of nature’s most magnificent phenomena. These life-giving, yet destructive forces are also creators of a rockhound’s treasure: agates. Although some agates form in sedimentary deposits, the majority are actually secondary deposits formed in lava.


Lava, the molten rock that reaches the surface of the earth through volcanic eruptions, often contains gas bubbles called vesicles. As the gas bubbles escape, the lava becomes frothy, resulting in a very light rock material filled with tiny bubbles of gas. This rock, pumice, is so light the rock can float in water! Do you have a ‘pumice stone’ at home to scrape the dead skin off your feet? Take a bucket of water and watch your stone float in water! This is a product of those vesicles with gas bubbles.

Photo Courtesy of Geologyin.com

Over time, and with proper conditions, a lava’s vesicles may be filled with fluid. Oftentimes that fluid is rich with dissolved chemicals, specifically silica. This is the main component of all quartz – both the crystalline and cryptocrystalline forms. As fluids flow through these vesicles, the silica concentration becomes supersaturated. This means that the amount of silica overwhelms the carrying capacity of the fluid. The silica forms a gelatin-like substance, and microscopic crystals may start forming. As more and more of this substance builds up within the vesicle, the most recognizable feature of agates, banding, may start forming. This is caused by different concentrations of trace elements in the silica. Remember, the most important rule of mineral identification is that color is not a good indicator, because any trace element can influence a color change!

 

This process will continue until the entire vesicle is filled in by cryptocrystalline silica. If there is enough fluid, the result will be an agate. If there is not enough fluid available, then an empty cavity may remain, allowing the microcrystals to form larger crystals, like those found in a geode. Erosional process will ultimately break down the surrounding rock material, releasing the harder, more resistant agate from its lava home.

Geology in Action!

A similar analogy for the process of agate formation is the salt crystal experiment. Try this cool experiment at home. Slowly add salt to a small cup of water until no more salt will dissolve. As the water evaporates away, watch as the salt crystals grow! When trying this at home, always make sure to have an adult help you. Another similar experiment is available here! (Note: this is how rock candy forms!)