Geysers—what exactly are they made of?

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Dakota Churchill, physical scientist with the U.S. Geological Survey.

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One of the most enticing attractions for visitors arriving in Yellowstone National Park every year is the park's iconic geysers—about half of all geysers in the world are in Yellowstone! As scientists, we are interested in understanding how geysers work for a variety of reasons. They are used as safe, small-scale models for volcanoes, and by understanding the processes that control geyser eruptions in Yellowstone, we improve not only our understanding of volcano and geyser dynamics, but of geothermal systems elsewhere. Previous editions of Yellowstone Caldera Chronicles have addressed many related topics such as why geysers are rare, and what causes them to erupt, in addition to their plumbing systems and water chemistry. In addition to understanding how they work, we also want to understand what they are made of, one reason being that by characterizing the composition of deposits formed by geyser eruptions, we can obtain a better understanding of when and how ore deposits, such as gold, are concentrated. And so today, we shed light on what makes up geyser cones.

First, a brief overview on how geysers form. It begins with groundwater flowing through rhyolitic rocks. These rhyolitic rocks mainly consist of silica (SiO₂) and are heated by a deep magma body beneath the Yellowstone Caldera. As hot groundwater flows through the rhyolitic rocks, it dissolves parts of the rock. The amount of silica that the groundwater can dissolve depends on several parameters, but mainly on temperature. At high temperatures, groundwater dissolves more silica from the rock than it could if it was at a lower temperature. When this water reaches the surface and erupts as a geyser, the silica-rich waters cool to the surrounding temperature and evaporate. Silica is left behind and forms a deposit of sinter, also known as Geyserite.

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When Geyserite is first deposited, it is composed of non-crystalline opal-A (with the "A" standing for amorphous), and roughly 10% of its weight is due to water. Opal-A is the same type of opal commonly thought of when one pictures opal gems, although the opal-A deposited by geysers does not appear iridescent like the gem variety. One intriguing aspect of sinter is that its mineralogy changes with time. Sinter matures progressively from opal-A, opal-A/C, opal-A/CT, opal-CT, opal CT + quartz, and finally quartz (with the "C" standing for the mineral cristobalite and "T" standing for the mineral tridymite).

Opal-A, cristobalite, tridymite, and quartz are all polymorphs of each other, meaning they have the same mineral composition: SiO₂; however, the way their atoms are arranged is distinct. Cristobalite, tridymite, and quartz are all crystalline minerals, meaning their atoms are organized in a defined pattern. In contrast, amorphous opal-A is not crystalline and therefore has no long-range order. Because there is a natural preference for order, as time passes, small patches of SiO₂ within the body of opal-A will organize themselves into a crystalline structure, forming cristobalite (opal-A/C) and tridymite (opal-A/CT). These patches grow and continue to mature, forming quartz, until the sinter loses all amorphous content and reaches full crystallinity (opal-CT, opal-CT+ quartz, quartz). Further, as the sinter transitions from amorphous opal-A into its crystalline polymorphs, water is squeezed out of the mineral's crystal structure, and so by the time sinter becomes fully quartz, its water content decreases to almost 0%.

Another important aspect of the formation of silica sinter is the interaction between the precipitating SiO₂ and the microbial mats that grow on these hydrothermal deposits. The microbial mats can be seen as colorful layers of organic material often coating the sinter. The microbial communities actually promote sinter deposition from groundwater that reaches the surface. As silica is constantly being deposited on the mats, the microbial mats eventually become silicified and can eventually form up to 50% of the sinter's volume.

Understanding the mineralogy of sinter, and how and why it changes with time, is important for a variety of reasons. For one, since the maturation from opal-A to quartz has been studied and documented at various geysers around the world, mineralogy can provide relative ages for the deposits, which improves our ability to interpret geyser growth over time. Furthermore, when the hot water underground is dissolving the silica from the rocks, it is also dissolving other elements as well. These elements can either get stuck in the sinter or be lost with the water. By analyzing the chemical composition of a geyser's sinter and water, we can learn about the conditions in which groundwater and rocks are interacting in Yellowstone's subsurface. Geysers are also economically valuable to society, because hydrothermal systems are the sites of many ore deposits. Finally, as the siliceous sinter forms, organic material can get trapped between its layers. Because the organic material originates from plants and microbes that grew over time and under a variety of environmental conditions, sinter deposits contain climate records stretching back to when glaciers covered Yellowstone.

By combining all these insights, these stunning natural phenomena can be used to tell the story of the hydrothermal history of an area.