Yellowstone National Park is home to the largest concentration of hydrothermal features in the world, including numerous geysers, hot springs, mudpots and fumaroles (steam vents). Scientists are interested in how these features change over time, not just from decade to decade but also from year to year.
Some are obviously expanding as the hot, mineral-laden water flows out of the ground, building deposits of various shapes and sizes. Sometimes, long-dormant features reactivate suddenly with expulsions that can collapse existing thermal vents. The thermal waters, depending on the chemistry of the rocks through which they percolated, precipitate minerals — primarily travertine (calcium carbonate from limestone) or sinter (opaline silica from quartz-rich rock). Deposition of these minerals is not uniform over the surface of a feature nor does deposition occur at a steady rate, ranging from millimeters to meters of change per year. This makes monitoring changes difficult.
Historic photographs are often used to monitor changes in Yellowstone’s thermal features. There are images from well over 100 years ago for some of the better-known thermal features within the park, like Giant, Castle and Old Faithful geysers. Some of these images show that significant changes have occurred, like at Mammoth Hot Springs, whereas others show little appreciable difference because the changes are occurring at rates that are not easily discernible.
Given that historical photos might not be able to capture very small changes, how can more precise measurements be collected? Walking up to an active geyser is very dangerous given the unstable ground and often unpredictable eruptions of thermal waters — people who approach or walk on thermal features have been severely injured and even killed. Even with safety in mind, scientists walking in thermal areas can cause damage to these delicate features, which Yellowstone National Park has pledged to protect “for the benefit and enjoyment of the people.” Taking measurements and obtaining imagery from a safe distance is thus important both for resource protection and safety. Hence, photography continues to be among the primary tools for documenting and monitoring change.
Structure-from-motion (SfM) photogrammetry is a method of photography that can provide the precise scale (size) of an object. Photogrammetry is the science of measuring a physical object through photography. A classic example is the use of overlapping aerial image pairs to map the Earth’s surface and generate contour lines that represent elevations on topographic maps. SfM photogrammetry relies on digital cameras and computer programs to build a 3D “virtual” representation of an object based on dozens, or even hundreds, of images. As an example, the technique has been utilized to better understand the rates of change of the travertine mounds at Mammoth Hot Springs based on photographs taken on aerial overflights. SfM photogrammetry can also be done from the ground. With high-resolution photography that includes scale bars of a precisely known size, SfM provides a way to measure a feature and compare it with other image data sets. These other data sets can be aerial photographs, historical photographs and even previous SfM models to measure how much change has occurred over time.
In 2019, Castle Geyser, Giant Geyser, Lone Star Geyser and Old Faithful were photographed for the creation of 3D models. Obtaining the needed photography for the models required walking a circular path around these features with a camera. Precisely calibrated controls of 4 or more aluminum 1 foot-scale bars with targets on their surfaces were positioned on the ground in safe locations near each feature and were visible within the photographs. The distance between the targets on each scale bar is precise to a fraction of a millimeter, which provides a very accurate scale for the final model. Using GPS data, a location was recorded for the camera and the scale bars. With the combination of accurate scale controls and positioning, it is possible to measure changes over time that are less than a millimeter!
The 3D data are also engaging to park visitors, even those who are visiting virtually. Because the geyser models show the entirety of a thermal feature, viewpoints can be shifted and viewers can even investigate the mouth of a geyser. This allows park staff to highlight some well-known features within the park, including details that park visitors might miss. These models are available for viewing by anyone via a dedicated webpage, so you can have a look from the comfort of your own home.
The collection of these data is simply a first step — there are many more geysers in Yellowstone to image. Repeat data will also be collected to assess how these features change over time, providing a view of geyser development that before was only possible in a gross sense using historical photographs. Stay tuned — we look forward to reporting on these studies in future editions of Yellowstone Caldera Chronicles.
Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory.
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