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Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Pat Shanks and Lisa Morgan, research geologist emeriti with the U.S. Geological Survey.

Monument Geyser Basin occurs west of Gibbon Canyon, south of Gibbon Geyser Basin, and topographically above Beryl Spring. This area is immediately west of and outside of the margin of Yellowstone caldera. The south-flowing Gibbon River separates the Lava Creek Tuff, which erupted during the formation of the caldera, and younger post-caldera rhyolitic lava flows to the east. The geyser basin occurs atop a thick section of Lava Creek Tuff cut by north-northwest-trending faults and fissures.

Monument Geyser Basin can be reached by hiking about 1 mile (1.6 km) from a trailhead on the west side of the Gibbon River.  The last half of the trail climbs steeply, gaining about 800 ft (244 meters) in elevation.  The elongated geyser basin is about 300 by 100 meters (984 by 328 ft), and the tan-white colors are hydrothermally altered Lava Creek Tuff capped with silica sinter deposits.  The hydrothermal alteration and sinter deposition is due to an acid-steam dominated hydrothermal system.  Many small hot springs and steam vents in the area occur as boiling pools, frying pans, mud pots, sulfur deposits, and fumaroles.  The fluids in active springs have temperatures from 71 to 91 °C (160 to 196 °F), acidic pH’s of 1.1–2.5, very low chloride from 0.5 to 3.0 ppm (parts per million), and high sulfate from 280 to 4000 ppm.  These acid-sulfate fluids are derived from vapors that contain steam, carbon dioxide, and hydrogen sulfide; the acidity is the product of carbonic acid and reaction of hydrogen sulfide with oxygen to form sulfuric acid.

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This area is unique in having about a half-dozen constructional sinter deposits, several of which are tall chimney- or spire-like features that are 1– 4 m (3–13 ft) wide at their base and up to 3 m (10 ft) in height.  These “monuments,” which give the basin its name, lie along a north-northwest trend, following the pattern of regional faults and fissures.  The tall narrow shape of the sinter spires is unusual compared to most other hydrothermal areas in Yellowstone, although they do have some similarity to hydrothermal spires found on the floor of northern Yellowstone Lake. The shape suggests that a glacially dammed lake or pond may have been present during their formation.  Submarine venting of buoyant hydrothermal fluids that precipitate silica can create tall structures similar to the sulfide chimney deposits—the famous “black smokers”—that develop from hydrothermal vents on the deep ocean floor.

Another unique feature of Monument Geyser Basin is that it caps a larger hydrothermal system along the northwest-trending set of faults or fissures that cuts through the hillside down to the Beryl Spring area along the Gibbon River, providing a three-dimensional view into the hydrothermal system. 

Beryl Spring is a strong, active alkaline-chloride hot spring pool, considered to be one of the hotter and more deeply sourced hydrothermal systems in Yellowstone based on boiling temperatures, a high chloride content of about 500 ppm, and pH in the 6.5–7.0 range.  Helium isotope studies and abundant carbon dioxide in the fluids support a deep-seated origin; temperature estimates for these types of fluids at depth are 350–400 °C (662–752 °F).

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Beryl Spring also is associated directly with vapor-dominated steam vents, one of which occurs directly behind the main pool—a testament to complicated hydrothermal plumbing that can result in vents with very different chemistry occurring right next to each other.  Deep-seated hot springs like Beryl commonly boil due to pressure decrease as the fluids ascend to the surface.  Boiling releases steam mixed with volatile gases, mainly carbon dioxide and hydrogen sulfide.  Additional vapor-dominated vents occur near Beryl Spring and as springs along the Gibbon River, including a troublesome vent beneath the road adjacent to Beryl Spring.  Nearby, to the northwest, is a hot spring with mixed high chloride and vapor-dominated fluids.  These low-density buoyant vapors have ascended along the northwest-trending faults through the thick section of Lava Creek Tuff, forming the vapor-dominated Monument Geyser Basin. 

As the vapors reach the surface at Monument Geyser Basin, cooling causes the steam to condense, and the carbon dioxide and hydrogen sulfide are absorbed into the hot liquid; both contribute to the strong acidity observed in the thermal area.  The hot acidic fluids intensely alter the high-silica Lava Creek Tuff, dissolving silica that reprecipitates as sinter and creating the silica spires and the white altered area that serves as a cap on the ascending vapors.  The result is a vapor-dominated zone beneath Monument Geyser Basin and the predominance of the low-chloride acidic hot springs in this fascinating area.