Journey into the roots of a hydrothermal vent system!

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

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You may have seen documentaries that feature bizarre-looking creatures living near submarine hydrothermal vents that spew high-pressure streams of cloudy looking water. Such sites are often associated with mid-ocean ridges, but there are also dozens of active hydrothermal vent fields on the floor of Yellowstone Lake. Have you ever wondered what it would be like to see what underlies these vents? Today's your lucky day!

Material filling the dissolved voids includes clasts of wall rock as well as beach sediments being washed into the voids. The lower structure on the left is about 0.7 m (2 ft) wide and 1.5 m (5 ft) high. Note the fracture alignment above the void structure on the right.

Multiple hydrothermal breccia pipes are exposed along the northern and western shores of Yellowstone Lake and provide clues into the shallow roots of the active vent systems. Geologically, they are referred to as breccia pipes because they are made up of broken bits of their diatom-rich lacustrine sedimentary host rock. The structures discussed here are generally small ranging from <1-3 m (3-10 ft) in diameter to as much as 1-5 m (3-16 ft) high. They have a cylindrical shape that widens toward the top where fluids are discharged.

The Yellowstone Lake breccia pipes exist because the slight inflation and subsidence associated with deformation of Yellowstone caldera causes the ground to develop fractures, which become pathways for hydrothermal fluids to travel. As breccia pipes form, the hydrothermal fluids dissolve the silica-rich host rock creating voids into which the adjoining host rock collapses. Continued up flow of hydrothermal fluids, rich in silica and hydrogen sulfide, causes quartz, sulfur, pyrite, other sulfides, and various clay minerals to fill in and recrystallize in small spaces between the broken host rock (kind of like grout between bathroom tiles).

The western shore of Yellowstone Lake hosts a young batch of breccia pipes. They form along northeast- and northwest-trending fractures that are related to ongoing caldera deformation of the Elephant Back Fracture Zone. Fluids flow up through existing fractures, which feed into the young and actively forming pipe structures. Several are in close proximity of each other and in various stages of formation.

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An inactive mineralized breccia pipe, informally referred to as the Black Dog hydrothermal breccia pipe, has been studied along the northern shore of Yellowstone Lake. It was once exposed in the bluffs along the shoreline, but erosive wave action caused the feature to topple out of the cliff face. The "black" in its name refers to its dark color, which reflects the finely disseminated pyrite and other sulfides (pyrrhotite, arsenopyrite) present in the siliceous breccia matrix (grout).

The Black Dog breccia body has characteristics similar to the breccia pipes now forming on the western shore. The Black Dog pipe, however, is a larger, resistant, more mature, and developed body that contains intensely mineralized breccia clasts in a mineralized matrix. The structure is located about 2 km (1.25 mi) west and outside of the Mary Bay explosion crater. Hydrothermal fluids ascended along joints in the base of the breccia body. In recent years, measured temperatures of fluids seeping out along these joints were about 18 °C (64 °F); an orange bacterial mat is associated with these seeps. Small, active hydrothermal springs exposed on the shoreline about 60 m (200 ft) to the east of the mineralized breccia pipe have diameters of 15 cm (6 in) with temperatures of 33-42 °C (91-108 °F) and neutral pH; this vent is covered with orange and green bacterial mats. These active springs likely represent the waning stages of a hydrothermal systems that probably had temperatures of at or above boiling on the lake floor.

The Black Dog breccia pipe has been slowly eroding due to strong wave action from Yellowstone Lake. Present exposures suggest that the breccia body was an open and vigorous hydrothermal system prior to emplacement of the Mary Bay explosion deposit about 13,000 years ago. The Mary Bay hydrothermal explosion breccia and its associated lower dark sand unit are found directly on top of the breccia body; both units are completely silicified and formed a local resistant knob suggesting that hydrothermal activity continued for some time after the explosion.

Breccia pipes are undoubtedly common in Yellowstone Lake below many of the active hydrothermal vents. The shorelines of Yellowstone Lake provide an opportunity to have a view into the shallow upper level of hydrothermal systems which feed hydrothermal vents we generally only can see as hot springs on the lake floor.