Evenly spaced columns in the Bishop Tuff (California, USA) as relicts of hydrothermal cooling

Abstract

A few square kilometers of the Bishop Tuff in eastern California (USA) have evenly spaced columns that are more resistant to erosion than the surrounding tuff owing to the precipitation of mordenite, a low-temperature (100-130 °C) zeolite. We hypothesize that the columns are a result of instabilities at the liquid water and steam interface as cold water seeped into the still-cooling Bishop Tuff. We use two methods to quantitatively assess this hypothesis. First, scaling shows which hydrodynamic instabilities exist in the system. Second, to account for the effects of multiphase flow, latent heat, and the finite amplitude and temporal evolution of these instabilities we use two-dimensional numerical models of liquid water infiltrating hot tuff. These tests highlight several features of boiling hydrothermal systems. (1)The geometry of at least some convection appears to be broadly captured by linear stability theory that neglects reactive transport, heterogeneity of the host rock, and the finite amplitude of instabilities. (2) Slopes >10% set the wavelength of convection, meaning that these columns formed somewhere with relatively gentle topography. (3) For permeabilities of >10 −13 m 2 , the wavelength of the instability changes through time, slowing infiltration, while for permeabilities <10 −15 m 2 , cooling is dominated by conduction. The spacing and stability of columns increase with higher vertical permeability and decrease with higher horizontal permeability. These columns are a rare window into hydrothermal processes that may be widespread.

Exposed columns in the Valles Caldera ignimbrites as records of hydrothermal cooling, Jemez Mountains, New Mexico, USA

“…The columns are generally vertical, have approximately uniform width, and are more resistant to erosion than the surrounding non-welded ignimbrite due to precipitation of an ultra-fine cement. Similar structures have been reported in the 760 kaold Bishop Tuff, California, USA (Randolph-Flagg et al, 2017) and in the Rio Caliente ignimbrite, Mexico (Wright, 1979(Wright, , 1981.…”

“…The formation of these minerals cements the ignimbrite matrix producing a coherent rock that Fenner (1948) termed sillar. However, as we document in this paper, columns in the Bandelier ignimbrites were cemented at much lower temperatures, which is also the case for the Bishop Tuff columns (Randolph-Flagg et al, 2017). Cas and Wright (1987) described features similar to the Bandelier Tuff columns in the Rio Caliente Ignimbrite, La Primavera caldera, Mexico and attributed them to steam pipes (Wright, 1979).…”

“…Experiments by Bish et al (1982) suggest that clinoptilolite to mordenite diagenesis occurs between 100 and 180 • C. Passaglia et al (1995) found that rhyolite tuff associated with domes and flows of Ponza Island, Italy, have been altered to a mordenitesmectite assemblage and concluded that alteration of the glass occurred by percolating meteoric waters, heated by post-volcanic events. Short-lived meteoric-sourced hydrothermal activity has also been recognized for the Bishop Tuff (Holt and Taylor, 1998;Randolph-Flagg et al, 2017) and, perhaps, in the much smaller 1980 Mount. St. Helens ignimbrite at the "pumice pond" (Seligman et al, 2018).…”

“…We show that the physical location (both geographically and in the deposit), field characteristics, chemical composition, and petrography of the columns in the Bandelier Tuff are inconsistent with the fossil fumarole hypothesis. We hypothesize, based on a previous study of similar columns in the Bishop Tuff (Randolph-Flagg et al, 2017), that the columns in the Valles Caldera ignimbrites record the geometry of surface water that boiled as it seeped into warm ignimbrite, but far below the temperatures of fumarolic alteration. We also consider the hypothesis that the columns formed due to a source of steam underneath (as per Cas and Wright, 1987;Lipman, 2018) and we also assess whether our model for downward heat and fluid transport can explain the observed vertical orientation, width, and spacing of the columns, and the inferred alteration temperature.…”

“…5c and e) and occasionally even steeper (to 45 o ), including a few sub-horizontal examples. Inclined and fanning column groups are more common in the Bishop Tuff examples (Randolph-Flagg et al, 2017). Columns are never fines-depleted and some show an annular rim with more clay-rich material that may postdate the initial hydrothermal alteration.…”