Simulations of large, wet volcanic eruptions

Citation: Van Eaton, A. R., M. Herzog, C. J. N. Wilson, and J. McGregor (2012), Ascent dynamics of large phreatomagmatic eruption clouds: The role of microphysics, J. Geophys. Res., 117, B03203, doi:10.1029/2011JB008892.

Lake Taupo, New Zealand. The eruptions from this volcano were so large that the erupted material left behind a huge space below the surface that caused the surface to subside and form a caldera, or a collapsed volcano. Afterwards, the lake filled in the hole created by the subsidence. Image source

Interactions between magma and surface water can play a significant role in the development of ash clouds in explosive volcanic eruptions. The authors of this study looked at such interactions by simulating volcanic eruptions with the computer model ATHAM (Active Tracer High-resolution Atmospheric Model). 2-dimensional simulations in ATHAM were run for a variety of different surface water contents, (between 0 and 40%) such as erupting into a lake. Their results indicate that increased water content can significantly alter the size and shape of the ash cloud. Previous studies have found that more surface water can shorten the height of the eruption cloud. However, this work by Van Eaton and others shows that that statement isn’t always true. For example, with the addition of more water (greater than 24%), simulations showed an eruption column partially collapsing and then dispersing into a high ash cloud. This shows that even smaller eruptions with column collapses can reach the same altitudes as larger eruptions, all thanks to the availability of moist air (see Figure 1). 

As part of this study, the authors used field data from a past eruption of Taupo volcano in New Zealand. The eruption occurred about 27,000 years ago and is known as the Oruanui supereruption. This particular eruption was chosen because it was a very large, wet eruption that dispersed ash far away from the volcano despite not having a very stable eruption column. We know that it was a wet eruption because of the abundance of ash aggregates in the deposits around the volcano. Ash aggregates are formed when wet ash collides with other ash particles in the air and then combine by sticking together, forming a larger particle. The authors used information about this eruption in ATHAM to determine how water content can impact an eruption and produce such widely dispersed ash patterns seen at Taupo volcano.

Figure 1: Cross-section of two eruptions into different atmospheres. With the availability of more water in a wet eruption, there is more exchange of heat and energy when water cools and becomes ice. These heat exchanges from phase changes impacts the size and shape of the eruption column by feeding or removing energy and giving the ash column the ability to rise higher. The grey box represents the troposphere, the atmospheric layer in which we all live. The black lines indicate different ash concentrations, increasing by an order of magnitude from an initial 0.01 g/m³ at the outer edge. 

Why do we want to know how water can impact an eruption? Well, as it’s been shown by the authors, water can play a significant role in plume development and therefore ash dispersal, which has implications for air traffic safety and accurately forecasting where the ash will go. For their simulations with water content above 10%, the ash column becomes increasingly unstable (or not buoyant) and some of the rising ash collapses back down toward the earth’s surface. Some of the collapsed ash rises back up into the atmosphere, similar to ash in a forest fire, and continues to disperse far away from the volcano. This type of eruptive style leads to more ash emplaced into the troposphere than an identical, stable eruption in which more ash is injected into the stratosphere. This leads to greater difficulty in forecasting ash dispersal, especially in an emergency context, where there is concern for aircraft safety or ground population.

Another important finding in this study is the effect of the tropopause on the maximum column height. The tropopause is a thermal boundary where eruption columns tend to stop rising and its height varies between 7 and 15 km high, depending on your location on earth. Ash often rises to the tropopause, at which point the ash levels out (think: smoke from a fire rising up and spreading out on the ceiling in a room). The fraction of ash that successfully penetrates through the tropopause can disperse in the stratosphere or fall back down. Large modeled eruptions in this study, regardless of their water content, were able to easily reach the tropopause, indicating that 1) the eruption output rate is a major controller on volcanic emissions and 2) the tropopause can play a significant role on the maximum column height reached by volcanic eruptions. 

Where are the geothermal resources in Southeast Idaho?

Figure 1. A summary of the geothermal systems in the Great Basin. The study area is focused in the NE area of the basin, in SE Idaho. From McCurry and Welhan (2012). 

Lots of magmatic heat resides below the surface around the Snake River Plain region in Idaho, as evidenced by the Yellowstone hot spot and regional volcanism. However, SE Idaho seems to lack obvious signs of thermal activity at the surface (Figure 1). The presence of the hot spot and other volcanics in the area should provide a reasonably good source for geothermal energy. If there is at least some magma body residing in the shallow crust to produce geothermal resources, then we expect to see some type of response at the surface (think hot springs like at Yellowstone). However, the expression of these geothermal resources at the surface in SE Idaho is not as strong as expected.

Three hypotheses are presented in this paper to explain this phenomenon. The first hypothesis states that there are no easily accessible magmatic heat sources in the area. This may be due to a lack of any magmas near the earth’s surface and instead are located too deep within the earth for us to access or detect. Also, it could be that any magmas that were once close to the surface had already erupted, preventing us from using them as a heat source today. Hypothesis 1 is unlikely because geotechnical seismic work indicates a significant magma storage exists in the mid- to upper-crust. This indicates that there is at least some magmatic fluid in the “shallow” crust.

The second hypothesis is that there is physically accessible magmatic heat but the amount of heat available is relatively low. This could be due to a low permeability layer (or in other words, a rock layer that prevents heat or fluids from travelling through it), preventing us from sensing the heat at the surface. Hypothesis 2 is also unlikely because previous work has demonstrated that the H2O content in the magma was 2-6%, which is comparable to other magma systems in the Basin and Range, and indicates the magma is not dry.

Figure 2. A conceptual model for the China Hat dome field and Blackfoot Reservoir rift zone. Modified from Autenrieth et al. (2011). This figure illustrates the movement of magma through faults toward the NE, away from the source. Original paper details the abbreviations. From McCurry and Welhan (2012). 

The third hypothesis states that there are geothermal systems in the area, but we don’t see them as well at the surface because the heat is reduced or diverted away. For example, a large, shallow water aquifer below the surface could absorb some of the heat that migrates toward the surface. Also, there may be fractures below the surface that allow the heat to migrate along the fracture paths away from the original magma source. Such a scenario may produce heat signs somewhere else in the area. Hypothesis 3 is favored due to the presence of a large groundwater system in the area that could dilute or divert thermal responses from deeper high-temperature magmatic fluids. Additionally, the study area contains west-dipping faults in the subsurface, allowing for magmatic fluids to travel away from its source (Figure 2).

Recent volcanic fields (less than 2.6 Million years old) in SE Idaho point towards a significant storage of magma and heat energy in the upper crust between 2 and 15 km deep. This region may be a strong candidate for future hydrothermal exploration work. However, the presence of a broad aquifer in the subsurface poses challenges to studying this type of resource where migration of magmatic heat is involved.

Paper: McCurry, M., and Welhan, J. (2012)Do Magmatic-Related Geothermal Energy Resources Exist in Southeast Idaho? GRC Transactions V36, p699-707.

http://www.researchgate.net/publication/235654596_Do_Magmatic-Related_Geothermal_Energy_ResourcesExist_in_Southeast_Idaho/file/79e41512510a13e1c4.pdf