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.
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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
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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.
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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.
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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.
