Explosive (plinian) eruptions are phenomena of violent, awe-inspiring proportions. For a plinian eruption to be possible, several factors have to coincide, including but not limited to: composition of magma, water content, and geomorphology (Freundt and Rosi, 1998). Plinian eruptions often include a combination of pyroclastic density currents, lahars, earthquakes, ash falls, and eruption columns, any one of which can cause devastating damage. Generally, the two constants in a plinian eruption are the presence of an eruption column (or several), and subsequent ash fall.

An eruption column can be broken into three parts: the gas thrust region, the convective region, and the umbrella region (Sparks, 1986). Put simply, the gas thrust region is the part of the column closest to the magma chamber where the expansion of gas has finally overcome a constraining force to explode upwards, the convective region is where air from the atmosphere is entrained and heated by the hot ash and gas exploding upward, and the umbrella region is where the gasses and ash start to equilibrate with the atmosphere (Sparks, 1986). Often, the umbrella cloud occurs at a point of atmospheric thermal boundary and is only still erupting upwards because of the momentum gained while traveling through the previous two regions (Woods and Wohletz, 1991).

To model such a dynamic and complicated feature of a plinian eruption, I stuck to the basics. The driving force of all plinian eruptions is the vesiculation of magmata volatiles in the magma chamber (Freundt and Rosi, 1998). This process of gas expansion creates the energy needed to initiate a plinian eruption, and in addition also continually “fuels” the column, resulting in a towering pillar of hot ash and gas that lasts anywhere from minutes to several hours (Carey and Sigurdsson, 1998). This steady fuelling creates a slight problem with regard to creating a comparable analogue model of an eruption column: the continual feeding of gas expansion in a reservoir is, as far as I’m aware, impossible to simulate in a laboratory setting. Because of this difficulty, a simple model has been constructed, using liquid nitrogen as the key component.

Due to an extremely low boiling temperature (-195.8°C), liquid nitrogen is unstable at room temperature (Nolan and Gish, 1996). This instability is what makes it possible to simulate the vesiculation of volatiles. If the liquid nitrogen is exposed to room temperatures, it rapidly evaporates into a gas. If it confined during the phase change process, the pressure in the confining container continuously builds until some part of the container breaks. Most of the time, if a strong enough container is used, the expansion of Nitrogen is enough so that, when the container finally breaks the built-up kinetic energy is enough to imitate the kinetic energy that drives a plinian eruption column (Carey and Sparks, 1986).

The largest example in living memory of a plinian-style eruption is the 1991 eruption of Mount Pinatubo in the Philippines (Fero et.al., 2009), shown above (photo: USGS). This event created several eruption columns that reached varying heights, culminating in the June 15th eruption that resulted in a column of estimated height of 40 km (Holasek et. al. 1996).e

The second part of this experiment has to do with studying the ash falls that stem from eruption columns. By the time entrained hot ash reaches the umbrella region of an eruption column, upward momentum is beginning to slow. As soon as gravity overcomes that momentum, every part of the column that is denser than air is going to start to descend (Veitch and Woods, 2001). The heaviest of the entrained material will fall the fastest, creating what are known as pyroclastic density currents (nuée ardents), very dangerous semi-buoyant avalanches of hot ash and rock (Branney and Kokelaar, 2002). The lighter a material is, the longer it can be held in the air and influenced by the weather, most specifically, wind. The settling of ash fall is often the only means to study a previous plinian explosion. Maps called isopleths are often created regarding ash clast size, layer thickness, and distance from the source of the eruption (Sarno-Wojcicki et. al. 1981). To simplify all these ash fall parameters, I will simply be looking at the dispersal patterns for two different types of balls to simulate two types of tephra, which is just a general name for the rocks deposited from a volcanic eruption (Koaguchi and Ohno 2001).