Monday, August 20, 2012

Eruptive Paleobarometers on Mars

One of the interesting topics the lab has been pursuing in the last couple of years is trying to gain a better understanding of the total atmospheric pressure on Mars. This obviously has implications for the potential stability of water on Mars surface, but also has a few important links to volcanology. Magmas ascending to the surface degas and provide a source of volatiles (e.g. water, carbon dioxide) to the atmosphere. Likewise the nature of the atmosphere can drastically alter the dynamics of eruptions.

Recently we conducted a series of experiments aimed at explaining one of the features observed by the rover Spirit at Home Plate and use it to constrain atmospheric pressure. This feature is a package of deformed sediments below a larger block of rock that has been interpreted as a bomb sag. On Earth, bomb sags are generated in explosive eruptions (often produced by magma-water interaction). During this sort of eruption base surges or pyroclastic flows are generated that accumulated deposits surrounding the blast region. Periodically larger chunks of rock (referred to as bombs) are ejected and follow near ballistic trajectories. They impact the accumulating deposit, deforming it, and leaving the impactor in place. This is then often covered by further accumulating deposits (see attached pictures and video).

Martian Bomb Sag Image
Inferred bomb sag on Mars. Courtesy: NASA/JPL.

The impact velocity of the bomb or clast is strongly controlled by the atmospheric density. A more diffuse atmosphere will lead to larger impact energies. By calibrating the impact energy required to produce a certain size bomb sag we can then deduce near surface atmospheric density and pressure if we hold other factors constant (things like gravity and size of the clast). With a ‘pumice-gun’ we were able to propel clasts in a controlled laboratory setting at sediment targets to do precisely this. (The pumice-gun propels the rocks using compressed gases --- the concept is probably familiar to all who have built a potato-gun).

Two important results came from this series of experiments. The first is that the energies implied to make the bomb sag at the Home Plate location requires atmospheric densities greater than 0.4 kg/m3 (roughly 20 times greater than mean present atmospheric density). Also to produce the continuous deformation of lower layers during the impact (which is diagnostic of a bomb sag) the sediment has to be saturated in water. This water can come from nearby environmental sources or from the explosion source as recondensed steam.

To learn more about these experiments see the associated video and release, and you can also look at some of the coverage of the Geophysical Research Letters manuscript.

Monday, August 13, 2012

The Electrodynamics of Volcanic Eruptions

Research in Progress

Josh Mendez
Graduate Student, Georgia Tech

Michoacán, México – 1953. Remigio Rita Morales and his friends stopped their afternoon game of marbles and looked up into the sky. The young P'urhépecha boys felt the ground shutter as jets of dark clouds engulfed the winter sky. Standing, the Remigio saw that the boiling gases and ash emanated from a large gash in a nearby corn field. Around the aperture, the surface moved up and down with every new explosion. The blasts echoed off the surrounding mountains—off Tacintaro, off Equijuata, off Tzirapan. On Feburary 20th, a few weeks after the P'urhépecha celebration of the New Fire (new year), a volcano was born. The new mountain was named Parhíkutini, the Place on the Other Side. 

The eruption was long; it lasted more than 9 years. Thick lava flows oozing out from two vents destroyed the town of San Juan Parangaricutiro, covering everything except for the steeples of an unfinished church. Only three people were killed in the eruption of Parhíkutini. However, it was not the sluggishly-flowing rivers of molten rock that claimed these lives (the lavas from Parhíkutini are so viscous once could easily outrun them). Death came from the skies; these unfortunate individuals were stuck down by powerful electric discharges resulting from accumulated charges in the volcano's eruptive column [1]. Volcano lightning.

USGS Photograph of volcanic lightning.

A number of other accounts regarding volcanic lightning can be found throughout human history.  Modern photography has allowed us to capture dazzling images of intricate electrical discharges enveloping clouds of hot ash and gas. However, other than of a purely observational nature, this phenomenon has received relatively little scientific attention.

Electric discharges, both natural and man-made, result when two regions accumulate huge amounts  opposite polarity charges. In a typical thunderstorm, for example, positive and negative charges exists at different heights within the thundercloud. These spatially separated charged regions forms an electric field within the cloud. The magnitude of the field, commonly expressed in volts per meter (V/m), is proportional to the charge density in each region. Now, we must remember that nature likes to minimize the energy of a system. As such, the electric field will try to bring opposite charges toward each other in order to neutralize them. Given that air is a relatively good insulator (meaning it does conduct electricity very well) electrical fields smaller than 100,000 V/m will have a difficult time  moving charge carriers around. However, larger pockets of charge will produce fields that will exceed the electrical breakdown value for air producing a massive, rapid flow of charge particles or a current. This is how you get a lightning strike [2].

The previous description of an electrical discharge holds true for volcanic lightning, however the mechanisms that produce the separation of charge within an eruptive column may be quite different from those within thunderclouds. Indeed, the physics behind the charging of ash particles is relatively unconstrained. While we by no means have a complete picture, we do have a number of clues that allow us to draw a rough sketch of the processes that cause eruptive columns become charged and eventually produce lightning.

For example, we know that when small particles, like ash or sand, collide and rub against each other they can acquire a net charge. This charging by rubbing is commonly known as the triboelectric effect. Indeed, in industrial settings where powders are moved around quite a bit, the triboelectric effect can be so powerful that it can produce sparks and explosions in the vicinity of combustible materials. As such, a number of researchers have devoted great effort to try to understand  what produces triboelectric charging at the microscopic scale and how negative effects can be mitigated [3] [4].

To test whether the triboelectric effect plays an important role in the charging of ash particles, Josh Méndez and Dan Arrington are conducting experiments using ash samples from three active volcanoes: Mount St. Helens (Washington, USA), Tungurahua (Tungurahua, Ecuador), Parhíkutini (Michoacán, México). By using  material from different volcanoes will explore how magma composition affects the magnitude of triboelectric charging. Mt. Saint Helens and Tungurahua have medium silica contents with subduction related input, typical of large stratovolcanoes. Parhíkutini, on the other hand, is a cinder cone volcano which produced magmas from a mantle source, with little subduction input and lower silica content [5].

To cause particles to collide we have developed an experimental apparatus similar to that used by Forward et al (2009) to explore triboelectric charging in other materials. The apparatus allows us to fluidize a certain quantity of and measure the resulting charge accumulated on the particles. Additionally, we are exploring how fast particles charge. At the time this blog entry was posted, experiments were currently underway and results were being prepared for   the 2012 American Geophysical Union meeting.

So, why understand volcanic lighting? The answer is simple: lightning can be detected from great (and safe) distances. By understanding the relationship between ash movements, chemistry, and lighting we may be able to probe the interior dynamics of an erupting volcano in a risk-fashion. Such a tool would indubitably be beneficial for hazard assessment, ash dispersal forecasts, in addition to gaining a better understanding of the global atmospheric electrical circuit.


[1] Paricutin, Mexico. Oregon State University.

[2] Rakov, V. and M. A. Uman (2007) Lightning: Physics and Effects. Cambridge University Press (January 8, 2007)

[3] Forward, K., D. Lacks, and R. M. Sankaran
 (2009). Triboelectric Charging of Granular Insulator Mixtures Due Solely to
 Particle-Particle Interactions
. Ind. Eng. Chem. Res. 2009, 48, 2309–2314

[4] Forward, K., D. Lacks, and R. M. Sankaran
 (2009). Methodology for studying particle–particle triboelectrification
 in granular materials. Journal of Electrostatics 67 (2009) 178–183

[5]  Verma, S. and T. Hasenaka (2003). Sr, Nd, and Pb isotopic and trace element geochemical constraints for a veined-mantle source of magmas in the Michoacán-Guanaj
uato Volcanic Field. Geochemical Journal, Vol. 38, pp. 43 to 65, 2004

Thursday, February 2, 2012

Comminution in Pyroclastic Density Currents

In explosive volcanic eruptions the grain size distribution strongly controls eruptive behavior. Examples include modifying the residence time of ash, as was illustrated in the Eyjafjallajökull eruptive episode, and altering the dynamics of pyroclastic density currents. Recently our lab has been exploring how modifications to grain size during an eruption can alter the dynamics, residence times, and ultimately the dispersal and inundation area for volcanic flows. There are numerous ways grain sizes can be modified, but we can roughly group these processes into aggregation (combining smaller particles to make bigger particles) and comminution (breaking larger particles into smaller particles). In this post we will explore some of our recent work on comminution, and in particular focus on low energy comminution. It is important to note that our experiments indicate that the style of particle breakup is energy dependent with low energy collisions breaking the bubble walls and septa in pumice. During low energy collisions and frictional grinding the particles removed from pumice are fine-grained ash leaving a progressively rounder pumice with more collisions. High-energy collisions break up particles in a macroscopic fashion and produce angular fragments.

Pyroclastic flow deposit from Mount St. Helens in 1980 showing some rounding of pumice clasts. USGS photograph.

You may be pondering, why consider particle breakup? While at face value the topic may appear somewhat obscure, the answer to this question really comes in two parts. First, particle breakup, especially low energy breakup, alters the textures of volcanic particles, and these textures can be measured in deposits. For instance, particles in pyroclastic density currents are typically rounder than fall deposits because small ash particles have been removed from the original pumice, little by little, to produce rounded pumice. By measuring the roundness of many particles we can constrain the dynamics in these currents, something we currently cannot do in real time due to the obvious hazard of placing equipment in the path of pyroclastic density currents. So particle breakup provides a metric for integrated energy in a flow.

Perhaps more importantly though, particle breakup manifestly changes the dynamics of volcanic flows. Pyroclastic density currents are mixtures of pumice, ash and volcanic gases that are denser than the surrounding atmosphere. You can typically think of them as hot, granular avalanches. They are particularly hazardous to those who live on the flanks of volcanoes because they are very energetic and fast moving. Particles in these currents can interact due to prolonged contact with each other (frictional contact) or through collisions. Both interactions are likely common in most pyroclastic density currents. The generation of greater amounts of fine particles increases the amount of particles that can be suspended by turbulence, enhances pore pressure in flows, creates larger granular drag on larger clasts, and ultimately makes flows go further. Moreover, fine particles can later be introduced to higher levels in the atmosphere due to secondary plumes.

To better understand the fate of fine ash in pyroclastic flows our group used information obtained from particle collision and frictional grinding experiments to create a constitutive relationship for ash production; basically a rule for the amount of ash produced based on the local concentration of particles and the energy of their collisions. The collision experiments used compressed gas to propel pumice at a target piece of pumice with a ‘pumice gun’, and used high-speed video to calculate the energy of the impact. A rotating cylinder filled with pumice is used to understand the prolonged frictional contact of particles. The constitutive relationship was then introduced into a computer simulation to understand the distance the flow travels and changes in particle concentration in pyroclastic density currents. We found that these numerous low energy collisions produce rounding like that observed at Mount St. Helens pyroclastic density current deposits, and the ash makes the flows travel further (an example simulation is shown below).

To learn more about this work see the following papers:

Dufek, J.  and  Manga, M. (2008) The In-Situ Production of Ash in Pyroclastic Flows. Journal of Geophysical Research, 113, B09207, doi:10.1029/2007JB005555.

Manga, M., Patel, A., and Dufek, J. (2011) Rounding of pumice clasts during transport: field measurements and laboratory studies. Bulletin of Volcanology, 73(3), 321-333.