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.
References:
[1] Paricutin, Mexico. Oregon State University.
http://volcano.oregonstate.edu/vwdocs/volc_images/img_paricutin.html
[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
