Friday, November 7, 2014

Volcanic eruptions are dangerous and hard to image. To learn about their fluid dynamics we often have to develop methods to understand the dynamics from the deposits left after an eruption. In the blog post below, one of our graduate students, Mary Benage, describes techniques she developed to better understand the dynamics of pyroclastic density currents in Ecuador.

Tying textures of breadcrust bombs to their transport regime and cooling history

At both Tungurahua and Cotopaxi volcanoes in Ecuador, there are unique pyroclasts (volcanology term for a rock formed from fragmented magma ejected out of a volcano) called breadcrust or cauliflower bombs. The reason for the name is the rocks have a breadcrusted appearance or a foamy appearance that resembles bread or cauliflower, respectively. Breadcrust bombs have a dense surface, called a rind. The rind is denser and has fewer bubbles than the interior of the rock (rind – low vesicularity, interior – high vesicularity). Breadcrust bombs range in composition and are generally tied to an eruption style called Vulcanian. There has been research on the texture and composition of breadcrust bombs (e.g. Wright et al., 2007, Giachetti et al., 2010) but previously there has not been a numerical model to examine how these bombs might form and what are the controlling factors in the formation of the crusted exterior of these volcanic rocks. We hypothesized that the thermal history of these breadcrust bombs plays a role in the rind thickness and morphology of the bombs. To test this hypothesis we developed a 3-part numerical model composed of (1) a macro-scale multiphase Eulerian-Eulerian-Lagrangian (EEL) model, (2) a clast-scale thermal and viscosity model, and (3) micro scale-bubble growth model. To test how the thermal history can affect the rind thickness, in the model we compare two end members of transport for a pyroclast: transport as a ballistic (parabolic trajectory out of the volcano) and transport in a pyroclastic density current (pyroclastic flow) with varying gas temperatures.

The macro-scale model is a complex fluid dynamics model that models the propagation and interaction of particles and gas (the two components that make up the pyroclastic density currents (PDCs) that flow down a volcano during eruption) as continuous phases, as a fluid. The two continua phases are solved using the conservation equations and many other constitutive equations (equations that tie the continua phases together and complete the conservation equations for the specific gas and particle phases). These two phases make up the Eulerian-Eulerian component of the model. The Lagrangian approach allows individual, larger pyroclasts to be tracked throughout the current. The Lagrangian approach is one-way coupled to the fluid phases. Through the Lagrangian component we can track pyroclasts through their transport regime (projectile pyroclast or pdc pyroclast) and thus the thermal history of the particle. The tracked environment of the particle is used in the clast-scale model that solves a one-dimensional heat equation (we are assuming the rocks are perfect spheres) coupled to a viscosity model. The viscosity model we use is from Giordano et al. (2008) that developed a model of magma viscosity from experiments on magma of varying composition, dissolved water concentration, and temperature. This is the part of the model where the cooling of the pyroclast is calculated and this depends on the path of the pyroclast such as its velocity or surrounding temperature. The third component of the model is the micro-scale bubble growth model that is dependent on the pyroclast composition, amount of dissolved water, temperature, and viscosity. We modify the bubble growth model of Proussevitch et al. (1993) for this part of the model. This component is crucial as it allows the model to quantify rind thickness, which is established as the radial component with the smallest bubbles.  

Through this 3-component model, we track and calculate how the transport (projectile pyroclast versus a pyroclast entrained in a pyroclastic density current) and thermal history of a pyroclast affects the temperature, viscosity, bubble size, and bubble growth rate that results in varying rind thicknesses. A clast that cools quickly has an increase in viscosity that slows or freezes bubble growth and a low-vesicularity rind is formed. If the clast stays relatively hot (its thermal environment is hot), then the bubbles are not restricted and grow to their a priori final size (numerically for the bubble growth model there is a calculated final bubble size). Through the model we determine the timescales of cooling and bubble growth are important in the formation of the rind. The morphology or lack of bubbles found in these breadcrust bombs can provide information about the cooling history or transport of these clasts. Our results show that not only does initial dissolved water content control rind thickness (which has already been shown through texture and chemical analyses) but also that the transport regime and surrounding environment (e.g. temperature) play a role in the rind thickness of these clast. The results of the model informs us that we need to examine breadcrust bombs more carefully as they may provide critical information about an eruption. We believe understanding the formation of these bombs can provide information about pre-eruptive conditions as well as transport history. There is still a lot of work to be done but the model suggests there is still more information these peculiar clasts hold about an eruption and the currents generated by the eruption. Further work will be mapping out these breadcrust bombs and analyzing their textures in detail at other locations.

We are currently applying this model to the deposits from the 2006 eruption of Tungurahua volcano in Ecuador. We will use the model to understand the cooling history and pyroclastic density dynamics of the eruption.

About Mary
Mary is currently in the fifth year of her PhD at Georgia Institute of Technology and is planning to defend her dissertation thesis in the next 6 months. Mary knew from a young age she wanted to study volcanoes. This was a result of growing up on the Bandelier Tuff, living in the nerdy town of Los Alamos, NM where everyone has a PhD, and having a desire to study science while being outdoors. Mary attended Mesa State College in Grand Junction, CO and majored in Geology and minored in Mathematics. She also participated in an NSF-REU program, attended geology field camp in Ireland, geophysics field camp in New Mexico, and the CSAV volcanology field camp in Hawaii. After graduating from Mesa State College (now Colorado Mesa University), she had a one-year post-baccalaureate job at Los Alamos National Lab (LANL) working in the Environmental group and Geophysics group. After her year off, she started graduate school at Georgia Tech working with advisor Dr. Josef Dufek. Mary has been fortunate to do field work at Tungurahua and Cotopaxi volcanoes in Ecuador, Mount St. Helens in Washington, USA, and Kos Plateau Tuff, Kos, Greece. Mary also loves to travel and tries to get out of the country once a year, loves to stay active running or hiking, wished she had more time to paint and draw, has a Little Sister from Big Brothers Big Sister of Atlanta, and is very blessed with amazing friends and family.

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