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Abstract

Deformation of anisotropic rocks, ice and metals mainly occurs by crystal-plastic mechanisms, where crystals dominantly deform by glide of dislocations in crystal slip planes. However, understanding and predicting the deformation of anisotropic rocks is not straightforward, especially because their behaviour under stress is very complex. This is mainly due to:

  1. the variety of deformation mechanisms that operate in polycrystalline materials,
  2. the range of deformation conditions (pressure, temperature, kinematics of deformation, etc.) that affect them, which in fact change the balance between deformation processes,
  3. the variety of resulting anisotropy and microstructures, which in turn alter the mechanical properties of the deforming rock

Two fundamental scientific problems nowadays are:

  1. how do different deformation mechanisms operate and interact in rocks?
  2. how are the rheological properties at the micro- scale control the large-scale rock behaviour?

Ice is one of the most common minerals found on the Earth’s surface. It has been widely used as an analogue for quartz- or olivine-rich rocks, as all these minerals deform following similar mechanisms resulting in non-linear flow laws. The study of polar ice gives us valuable information as we can directly link the measured strain rate in ice sheets with the microstructures observed from ice drill cores and the seismic velocities estimated in ice sheets. By numerical simulations of ice microstructure evolution during deformation and recrystallisation, we demonstrate that the mechanical anisotropy at the crystal scale is transferred to the scale of the polycrystalline rock. Mechanical anisotropy has an enormous impact on understanding and predicting localisation of deformation at different scales (Jansen et al., 2016; Llorens et al., 2016), rock strength (Llorens et al., 2017) and seismic velocity estimations (Llorens et al., 2020). These are all key for forecasting ice-sheet flow to the oceans, and for understanding past climate changes and predicting future ones. Moreover, the lessons learned from the study of polar ice microstructures and how they control physical properties can be applied to other geological problems, such us understanding flow of mantle and lower crustal rocks as well as evaporites.

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  • Lecturer: Dr. Dario Pedrazzi, Geociencias Barcelona-CSIC
  • Date: 26 January 2021; 12:00 h
  • Venue: Online (via Conecta: http://bit.ly/3ioG6fS)
  • Further information:Dr.Juan Alcalde (GEO3BCN-CSIC)

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Abstract

In 431 AD, the Mayan civilization was devastated by the violent eruption of the Ilopango volcano, located in what is now El Salvador. The eruption called TBJ, “Tierra Blanca Joven-Young White Earth” due to the characteristic whitish color of the deposits, produced a total volume of magma of approximately 55 km3, with the formation of dense and dilute Pyroclastic Density Currents, with associated caldera collapse and the formation of a coignimbritic column that reached the stratosphere.

The recently work published by Smith et al. (2020) has allowed to accurately dated this eruption to AD 431. (with a margin of error of two years), thanks to the identification of ash fragments from the TBJ eruptive column found inside an ice core in Greenland. This dating has been correlated with the interval obtained with the Carbon 14 method on the remains of trees "in situ", i.e. found in the deposits of the eruption. The sulfur content of volcanic origin found in the cores of Polar ice has also revealed a decrease in global temperatures of around half a degree Celsius in the years after the eruption.

Thanks to the use of a 3D model, it has been possible to estimate the dispersion of the ash and the height of the eruptive column capable of reaching 45 kilometers, transporting volcanic products for more than 7,000 km to polar latitudes.

Further information

Smith, V., et a l'(2020)., The magnitude and impact of the 431 CE Terra Blanca Jove eruption of Ilopango, El Salvador. PNAS. doi: https://www.pnas.org/cgi/doi/10.1073/pnas.2003008117

Pedrazzi, D., Sunye-Puchol, I., Aguirre-Díaz, G., Costa, A., Smith, V. C., Poret, M., … Gutiérrez, E. (2019). The Ilopango Tierra Blanca Joven (TBJ) eruption, El Salvador: Volcano-stratigraphy and physical characterization of the major Holocene event of Central America. Journal of Volcanology and Geothermal Research, 377, 81–102. https://doi.org/https://doi.org/10.1016/j.jvolgeores.2019.03.006

Suñe-Puchol, I., Aguirre-Díaz, G. J., Pedrazzi, D., Dávila-Harris, P., Miggins, D. P., Costa, A., … Hernández, W. (2019). The Ilopango caldera complex, El Salvador: Stratigraphic revision of the complete eruptive sequence and recurrence of large explosive eruptions. Journal of Volcanology and Geothermal Research, 374, 100–119. https://doi.org/https://doi.org/10.1016/j.jvolgeores.2019.02.011

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