Improving the longer-term record of explosive volcanism in the Mexico City region: implications for understanding volcanic hazards from diverse eruptions

Hodgetts, Alastair G.E. ORCID: https://orcid.org/0000-0002-7559-7660 (2022). Improving the longer-term record of explosive volcanism in the Mexico City region: implications for understanding volcanic hazards from diverse eruptions. University of Birmingham. Ph.D.

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Abstract

Eruption records are crucial to understanding volcanic output and eruptive processes over time. However, due to a diminishing longer-term record of explosive eruption deposits, often due to burial, erosion and cover, volcanic histories are regularly incomplete thus limiting our understanding of eruptions and their effects through time. This is particularly detrimental for densely populated regions at risk from volcanic activity.
Mexico City is one of the largest populations in the world and sits within one of the major intracontinental volcanic arcs. Risk from diverse eruptions is high, however eruption records are limited due to poor exposure. This thesis, therefore, improves upon these records by combining stratigraphic, geochemical, and geochronological results.
To achieve this, a c.500 m lacustrine core from Lake Chalco, Mexico City is probed in order to construct a detailed tephrastratigraphy and volcanic stratigraphy of the succession. It is found that volcanic deposits account for 59.81% of the composite core, comprising a maximum of 603 tephra layers, along with laharic and coarser-grained volcaniclastic facies and lavas. Volcanic output is assessed using a novel methodology and reveals that the frequency of explosive eruption deposits in the lake has generally waned over time.
Fieldwork was conducted at several volcanoes, including Popocatépetl, Iztaccíhuatl, and Tláloc-Telapón, in order to sample and obtain glass geochemical data to fingerprint possible volcanic sources of explosive eruption deposits at Lake Chalco. All three volcanoes revealed distinct fingerprints and supplementary 40Ar/39Ar dates extend the older eruptive histories of these volcanoes.
Core tephras were also geochemically investigated, with 137 layers analysed for major element glass geochemistry. It is revealed that Nevado de Toluca and La Malinche volcanoes are not chemically distinct from Iztaccíhuatl, so the three volcanoes have to be grouped for correlation with the core. Of the 137 tephras analysed at Lake Chalco; 35 are from Popocatépetl, 43 from either Iztaccíhuatl, Nevado de Toluca or La Malinche; 1 from Tláloc-Telapón; 17 from the nearby monogenetic Sierra Chichicnautzin Volcanic Field; 3 from Los Humeros caldera, Mexico and 1 from Atitlán caldera, Guatemala.
Amongst the analysed tephras, two well-dated and widespread eruptions (isochrons) are correlated into the Chalco succession: (1) the Xáltipan ignimbrite from Los Humeros and (2) the Los Chocoyos (LCY) eruption from Atitlán. Further to these chronological markers, 12 samples (plagioclase and groundmass separates) have been submitted for 40Ar/39Ar dating, although results are yet to be finalised.
This study significantly builds upon the eruptive history of the Mexico City region, identifying and characterising many newly identified eruption layers as well as correlating others. The results of this thesis are of direct relevance to hazard assessment for Mexico City, providing a longer-term assessment of explosive volcanism that has directly impacted the region in the past. The methodology used in this study may also be replicated in other regions, and sedimentary successions containing volcanic deposits.

Type of Work:	Thesis (Doctorates > Ph.D.)
Award Type:	Doctorates > Ph.D.
Supervisor(s):	Supervisor(s) Email ORCID Watt, Sebastian F.L. UNSPECIFIED orcid.org/0000-0001-9734-4210 Smith, Victoria C. UNSPECIFIED orcid.org/0000-0003-0878-5060 Branney, Michael J. UNSPECIFIED UNSPECIFIED
Licence:	All rights reserved
College/Faculty:	Colleges (2008 onwards) > College of Life & Environmental Sciences
School or Department:	School of Geography, Earth and Environmental Sciences, Department of Earth and Environmental Sciences
Funders:	Natural Environment Research Council
Subjects:	Q Science > QE Geology Q Science > QH Natural history
URI:	http://etheses.bham.ac.uk/id/eprint/13108

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