Chemical Fingerprinting of Explosive Eruptions from Mocho-Choshuenco Volcano, Chile

Abstract:

Mocho-Choshuenco (39°55’S 72°2’W) is a poorly studied volcanic complex in southern Chile. Previous work on the volcanic deposits identified three post-glacial (<17 ka) explosive eruptions with volumes ≥1km³. We have re-examined the field stratigraphy and carried out extensive geochemical analyses on the deposits to fingerprint and correlate the deposits to establish a detailed, high-resolution, post-glacial explosive history of the volcano. This new data indicates that there have been four large (volume ≥1km³) explosive eruptions, 12 more minor events from the main edifice, and over 40 minor scoria cone eruptions from the flanks of the volcano. No other volcano in southern Chile is known to have had as many large eruptions in the Holocene, indicating that Mocho-Choshuenco is potentially one of the most hazardous volcanoes in southern Chile.The eruption history of the volcanoes in southern Chile is largely based on field observations. However, tephra deposits are not well preserved in this region, making field mapping difficult and therefore a detailed record of the frequency and magnitude of the Mocho-Choshuenco eruptions is lacking. This work used detailed geochemistry of units within numerous outcrops (ca. 400) both proximally and distally around the volcano to correlate the units and assemble a detailed stratigraphy for the volcano. Since many of the deposits were weathered, and many others were microlite-rich, we could not rely on glass chemistry alone to correlate sequences and hence also used Fe-Ti oxide compositions.

This research has generated a large geochemical dataset of the compositions of tephra from the region (>2500 glass and >2800 Fe-Ti oxide analyses), which can now be used to correlate Mocho-Choshuenco tephra layers in the region. Chronological constraints of these units mean that these tephra layers can be used to constrain the eruptive history at other volcanoes in the region. Tephrochronology is also being used to improve the age models of lake cores, which are being used to explore earthquake frequency and the relationship between eruption frequency and climate.