IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3A2_3F-O11Room A3Date/Time: July 23 12:00-12:15

Tephra in Argentina establishes postglacial eruptive history of Laguna del Maulevolcanic field in Chile
Judy Fierstein1 , Patricia Sruoga2 , Alvaro Amigo3 , Manuela Elissondo2 , Mario Rosas2
1U.S. Geological Survey, USA, 2 SEGEMAR, Argentina, 3 SERNAGEOMIN, Chile
E-mail: jfierstn@usgs.gov

The Laguna del Maule (LdM) volcanic field covers 500 km2 of rugged glaciated terrain with Quaternary lavas
and tuffs on both sides of the Argentina-Chile border in the Southern Volcanic Zone. At least 130 separate
vents erupted >350 km3 of products since 1.5 Ma (Hildreth et al., 2010). These include a ring of 36 postglacial
rhyolite and rhyodacite coulees and domes that erupted from 24 separate vents and encircle the lake, suggesting
persistence of a large magma reservoir. The young, glassy, and largely non-overlapping lavas are difficult to date by
40Ar/39Ar. But whole-pumice and microprobe chemistry of downwind fallout in Argentina, and radiocarbon datingof intercalated soils, establish eruptive stratigraphy and timing of events for the postglacial eruptions at Chilean
vents around Laguna del Maule.
A tri-country collaboration among the geological surveys of the United States. Chile, and Argentina, have now
established that a wide area east of the volcanic field was blanketed by at least 3 plinian explosive eruptions from
LdM sources, and by at least 3 more modest, but still significant, eruptions. In addition, an ignimbrite from the LdMBarrancas vent complex on the border near the SE corner of the lake traveled at least 15 km from source and now
makes up a pyroclastic mesa at least 40 m thick. This ignimbrite (72-75% SiO2) preceded a series of fall deposits
that are correlated with eruption of several lava flows that built the Barrancas complex.
Recent 14C dates suggest that most of the preserved LdM fallout eruptions were between 7 ka and 3 ka. However,
the oldest and perhaps largest fall unit yet recognized is correlated with the Los Espejos rhyolite lava flow thatdammed the lake and yields a 40Ar/39Ar age of 19 ka and a (calibrated) radiocarbon age of 14 ka. Pumice clasts
as large as 8.5 cm and lithics to 4 cm were measured 32 km ENE of source. It is the only high-silica rhyolite(75.5-76% SiO2) fall layer yet found, correlates chemically with the Los Espejos rhyolite lava flow, and includes
distinctive olivine-bearing lithics that are correlated with mafic lavas which underlie the Espejos vent. Extremely
frothy pumice found near the vent is also consistent with the bubble-wall shards and reticulite pumice distinctive of
the correlative fall deposit.
Another large rhyolite fall deposit (74.5% SiO2), 4 m thick 22 km E of source, has pumice clasts to 9.5 cm andincludes ubiquitous coherent clasts of fine, dense soil that suggests it erupted through wet ground; 14C dates(uncalibrated) yield ages <7 ka. Stratigraphic details suggest that pulses of fallout were accompanied by small
pyroclastic flows. Ongoing field and lab work continues to build the LdM postglacial eruptive story. The numerous
postglacial explosive eruptions from the LdM field are of significant concern because of ongoing 30 cm/year uplift
along the western lakeshore, as measured by InSAR.

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