Geologic Background:
In south-central Chile, the Benioff zone dips
25° and has been the site of large, shallow
earthquakes. Navados de Chillan is about 100 km inland
from the epicenter of a great earthquake in 1835, and
several hundred kilometers south of the epicenters of
several more great earthquakes of the 19th and 20th
centuries.
Voluminous pyroclastic-flow deposits reach from the
Nevados de Chillan area to the central valley of
south-central Chile, and a 8 km x 10 km caldera has been
suggested at Nevados de Chillan. However, only a 7 km x
4.5 km diameter caldera is clear. About 10 cones and
domes are aligned along a northwest- trending axis
through Nevados de Chillan Caldera, of which Volcan Nuevo
is the youngest.
The compound volcano of Nevados de Chillan is one of
the most active volcanoes of the Central Andes of Chile.
Three late-Pleistocene to Holocene stratovolcanoes were
constructed along a NNW-SSE line within nested
Pleistocene calderas, which produced ignimbrite sheets
extending more than 100 km into the Central Depression of
Chile. The largest stratovolcano, 3,212-m-high Cerro
Blanco, is located at the NW end of the group, and
3,089-m-high Volcan Viejo (Volcan Chillan), which was the
main active vent during the 17th-19th centuries, occupies
the SE end. The new Volcan Nuevo stratovolcano formed
beginning in 1861 between the two volcanoes, and has been
the most active vent since, growing to exceed Volcan
Viejo in altitude.
Historic Activity:
- Eruptions in the 1600's, 1750, and 1835.
- The 1861 eruption commenced a few months after a
disastrous earthquake near Mendoza, Argentina, and the
simultaneous extenguishing of Volcan Antuco ( a few
hundred kilometers from Nevados de Chillan). The vent for
the 1861 eruption was Cerro Negro at the NW foot of Cerro
Blanco. This eruption continued into 1862.
- Eruptions in 1864-1865, 1872, 1877, 1883, 1891, and
1898.
- A small Strombolian eruption at Nevados de Chillan
began on August 6, 1906, 10 days before a strong
earthquake that destroyed Valparaiso. Volcan Nuevo formed
during this eruption.
- Eruptions in 1923, 1929, 1934, and 1945-1947.
Recent Activity:
- New(?) fumaroles have been observed since 1965,
especially on the slopes of Volcan Nuevo.
- Remarkable gas emissions occurred in July 1973 along
the axis of the caldera, and by December 1973 a new cone
(dome) began to form on the slope of Vulcan Nuevo.
- Small, rhythmic phreatomagmatic eruptions ended about
June 1983, and only a few explosions had been reported as
of 1988.
Data Sources:
- Smithsonian's SEAN Bulletin (V. 4, No. 7)
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