Geologic Background:
Altar is a stratovolcano that has been extinct
for nearly 1 million years. It is located 25 km E of
Riobamba, and is the fifth largest volcano in Ecuador.
Its caldera contains a large glacial lake 1 km long and
350 m wide at an elevation of 4,170 m. Six prominent
peaks rise above the lake below. The W side of the
caldera is open; a large collapse and subsequent erosion
by glaciers appear to have produced its horseshoe shape.
The volcano is mainly composed of andesite and rhyolite,
cut by andesitic dike. Two layers of andesitic breccia
make up the highest portion of the summit.
Historic Activity:
- None
- Indian legends report that the top of Altar collapsed
after seven years of activity in about 1460, but the
caldera is considered to be older than this.
Recent Activity:
- None
- On Oct. 13, 2000, a large block of andesite breccia
fell from one of the peaks surrounding the caldera, in to
the caldera lake. A large volume of water was ejected
from the caldera and formed a debris flow down west flank
of volcano. Thirteen people (and numerous livestock) were
killed by the debris flow.
Data Sources:
- Global Volcanism Network (V. 25, No. 10 ).
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