La
Garita Caldera is a large volcanic
caldera located in the San Juan volcanic field in the
San Juan Mountains in
southwestern Colorado
, United States
, to the west of the town of La Garita, Colorado. The eruption
that created the La Garita Caldera was, perhaps, the largest
known explosive eruption in all of Earth's history (the
Siberian Traps may have been larger
but the cause is still being debated).
Date
The La
Garita Caldera is one of a number of calderas that formed during a
massive ignimbrite flare-up in Colorado, Utah
and Nevada
from 40–25
million years ago, and was the site of truly enormous eruptions about 28–26 million years ago,
during the Oligocene Epoch.
Province
The area
devastated by the La Garita eruption is thought to have covered a
significant portion of what is now Colorado, and ash could have
fallen as far as the east coast of North
America and the Caribbean
.
Size of eruption
The scale of La Garita volcanism was far beyond anything known in
human history.
The resulting deposit, known as the Fish Canyon Tuff, has a volume of
approximately , enough material to fill Lake Michigan
(in comparison, the May 18, 1980 eruption of
Mt.
St. Helens was only in volume).
By
contrast, the most powerful human-made explosive device ever
detonated, the Tsar Bomba or
Emperor Bomb
, had a yield of 50 megatons, whereas the eruption
at La Garita was approximately 105 times more
powerful. It is possibly the most energetic event on
Earth since the Chicxulub
impact
, which was 50 times more powerful.
Geology
The Fish Canyon Tuff, made of
dacite, is
known to be remarkably uniform in its petrological composition and
forms a single cooling unit despite the huge volume. Dacite is a
silicic volcanic rock common in explosive eruptions, lava domes and
short thick lava flows. There are also large intracaldera lavas
composed of
andesite, a volcanic rock
compositionally intermediate between
basalt
(poor in silica content) and dacite (higher silica content) in the
La Garita Caldera.
The caldera itself, like the eruption of Fish Canyon Tuff, is quite
large in scale. It is oblong shape . Most
supervolcano calderas of explosive origin are
slightly ovoid or oblong in shape. Because of the vast scale and
erosion, it took scientists over 30 years to fully determine the
size of the caldera. La Garita can be considered a "supervolcano",
albeit an
extinct one.
La Garita is also the source of at least 7 major eruptions of
welded
tuff deposits over a time span of 1.5
million years since the Fish Canyon Tuff eruption. The caldera is
also known to have extensive outcrops of a very unusual lava-like
rock made of dacite that is very similar to that of the Fish Canyon
Tuff. This rock, which has characteristics of both lava and welded
tuff, was erupted probably shortly before the Fish Canyon Tuff. The
lava-like rock has been interpreted as having erupted as thick
spatter during low-energy
lava
fountaining. The lava-like rock is also voluminous — up to
.
See also
References
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