La Garita Caldera is a large
volcanic3. Conduit
4. Base
5. Sill
6. Dike
7. Layers of ash emitted by the volcano
8. Flank| 9. Layers of lava emitted by the volcano
10. Throat
11. Parasitic cone
12. Lava flow
13. Vent
14. Crater
15...
calderaA caldera is a cauldron-like volcanic feature usually formed by the collapse of land following a volcanic eruption such as the ones at Yellowstone National Park in the US and Glen Coe in Scotland. They are sometimes confused with volcanic craters. The word comes from Portuguese caldeira, and this...
located in the San Juan volcanic field in the
San Juan MountainsThe San Juan Mountains are a rugged mountain range in the Rocky Mountains in southwestern Colorado. The area is highly mineralized and figured in the gold and silver mining industry of early Colorado. Major towns, all old mining camps, include Creede, Lake City, Silverton, Ouray, and Telluride...
in southwestern
ColoradoColorado is a U.S. state located in the Rocky Mountain region of the United States of America. It may also be considered to be part of the Western and Southwestern regions of the United States. Colorado entered statehood in 1876 and was nicknamed the “Centennial State”...
,
United StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
, 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 TrapsThe Siberian Traps form a large igneous province in Siberia. The massive eruptive event spans the Permian-Triassic boundary, about 251 to 250 million years ago, and was essentially coincident with the Permian–Triassic extinction event in what was one of the largest known volcanic events of...
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
ignimbriteAn ignimbrite is the deposit of a pyroclastic density current, or pyroclastic flow, a hot suspension of particles and gases that flows rapidly from a volcano, driven by a greater density than the surrounding atmosphere...
flare-up in
ColoradoColorado is a U.S. state located in the Rocky Mountain region of the United States of America. It may also be considered to be part of the Western and Southwestern regions of the United States. Colorado entered statehood in 1876 and was nicknamed the “Centennial State”...
,
UtahUtah is a western state of the United States. It was the 45th state admitted to the Union, on January 4, 1896. Approximately 80 percent of Utah's 2,736,424 people live along the Wasatch Front, centering around Salt Lake City. In contrast, vast expanses of the state are nearly uninhabited, making...
and
NevadaNevada is a state located in the western region of the United States. The capital is Carson City and the largest city is Las Vegas. The state's nickname is Silver State, due to the large number of silver deposits that were discovered and mined there...
from 40–25 million years ago, and was the site of truly enormous eruptions about 28–26 million years ago, during the
OligoceneThe Oligocene is a geologic epoch of the Paleogene period and extends from about 33.9 million to 23 million years before the present. As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...
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 AmericaNorth America is the northern continent of the Americas, situated in the Earth's northern hemisphere and in the western hemisphere. It is bordered on the north by the Arctic Ocean, on the east by the North Atlantic Ocean, on the southeast by the Caribbean Sea, and on the west by the North Pacific...
and the
CaribbeanThe Caribbean is a region consisting of the Caribbean Sea, its islands , and the surrounding coasts...
.
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 MichiganLake Michigan is one of the five Great Lakes of North America, and the only one located entirely within the United States. The second largest of the Great Lakes by volume The third largest of the Great Lakes by surface area , it is bounded, from west to east, by the U.S. states of Wisconsin,...
(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 10
5 times more powerful. It is possibly the most energetic event on Earth since the
Chicxulub impactThe Chicxulub crater is an ancient impact crater buried underneath the Yucatán Peninsula in Mexico. Its center is located near the town of Chicxulub, after which the crater is named — as well as the rough translation of the Mayan name, "the tail of the devil." The crater is more than...
, which was 50 times more powerful.
Geology
The Fish Canyon Tuff, made of
daciteDacite is an igneous, volcanic rock. It is intermediate in compositions between andesite and rhyolite. The relative proportions of feldspars and quartz in dacite, and in many other volcanic rocks, are illustrated in the QAPF diagram...
, 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
andesiteAndesite is an igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. The mineral assemblage is typically dominated by plagioclase plus pyroxene and/or hornblende. Magnetite, zircon, apatite, ilmenite, biotite, and garnet are common accessory minerals. Alkali...
, a volcanic rock compositionally intermediate between
basaltBasalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey.On Earth, most...
(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
supervolcanoA supervolcano or super volcanic eruption is a volcanic eruption with ejecta greater than 1,000 cubic kilometers , which is substantially larger than any volcanic event in historic times. Supervolcanoes can occur when magma in the Earth rises into the crust from a hotspot but is unable to break...
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
tuffTuff is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption. Tuff is sometimes called tufa, particularly when used as construction material, although tufa also refers to a quite different rock. - Volcanic ash :The products of a volcanic eruption are...
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 fountainA lava fountain is a volcanic phenomenon in which lava is forcefully but non-explosively ejected from a crater, vent, or fissure. Lava fountains may reach heights of up to . They may occur as a series of short pulses, or a continuous jet of lava. They are commonly seen in Strombolian and...
ing. The lava-like rock is also voluminous — up to .
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