The San Juan volcanic field is part of the
San Juan Mountains
The San Juan Mountains is a high and rugged mountain range in the Rocky Mountains in southwestern Colorado and northwestern New Mexico. The area is highly mineralized (the Colorado Mineral Belt) and figured in the gold and silver mining industry ...
in southwestern Colorado. It consists mainly of volcanic rocks that form the largest remnant of a major
composite volcanic field that covered most of the southern
Rocky Mountains
The Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch in straight-line distance from the northernmost part of western Canada, to New Mexico ...
in the Middle
Tertiary
Tertiary ( ) is a widely used but obsolete term for the geologic period from 66 million to 2.6 million years ago.
The period began with the demise of the non-avian dinosaurs in the Cretaceous–Paleogene extinction event, at the start ...
geologic time.
[Steven, Thomas A.; Lipman, Peter W. (1976)]
"Calderas of the San Juan Volcanic Field, Southwestern Colorado"
''U.S. Geological Survey Professional Papers''. Washington, DC: U.S. Government Printing Office. 958: 1–35. Retrieved 2012-05-16. There are approximately fifteen calderas known in the San Juan Volcanic Fields; however, it is possible that there are two or even three more in the region.
The region began with many composite volcanoes that became active between 35 and 40 million years ago, with peak activity in the time period around 35-30 million years ago.
Around this time the activity began to include explosive ash-flow eruptions. Many of these volcanoes experienced
caldera collapse
A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is ...
, resulting in the fifteen to eighteen
caldera volcanoes in the region today.
Phases of Volcanism
The San Juan volcanic field experienced two phases of
volcanism
Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics, and volcanic gases erupt through a break in the surface called a ...
.
The earlier volcanism took place during the
Oligocene
The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but t ...
age of the
Paleogene
The Paleogene ( ; also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period million years ago ( Mya) to the beginning o ...
Period. It produced largely intermediate composition
lava
Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or ...
s and breccias, together with ash flow
tuff
Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock ...
s reflecting
differentiation of the original magma.
[ The precaldera intermediate volcanic rocks include the Conejos Formation in the southeastern part of the field. Intermediate volcanism did not cease with caldera eruptions, and included such voluminous intermediate volcanic rocks as the Huerto Andesite.
The later volcanism took place from ]Miocene
The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...
to Pliocene
The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58[Neogene
The Neogene ( ), informally Upper Tertiary or Late Tertiary, is a geologic period and system that spans 20.45 million years from the end of the Paleogene Period million years ago ( Mya) to the beginning of the present Quaternary Period Mya. ...](_blank)
Period. It was characterized by bimodal volcanism
Bimodal volcanism is the eruption of both mafic and felsic lavas from a single volcanic centre with little or no lavas of intermediate composition. This type of volcanism is normally associated with areas of extensional tectonics, particularly rif ...
, producing both low-silica alkaline flows and high-silica rhyolites
Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral ...
. It is usually interpreted as a partial melt of the lower crust that was erupted onto the surface.[
]
Notable calderas
Economic Impact
The San Juan volcanic field has been a historically important mining district, producing lead, zinc, copper, gold, and
silver. The ores were mostly deposited in and near calderas that experienced significant postcaldera activity. Ore veins were concentrated in fractures associated with caldera activity and in postcaldera intrusive bodies. The greatest mineralization took place near the youngest and most silicic intrusions of each caldera cycle.
See also
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References
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Volcanic fields of Colorado
San Juan Mountains (Colorado)
Extinct volcanoes
Geology of the Rocky Mountains
Landforms of Mineral County, Colorado
Oligocene volcanism
Miocene volcanism
Pliocene volcanism
Oligocene North America
Miocene North America
Pliocene North America
Geologic formations of Colorado
Neogene Colorado
Paleogene Colorado
Calderas of Colorado
Oligocene calderas