Fish Canyon tuff
Encyclopedia
Tuff
Tuff 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. Rock that contains greater than 50% tuff is considered...
resulting from one of the largest known explosive eruptions on Earth, estimated at 5,000 cubic kilometers. (see List of largest volcanic eruptions) The eruption was centered at La Garita Caldera
La Garita Caldera
La Garita Caldera is a large volcanic caldera located in the San Juan volcanic field in the San Juan Mountains near the town of Creede in southwestern Colorado, United States. It lies to the west of the town of La Garita, Colorado...
in southwest Colorado
Colorado
Colorado is a U.S. state that encompasses much of the Rocky Mountains as well as the northeastern portion of the Colorado Plateau and the western edge of the Great Plains...
. The tuff can be assured to belong to one eruption due to its high chemical consistency (SiO2=bulk 67.5-68.5% (dacite
Dacite
Dacite is an igneous, volcanic rock. It has an aphanitic to porphyritic texture and is intermediate in composition 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...
), matrix 75-76% (rhyolite
Rhyolite
This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellite system, see Rhyolite/Aquacade.Rhyolite is an igneous, volcanic rock, of felsic composition . It may have any texture from glassy to aphanitic to porphyritic...
) and consistent phenocryst
Phenocryst
thumb|right|300px|[[Granite]]s often have large [[feldspar|feldspatic]] phenocrysts. This granite, from the [[Switzerland|Swiss]] side of the [[Mont Blanc]] massif, has large white [[plagioclase]] phenocrysts, [[triclinic]] [[mineral]]s that give [[trapezium|trapezoid]] shapes when cut through)...
content (35-50%) and composition (plagioclase
Plagioclase
Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
, sanidine
Sanidine
Sanidine is the high temperature form of potassium feldspar 4O8. Sanidine most typically occurs in felsic volcanic rocks such as obsidian, rhyolite and trachyte. Sanidine crystallizes in the monoclinic crystal system. Orthoclase is a monoclinic polymorph stable at lower temperatures...
, quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
, biotite
Biotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
, hornblende
Hornblende
Hornblende is a complex inosilicate series of minerals .It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole....
, sphene, apatite
Apatite
Apatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite, chlorapatite and bromapatite, named for high concentrations of OH−, F−, Cl− or Br− ions, respectively, in the crystal...
, zircon
Zircon
Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y...
, Fe-Ti oxide
Oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
s are the primary phenocrysts). This tuff and eruption is part of the larger San Juan volcanic field
San Juan volcanic field
The San Juan volcanic field, located in southwestern Colorado, contains two distinct phases of volcanism. The earlier volcanism is Oligocene in age, and consists of largely intermediate composition lavas and breccias. A few ash flow tuffs also constitute this phase of volcanism. Later volcanism is...
and Mid-Tertiary ignimbrite flare-up
Mid-Tertiary ignimbrite flare-up
The Mid-Tertiary ignimbrite flare-up, as the name implies, is a dramatic period of volcanic eruptions in mid-Tertiary time, approximately 25-40 million years ago, centered in the western United States...
.
See also
- TuffTuffTuff 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. Rock that contains greater than 50% tuff is considered...
- IgnimbriteIgnimbriteAn 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....
- Mid-Tertiary ignimbrite flare-upMid-Tertiary ignimbrite flare-upThe Mid-Tertiary ignimbrite flare-up, as the name implies, is a dramatic period of volcanic eruptions in mid-Tertiary time, approximately 25-40 million years ago, centered in the western United States...
- World's largest eruptions