# Amalia Tuff

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The **Amalia Tuff** is a [geologic formation](/source/Geologic_formation) exposed in and around [Questa, New Mexico](/source/Questa,_New_Mexico).[1] It has a [radiometric age](/source/Radiometric_dating) of 25.39 ± 0.04 million years, corresponding to the [Oligocene](/source/Oligocene) [epoch](/source/Epoch_(geology)).[2]

## Description

The Amalia Tuff is a weakly to densely welded ash-flow [tuff](/source/Tuff) erupted during the formation of the Questa [caldera](/source/Caldera) 25.39 ± 0.04 million years ago ([Ma](/source/Ma_(unit))),[2] which erupted some 500-1000 km3 mi3 of [magma](/source/Magma).[3] It originally formed an [outflow sheet](/source/Outflow_sheet) 20-50 meter feet thick[1] that extended as far as 45 kilometers (28 miles) to the southwest and 25 kilometers (16 miles) to the northeast. Within the caldera the tuff pooled to a depth of 1000-2000 meter feet. The tuff appears to be a single cooling unit.[4]

The tuff is silicic and alkaline, though its [silica](/source/Silica) content of 77–80% likely reflects some secondary silicification. It contains 10–20% [quartz](/source/Quartz) and [alkali feldspar](/source/Alkali_feldspar) [phenocrysts](/source/Phenocryst). Rare occurrences of a [vitrophyric](/source/Vitrophyric) tuff at the base of the main bed contain [ferrohedenbergite](/source/Hedenbergite) and [fayalite](/source/Fayalite), while the upper beds sometimes preserve sodic [amphibole](/source/Amphibole) phenocrysts. The intracaldera tuff contains lenticular [megabreccia](/source/Megabreccia) blocks up to 1,000 meters (0.62 miles) across. The tuff includes a lithic-rich lower [facies](/source/Facies_(geology)) that is nonwelded to partially welded and up to 30 meters (98 feet) thick. This contains up to 5% [andesite](/source/Andesite) and sparse [Proterozoic](/source/Proterozoic) rock fragments. This grades into the main bed of the formation.[1]

[Ar-Ar dating](/source/Ar-Ar_dating) of the tuffs and associated [plutons](/source/Pluton) indicates the [magma](/source/Magma) that formed the tuff was not generated in place in the upper crust.[2] Isotope evidence suggests the original [basaltic](/source/Basalt) magma assimilated considerable crust material and underwent [crystal fractionation](/source/Crystal_fractionation_(geology)) at a deeper level of the crust than that of the [magma chamber](/source/Magma_chamber) responsible for the Questa eruption.[5]

## History of investigation

The unit was first designated the Amalia Formation in 1956 by P.F. McKinlay. [6] As originally defined, it included basalt flows and sedimentary beds dating from before the Questa eruption. These were removed from the unit definition by Lipman in 1983, who restricted the unit to the densely welded tuff outflow sheet from the Questa eruption and renamed the unit the Amalia Tuff.[4]

## Footnotes

1. Lipman & Reed 1989.

1. Zimmerer & McIntosh 2012.

1. Johnson, Shannon & Fridrich 1989.

1. Lipman 1983.

1. Johnson & Lipman 1988.

1. McKinlay 1956.

## References

- Johnson, Clark M. & Lipman, Peter W. (September 1988). "Origin of metaluminous and alkaline volcanic rocks of the Latir volcanic field, northern Rio Grande rift, New Mexico". *Contributions to Mineralogy and Petrology*. **100** (1): 107–128. [Bibcode:1988CoMP..100..107J](https://ui.adsabs.harvard.edu/abs/1988CoMP..100..107J). [doi:10.1007/BF00399442](https://doi.org/10.1007/BF00399442)
- Johnson, C.M.; Shannon, J.R.; Fridrich, C.J. (1989). "Roots of ignimbrite calderas: Batholithic plutonism, volcanism, and mineralization in the Southern Rocky Mountains, Colorado and New Mexico". *New Mexico Bureau of Mines and Mineral Resources Memoir*. **46**: 275–302.
- Lipman, P.W. (1983). ["The Miocene Questa Caldera, northern New Mexico; relation to batholith emplacement and associated molybdenum mineralization"](https://www.researchgate.net/publication/262261926). *The genesis of Rocky Mountain ore deposits; changes with time and tectonics*. Society of Exploration Geologists. pp. 133–148. Retrieved 26 July 2020.
- Lipman, P.W. & Reed, J.C. Jr. (1989). ["Geologic map of the Latir volcanic field and adjacent areas, northern New Mexico"](https://pubs.usgs.gov/imap/i-1907/downloads/I-1907textonly_508.pdf). *U.S. Geological Survey Miscellaneous Investigations Series Map*. **I-1907**. Retrieved 26 July 2020.
- McKinlay, P.F. (1956). ["Geology of the Costilla and Latir Peak quadrangles, Taos County, New Mexico"](https://geoinfo.nmt.edu/publications/monographs/bulletins/downloads/42/Bulletin042.pdf). *New Mexico Bureau of Mines and Mineral Resources Bulletin*. **42**. Retrieved 11 November 2020.
- Zimmerer, M. J. & McIntosh, W. C. (18 May 2012). "The geochronology of volcanic and plutonic rocks at the Questa caldera: Constraints on the origin of caldera-related silicic magmas". *Geological Society of America Bulletin*. **124** (7–8): 1394–1408. [Bibcode:2012GSAB..124.1394Z](https://ui.adsabs.harvard.edu/abs/2012GSAB..124.1394Z). [doi:10.1130/B30544.1](https://doi.org/10.1130/B30544.1)

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Adapted from the Wikipedia article [Amalia Tuff](https://en.wikipedia.org/wiki/Amalia_Tuff) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Amalia_Tuff?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.