Tephrochronology By Suzanne ubick What is tephrochronology Tephrochronology as used here is the study of tephra layers volcanic ash beds and tuffs for purposes of correlation and dating of sediments rocks and structures ID: 767506
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Tephrochronology By Suzanne ubick
What is tephrochronology? “Tephrochronology as used here is the study of tephra layers – volcanic ash beds and tuffs – for purposes of correlation and dating of sediments, rocks, and structures.” Sarna-Wojcicki , A.M. and J.O. Davis. 1991. Quaternary tephrochronology. In The Geology of North America , Vol. K-2: 93.
Tephrochronology subdisciplines Tephrostratigraphy : correlation of tephra layers by physical and chemical characteristics and stratigraphic sequences Tephrochronometry : numerical age determination of tephra layers Used in tandem for correlation of layers and indirect dating
What is tephra? (Tuff is cemented or otherwise consolidated tephra) “A collective term for all clastic volcanic materials which during an eruption are ejected from a crater or some other kind of vent and transported through the air; includes volcanic dust, ash, cinders, lapilli, scoria, pumice, bombs, and blocks.” American Geological Institute. 1962. Dictionary of Geological Terms . New York: Dolphin Books. Sarna-Wojcicki and Davis (1991) expand the definition to include all pyroclastic material ejected from a volcanic vent, including pyroclastic flow material.
Types of tephra
Tephra: pyroclastic flow
Tephra components Pumice Glass shards Crystalline minerals and crystal fragments Lithic fragments Clasts, bioclasts , organic sediments, chemical precipitates, cementing materials
Tephra generator Undersea volcano, Tonga, March 2009
Tephra Generator Mount St. Helens, 1980
Tephra Generator Mount Montserrat, Caribbean, 1997
What is the Quaternary?
What is the scientific basis for tephrochronology? “Tephra layers have unique characteristics, physical or chemical, by which they can be distinguished one from another, and by which they can be consistently and correctly identified.” Sarna-Wojcicki and Davis, 1991a. Example on next slide: Layered scoria, pumice, and ash strata in an alpine meadow near Williwakas Glacier, on the southeast flank of Mt. Rainier. The dark reddish scoria and pumice, and the dark brownish ash layers, are all from Mt. Rainier eruptions. The paler ash deposits come from at least one other volcano.
Why are tephras good tools? Each eruption of each volcano produces a different tephra Because each magma chamber has different proportions of constituents Because each eruption, by removing some minerals - following Bowen’s reaction series - changes the composition of the magma left behind, and increases the viscosity Because the physical properties of each successive magma affect the kind of tephra that is produced Because the chemical composition of each successive magma affects the tephra it will produce (e.g. color, reactivity) Each volcano’s successive eruptions have smaller differences than those of different volcanos of the same age and region
Bowen’s reaction series
Important US Quaternary tephras Mazama ash bed: 6850 BP Rockland ash bed: 0.4 Ma Lava Creek B: 0.62 Ma Bishop Ash Bed: 0.74 Ma Huckleberry Ridge ash bed: 2.0 Ma
What laboratory techniques are used for preparation of tephra samples? Removal of organic material Sieving Heavy liquid flotation to extract shards ranging from sand-size to microscopic cryptotephra Source: Tephra Lab. 2015.
What techniques are used for analysis? Physical determination of characteristics, e.g. microscopic examination of ash to determine the shapes and sizes of glass shards
What techniques are used for analysis? Microscopic examination to identify Crystal fragments Crystalline minerals, e.g. feldspars, quartz, biotite, apatite Lithic fragments of rock chambers Inclusions: clasts, bioclasts , organic sediments, chemical precipitates and cementing materials incorporated into tephra during transportation, deposition, and reworking of tephra
The polarizing microscope
Mineral components in tephra: aegerine , unique to Tuhuara Tephra, New Zealand
Using birefringence to identify minerals: pyroxene and feldspar in glassy matrix
What techniques are used for analysis? Precise chemical analyses of glass and mineral grains by Electron microprobe X-ray fluorescence Atomic absorption Plasma atomic emission spectrometry Chemical fingerprints of tephras can be produced in this way
What techniques are used for analyzing tephra? Dating techniques, including Laser-fusion 40Ar/39Ar analysis Fission-track dating of zircons and glass shards Hydration Thermoluminescence Uranium-series dating
What are the analytical uncertainties – precision (laboratory)? Failure to recognize leaching, Aeolian and hydraulic sorting, and chemical and physical weathering during collection Failure to use large enough samples of young tephras for dating; young tephras have accumulated only small quantities of radiogenic argon
What are the analytical uncertainties – accuracy (field)? Tephras can be easily reworked, making it difficult to distinguish layers close to the eruptive source. Care must be taken to ensure homogeneity of the layer before specimen collection Contamination during collection (sloppy field techniques!) Physical, mineralogic and chemical characteristics can change with distance from source, depositional environment, and time
Which materials are most and least useful for this technique? Tephras from Plinian eruptions are most useful, as very large amounts of material are produced, which may be carried far and deposited widely Tephras containing large amounts of potassium-rich minerals: sanidine , biotite, honblende , and plagioclase feldspar Small eruptions of lava are least useful, as they are highly localized
What are the strengths of tephrochronology? Given that tephras are easily dated, materials or structures positioned above or below the tephra can be set into relation with an absolute date Materials or structures positioned between tephra layers can be placed between two absolute dates The source of the tephra need not be known in order to carry out correlations Tephras can be collected from ice cores, varves , and deepsea oozes to identify deep time events
What are the weaknesses of tephrochronology? Larger ejecta are not carried long distances Tephra is usually carried downwind, so that upwind areas must be dated by correlation with other techniques Tephra composition can change (by sorting and contamination) with distance from source, place of deposition, and age Most airfall tephras only accurate to 200km from source
What age ranges are covered by tephrochronology? Depending on the age of the tephra, and the dating method used, the potential age range is from the present to 4.6Ga.
What are tephrochronology’s applications? To provide stratigraphic and age control when synthesizing tectonic events and determining regional stratigraphy To assist in determining crustal motions To calibrate other dating methods To correlate continental and marine faunal and isotopic stages To provide relative ages, calibrate and correlate events in human history and prehistory by anthropologists and archaeologists *Very important: precise correlations between deep-ocean, uplifted onshore marine, and continental stratigraphic sequences. No other technique is so good. Potential global correlations as techniques improve for detection and identification of very fine tephra in sediments worldwide
Case study 1: Sacramento-San Joaquin Delta, CA Maier et al. (2014) used four tephras to correlate subsurface facies at 27 sites in the Sacramento-San Joaquin Delta The tephras were Wildcat Grade, Loleta , Rockland, and Hood Three facies were recognized and described Wildcat Grade is not yet fully interpreted
Sacramento-San Joaquin Delta, tephra correlation panel
Future work in Sacramento-San Joaquin Delta project The chronostratigraphy of the region can be mapped in fine detail Buried sediments can be assigned reliable ages Assessment of geotechtonic risks can be carried out
Case Study 2: Hey Fatty Boom Boom !
Dating an hypothesized microevolutionary event in modern humans: Ubick 2014. Something happened to Homo sapiens before their second great dispersal from Africa 60,000 years ago This event gave them an adaptive edge that allowed them to replace, displace, or absorb all other human species on Earth I argue that an environmentally-driven microevolutionary event increased Homo sapiens female fertility by increasing their ability to gain body fat The lower time boundary can be inferred by tephrochronology: ash from the Mt. Toba eruption of ~72,000 years ago has been recovered from East Africa – the homeland of all modern humans on Earth today.
Mt. Toba eruption ashfall range
Image sources Tonga volcano: http:// i.telegraph.co.uk/multimedia/archive/03163/tonga-volcano_3163001b.jpg Mount Monserrat: http :// iavceicev.clas.asu.edu/images/montserrat.jpg Mount St. Helens: https:// geohazards.community.uaf.edu/files/2014/03/738-567-mt-st-helens.jpg Types of tephra: http:// image.slidesharecdn.com/13398107-volcanoes-a-presentation-140321050223-phpapp01/95/13398107-volcanoesapresentation-12-638.jpg?cb=1395391971
Image sources continued Pyroclastic flow : http:// 9.thumbs.scribol.com/4/sites/default/files/images/scaryflow.jpg?v=1 Quaternary Period: http:// www.buzzle.com/images/infographics/quaternary-period-timescale.jpg Mt. Rainier tephra layers: http :// pubs.usgs.gov/bul/1326/report.pdf Dating chart http:// www.geocraft.com/WVFossils/DatingMethods.html Aegirine http:// www.teara.govt.nz/en/photograph/8732/minerals-from-mayor-island
Yet more image sources Pyroxene and feldspar: Geosec slides. http:// www.geosecslides.co.uk/index.php?s=Viewing More minerals: Geosec slides. http:// www.geosecslides.co.uk/minerals/MIAindex0.4.html Pompeii victims: https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcTbMnZHw4ntqLR0aQUnK-Qx3c__ dstf48H49hZIgrRT-hm5hy3n2w
Bibliography American Geological Institute. 1962. Dictionary of Geological Terms . New York: Dolphin Books. Tephra Lab. 2015. Tephrachronology - extracting and preparation of samples. http://www.uib.no/en/earthlab/81502/tephrachronology-extracting-and-preparation-samples Maier , K.L., et al., Quaternary tephrochronology and deposition in the subsurface Sacramento-San Joaquin Delta,California , U.S.A., Quaternary Research (2015), http://dx.doi.org/10.1016/j.yqres.2014.12.007 Mullineaux , D.R. 1974. Pumice and other pyroclastic deposits in Mt. Rainier National Park, Washington . Geological Survey Bulletin 1326. http:// pubs.usgs.gov/bul/1326/report.pdf Sarna-Wojcicki , A.M. and J.O. Davis. 1991. Quaternary tephrochronology. In The Geology of North America , Vol. K-2: 93-116 Ubick, S. 2014. Grey cells or fat cells? A speculative hypothesis for the success of Homo sapiens. Unpublished senior honors thesis. https://www.academia.edu/15771416/Grey_cells_or_fat_cells_A_speculative_hypothesis_for_the_success_of_Homo_sapiens
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