Transcript Tephrochronologyx

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
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Tephrostratigraphy: correlation of tephra layers by physical and
chemical characteristics and stratigraphic sequences

Tephrochronometry: numerical age determination of tephra layers
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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
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Glass shards
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Crystalline minerals and crystal fragments
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Lithic fragments
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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
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Mazama ash bed: 6850 BP
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Rockland ash bed: 0.4 Ma

Lava Creek B: 0.62 Ma
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Bishop Ash Bed: 0.74 Ma
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Huckleberry Ridge ash bed: 2.0 Ma
What laboratory techniques are used for
preparation of tephra samples?

Removal of organic material

Sieving
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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?
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Microscopic examination to identify
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Crystal fragments

Crystalline minerals, e.g. feldspars, quartz, biotite, apatite
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Lithic fragments of rock chambers
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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?
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Precise chemical analyses of glass and mineral grains by
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Electron microprobe
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X-ray fluorescence
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Atomic absorption
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Plasma atomic emission spectrometry
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Chemical fingerprints of tephras can be produced in this way
What techniques are used for
analyzing tephra?
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Dating techniques, including
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Laser-fusion 40Ar/39Ar analysis
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Fission-track dating of zircons and glass shards
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Hydration
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Thermoluminescence
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Uranium-series dating
What are the analytical uncertainties –
precision (laboratory)?
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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)?
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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
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Contamination during collection (sloppy field techniques!)
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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?
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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?
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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
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Tephras can be collected from ice cores, varves, and deepsea
oozes to identify deep time events
What are the weaknesses of
tephrochronology?
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Larger ejecta are not carried long distances

Tephra is usually carried downwind, so that upwind areas must be
dated by correlation with other techniques
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Tephra composition can change (by sorting and contamination)
with distance from source, place of deposition, and age
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Most airfall tephras only accurate to 200km from source
What age ranges are covered by
tephrochronology?
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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?
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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
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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
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Maier et al. (2014) used four tephras to correlate subsurface facies
at 27 sites in the Sacramento-San Joaquin Delta
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The tephras were Wildcat Grade, Loleta, Rockland, and Hood
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Three facies were recognized and described
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Wildcat Grade is not yet fully interpreted
Sacramento-San Joaquin Delta,
tephra correlation panel
Future work in Sacramento-San Joaquin
Delta project
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The chronostratigraphy of the region can be mapped in fine detail
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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
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Tonga volcano:
http://i.telegraph.co.uk/multimedia/archive/03163/tongavolcano_3163001b.jpg
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Mount Monserrat:
http://iavceicev.clas.asu.edu/images/montserrat.jpg
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Mount St. Helens:
https://geohazards.community.uaf.edu/files/2014/03/738-567-mt-sthelens.jpg
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Types of tephra: http://image.slidesharecdn.com/13398107volcanoes-a-presentation-140321050223-phpapp01/95/13398107volcanoesapresentation-12-638.jpg?cb=1395391971
Image sources continued
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Pyroclastic flow:
http://9.thumbs.scribol.com/4/sites/default/files/images/scaryflow.jpg?v
=1
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Quaternary Period:
http://www.buzzle.com/images/infographics/quaternary-periodtimescale.jpg
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Mt. Rainier tephra layers:
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Dating chart
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http://www.geocraft.com/WVFossils/DatingMethods.html
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Aegirine
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http://www.teara.govt.nz/en/photograph/8732/minerals-from-mayorisland
http://pubs.usgs.gov/bul/1326/report.pdf
Yet more image sources
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Pyroxene and feldspar: Geosec slides.
http://www.geosecslides.co.uk/index.php?s=Viewing
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More minerals: Geosec slides.
http://www.geosecslides.co.uk/minerals/MIAindex0.4.html
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Pompeii victims: https://encryptedtbn0.gstatic.com/images?q=tbn:ANd9GcTbMnZHw4ntqLR0aQUnKQx3c__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-preparationsamples

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_f
or_the_success_of_Homo_sapiens
A nice quick stop site for dating methods:
http://datingthepast.org/