Revisions in Geochronology of Yellowstone and Surrounding Areas

For over a century, geologists have looked to lava flows and ash tuffs to determine the date of the first volcanic activity at Yellowstone National Park. As one of the most renowned supervolcanoes in the world, understanding the geochronology of Yellowstone is directly relevant to the lives of Americans. Determining how the volcano erupted in the past and on what timeframe sheds light on what humanity can expect in the future. Through exploring a history that dates back over 2 million years has been a daunting task, modern technology has improved the dating methods available to geologists. This report will explore how developments in dating methods have contributed to the understanding of the geochronology of Yellowstone. Through an examination of the stratigraphy, geomagnetic analysis, and chemical dating, the merits of the 40Ar/39Ar dating technique for providing an updated review of the geochronology of the volcanic formations at Yellowstone will be supported.

Yellowstone Geological Overview

Developments in aeromagnetic mapping provide updated data on the geological framework of Yellowstone National Park. As Finn and Morgan (2001) note, Yellowstone has widespread volcanic rock along with Eocene and andesitic volcanic rock (p. 208). The sediment layer provides a special point of consideration for scientists who might wish to analyze the geochronology of Yellowstone formations. As Finn and Morgan note, while a lot of Yellowstone is covered by young sediments, Quaternary volcanic rocks, and other obstructive features, aeromagnetic data enables researchers to penetrate these superficial layers in order to examine the subsurface rocks in the area (Finn and Morgan, 2001, p. 208). Revelations from aeromagnetic mapping have aided in dating rhyolitic ignimbrite deposits in Yellowstone.

Finn and Morgan provide a further overview of the tectonic features of Yellowstone. As they note, the volcanic field in the area has developed in three cycles over a period lasting 2 Myr (Finn and Morgan, 2001, p. 208). The oldest recorded eruption is the Huckleberry ridge eruption. As the researchers note, this first eruption took place at 2.1 Ma and was distributed across the Yellowstone Plateau, producing over 2500 km3 of pumice and ash deposits that make up the Huckleberry Ridge Tuff (Finn and Morgan, 2001, p. 208). The second oldest recorded eruption was the Mesa Falls eruption. It is estimated that this eruption took place at about 1.3 Ma and produced around 500 km3 of pumice and ash that make up Mesa Falls Tuff and which resulted in the formation of Henry’s Fork caldera (Finn and Morgan, 2001, p. 208). Finally, the third, and final, recorded eruption contributing to the Yellowstone formation was the Lava Creek eruption. Taking place around 1.23 Ma and recontinuing in a subsequent cycle around 0.63 Ma, creating the Lava Creek Tuff (Finn and Morgan, 2001, p. 208). Together, the Huckleberry Ridge Tuff, the Mesa Falls Tuff, and the Lava Creek Tuff compose the Yellowstone Caldera.

Further, the researchers describe the three stages through which the Yellowstone caldera was formed. During Stage 1, basalt and rhyolite flows erupted in intermittent intervals (Finn and Morgan, 2001, p. 208). During Stage 2, abrupt, large-volume, ignimbrite and ash eruptions occurred that formed the calderas (Finn and Morgan, 2001, p. 210). Next, during stage 3, flows of rhyolitic lava filled the caldera and surrounding terrain (Finn and Morgan, 2001, p. 210). Finally, during stage 4, basaltic volcanism covered the formed the caldera through both solfatara and hot spring activity (Finn and Morgan, 2001, p. 210). While the four stages provide geological evidence that has been useful in dating volcanic activities, they also contribute to obstructions that interfere with traditional dating methods.

Historical Development of Methods in Geochronology

Before examining the particular problems related to the geochronology of Yellowstone, it is necessary to examine the tools that are available to researchers. The effort to date the volcanic formations at Yellowstone parallels the efforts for accurate assessments that exist across the field of geology. As Dosseto, Turner, and Van Orman note, Norman L. Bowen pioneered the study of igneous rocks in his 1928 publication The Evolution of Igneous Rocks (Dosseto, Turner, and Van Orman, 2010, p. 1). However, the main limitation that geologists faced at the beginning of the center, there was a lack of radiometric dating methods (Dosseto, Turner, and Van Orman, 2010, p. 1). Thus, while geologists were able to collect a wide variety of data, they were unable to conduct a thorough analysis of samples to determine the age of formations.

The authors further discuss how geomechanical analysis contributed to the ability of geologists to understand volcanic formations. Utilizing geomechanical analysis in combination with numerical models and field experiments, researchers were able to determine the relationships between plate tectonics and magma formations (Dosseto, Turner, and Van Orman, 2010, p. 2). As they note, numerical models are especially useful because they enable researchers to understand the ways that magma generates and erupt (Dosseto, Turner, and Van Orman, 2010, p. 2). However, a drawback of numerical models is that an incorrect variable can render inaccurate results. Further, this method of analysis enabled researchers to conduct in-depth petrological and geomechanical studies of individual rocks and crystals, which revealed that magmas carry mixtures from various sources and various ages (Dosseto, Turner, and Van Orman, 2010, p. 2). These findings demonstrated the difficulty in isolating certain types of rock for analysis. Yet, an advantage was that it provided a source for determining the age of the samples.

No more than two decades ago, the process of dating has seen major improvements through the introduction of new dating techniques. For example, mass spectrometry provides an accurate assessment of the radioactive isotopes of the U and Th decay series, enabling researchers to use in-situ dating on extremely small amounts of miners that could be as mall as several tens of m (Dosseto, Turner, 2010, p. 2). Combined with traditional field observations, these methods yielded greater results in constructing geochronology for volcanic formations. For example, data on the kinetics of the migration of elements in minerals in combination with natural observations led to the realization that chemical mixtures in crystals could be used to assess the time involved in the magmatic process (Dosseto, Turner, 2010, p. 3). These developments made it possible for the first time for scientists to determine how long magma was stored and explore other related questions that would enable them to date rocks and magma.

Today, radioactive dating is the gold standard for assessing the geochronology of formations. Through this process, decay chains that consist of radioactive systems decay, emitting alpha or beta emissions that can be registered by equipment (Dosseto, Turner, 2010, p. 3). By measuring the age of rock through the measurement of radioactive decay, geologists have been able to calibrate sequences of magmatic and volcanic events (Dosseto, Turner, and Van Orman, 2010, p. 2). Further, radioactive dating has enabled researchers to estimate the rates of magma production and evaluation without the necessity of obtaining large samples (Dosseto, Turner, and Van Orman, 2010, p. 2). As a result, radioactive dating is among the most prevalent method for determining the age of rocks and geological formations of the earth. Yet the main source of contention is in determining which isotopes yield the most accurate results. As this research will demonstrate, this controversy carries over to the debate on the geochronology of the volcanic formations at Yellowstone National Park.

Stratigraphy Limitations

The challenges of correctly dating volcanic activity at Yellowstone are especially apparent when the limits of obtaining a consistent stratigraphic record are examined. One method of analyzing the age of layers is to construct a numerical model that analyzes the ash-flow surrounding volcanic formations. These models are also significant because they enable researchers to identify the eruptive centers and the direction of the flow of the lava (Armstrong et al., 1975, p. 227; Bonnichsen et al., 2008, p. 235). Yet, while the analysis of ash-flow can be beneficial in dating formations, Riehle, Miller, and Paquereau-Lebti (2010) point out the limitations of these models. As the authors note, traditional models based on density profiles assume this ash-flow tuffs hinge upon the assumption that cooling from the bottom of the substrate through the process of conduction causes uniform temperatures (Riehle, Miller, and Paquereau-Lebti, 2010, p. 119). Factors that can lead to various temperatures include: 1) the existence of a cooler deposit that cools down the temperature of the surge deposit, 2) an interval before a second deposit that enabled temperature disparities to emerge, and 3) disparities in the density profile caused by periodic cooling intervals (Riehle, Miller, and Parquereau-Lebti, 2010, p. 119). As these findings imply, models that consider density in order to assess the date of eruptions can obtain disparate results that fail to consider variables that might distort the data.

Additionally, the attempt to assess the mantle mass transfer along Yellowstone has yielded varied results. For example, in one geomechanical study, McCurry and Rodgers (2009) estimate the mantle mass transfer along the Yellowstone-Snake River Plain. Through their estimations, the maximum amount of crust involved in generating rhyolites was between 12% and 40% (McCurry and Rodgers, 2009, p. 96). Further, in their treatment of the evolution of young rhyolites on the hotspots located on the Yellowstone Plateau, Vazquez, Kyriazis, Reid, Sehler, and Ramos (2008) discuss how the replenishment of cooling magma reservoir was replenished with new rhyolite (p. 186). There are methods of adjusting for these variations. For example, as Sears, Hendrix, Thomas, and Fritz (2009) demonstrate through their research, the formations in Southwest Montana can serve a control for current stratigraphic records of the Yellowstone hotspot (p. 258). The hotspot activity along the Sixmile Creek Formation is believed to date back between 17 Ma to 2 Ma (Sears, Hendrix, Thomas, and Fritz, 2009, p. 258). As Jean, Hanan, and Shervais note, the Snake River Plain represents 17 m.y. of volcanic activity (Jean, Hanan, and Shervais, 2013, p. 119). By using these figures as a baseline, researchers can attempt to adjust for variations in their models. However, without a solid baseline, the results obtained from these methods are less reliable.

Geomagnetic Dating

Brad Singer (2012) discussed how geomagnetic instability contributed to reversals and excursions of the Earth’s geomagnetic field and helped to create markers that could be utilized for dating purposes. As Singer notes, geomagnetic field reversals and excursions have covered a period of the Earth’s history that spans 2.6 Ma (Singer, 2012, p. 29). Further, Singer defines excursions as “brief periods of <104 years during which virtual geomagnetic poles (VGPs) – essentially ‘snapshots’ of paleofield geometry – shift from the geocentric axial dipole beyond the range of secular variation” (Singer, 2012, p. 29). The reason that obtaining an accurate chronology that takes reversals and excursions into account is that it enables climate proxy records from regions that are spatially distinct to be compared, it informs theoretical arguments and numerical models that rely upon the frequency, duration, and geometry of reversals, and it contributes to enhanced cosmogenic isotopes (Singer, 2012, p. 30). Because of their role in establishing the variables in other models geomagnetic reversals and excursions are critical to geochronological dating. However, while geomagnetic dating is useful for dating, it typically used in combination with forms of radiometric dating, which will be discussed in the following section.

Advancements in Radiometric Dating

As Singer (2012) notes, geochronological data are primarily derived in two ways. The first is through radioisotopic methods, such as 40Ar/39Ar that obtain the age of lava flows by recording short periods of magnetic field states (Singer, 2012, p. 30). The second is through astrochronologic age models that are obtained from the cores of marine sediment where the magnetic field direction is well documented (Singer, 2012, p. 30). These methods have confirmed the benefits of analyzing lava flow in order to date volcanic eruptions. Particularly, 40Ar/39Ar dating determines that since lava flows possess an intermediate polarity state, countering the belief that lava flows can’t provide a sound record of reversals or excursions (Singer, 2012, p. 30). The main limitation of stratigraphic data is that it utilizes Ar methods, yet fails to produce data on the ages of eruptions (Vazquez and Reid, 2002, 284; Simon and Reid, 2005, 130). Thus, 40Ar/39Ar dating contributes greatly to geomagnetic dating by proving the veracity of the data.

Singer also describes the contributions of K-Ar dating. Developed in the early 1960s, the Pilo-Pleistocene Geomagnetic Polarity Time Scale proposed a sequence of normal and reverse polarity periods, called chrons, based on K-Ar dating (Singer, 2012, p. 30). However, a shortcoming of this method was that it was difficult to determine the length of the chrons because there was insufficient data to determine the timing of the polarity reversals between chrons (Singer, 2012, p. 30). Thus, K-Ar dating is less commonly used in the dating of lava flows.

Through research on the magmatic histories of zircons in silicic magmas, Simon, Renne, and Mundil (2007) explain the discrepancies between measurements of eruption age. Specifically, the authors examine the applications of the U-Pb radioisotope dating method to the mineral zircon. As they note the main benefit of the method is that it has greater precision than other radioisotope methods that are used for the purpose of geochronology (Simon, Renne, and Mundil, 2007, p. 183). Further, the researchers justify its common application to the dating of zircon. As they note, U-Pb geochronometry works well with zircon because zircon excludes Pb during the process of crystallization, which reduces the corrections that have to be made for Pb (Simon, Renne, and Mundil, 2007, p. 182). The ability to accurately date zircons is especially significant from the standpoint of volcanic geochronology.

An accurate method of dating zircon enables researchers to gain confidence in their ability to establish a date for volcanic activities. Because Zircon is uniformly found in silicic igneous materials that contribute to the establishment of time-stratigraphic markers (Simon, Renne, and Mundil, 2007, p. 182). However, there are limitations to efforts at dating through U-Pb geochronology. First, when samples are examined near the surface, zircon crystals become less reliable because the diffusion process is slower (Simon, Renne, and Mundil, 2007). These limitations also account for the discrepancies between U-Pb geochronology and other dating methods.

The implications discrepancies between U-Pd geochronology and alternative forms of dating to the established chronology for Yellowstone can especially be demonstrated in a comparison with 40Ar/39Ar dating. As the researchers explain, diagrams of Yellowstone reveal excess 40Ar that likely originate from the melt inclusions (Simon, Renne, and Mundil, 2007, p. 184). The result is that scatter data obtained for dating might have excess Ar, which can alter the results. However, the researchers found that once samples were adjusted to reduce excess Ar, measurements obtained results that were similar to the results obtained by other measurements (Simon, Renne, and Mundil, 2007, p. 184). However, there are still variations between argon measurements and ages obtained from zircon. Measurements of zircon growth place silicic magmas several 100 ka before recorded eruption dates (Simon, Renne, and Mundil, 2007, p. 192). As a result, the researchers note that U-Pb zircon age that is calculated deviates from the eruption age by 30-300 ka (Simon, Renne, and Mundil, 2007, p. 192). For this reason, Ar dating is viewed to be more reliable than U-Pb geochronology for dating the silicic extrusions at Yellowstone.

Bindeman, Valley, Wooden, and Persing (2001) also address the advantages of Ar dating over U-Pb geochronology. Through in situ measurements of U-Pb age in a sample of Pleistocene zircons obtained from the Yellowstone Caldera, the researchers noted that long residence times of over 100 kyr of zircons were reported in large silicic magma reservoirs (Bindeman, Valley, Wooden, and Persing, 2001, p. 203). Yes, large volume Lava Creek Tuff and Huckleberry Ridge Tuff magmas did not possess long residence time, and only eruption-age zircons were found in the tuffs (Bindeman, Valley, Wooden, and Persing, 2001, p. 203). However, while there is a discrepancy between the age of the peak and the results from the zircons measurements, the Ar geochronology corresponded with the young age of the peak (Bindeman, Valley, Wooden, and Persing, 2001, p. 203). These findings further confirm the superiority of Ar dating.

However, the researchers present many explanations for the discrepancies obtained from the zircon measurements. As they assert, the zircon was recycled from rocks that predated the eruptions at Yellowstone (Bindeman, Valley, Wooden, and Persing, 2001, p. 203). Thus, the presence of recycled zircons in samples can distort the age of dated zircon samples. For this reason, 40Ar/39Ar has been relied upon to bring new insights into the geochronology of Yellowstone.

According to Ellis, Mark, Pritchard, and Wolff (2012), Huckleberry Ridge Tuff is composed of three distinguishable components, labeled as A, B, and C (p. 34). Distinguishing between these three sections is significant for determining the timeline of the Huckleberry Ridge eruption. If a single eruption did occur in the same time period, then this should be verified by dating methods. Thus, the researchers utilize 40Ar/39Ar dating to test samples from A, B, and C, determining that members A and B yielded ages that fell within the set confidence level while member C produced divergent results that fell outside of the confidence level (Ellis, Mark, Pritchard, and Wolff, 2012, p. 38). Further, they found that the age of members demonstrates that it produced an eruption at least 6 kyr prior to members A and B (Ellis, Mark, Pritchard, and Wolff, 2012, p. 38). Thus, the researchers produced evidence that refuted previous dates for member C by demonstrating that it took place at a separate time.

Further, the research investigated the date of member C by placing it on a timescale to produce a magnetostratigraphic record for the samples. As they noted, members A and B possessed palaeomagnetic inclinations and declinations that ranged outside of the scale (Ellis, Mark, Pritchard, and Wolff, 2012, p. 39). However, the researchers failed to obtain sufficient data for member C (Ellis, Mark, Pritchard, and Wolff, 2012, p. 39). Thus, in the scope of their report, the researchers were unable to obtain geomagnetic support for their thesis. This research further strengthens the results that yielded from 40Ar/39Ar dating.

Conclusion

Radioactive dating has contributed greatly to enabling geologists to construct accurate geochronology for the volcanic eruptions at Yellowstone national park. As an analysis of the methods of developing a stratigraphic record reveal, the assumptions that rock formations remain steady over time have led to simplifications in dating methods. Yet, because new layers are constantly formed in gaps of older layers, efforts to date a formation simply through examining the layers of the formation can be skewed. Further, while U-Pb has often be utilized as an efficient method of dating zircons, which are prevalent in the Yellowstone volcanic formations, this method has proven to be inaccurate because of the instability of zircon deposits. However, 40Ar/39Ar dating has emerged as the most prevalent form of dating and is widely credited for its accuracy. As this method of dating reveals, the geochronology of Yellowstone should be adjusted to accommodate the theory that there were multiple eruptions that led to the development of Hickory Ridge Tuff rather than one. Though this finding, geologists can revise the timescale of eruptions in order to determine that they more frequently and earlier than previous estimates established.

References

Armstrong, R.L., Leeman, W.P., Malde, H.E., 1975. K-Ar dating quaternary and Neogene rocks of the Snake River Plain, Idaho: American Journal of Science, v. 275, 225-251.

Bindeman, I.N., Valley, J.W., Wooden, J.L., Persing, H.M., 2001, Post-caldera volcanism: In situ measurement of U-Pb age and oxygen isotope ratio in Pleistocene zircons from Yellowstone caldera, v. 189, p. 197-206.

Bonnichsen, B., Leeman, W.P., Honjo, N., McIntosh, W.C., Godchaux, M., 2008, Miocene silicic volcanism in southwestern Idaho: Geochronolgy, geochemistry, and evolution of the central Snake River Plain: Bulletin of Volcanology, v. 70, 315-342.

Dosseto, A.T., Turner, S.P., and Van Orman, J.A., 2010, Timescales of magmatic processes: From core to atmosphere: Hoboken, NJ, Wiley-Blackwell, 255 p.

Ellis, B., Mark, D.F., Pritchard, C.J., and Wolff, J.A., 2012, Temporal dissection of the Huckleberry Ridge Tuff using the 40Ar/39Ar dating technique: Quaternary Geochronology, v. 9, p. 34-41.

Finn, C.A., and Morgan, L.A., 2001, High-resolution aeromagnetic mapping of volcanic terrain, Yellowstone National Park: Journal of Volcanology and Geothermal Research, v. 115, p. 207-231.

Jean, M.M., Hanan, B.B., and Shervais, J.W., 2014, Earth and Planetary Science Letters, v. 389, p. 119-131.

McCurry, M., and Rodgers, D.W., 2009, Mass transfer along the Yellowstone hotspot track I: Petrologic constraints on the volume of mantle-derived magma: Journal of Volcanology and Geothermal Research, v. 188, 86-98.

Riehle, J.R., Miller, T.F., and Paquereau-Lebti, P., 2010, Compaction profiles of ash-flows tuffs: Modeling versus reality: Journal of Volcanology and Geothermal Research, v. 195, p. 106-120.

Sears, J.W., Hendrix, M.S., Thomas, R.C., and Fritz, W.J., 2009, Stratigraphic record of the Yellowstone hotspot track, Neogene Sexmile Creek Formation grabens, southwest Montana: Journal of Volcanology and Geothermal Research, v. 188, p. 250-259.

Simon, J.I., and Reid, M.R., 2005, The pace of rhyolite diffentiation and storage in an ‘archetypical’ silicic magma system, Long Valley, California. Earth and Planetary Science and Letters, v. 235, 123-140.

Simon, J.I., Renne, P.R., and Mundil, R., 2008, Implications of pre-eruptive magmatic histories of zircons for U-Pb geochrology of silicic extrusions: Earth and Planetary Science Letters, v. 266, p. 182-194.

Singer, B.S., 2012, A Quaternary geomagnetic instability time scale: Quaternary Geochronology, v. 21, p. 29-52.

Vazquez, J.A., Kyriazis, S.F., Reid, M.R., Sehler, R.C., and Ramos, F.C., 2008, Thermochemical evolution of young rhyolites at Yellowstone: Evidence for a cooling but periodically replenished postcaldera magma reservoir: Journal of Volcanology and Geothermal Research, v. 188, p. 186-196.

Vazquez, J.A., and Reid, M.R., 2002, Time scales of magma storage and differentiation of voluminous high-silica rhyolites at Yellowstone caldera, Wyoming: Contributions to Mineral and Petroleum, v. 144, p. 374-285.