Obsidian Flow Research Articles

Cooling dynamics of spatter-fed phonolite obsidian flows on Tenerife, Canary Islands

Relaxation geospeedometry has been applied to two series of clastogenic obsidian flows on Tenerife to determine their thermal history across the glass transition. The phonolite flows investigated were both generated by lava fountaining activity followed by rheomorphism of the deposits. The detailed sampling resolution within the two series enabled an accurate quantification of their thermal history. Cooling rates within the investigated spatter-fed flows vary over more than two orders of magnitude. The highest cooling rates of 0.39 K/min were modeled for the central vesiculated part of one flow. The dense basal obsidian layers of both flows were cooled at substantially lower rates of 0.0042 and 0.0028 K/min, respectively. There appears to be an influence of in-situ vesiculation processes on the thermal budget of the investigated flows. In addition, the slow cooling rates for the basal portions of both flows seem to be associated with a stage of thermal buffering. Continual advective heat transport of hot material along a basal shear plane may sustain elevated temperatures associated with (quasi-) isothermal annealing within this “décollement”. Numerical simulations based on conductive heat loss concepts fail to resolve the cooling history quantified through relaxation geospeedometry for the investigated flows. The effects of vesiculation and thermal annealing on the cooling behavior of the clastogenic flows across the glass transition are discussed in the light of these new data. In addition, viscometric data on these phonolites are used to correlate the known cooling rates to viscosities at the glass transition.

Neutron Activation Analysis of Obsidians from Quarries of the Central Quaternary Trans-Mexican Volcanic Axis

Obsidian samples from 13 Mexican quarries (Sierra de Pachuca and Zacualtipan, Hidalgo, Zaragoza and Oyameles, Puebla, Altotonga Veracruz and eight sites of the Zinaparo — Varal — Churintzio Hills region) have been analyzed by NAA. The concentration of 19 elements, major and trace, are reported. Excepting Altotonga Veracruz, the chemical composition of other obsidian flows was found homogenous. The region of the Varal obsidian was delimited. An excellent correlation between ytterbium and lutetium contents in obsidians was found.

Obsidian Characterization by Laser Ablation ICP-MS and its Application to Prehistoric Trade in the Mediterranean and the Near East: Sources and Distribution of Obsidian within the Aegean and Anatolia

For geological studies, interest in mass spectrometry with an inductively coupled plasma as an ion source and its association with laser ablation as a sample introduction technique (LA-ICP-MS) has steadily increased during the past few years and is now being developed in other fields such as archaeology. After a description of the analytical procedure and the calculation method, we show the potential of this technique to characterize, almost non-destructively, archaeological artefacts. Among the 70 elements that could be routinely analysed by LA-ICP-MS with detection limits below the ppm level, we choose to determine the more critical ones in order to evaluate the geochemical models of the magmatic process (major elements, rare earths and some transition elements). A detailed survey of Cappadocian obsidian flows is given, and includes the characterization of nine different sources. Evidence of complex trade activities is clearly shown by the obsidian tools found at different prehistoric sites (from Neolithic to Bronze Age levels) in the Mediterranean and the Near East. New results obtained on some archaeological sites located in Turkey, Syria and Cyprus are presented. They show the importance of Cappadocian sources in obsidian trade. Our results show that LA-ICP-MS allows a non-destructive analysis of archaeological objects and that it combines the advantage of the different classical methods used to characterize obsidian sources (mainly XRF and INAA) with high sensitivity and rapidity. Thus LA-ICP-MS appears to be a very powerful analytical tool and, at this time, this technique is the only one which can non-destructively determine such an important number of elements with such low detection limits.

Constraints on rheology of obsidian lavas based on mesoscopic folds

The geometry of mesoscopic single and multiple layer folds in rhyolitic obsidian flows is investigated. Folds are composed of obsidian embedded in a matrix of pumice. Folds form by buckling processes as indicated by discontinuous deformation between obsidian and pumice layers and by the geometries of wavetrains. Buckling occurs through a continuum of styles controlled largely by the thickness ratio of pumice to obsidian ( N). Styles of folds include chevron, harmonic, polyharmonic, disharmonic and single-layer assemblages. Harmonic and chevron folds are observed for small values of N. For large values of N, folds buckle independently of one another and form disharmonic and single-layer assemblages. Wavelength-to-thickness ratios of single-layer folds are compared to theoretically predicted ratios for Newtonian and power law fluids as a means of estimating shear viscosity ratios of obsidian and pumice. While all folds indicate that bubble-free rhyolite is more viscous during flow than bubbly rhyolite, estimates of shear viscosity ratio based on Newtonian theory (∼10–500), may exceed estimates based on power law rheologies by more than an order of magnitude. Newtonian buckling theory involves a number of simplifications and does not account for the possibly complex rheology of bubble-bearing rhyolite.

Trace Element Characteristics of Central Anatolian Obsidian Flows and Their Relevence to Pre‐History

AbstractThis report concerns the chemical characterization of obsidian sources from Central Anatolia by neutron activation analysis. The sources covered in this study were Göllü Dağ, Hotamiş Dağ, Köru Dağ, Nenezi Dağ, and Çatköy. The study reported here was undertaken with the object of obtaining an accurate “fingerprint” of the compositions of obsidian sources that would permit tracing the origin of archaeological obsidian artifacts to their sources, and in particular, archaeological obsidian recovered from prehistoric sites in Israel. The extent of the sampling within the sources was more thorough than hitherto reported and the results are of high precision and accuracy.

Obsidian provenancing and magmatic fractionation in central oregon

AbstractIn many instances, geologically distinct obsidian flows located within even a relatively small geographic area can be uniquely identified by their chemical composition. This happens to be true for several obsidian sources from central Oregon. Internally each obsidian locality is chemically homogeneous, but the obsidian rocks from different collection sites exhibit chemical differences. Based on the geochemical variations and on K/ Ar dating of the end members of the chemical differentiation trend, these differences are related to the fractionation of a single Late Miocene magma chamber, dated at 6.5 Ma. By understanding the underlying causes of the chemical differences, constraints are disclosed that will govern the possible chemical variations of other, as yet unidentified but related obsidian flows. These can be useful for identifying the possible natural sources of obsidian artifacts which do not match known obsidian sources, and for suggesting possible geographic areas where these as yet undiscovered obsidian flows may be found. © 1993 John Wiley & Sons, Inc.

Obsidian-bearing lava flows and pre-Columbian artifacts from the Ecuadorian Andes: First new multidisciplinary data

All known outcrops of obsidian flows in the Cordillera Real (Ecuador) have been mapped and sampled to reconstruct their eruptive history using geological observations, age determinations, and trace element data. Our results bear also on the recognition of the sources of obsidian artifacts, used in pre-Columbian tools, and on the reconstruction of ancient trading patterns in Ecuador. Three obsidian flow groups were identified on the basis of flow structures and state of preservation. The groups are further defined by differences in radiometric age, fission-track measurements made at two laboratories (Pisa, Italy, and Campinas, Brazil), and different trace element patterns, determined by neutron activation analysis (Pavia, Italy). The oldest obsidian flows form the upper part of the Basal Volcanic Complex (BVC), the basement of Quaternary Ecuadorian stratovolcanoes. Their ages fix the upper limit of the BVC at 1.5 Ma, in the central Cordillera Real. The two more recent episodes of obsidian rhyolitic volcanism are date at ∼0.85 Ma and slightly less than 0.2 Ma, corresponding in age to the present volcanic arc.

Magmatic inclusions in the Holocene rhyolites of Newberry Volcano, central Oregon

The 1350 years B.P. Big Obsidian Flow (BOF) at Newberry Volcano in central Oregon contains a wide variety of mafic magmatic inclusions. Although little grain size variation is observed within single inclusions, within the suite textures vary continuously from fine‐grained (“quench”) to coarse‐grained (“cumulate”). The BOF inclusion suite, therefore, defies conventional genetic classifications. Detailed petrographic and geochemical observations require a new model for inclusion formation. Only a few of the finer‐grained inclusions have margins suggestive of liquid‐liquid interaction. Interstitial glass content (≈ 20 vol %) and composition (high‐silica rhyolite) are approximately constant throughout the suite. Mineral compositional zoning is greater in the coarse‐grained than fine‐grained inclusions. Whole inclusion compositions vary significantly but do not correlate well with textural diversity. We interpret the BOF inclusion suite to be a heterogeneous sample set of whole inclusions and parts of disaggregated inclusions. The compositional variability of the suite can be explained by a combination of preentrapment (hybridization) and postentrapment (interstitial liquid loss/gain) processes. Preentrapment hybridization involved mixing of basaltic andesite magma similar to Holocene flank flows and up to 30% rhyolitic liquid. The BOF inclusions may demonstrate the recent existence of mafic magma in the Newberry system. Inclusion features characteristic of small ΔT are consistent with the crystal‐free nature of the BOF, suggesting that the rhyolite may have been superheated. If true, a crystal fractionation origin for the BOF rhyolite is unlikely. The disaggregation of grain size and compositionally zoned inclusions, which produced the observed textural diversity in the BOF suite, compounds problems of interpreting mafic inclusions in silicic volcanic rocks and, more so, in plutonic equivalents.

Initial results from VC‐1, First Continental Scientific Drilling Program Core Hole in Valles Caldera, New Mexico

Valles Caldera 1 (VC‐1) is the first Continental Scientific Drilling Program (CSDP) core hole drilled in the Valles caldera and the first continuously cored well in the caldera region. The objectives of VC‐1 were to penetrate a hydrothermal outflow plume near its source, to obtain structural and stratigraphie information near the intersection of the ring fracture zone and the precaldera Jemez fault zone, arid to core the youngest volcanic unit inside the caldera (Banco Bonito obsidian). Coring of the 856‐m well took only 35 days to finish, during which all objectives were attained and core recovery exceeded 95%. VC‐1 penetrates 298 m of moat volcanics and caldera fill ignimbrites, 35 m of precaldera volcaniclastic breccia, and 523 m of Paleozoic carbonates, sandstones, and shales. A previously unknown obsidian flow was encountered at 160 m depth underlying the Battleship Rock Tuff in the caldera moat zone. Hydrothermal alteration is concentrated in sheared, brecciated, and fractured zones from the volcaniclastic breccia to total depth with both the intensity and rank of alterations increasing with depth. Alteration assemblages consist primarily of clays, calcite, pyrite, quartz, and chlorite, but chalcopyrite and sphalerite have been identified as high as 450 m and molybdenite has been identified in a fractured zone at 847 m. Carbon 13 and oxygen 18 analyses of core show that the most intense zones of hydrothermal alteration occur in the Madera Limestone above 550 m and in the Madera and Sandia formations below 700 m. This corresponds with zones of most intense calcite and quartz veining. Thermal aquifers were penetrated at the 480‐, 540‐, and 845‐m intervals. Although these intervals are associated with alteration, brecciation, and veining, they are also intervals where clastic layers occur in the Paleozoic sedimentary rocks.

Structure and emplacement of a rhyolitic obsidian flow: Little Glass Mountain, Medicine Lake Highland, northern California

Many rhyolitic obsidian flows show consistent stratigraphic relations among textural units exposed in the flow fronts of undissected flows and in cross sections of older flows. The stratigraphy of the Holocene Little Glass Mountain rhyolitic obsidian flow consists of (from bottom to top): air-fall tephra deposits, basal breccia, coarsely vesicular pumice, obsidian, finely vesicular pumice, and surface breccia. Slightly crystalline rhyolite occurs near the vent areas. A model of obsidian-flow emplacement based on these textural relations is presented. This stratigraphy may reflect the distribution of volatiles within the magma source region, with the interlayered contact between coarsely vesicular pumice and overlying obsidian indicating stratification of volatiles in the magma body. The distribution of pumiceous and glassy zones along with the orientations of flow banding can be used to map the surface structure of rhyolitic obsidian flows. This complex structure, as mapped on the Little Glass Mountain flow, reflects both the initial flow stratigraphy and subsequent deformational processes. During emplacement of the lava, three processes disrupt the initial flow configuration: the rise of coarse pumice diapirs from the base of the flow, the inward propagation of fractures in areas of extension, and surface folding in sites of flow-parallel compression. Subvertical flow banding in vent areas indicates that fracturing accompanies the emergence of lava; most of the observed upper surface of a dome originates as a fracture plane. The structure of domes that form over vent areas may reflect the orientation of dike-like conduits as well as the local state of stress during extrusion.

Gravity instability in the Holocene Big and Little Glass Mountain rhyolitic obsidian flows, northern California

Field relations on the surfaces of the Holocene Big and Little Glass Mountain rhyolitic obsidian flows in northern California suggest that regularly spaced areas of coarsely vesicular pumice are diapirs which rose from the base of the flow in response to a density inversion inherent in the flow stratigraphy. The flows contain three lithologic units, defined on the bases of color, texture, and specific gravity, which are arranged in the following vertical sequence: a basal layer of coarsely vesicular, brownish grey pumice ( ρ∼- 0.8 to 2.0), a central core of black, glassy obsidian ( ρ ∼- 2.2), and a surface crust of white to pinkish grey, finely vesicular pumice ( ρ ∼- 2.0). The flows were modeled as multilayered fluid systems with unstable density stratifications, and the spacings of coarse pumice diapirs were equated with the dominant wavelengths of viscous folding theory. Wavelengths of 45 m, 56 m, and 104 m were calculated using measurements of unit thicknesses and estimates of viscosity ratios for the three lobes. Dome spacings of 43 m, 70 m, and 60 m were measured on the same three flow lobes. Discrepancies between predicted and measured spacings are attributed to local variations in thicknesses and viscosities of the flow units. Elongation and surface folding of diapirs are cited as evidence that they emerged before the flows stopped moving.

Surface folding and viscosity of rhyolite flows

Regularly spaced ridges on rhyolite flows are analyzed through the use of a surface-folding model that was first applied to ropy structures on pahoehoe basalt flows. The requirement that there be a strong folding instability to produce regularly spaced ridges places constraints on three dimensionless parameters related to the properties of the lava and the geometry of the channel: R > 35, S 28. R is the ratio of surface to interior viscosities, S is a ratio between the stress due to the weight of the lava and the compressive stress due to folding, and L dγ is a dimensionless form of the ridge spacing. Estimates of strain rates and measurements of ridge spacings and thicknesses of thermal boundary layers of flows allow these three parameters to be calculated independently for a given flow lobe. For the Big Glass Mountain rhyolitic obsidian flow in northern California, R ≅ 104, S 44. This compatibility between theory and observation supports the folding interpretation for ridges. Furthermore, the model allows calculation of the minimum viscosity of many flows for which such data are otherwise unavailable. The viscosities of a dacite flow in Chile and of a. possible lava flow on Mars are calculated as examples.

Fission Track Dating of the Obsidian of Lipari Island (Italy)

OBSIDIAN was widely traded in prehistoric times, and it is of particular interest to archaeologists to reconstruct the trading patterns1–5. One possible source of obsidian for prehistoric man is Lipari Island (Eolie Archipelago, Italy) and we have dated obsidian flows there for two chief reasons: first, to find out which of these flows could have been used by primitive man, and second, because we wished to identify the artefacts that have been found at various sites in the Mediterranean basin.

Geochemical examination of obsidians from Newberry Caldera, Oregon

Five obsidian flows located in Newberry Caldera, Oregon, were systematically sampled, and the samples have been chemically analyzed to examine the homogeneity in a single large flow and to study chemical differences among the flows. The largest flow, the Big Obsidian Flow, is highly homogeneous chemically. The five flows are very similar in composition, indicating a common source. Small chemical differences among the flows do exist, but the geologic cause of the differences is not determined.

Paleomagnetism, Geochemistry, and Potassium-Argon Ages of the Rio Grande de Santiago Volcanics, Central Mexico

Seventy-two oriented cores of 2.5 cm diameter have been drilled from seven bodies comprising part of a sequence of ignimbrite sheets interdigitated with basalts and basaltic andesites, in the Grand Canyon of the Rio Grande de Santiago of the Mexican Cordilleran Volcanic Province. Field observations and whole-rock analyses reveal two groups: an olivine basalt-high alumina basalt-basaltic andesite series, and a sequence of rhyolitic ignimbrite sheets, with some rhyolitic and obsidian lava flows. Crystal separation, partial melting of oceanic tholeiite, and simple mixing of basalt and oceanic sediments or granodiorite cannot acceptably explain the observed fractionation trends. The favored origin is a partial melting of a second or third stage product, such as a basalt or amphibolite enriched in K, Rb, Th, and Pb, relative to oceanic tholeiite. The observed fractionation trends are parallel to those of the andesite-dacites of the Mexico City region, but the Rio Grande de Santiago rocks have a systematically higher K 2 O/SiO 2 ratio, which can be explained by a greater magma generation depth. The paleomagnetic polarity sequence with decreasing age is R,R,N,R,R,N,N, providing easily measurable markers of high potential stratigraphic value. The mean virtual geomagnetic pole is not significantly different from the present spin axis. The potassium-argon analyses show that the sampled section contains two distinct volcanic episodes, from approximately t = 4.6 to 5.5. m.y. and from approximately t = 8.7 to 9.5 m.y. The polarities recorded by the youngest volcanic cycle are consistent with the predicted polarity time scale of Heirtzler and others (1968), while the older cycle polarities can be explained by minor modification of the predicted polarity scale, to include a short period of reversed polarity between 9.0 and 9.5 m.y., within the normal polarity anomaly 5 (t = 8.79 to 9.94 m.y.) of the time scale.

X-Ray Fluorescence of Rock Samples as Applied to Geological Problems

Three methods of sample preparation of rock specimens have been evaluated for precision and accuracy. A fusion technique was found to give the best precision. Standard deviations (1 σ) using this method and expressed as percent of the amount present for several elements are: Al 0.83, Si 0.64, K 0.43, Ca 0.43, Mn 0.48, Fe 0.24. Three recent studies of compositional variation were performed on volcanic rocks. Analyses of samples from the Hopi Buttes, Arizona, on obsidian flows from Newberry Caldera, Oregon, and on igneous rocks collected from grids or linear traverses give data which, in each case, are useful in making significant interpretations about the geologic history of the rocks concerned. These examples illustrate the rapidity and high quality of quantitative chemical analyses which can be obtained by application of x-ray fluorescence analytical techniques.

Age of the obsidian flow at Glass Mountain, Siskiyou County, California

Age of the obsidian flow at Glass Mountain, Siskiyou County, California

The velocity of the big obsidian flow, Bend, Oregon

Newberry Crater is about 20 miles southeast of Bend, Oregon. It is a caldera and it has been studied and described by Howel Williams. In this caldera there are three obsidian flows. The most spectacular and interesting of these is the Big Obsidian Flow. This obsidian shows its original flow‐features more distinctly, the author believes, than any other obsidian in the United States. It is unweathered and uneroded. It is about a mile long and about 115 feet thick, with an angle of inclination of about 6°. When in motion its specific gravity was not far from 2.5, and, according to the experimental work of A. A. Leontleva, its viscosity was approximately 109 poises.