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Nouvelles recherches sur l'obsidienne de Cheikh Hassan (Vallée de l'Euphrate, Syrie) au Néolithique

New research on the obsidian of Sheikh Hassan (Euphrates Valley, Syria) in the Neolithic

Formation of hydrotalcite-like compounds during R7T7 nuclear waste glass and basaltic glass alteration

Alteration experiments have been performed using RTT7 and synthetic basaltic glasses in MgC1E-CaClz salt solution at 190~ The duration of experiments ranged from 0.25 to 463 days. The alteration products were studied by Scanning Electron Microscope (SEM), Scanning Transmission Electron Microscope (STEM), X-ray diffraction (XRD) and Electron Spectrometry for Chemical Analysis (ESCA). For both glasses, the early alteration product is a hydrotalcite-like compound (MgrAlzCO3(OH)~6' 4H20) in which HPO42 , SO42- and C1- substitutes for CO32-. The measured basal spacing is 7.68 A for the hydrotalcite formed from R7T7 glass and 7.62 ~ for the hydrotalcite formed from basaltic glass which reflect the high A1/A1 + Mg ratios x (0.34 -< x -< 0.46). The chemical microanalyses show that the hydrotalcite is subsequently covered by a silica-rich gel which evolves into saponite after a few months. These results support the use of basaltic glasses alteration patterns in Mg-rich solution, to understand the long-term behavior of R7T7 nuclear waste glass.

Stratigraphy, structure, and volcanic evolution of the Pico Teide-Pico Viejo formation, Tenerife, Canary Islands

The latest cycle of volcanism on Tenerife has involved the construction of two stratovolcanoes, Teide and Pico Viejo (PV), and numerous flank vent systems on the floor of the Las Cañadas Caldera, which has been partially infilled by magmatic products of the basanite-phonolite series. The only known substantial post-caldera explosive eruption occurred ∼2 ka bp from satellite vents at Montaña Blanca (MB), to the east of Teide and at PV. The MB eruption began with extrusion of ≈0.022 km3 of phonolite lava (unit I) from a WNW-ESE fissure system. The eruption then entered an explosive subplinian phase. Over a 7–11 hour period, 0.25 km3 (DRE) of phonolitic pumice (unit II) was deposited from a 15 km high subplinian column, dispersed to the NE by 10 m/s winds. Pyroclastic activity also occurred from vents near PV to the west of Teide. Fire-fountaining towards the end of the explosive phase formed a proximal welded spatter facies. The eruption closed with extrusion of small volume domes and lavas (≈0.025 km3) at both vent systems. Geochemical, petrological data and Fe-Ti oxide geothermometry indicate the eruption of a chemically and thermally stratified magma system. The most mafic and hottest (≈875°C) unit I magma can yield the more evolved and cooler (755–825°C) phonolites of units II and III by between 7 and 11% fractional crystallization of an assemblage dominated by alkali feldspar. Analyses of glass inclusions from phenocrysts by ion microprobe show that the pumice was derived from the water-saturated roof zone of a chamber containing 3.0–4.5 wt.% H2O and abundant halogens (F≈0.35wt.%). Hotter, more mafic tephritic magma intermingled with the evolved phonolites in banded pumice, indicating the injection of mafic magma into the system during or just before eruption. Reconstruction ot the event indicates a small chamber chemically stratified by in situ (side-wall) crystallization at a depth of 3–4 km below PV. Although phonolite is the dominant product of the youngest activity of the Teide-PV system, there has been no eruption of phonolitic magma for at least 500 years from teide itself and for ∼2000 years from the PV system. Therefore there could be a large volume of highly evolved, volatile-rich magma accumulating in these magma systems. An eruption of fluorine-rich magma comparable with MB would have major damaging effects on the island.

Prehispanic Obsidian Workshops at the Zacualtipan, Hidalgo Obsidian Source: A Methodological Essay (Abstract)

Cuaderno de trabajo No. 1, Centro Regional Hidalgo, Edited by INAH, Mexico

Acocella, Tomozawa & Watson, 1984

The nature of dissolved water in sodium-silicate glasses and its effect on various properties

Acquafredda and Paglionico, 2004

SEM-EDS microanalysis of microphenocrysts of Mediterranean obsidians: a preliminary approach to source discrimination

SEM-EDS non-destructive analysis allows the source discrimination of the various microphenocrysts in obsidian rocks of the Mediterranean. Samples of six Mediterranean sources, Monte Arci (Sardinia), Palmarola, Lipari, Pantelleria, Gyali and Melos were studied using SEM-EDS non-destructive techniques in order to obtain a data base of their microphenocryst content. The results demonstrate that provenance of an obsidian can be distinguished on the basis of its petrography, in particular by quantitative analyses of Fe-Mg microphenocrysts; this information is particularly useful when coupled with data obtained by other non-destructive techniques such as glass microanalysis of artefacts (Acquafredda et al., 1999). Such investigations, carried out by SEM, an analytical technique relatively accessible to geoarchaeologists, are relatively rapid, effective and above all non-destructive.

Proposal of a non destructive analytical method by SEM–EDS to discriminate Mediterranean obsidian sources

Chemical characterization of obsidians from different Mediterranean sources by non-destructive SEM–EDS analytical method

The aim of our research is to check the SEM-EDS non-destructive analytical method for discriminating and locating the possible sources of obsidian artefacts. Moreover, in order to obtain a significant discrimination of Mediterranean obsidians, we analysed samples collected from outcrops of the major sources: Monte Arci (Sardinia) and Palmarola, Lipari, Pantelleria, Gyali and Melos islands. All samples were analysed by both XRF (whole rock) and SEM-EDS (glass and microliths-microphenocrysts). The XRF analysis reveals that major elements discriminate obsidian. The discrimination using major elements is very useful because the amount of trace elements is lower than the detection limit of a Si(Li) ED Detector. The major elements, particularly SiO2, Al2O3, CaO, Na2O and K2O, of obsidian glass discriminate the six main Mediterranean sources. Our work demonstrates, therefore, the possibility of discriminating different provenance of obsidian artefacts using SEM-EDS, by means of a relatively rapid, effective and above all non-destructive method.

Report prepared for the California Department of Transportation, Office of Environmental Analysis, Sacramento, California

On the temperature dependence of cooperative relaxation properties in glass-forming liquids

Numerical Dating of Pinedale Fluvial Terraces Near West Yellowstone, Montana Using Obsidian Hydration Dating Techniques (Abstract)

An Analysis of Post-Pinedale Fluvial Terraces Near West Yellowstone, MT, Using Obsidian Hydration Dating Techniques (Abstract)

Obsidian-Hydration Dating of Reworked Sediments in the West Yellowstone Region, Montana

This study evaluates obsidian-hydration dating in postglacial fluvial terraces cut into an outwash plain near West Yellowstone, Montana. Fluvial transport fractures obsidian grains. However, some old hydration rinds may be preserved, thus, a grain may record several fracturing events. The most recent fracturing event at West Yellowstone is recorded in surface sediments from all of the terraces, which were cut in a shorter period of time than the technique can discern. They formed about 19,000 ± 1000 yr ago, using published hydration-rate estimates and a mean rind thickness of 6.34 ± 0.14 μm (1 SE). Alternatively, the application of published hydration-rate constants for the Obsidian Cliff flow with an estimated effective hydration temperature of 1.4°C yield an age of 24,400 ± 1100 yr (1 SE). Thicker rinds record fracturing during Bull Lake glaciation and cooling cracks from the emplacement of several source flows. Much of the observed spread in rind thicknesses (6.34 ± 1.69 μm: 1 SD) is probably the result of chemically induced variations in hydration rate. Terrace ages based on a single rind would range from 13,000 to 39,000 yr (±1 SD). Therefore, it is inappropriate to (1) use a set of hydration-rate constants determined from a single sample to calculate ages for multiple artifacts or geological samples, (2) date an archaeological or geological event on the basis of a single artifact, or (3) generate a chronostratigraphy on the basis of individual dates as a function of depth. Multiple evaluations of source chemistry and hydration rates and multiple rind measurements are required to date fracturing events.

The principles of chemical corrosion of glasses are reviewed. The influence of temperature, glass composition, glass homogeneity and corrosion media are related to observed corrosion rates. A comparison of the durability of man-made and natural glasses is offered and the corrosion rates of tektites and obsidians are discussed. Natural glasses are not likely to advance our present understanding of corrosion theory.

Glass Corrosion Theories: A Tool for Understanding the Past, Designing for the Present, and Predicting the Future

There is renewed appreciation for understanding glass corrosion mechanisms due in large part to the urgency of nuclear waste management. Although the ultimate goal - to predict specific corrosion performance - has not yet been reached, theories continue to be fine tuned so that this objective now appears attainable. However, even at our present level of understanding, chemical durability theory is a vital tool in reading the secrets of the past as contained in both ancient and natural glasses, designing products such as optical waveguides for a new technological age and predicting how those products, such as nuclear waste glass, will perform - far into the future.

Adams, Houghton and Fagents, 2006

The transition from explosive to effusive eruption regime: the example of the 1912 Novarupta eruption, Alaska

Debitage Analysis: Lithic Technology and Interpretations of an Archaic Base Camp Near Moquino, New Mexico

Master's Thesis, Eastern New Mexico University: Portales, New Mexico

Ades, Toganidis and Traverse, 1990

High-temperature Optical-spectra of Soda Lime Silica Glasses and Modelization In View of Energetic Applications

Abstract also appears in International Association/or Obsidian Studies Newsletter, 7:7, 1992

The structural relaxation kinetics of a silica glass were measured by following the IR structural band positions, which are directly correlated with the average Si–O–Si bond angle as well as with the fictive temperature of the glass, as a function of heat-treatment time, temperature and the water vapor pressure. Both surface relaxation and bulk relaxation kinetics were determined by measuring the IR reflection and absorption band positions, respectively. The surface relaxation was much faster than the bulk relaxation and had a smaller activation energy. Also, both relaxation kinetics were faster in the presence of water vapor. The apparent bulk relaxation time determined from the IR absorption band shift was a composite relaxation time consisting of both the relaxation time of the water-catalyzed near surface layer and the true bulk relaxation time of the glass interior which is unaffected by water vapor. The true bulk relaxation time was evaluated and found to have an activation energy consistent with that of the viscous flow.

Infrared (IR) spectroscopy has been used to interpret structural changes in silica glasses. Specifically, a shift of the Si–O stretching band in IR spectra is used to monitor changes in average Si–O–Si bond angle in the glass structure. A similar structural change is induced by the change of fictive temperature, hydrostatic pressure or compressive stress, with the average Si–O–Si bond angle decreasing with the increase of these parameters. It is anticipated that these similar structural changes would produce a similar change in glass properties. In order to confirm this expectation, HF etch rates of silica glasses were measured as a function of fictive temperature and stress. The experimental results on HF etch rates, together with changes in other glass properties in the literature, were compared with the change in glass structure revealed by IR spectroscopy. It was found that the similar structural change is accompanied by the consistent changes in a variety of glass properties. Monitoring the IR spectra of a silica glass sample, therefore, can be used to deduce changes in glass properties.

Agarwal, Davis and Tomozawa, 1997

Infrared spectroscopy was used to determine fictive temperature of silica glasses. Positions of both the fundamental structural band at ∼ 1122 cm−1 monitored in infrared (IR) reflection mode and an overtone of this band at ∼ 2260 cm−1 monitored in IR transmission mode were found to be directly correlated with the glass fictive temperature. At any particular fictive temperature, the equilibrium structural band positions were found to be independent of the impurity content, such as hydroxyl, in different types of silica glass. From band shifts, the average SiOSi bond angle was computed to decrease by about 1.3° when the fictive temperature increased from 950 to 1400°C. Using this method, fictive temperatures of various as-received silica glasses were determined.

Source Provenance of Obsidian Artifacts from the Early Stone Age (ESA) Site of Melka Konture, Ethiopia

1647-1650

The source provenance of 10 Early Stone Age artifacts from the localities in Melka Konture has been determined by EDXRF. Results show that the early to mid-Pleistocene makers of the artifacts derived the raw material from a source located in their proximity, supporting the previously proposed short distance transport of raw material for the time period.

Aguillon-Robles, Aranda-Gomez and Solorio-Munguia, 1994

Aranda-Gomez, Jose Jorge

Solorio-Munguia, Jose Gregorio

Geologia y tectonica de un conjunto de domos riolíticos del ologoceno medio en el sur del estado de San Luis Potosi, México

La riolita con topacio y, o en su lugar, estano, es comun en la Republica Mexicana y forma un extenso cinturon que se prolonga hasta los Estados Unidos de America. Estas rocas tienen interes especial porque son productos caracteristicos de fusion parcial de cortez a continental precambrica, derramados durante periodos de actividad tectonica extensiva. Ademas, en algunos lugares existe mineralizacion economic a asociada a ellas. En el Estado de San Luis Potosi existen afloramientos de riolita topacífera dentro del campo volcánico del mismo estado (CVSLP), el cual se forma durante el Oligoceno medio (32-26 Maj. Las zonas topacíferas estan localizadas cerca de los conductos volcánicos, o bien en los derrames de lava de la Riolita San Miguelito (K-Ar: 29 ± 0.8 Ma ). Las rocas riolíticas estudiadas contienen fenocristales de cuarzo, sanidino, plagioclasa sodica, biotita y minerales opacos: su alto contenido de flúor, lo atestigua el topacio cristalizado en fase de vapor en compañia de minerales de estano . Los domos EI Gato y Cerro Silva, localizados cerca del poblado de Tepetate, a unos 40 km al surponiente de la ciudad de San Luis Potosi, fueron formados por erupciones pasivas continuas, lo que originó foliación de flujo vertical en las partes centrales de las estructuras, y subhorizontal en la parte intermedia . Hacia los frentes de los derrames de lava, la foliación se hace casi vertical. Estas rocas tienen un contenido promedio aproximado de 75% de Si02, 13% de Ab03, 8-9% de Na20+K20 y 1% de CaO. La riolita topacífera es roca metaluminosa a ligeramente peraluminosa.

Lopez-Martinez, Margarita

Ignimbrites of the central Méxican volcanic belt - Amealia and Huichapan calderas, states of Querétaro and Hidalgo

Magmatism and tectonics of central and northwestern Mexico - A selection of the 1997 IAVCEI General Assemply excursions

Aguirre-Diaz, Gerardo J., Aranda-Gomez, Jose Jorge, Carrasco-Nunez, Gerardo, Ferrari, Luca, Universidad Nacional Autonoma de Mexico, Instituto de Geologia, Excursion, (1)

The Amealco and Huichapan calderas lie in the central sector of the Mexican Volcanic Belt (MVB) (Figure 1). These two calderas erupted voluminous and widespread ignimbrites at different times during the Pliocene: the Amealco and Huichapan tuffs, at 4.7 and 3.5 Ma, respectively, and both deposits overlap in space. During the fieldtrip we will examine the Amealco and Huichapan tuffs. These ignimbrites are generally welded, lowaspect ratio deposits that are spectacularly exposed in several canyons around the calderas. We will be able to observe the entire pyroclastic-epiclastic sequence associated with each major tuff, including pumice fall, surge, ash-flow, and mud-flow deposits, as well as welded and unwelded ignimbrites. One of these canyons exposes about 150 m of both the Amealco and Huichapan tuffs. We will examine the ignimbrites at their vents, proximal, medial and distal facies. Amealco deposits are particularly interesting because they are trachyandesitic-trachydacitic and show evidence of magma mingling in each of the three.,major ignimbrites that compose the Amealco Tuff. We will visit both the Amealco ( 1 1 km diameter) and Huichapan (10 km diameter) calderas, which are well exposed and have good access.

Ahmad, Chaudhary and Qureshi, 1981

An instrumental neutron activation analysis technique has been developed for the simultaneous determination of up to 30 elements including major, minor and trace elements in Obsidian Rock (a proposed NBS-SRM-278). INAA method involves both short and long irradiations followed by gamma-ray activity measurement with a Ge(Li) detector. The accuracy of the procedure has been checked by analyzing IAEA Reference Materials.

The Chemical Composition of Rhyolitic Obsidian and Its Effect on Hydration Rate: Some Archaeological Evidence

Master's Thesis, Department of Anthropology, University of California: Los Angeles, California

9503-9510

Direct Observation of Water Speciation in Rhyolite at Temperatures Up to 850 Degrees C (Abstract)

Journal of the Royal Statistical Society, Series B (Methodological)

vol. 44, num. 2

Spherulites consisting of fibrous alkali feldspar and silica minerals are produced by devitrification of rhyolite glass under hydrothermal conditions. The alkali feldspars (Ab72.5Or23.0An4.5, Ab81.7Or14.0An4.3) in spherulites from two localities in Japan consist of triclinic anorthoclase showing fine cross-hatched twinning and monoclinic sanidine showing fine cross-hatching not attributable to twinning. The cross-hatching, which corresponds to albite and pericline twinning, is produced in the process of transition from a monoclinic to a triclinic phase. The spherulite may develop at a temperature lower than about 200°C because the co-existing silica mineral is not quartz, but metastable tridymite. According to the phase diagram of the alkali feldspars by MacKenzie (1952), the alkali feldspars should have been triclinic during growth. However, the textures show that the alkali feldspar grew as a disordered monoclinic phase. Because of the high growth rate, the Al/Si disordered structure was produced during growth and afterwards transformed into a triclinic structure with cross-hatched twinning.

The classical simple limiting case of diffusion is described by Fick's equation, with a constant diffusion coefficient: D2Vc= dc/bt. Diffusion of small molecules into or through a polymer often follows a niorc complicated pattern.

Abstracts, International Symposium on Archaeometry, 2-6 Apri/1990, Heidelberg, Germany

Allen, Ramirez and Wolleben, 1977

Chemical Characterization of Obsidian from Central Mexico and Its Application to Archaeological Investigations (Abstract)

Archaeology of the Lapita Cultural Complex: A Critical Review

P.V. Kirch and T.L. Hunt. Thomas Burke Memorial Washington State Museum Research Report No. 5: Seattle, Washington

On certain ancient devitrified pitchstones and perlites from the Lower Silurian District of Shropshire

Aguirre-Martínez, P.I.

Determination of the provenance of obsidian samples collected in the archaeological site of San Miguel Ixtapan, Mexico State, Mexico by means of neutron activation analysis

1309-1316

The Mechanisms for Hydrothermal Leaching of Glass and Glass-Ceramic Nuclear Waste Forms

The very different chemical constituents contained in the waste glass lead to a specific leaching behavior under hydrothermal conditions. The results are still difficult to interpret in detail but the mechanism in deionized water resembles the one where a protective layer is formed by some of the glass constituents which determine the further release of material from the sample surface. Magnesium, Al, Zn, and Ti were found to contribute considerably to the layer formation in addition to waste constituents such as rare earths and U, Zr. The former elements have already been used for glass frit formulations in order to better digest the waste oxides. They might also be helpful, if used at appropriate concentration levels, to increase the leach resistance of the waste form

Obsidian: Rohstoff-Werkstoff-Handelsgut. Eine Naturmaterial-Industrie in der Frühgeschichte

Obsidian: Commodity Material. A natural material industry in the early history

Abstract from a paper presented at the Society for California Archaeology Annual Meeting, April, 1990, Redwood City, California

Obsidian dating and source exploitation studies in Africa: Implications for the evolution of human behavior

Obsidian occurs in the eastern Rift Valley region of Ethiopia, Eritrea, Kenya, and Tanzania, and in a restricted region of West Africa, in Cameroon. Source locations and chemical compositions are best documented in Kenya and Tanzania, and significant advances are now being made in Ethiopia. Several sites have been dated by obsidian hydration, one by fission tracks and four by radiogenic argon isotope ratios. Hydration dating appears to be limited to sites younger than 120,000 years. Although the most advanced methods of hydration dating have not been applied in this region, radiometric dating demonstrates that the Middle to Later Stone Age transition in eastern Africa is substantially older than the Middle to Upper Paleolithic transition in western Eur asia. Obsidian was first used for flaked stone-tool manufacture during the Early Stone Age (Oldowan), more than 1.7 million years ago. Rare instances of long-distance transport of obsidian occurred during the Oldowan at Olduvai Gorge and during the early Acheulean in Kenya and Ethiopia. However, substantial amounts of obsidian were not transported long distances until the second half of the Middle Stone Age (MSA). The increase in long-distance transport of obsidian during the late MSA reflects the African origin and development of large-scale regional social networks, which is an important milestone in the evolution of modern human behavior. Patterns of obsidian use in Kenya and Tanzania during the Holocene may reflect adaptations to changing environments by hunter-gatherers, and exchange patterns and territorial boundaries of Neo lithic pastoralists. Obsidian in Egyptian sites as early as the Predynastic era reflect long-distance contacts with Ethiopia and Eritrea.

Advances in Obsidian Glass Studies: Archaeological and Geochemical Perspectives

The basis and method of dating obsidian artifacts by determination of the samples hydration rate are discussed. Its advantages in comparison with radiocarbon dating are also discussed.

Abstracts, Ninth Congress of the International Union for Quaternary Research

Obsidian and Its Prehistoric Distribution in Melanesia, in The Proceedings of a Symposium on Scientific Methods of Research

The Study of Ancient Chinese Bronzes and Southeast Asian Metal and Other Artifacts

Ambrose, Bird and Duerden, 1981

W.R. Ambrose and Duerden. Australian National University: Canberra, Australia

Papua New Guinea - Admiralty Islands

Ambrose, Duerden and Bird, 1981

Papua New Guinea - Admiralty Islands

237(5349)

Polynesia - Solomon Islands

Reepmeyer, C.

Possible obsidian sources for artifacts from Timor: narrowing the options using chemical data

Measurements made at the Australian National University using laser ablation ICPMS show that none of the 88 analyzed obsidian artifacts from East Timor match either the known Papua New Guinea or the five Island SE Asian source samples in our ANU collections. There is a coastal journey of more than 3000 km between the occurrence of obsidians from the Bismarck Archipelago volcanic province of Papua New Guinea and the Sunda-Banda Arc volcanic chain, yet obsidian artifacts from the two important PNG sources of Talasea and Lou Island are found at coastal Bukit Tengkorak in eastern Sabah at a similar distance along with material that has no known source. Timor lies south of the eastern section of the active volcanic Banda Arc island chain but it is within range of possible rhyolite sources from there. Although there is a continuous chain of around 60 active volcanoes stretching from west Sumatra to the Moluccas most are basaltic to andesitic with few areas likely to produce high silica dacite–rhyolite deposits. This does not exclude the possibility that the volcanic landscapes may contain obsidian, but without detailed survey and chemical analysis of sources from the Sunda-Banda Arc the attribution of the Timor obsidian artifacts remains to be demonstrated. Timor may seem to be an unlikely source for the presence of obsidians as it lacks reports of the silica-rich rhyolite volcanic centers necessary toproduce this material. Despite the absence of detailed survey and analysis of Indonesian obsidian sources, especially from the volcanically active Banda Arc, this paper presents evidence that one of two obsidian sources is clearly from Timor while the other, with less certainty, is also from an unknown local source

Characterization of Archaeologically Significant Obsidian Sources in Oregon by Neutron Activation Analysis

Ambroz, Glascock and Skinner, 2001

Occasional Paper No. 10, Institute of Archaeology: London, England

Ammerman, Cesana and Terrani, 1990

Ammerman, Matessi and Cavalli-sforza, 1978

Some New Approaches to the Study of the Obsidian Trade in the Mediterranean and Adjacent Areas

Possible consequences of compositon gradients in basalt glass adjacent to olivine phenocrysts

Prehistoric Two-Way Voyaging Between New Zealand and East Polynesia: Mayor Island Obsidian on Raoul Island, and Possible Raoul Island Obsidian in New Zealand

Studies in Volcanology: A Memoir in Honor of Bowel Williams

R.R. Coats, R.L. Hay, and C.A. Anderson. Geological Society of America Memoir

Anderson, Tiffany and Nelson, 1986

The pitchstones of Mull and their genesis, with notes on the rock-species leidleite and inninmorite by H. H. Thomas and E. B. Bailey

Crease Structures: Indicators of Emplacement Rates and Surface Stress Regimes of Lava Flows

Crease structures are features commonly found on lava flow surfaces and consist of a fracture with curved walls that extend outward from a linear valley. These crease structures are found on flows of nearly all compositions and crystallinities. We have mapped the distributions of crease structures on many flows in the western United States and found that (1) axial length is not dependent upon composition and crystallinity; (2) adjacent crease structures are generally aligned in an en echelon pattern; (3) crease structures located adjacent to flow margins are generally perpendicular to these margins; and (4) at Mount St. Helens, Washington, large lobe-bisecting crease structures are found on lobes situated on slopes of less than 20 degrees. A primary surface feature found on many crease structures is striations. Striations are sets of long stripes on the walls of the crease structure that extend approximately parallel to the axis of the central valley, and they appear to be analogous to those found on the faces of cyclically fractured basalt columns. Observations of developing crease structures on 6 of the nearly 20 Mount St. Helens dome lobes show that they form throughout the extrusion of flows situated on slopes of less than 20 degrees, but only at the very beginning and/or end of extrusion of flows on steeper slopes. These observations imply that crease structures form when the lava flow is forced to spread laterally, either as the flow advances over a flat area, or as the down-slope movement stagnates near the end of extrusion (causing the rate of spreading to exceed the downslope rate of flow). This lateral spreading of lava results in the concentration of tensile stress along a line oriented perpendicular to the direction of spreading. The cooled crust of the extrusion is therefore torn apart about this line of tensile stress concentration, forming a central valley that exposes hot, ductile material from the flow interior to the atmosphere. The presence of striations on many crease-structure walls implies that the emplacement mechanism is similar to that suggested for columnar basalts, where each striation is produced by incremental fracturing.

Surface characteristics of steep-sided domes of Nenus and terrestrial silici domes: a comparision

A silicic composition for steep-sided Venusian domes has recently been proposed on the basis of similarities in thickness, planform, and gross topographic profile to terrestrial rhyolitic and dacitic domes [1-4]. However, there are significant morphologic, volcanologic, and remote sensing characteristics that do not support direct comparison. For example, domes on Venus lack typically observed structures and morphological units, are an order of magnitude or more larger in volume and have smoother surfaces than terrestrial silicic lava domes In order to evaluate the validity of the silicic model for Venusian domes, we measured and analyzed topographic profiles, boulder-size distributions, and structural elements present on the surfaces of recent rhyolitic and dacitic domes in the western United States. These data show that steep-sided domes on Venus lack the progression in surface morphologies, characteristic large block size, and topographic variability found on terrestrial dome surfaces. Our study suggests that these discrepancies preclude direct comparison of domes on Venus and Earth, and that a re-evaluation of the silicic model for the origin of Venusian domes is required.

Boulders are a ubiquitous but often neglected feature present on silicic lava flows and domes. As part of an analysis of the emplacement history of silicic extrusions, we determined boulder size distributions on young lava flows and studied the development of boulders on the active Mount St. Helens and Mount Unzen lava domes. We find measurable variations in average boulder size across a single extrusion and between different extrusions that are related to changes in emplacement conditions during eruption and flow emplacement. Our study concentrates on two large eruptive centers in California. The Medicine Lake Highland volcano contains several rhyolitic and dacitic lava domes with volumes ranging from <3000 m3 to >1 km3 [1,2]. Many of the domes have no vegetation present on their surfaces and have been emplaced within the past 10,000 years [2]. The Inyo dome chain consists of 5 rhyolitic domes erupted along an en echelon dike [3]. Obsidian, Deadman, and Glass Creek domes were erupted 550-650 years ago with volumes ranging from 0.04 km3 to 0.3 km3

Block size distributions on silicic lava flow surfaces: implications for emplacement conditions

1258–1267

Examples of Tools Showing Craft Specialization Activities in the Harrappean of Pakistan and the Neolithic of Turkey (Abstract)

Abstract from a paper presented at the 55th Annual Meeting of the Society for American Archaeology, April 18-22, Las Vegas, Nevada

2075–2080

Andrews and Gallareta-Negrón, 1986

Gallareta-Negrón, Tomás

Cervera-Rivero, P.

Paper presented at the 62nd Annual Meeting of the Society for American Archaeology, Nashville, Tennessee

Craftsman skill and specialization: Investigating the craft production of prehispanic obsidian blades at Xochicaloo and Teotihuacan

Ph.D. dissertation, Department of Anthropology, Pennsylvania State University, University Park, Pennsylvania

Pathways to Prismatic Blades: A Study in Mesoamerican Core-Blade Technology

Kenneth G. Hirth and Bradford W. Andrews, Monograph 45, Cotsen Institute of Archaeology, University of California. Los Angeles.

Report to Foundation for the Advancement of Mesoamerican Studies, Inc. (FAMSI).

http:// www.famsi.org/reports/01029/index.html