The geochemistry of obsidian provides a unique tool for both archaeologists and geologists. Geochemical data from archaeological artifacts can be matched with those from known obsidian sources to indicate the parent source of origin for each artifact, and they also may indicate the presence of unknown geological sources for obsidian. Archaeologists are interested in locating all sources of artifact-quality obsidian and in identifying geochemical types on a scale that separates geographic obsidian sources and matches them with artifacts. Archaeologists then use geochemical data to reconstruct prehistoric trade/exchange relationships and group mobility patterns through time. Geologists are interested in obsidian geochemistry (1) to understand intraflow and interflow compositional variations in order to gain insight into magmatic and flow process and (2) to compare obsidians on a regional, or global, scale to provide insight into major problems of tectonics and crustal evolution of the earth. Geochemical data are thus crucial to both archaeology and geology, but archaeological provenance studies may be compromised by a lack of knowledge about the genesis of obsidians. Obsidian sources are of several geologic types; we draw examples from Obsidian Cliffs, Oregon, and Casa Diablo, California, Borax Lake, California, Brown’s Bench, Nevada/Idaho, to illustrate, respectively, geochemical homogeneity or heterogeneity in obsidian flows, the geologic processes of magma mixing, and chemical zonation in ash-flow tuffs and to discuss the ways each geologic situation may affect provenance studies. We emphasize that appreciation of these geologic processes is requisite to sound execution of provenance analysis in geoarchaeology.

Abstract This discussion of Quaternary volcanism in the western conterminous United States in part is similar to our earlier paper (Smith and Luedke, 1984) concerned with the late Cenozoic volcanism for the same region. One important difference between the two papers is the much shorter geologic time interval covered here (about the last 1.6 m.y.); in contrast, our earlier paper considered volcanism for a time span covering about the last 16 m.y. Both this and our earlier paper are based on a comprehensive study utilizing hundreds of references and five maps compiled and published in the U.S. Geological Survey Miscellaneous Investigations Series (Maps I-1091-A through E) at the scale of 1:1,000,000 (Luedke and Smith, 1978a, 1978b, 1981, 1982, and 1983). For compilation purposes, the volcanic rocks were grouped into five major types based primarily on their known or assumed silica content; the ages of the volcanic rocks arbitrarily were divided into three time frames of about 5 m.y. each, and the distribution and extent of volcanic fields were shown with and without vents. The information from those five maps, excluding vent data, was combined into a composite map at the scale of 1:2,500,000 (Luedke and Smith, 1984) thereby providing an overall data base for late Cenozoic volcanism. It is not the intent of this brief review of Quaternary volcanism to discuss major volcano-tectonic or magmatic events, particularly considering the relatively short geologic time period. Such events, well

Calderas related to ash-flow sheets show a positive correlation between caldera area and ejecta volume; this correlation places constraints on magma drawdown during eruption and implies a systematic relationship between these parameters and magma volume of the chamber. Caldera areas range from 1 to 10 4 km 2 ; the volume of ejecta from caldera sites ranges from 1 to 10 4 km 3 , and the volume of the related magma chambers is thought to range from 10 to 10 5 km 3 . A tentative correlation between ejecta volume and the time required to produce that volume reveals an approximate production rate of 10 −3 km 3 /yr. This seems to hold for relatively small eruptions, such as the ash-flow and related pyroclastic eruptions of as little as 10 −3 km 3 that are associated with central-vent volcanoes, even though volumes below about 1 to 10 km 3 are not related to caldera formation. The correlation also holds for large eruptions up to the limits for cumulative ejecta volumes and composite batholith emplacements. Precaldera magma chambers probably all have physical and chemical gradients and measurable variations in chemistry and mineralogy. These variations are revealed in the pyroclastic deposits, if the eruption taps to the maximum eruptible level, and give insight into differentiation processes. Compositional contrasts are commonly greater in small-volume central-vent systems than in large-volume ring-fracture systems, but all systems tend to become more mafic with depth. Successive caldera-forming eruptions from the same system commonly become more mafic with time. This is possibly due to two effects: (1) decreasing thermal input and (2) progressive depletion in “residual” elements. However, new thermal inputs, probably in the form of mafic primitive magma from the mantle, take place intermittently throughout the volcanic life of the system and may extend, as a waning influence, into and beyond the crystallization stage of the pluton. Very strong resurgence of primitive magma may reactivate large-volume systems to begin new cycles of magmatic activity. In common stratovolcanic systems (Crater Lake, Oregon) that rarely fractionate to high-SiO 2 rhyolite, alternating progression and regression of chemical trends with time is dominated by major-element variations. In high-SiO 2 rhyolite magmas (Bandelier Tuff, New Mexico), minor elements may sometimes show striking alternating enrichments and depletions with successive eruptions from the most-fractionated boundary layer at the top of the magma chamber. In the Bandelier magma, many elements such as Nb, Ta, U, Th, Cs, Rb, Li, Sn, Be, B, W, Mo, F, CI, Pb, Zn, Sm, and the heavy rare-earth elements concentrated upward in the system, whereas other elements such as Ba, Sr, Eu, Ti, Cr, Co, Sc, Au, and Cu concentrated downward. For magmas in general, the direction and amount of concentration of both groups of elements are a function of both the initial composition of the parent magma and the effects of the dominant fractionation process operating at any given time. Thus the permissibility of mineralization by or ore formation involving those elements that are dependent on magmatic concentration may be directly related to specific stages in the volcanic history of any given system. Any ash-flow sheet holds basic clues for the behavior of certain elements in that system at a specific time and, seen in conjunction with the composition of magmas erupted from the system at other times, suggests concentration trends that provide insights into the controlling magmatic processes.

Paleomagnetic and potassium-argon studies support geologic evidence that the lower member of the Bandelier Tuff was deposited 1.4 m.y. ago. The upper member erupted about 1.0 m.y. ago and was followed by caldera collapse which formed the 12- to 14-mile diameter Valles Caldera. Postcaldera activity which resulted in the eruption of rhyolite domes and pyroclastic material, has occurred at about 0.9, 0.7, 0.5, and 0.4 m.y. ago, with later undated eruptions that were estimated at about 0.1 m.y. ago. These data from the Valles Caldera are the basis for the previously published age revision of the Brunhes-Matuyama geomagnetic polarity epoch boundary from 1.0 to 0.7 m.y. ago, and they were used to define the Jaramillo normal polarity event at about 0.9 m.y. ago (Doell and Dalrymple, 1966).

Resurgent cauldrons are defined as cauldrons (calderas) in which the cauldron block, following subsidence, has been uplifted, usually in the form of a structural dome. Seven of the best known resurgent cauldrons are: Valles, Toba, Creede, San Juan, Silverton, Lake City, and Timber Mountain. Geologic summaries of these and Long Valley, California, a probable resurgent caldera, are presented. Using the Valles caldera as a model, but augmented by information from other cauldrons, seven stages of volcanic, structural, sedimentary, and plutonic events are recognized in the development of resurgent cauldrons. They are: (I) Regional tumescence and generation of ring fractures; (II) Caldera-forming eruptions; (III) Caldera collapse; (IV) Preresurgence volcanism and sedimentation; (V) Resurgent doming; (VI) Major ring-fracture volcanism; (VII) Terminal solfatara and hot-spring activity. These stages define the terminal cycle of resurgent cauldrons, which in the Valles caldera spanned more than 1 million years. The known and inferred occurrence of the seven stages in the eight cauldrons discussed, together with some time control in four cauldrons, indicates that resurgent doming is early in the postcollapse history; hence, it seems part of a pattern and not fortuitous. Doming of the cauldron block by magma pressure is preferred to doming by stock or laccolithic intrusion, although these processes may be subsidiary. Magma rise that produces doming may be explained in several ways, but the principal cause is not known. Nor is it known why some otherwise similar calderas do not have resurgent domes, although size and thickness of the cauldron block and the degree to which it was deformed during caldera collapse may be factors. All known resurgent structures are larger than 8 miles in diameter and are associated with silicic and, presumably, high-viscosity magmas. Genetically, resurgent cauldrons belong to a cauldron group in which subsidence of a central mass takes place along ring fractures and is related to eruption of voluminous ash flows, thereby differing from Kilauean-type calderas. It is proposed that typical Krakatoan-type calderas differ in that collapse is chaotic and ring fractures are not essential to their formation. Krakatoan calderas typically occur in the andesitic volcanoes of island arcs or the eugeosynclinal environment, and their sub-volcanic analogues are not known, whereas resurgent and related Glen Coe-type cauldrons are more common in cratonic or post-orogenic environments as are their sub-volcanic analogues — granitic ring complexes. Granitic ring complexes, such as Lirue, Sande, Ossipee, and Alnsjø, are probably the closest sub-volcanic analogues of resurgent calderas. The source areas of most of the ash-flow sheets of western United States and Mexico are yet to be found. It is suggested that many of them will prove to be resurgent structures. Present evidence suggests that ore deposits are more commonly associated with resurgent cauldrons than with other cauldron types.