Whether the volcanism of the Columbia River Plateau, eastern Snake River Plain, and Yellowstone (western U.S.) is related to a mantle plume or to plate tectonic processes is a long-standing controversy. There are many geological mismatches with the basic plume model as well as logical flaws, such as citing data postulated to require a deep-mantle origin in support of an “upper-mantle plume” model. USArray has recently yielded abundant new seismological results, but despite this, seismic analyses have still not resolved the disparity of opinion. This suggests that seismology may be unable to resolve the plume question for Yellowstone, and perhaps elsewhere. USArray data have inspired many new models that relate western U.S. volcanism to shallow mantle convection associated with subduction zone processes. Many of these models assume that the principal requirement for surface volcanism is melt in the mantle and that the lithosphere is essentially passive. In this paper we propose a pure plate model in which melt is commonplace in the mantle, and its inherent buoyancy is not what causes surface eruptions. Instead, it is extension of the lithosphere that permits melt to escape to the surface and eruptions to occur—the mere presence of underlying melt is not a sufficient condition. The time-progressive chain of rhyolitic calderas in the eastern Snake River Plain–Yellowstone zone that has formed since basin-range extension began at ca. 17 Ma results from laterally migrating lithospheric extension and thinning that has permitted basaltic magma to rise from the upper mantle and melt the lower crust. We propose that this migration formed part of the systematic eastward migration of the axis of most intense basin-range extension. The bimodal rhyolite-basalt volcanism followed migration of the locus of most rapid extension, not vice versa. This model does not depend on seismology to test it but instead on surface geological observations.

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Abstract Before the time of the Laramide orogeny, an active orogenic and magmatic system was more or less continuous along the continental margin of western North America and had long dominated the Cordileran geologic framework; Laramide events reflected a major break in that continuity and were unusual in several respects. Contractional Laramide orogenesis affected a very wide zone, with deformation and foreland uplift extending nearly to the middle of the continent. Laramide magmatism, too, although discontinuous along strike of the orogenic region, extended locally far eastward. These events may have occurred in response to rapid westward drift of the North American Plate and extreme flattening of the Farallon subduction zone (Dickinson and Snyder, 1978).

Abstract Island Park, situated between the northeastern end of the Snake River Plain and the western margin of the Yellowstone Plateau, is traversed by U.S. 20-191 between Ashton, Idaho, and West Yellowstone, Montana (Fig. 1) and, in part, by the Mesa Falls Road (Idaho 47). All of the physiographic and geologic features described in this guide can be reached during the summer months by passenger car on those two paved highways or on short gravel roads that extend from them. During the winter, U.S. 20-191 remains open, but other winter travel is possible only by skis or skimobile. One quarry locality described later is privately owned but has generally been freely accessible.

Abstract A volcanic plateau constructed by late Pliocene and Quaternary eruptions spans the Continental Divide and occupies the central part of Yellowstone National Park, mainly in northwestern Wyoming but overlapping into eastern Idaho and southwestern Montana. The National Park is accessible by road through five entrances, reached by U.S. 20 from the west and east, from the north and south, and U.S. 212 from the northeast (Fig. 1). The localities described in this guide are all on or adjacent to the park road system and can be reached, during the summer months (usually early May through late October), by passenger car or, during the winter, by skis or skimobile. A nominal entry fee for vehicles is valid in both Yellowstone and Grand Teton national parks. Special caution should be exercised in visiting the thermal 89 areas; they are both delicate and dangerous. Designated walk-ways are provided in the thermal areas described here, and posted park regulations should be followed. Specimen collecting in the national park is allowed only by permit, which can be issued by the National Park Service only in advance of a collecting trip.

Many source areas for voluminous ash flows have histories of repeated catastrophic eruption and caldera-forming collapse within a few million years or less. Examples range in complexity from simple calderas related to eruption of a single cooling unit, through sequential collapses of the same caldera related to successive eruptions of separate cooling units, and nested calderas related to eruption of successively smaller cooling units between successively longer time intervals, to complexes of overlapping calderas related to eruption of several cooling units from overlapping source areas within a large volcanic field. Some caldera complexes are related to collapse of adjacent, simultaneously active ring-fracture zones by immediately successive ash-flow eruptions to form composite sheets. Despite this wide range in complexity, a basically similar primary sequence of events in each area reflects the formation of a large volume of magma, at least part of which rose to form shallow epizonal magma chambers. In some instances, only a single high-level chamber formed and produced ash-flow eruption and collapse one or more times. In others, several high-level chambers formed either simultaneously or in succession; where several chambers were active simultaneously, major ash-flow eruption and collapse at one chamber may have triggered events in an adjacent one. Variations in the pattern of ash-flow–caldera relations reflect variations in relative volumes of magma in the chambers, of tectonic controls on intrusions and the timing of eruptions, and of the continuity of magma generation beneath a volcanic region. These diverse patterns represent variations on R. L. Smith’s concept that voluminous ash-flow eruptions and related caldera formation are surficial expressions of the degassing, but not the total emptying, of large magma chambers at high levels in the Earth’s crust. Study of these systems offers unique insights into the physical and chemical processes and evolution of silicic magmas.

The Great Basin is a tectonically youthful region that shares some features with the Columbia Intermontane region but is separated from the more mature southern part of the Basin and Range province by a zone of active seismicity and geophysical contrasts. Sedimentary, physiographic, and structural features show that during the past 17 m.y., extensional linear normal faulting has been active in the Great Basin region, and extension also is indicated by numerous dikes in the High Lava Plains and the Columbia Plateau. Cumulative tectonic extension in the Great Basin is more than 100 km. Since about 14 m.y. ago, tectonic activity in the Great Basin region has tended to become progressively more concentrated toward the margins, and extension has been taken up by a wide transform zone along the High Lava Plains. Within several tens of kilometres north of the High Lava Plains of Oregon and Idaho, cumulative extension is generally less than a few kilometres and has been nearly inactive since about 14 m.y. ago. Volcanism in the past 17 m.y. has been characterized by basaltic and bimodal rhyolite-basalt suites. Between 17 and 14 m.y. ago, the predominant volcanism was basaltic, being somewhat alkalic and of relatively small volume in the central Great Basin, more voluminous and less alkalic northward into the plateaus of southern Oregon and the High Lava Plains, and extremely voluminous and tholeiitic in the Columbia Plateau. Since about 14 m.y. ago, basaltic and bimodal volcanism has occurred throughout the Great Basin region but generally has tended to erupt in successively narrower zones near its margins, probably in direct correspondence to the increasing concentration of normal faulting toward these margins. The High Lava Plains have been characterized during this same time by two linearly propagating volcanic systems, in which major cycles of rhyolitic volcanism have been initiated successively farther northwest and northeast. These two volcanic systems have propagated away from a region in the center of the High Lava Plains at about the same rate that faulting and volcanism in the Great Basin have been concentrated toward its margins. A model that accounts for this evolution relates tectonic extension to the regional stress fields that result from the motions and changes in the interactions of the North American, Pacific, and Farallon lithospheric plates. In this model, geophysical and volcanic features of the region are interpreted to be due to a chain of heating events caused by this extension but conditioned by the stress and thermal history of the continental plate. Stress relief at the base of the lithosphere causes basaltic magma generation of varying amounts and at varying depths in the upper mantle, depending on the thickness and history of the overlying crust. The generation of basaltic magmas and their intrusion into and through the crust during continued extension have increased regional heat flow, lowered the rigidity of the lithosphere, caused crustal thinning, produced flowage and decreased seismic velocities in the upper mantle, caused regional uplift by thermal expansion, and produced rhyolitic magmas by localized partial melting of the lower crust. According to the model, initial rifting occurred between 17 and 14 m.y. ago when northward migration of the Mendocino triple junction caused the continental-margin subduction zone to become short enough to allow partial coupling between two zones of transform displacement of the Pacific and North American plates. The increased coupling between these two zones caused extension in the North American plate perpendicular to the continental margin. Since about 14 m.y. ago, continued tectonic extension and basaltic magma generation have (1) caused a wide zone of oblique extension to become successively hotter and less rigid near the zone’s central axis, (2) increasingly concentrated brittle deformation and high-level magmatism outward toward the margins of the Great Basin region, and (3) produced concentrated zones of extension and crustal melting at the intersections of the resulting marginal zones with the transitional northern transform boundary of the extending region. This accounts for the symmetrically propagating volcanic systems of the High Lava Plains. The Yellowstone melting anomaly, whose locus was controlled initially by an old structural boundary, was favorably oriented to be augmented by shear melting at the base of the lithosphere; it has become self-sustaining because of the initiation of a thermal feedback cycle and the development of a root in the mantle by inward flow around a dense, sinking, unmelted residuum.

Abstract Commercial geothermal power plants throughout the world are located in areas of recent silicic volcanism, but the obvious supply of heat associated with Hawaii's active basaltic volcanoes is attractive as a possible future source of power. Between April 6 and July 9, 1973, the first deep borehole at the summit of an active volcano was drilled at Kilauea Volcano, Hawaii. The 1,262-m-deep hole was drilled to test predictions based on surface geophysical surveys and to obtain information on the hydrothermal regime above a postulated magma reservoir. Surface deformation during inflations by magma and deflations (via eruptions) had been interpreted to indicate a complex magma-reservoir system 2 – 4 km below Kilauea's summit. An electromagnetic sounding survey had defined a dome-shaped zone of low resistivity 900–2,000 m below the summit. This zone was believed likely to contain saline groundwater domed into a hydrothermal convection cell.