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Great thicknesses of eolian dune deposits of early Oligocene age crop out in the Chuska Mountains of northwestern New Mexico-Arizona (as much as 535 m thick) and in the Mogollon-Datil volcanic field of western New Mexico-Arizona (as much as 300 m thick). 40 Ar/ 39 Ar ages of intercalated volcanic rocks indicate eolian deposition in these areas was approximately synchronous, with eolian accumulation beginning regionally at ca. 33.5 Ma and ending at ca. 27 Ma. Probable eolian sandstone of Oligocene age 483 m thick is also present in the subsurface of the Albuquerque Basin of the Rio Grande rift. The beginning of eolian deposition on the Colorado Plateau corresponds closely to the beginning of eolian (loessic) deposition in the White River Group of the Great Plains and major Oi1 glaciation in Antarctica, suggesting possible global paleoclimatic control. Successions of Oligocene eolian sandstone on the Colorado Plateau are thicker than all of the better known Upper Paleozoic-Mesozoic eolianites in the region, except the Jurassic Navajo Sandstone. We suggest that the widely separated Oligocene eolianites in the Colorado Plateau region were probably originally continuous, and thus are erosional remnants of an extensive (∼140,000 km 2 ), regional sand sea (the Chuska erg). This interpretation is based on: (1) comparison with thickness trends of older eolianites in the Colorado Plateau region, (2) evaluation of regional topographic gradients of modern ergs, and (3) hydrologic modeling of a 300- to 400-m–thick zone of saturation that existed during eolian deposition in the Chuska Mountains. The Chuska erg represents the final episode of Paleogene aggradation on the central and southern Colorado Plateau. Aggradation was driven primarily by trapping of fluvial sediments on the plateau by development of major volcanic fields along the eastern plateau margin. These volcanic fields blocked earlier Laramide drainages that had previously transported sediments eastward off the plateau. Following a shift to widespread eolian deposition at ca. 33.5 Ma, constructional volcanic topography induced eolian accumulation upwind of developing volcanic fields. Stratal accumulation rates (not decompacted) of eolian deposits were ∼28–82 m/m.y. The reconstructed top of the Chuska erg would lie at a present-day elevation of ∼3000 m or more, and provides a datum for assessing subsequent erosion on the Colorado Plateau. Major exhumation (≥1230 m) occurred during the late Oligocene and early Miocene, following the end of Chuska deposition and prior to the onset of Bidahochi Formation deposition at ca. 16 Ma on the south-central part of the plateau. The Bidahochi Formation attained a thickness of ∼250 m by ca. 6 Ma, followed by ∼520 m of late Miocene and younger erosion in the valley of the Little Colorado River. The depth of late Oligocene-early Miocene (ca. 26–16 Ma) exhumation of the central and southern Colorado Plateau thus was more than twice that of the late Miocene-Holocene (ca. 6–0 Ma). The timing of initial deep erosion in the Colorado Plateau-Southern Rocky Mountains region suggests the beginning of major epeirogenic rock uplift occurred during post-Laramide magmatism.

Three lithostratigraphic units have been distinguished in the volcanic succession of the basal complex of Fuerteventura Island. These units are, from bottom to top: the submarine volcanic group, the transitional volcanic group, and the subaerial volcanic group. These three groups record the submarine growth and emergence of the island. The volcanism is represented by ultra-alkaline and strongly alkaline igneous series. The igneous activity was due to the presence of an anomalous zone in the sublithospheric mantle, the low density of which also caused uplift of the Mesozoic oceanic crust. Two extensional phases and an intervening contractional phase developed coeval to the generation of the volcanic succession. The submarine volcanic group was deposited in the hanging wall basin of a large listric extensional detachment directed toward the SSW. The transitional volcanic group was syntectonic with respect to a late inversion of the listric detachment. Finally, the subaerial volcanic group resulted from a second episode of WNW extension. This study of the evolution of the basal complex of Fuerteventura serves as the basis for a tectonic model of submarine growth and emergence of volcanic islands.

The Camargo volcanic field is the largest mafic alkalic volcanic field in the Mexican Basin and Range province, and the relationship between volcanism and normal faulting is especially strong. The Camargo volcanic field lies in the northern part of the province, midway between the Sierra Madre Occidental and Trans-Pecos Texas. It is formed by Pliocene–Pleistocene (4.7–0.09 Ma) intraplate mafic alkalic volcanic rocks, some of which contain peridotite, pyroxenite, and granulite xenoliths. The volcanic field covers ∼3000 km2 and has an estimated volume of ∼120 km3 erupted from >300 recognized vents. Twenty-six new 40Ar/39Ar age determinations for the Camargo volcanic field and its environs show that volcanic activity began in the southwest part of the field and shifted toward the northeast at ∼15 mm/yr. The average magmatic eruption rate during growth of the field was ∼0.026 km3/k.y. The Camargo volcanic field lies within an accommodation zone with west-dipping faults and east-tilted blocks to the north and east-dipping faults and west-tilted blocks to the south. These faults are expressed in the volcanic field by a N30°W-trending graben with scarps up to ∼100 m high through its central part. Volcanism and faulting were at least in part coeval, and younger volcanic products commonly drape fault scarps that cut earlier lavas. Normal faulting is bracketed between 4.7 and 2.1 Ma and may have also migrated northeastward. Estimated vertical slip rates on four Pliocene faults range from 0.03 mm/yr, a likely long-term rate, to 1.67 mm/yr, interpreted as a short-term rate operative during periods of active faulting. Northwest-striking normal faults that cut alluvial-fan deposits and Pleistocene lavas in the northern Camargo volcanic field and geomorphic evidence for recent uplift to the south of the volcanic field suggest that the region is still extending.

Lassen Peak is a large volcanic dome of late Pleistocene age at the southern end of the Cascades arc. This dacitic dome has a complex phenocryst assemblage of mixed-magma origin and has been effectively dated for the first time using the 40 Ar/ 39 Ar technique. Emplacement of the dome occurred after an advance of late Pleistocene glaciers from an older volcanic cone to the southwest and from a large volcanic plateau to the east. Emplacement of the Lassen Peak dome before the latest major Pleistocene glaciation—correlated by previous workers with the Tioga glaciation of the Sierra Nevada—formed a high peak that intercepted some of the precipitation that previously had fed the plateau ice cap, forming a 10-km-long glacier from a cirque on the northeast side of the peak. The weighted mean 40 Ar/ 39 Ar age of the dacite of Lassen Peak is 28.3 ± 2.7 ka. Comparison of its remanent magnetic direction to that of sedimentary deposits at Mono Lake, California, indicates a correlation to strata of 27 ± 1 ka. The dacite of Kings Creek, which erupted from the site of Lassen Peak before a glacial advance that predated the Lassen Peak dome, overlies peat that has a U-Th-calibrated 14 C age of 37.6 ± 0.2 ka. The dacite of Kings Creek yielded a 40 Ar/ 39 Ar age of 32 ± 17 ka. The paleomagnetism of this dacite is similar to that of Mono Lake sediments of 35 ± 1 ka age. Thus, a late Pleistocene glacial advance probably began in the Lassen region between about 35 and 27 ka.

Clastic sedimentary deposits and associated volcanic rocks record the progress of Tertiary extension in the Mexican state of Sonora. These deposits accumulated within basins located throughout the eastern two-thirds of the state. The rocks are slightly indurated conglomerates and sandstones; clast types reflect the local highland exposures. Volcanic units that bound and are interlayered with the sedimentary rocks provide stratigraphic and time markers of basin evolution and, by inference, of marginal fault development. This record has been examined in an east-west belt across south-central Sonora. The volcanic rocks occur in three distinct associations. At the base and interbedded within the lower portions of the sedimentary sections are lava flows of dominantly basaltic andesite composition. In most localities the overlying clastic sedimentary strata are conformable with these lava flows, and we conclude that the magmatism was triggered by early faulting along the basin margins. A second association present near the base of the sections comprises massive lava domes and flows of intermediate composition that contain distinctive phenocrysts of dark brown amphibole. The domes apparently formed when viscous lava welled up along developing marginal basin faults. In one case a transition was observed over a short distance from a structureless dome to a thick lava flow that is conformably interbedded with, and sheds clasts laterally into, the sedimentary section. A third volcanic association comprises layers of rhyolitic lava flows and ignimbrites that overlie the coarsest and thickest lower portions of the sedimentary sections. K-Ar ages for volcanic rocks in the two oldest of these settings indicate that each Tertiary basin had a distinct history with no apparent regional geographic pattern. In the Rio Yaqui basin, where exhumation by the major river of Sonora has exposed a full section of the volcanic and clastic sequences, extension was possibly as old as 27 Ma. Elsewhere, deposition of sediments was underway in most basins by 24 Ma, and in all basins by 20 Ma. K-Ar ages of the younger rhyolitic volcanic rocks fall within a narrow range from 12.8 to 10.5 Ma. These ages provide a younger time limit to deposition of the coarser and thicker portions of the clastic sedimentary sections. The mafic lava flows, dominantly basaltic andesite, are similar in major-element composition to mafic lava flows that cap felsic sections within the Sierra Madre Occidental volcanic field of western Mexico and the mid-Tertiary Datil-Mogollon volcanic field in southwestern New Mexico. They are higher in silica and lower in total alkalies than younger capping mafic lava flows within the Gulf of California extensional province of western Sonora and Neogene basaltic lava flows in the Datil-Mogollon field. They are similarly distinct from basaltic dikes and lava flows that are clearly associated with Basin and Range faulting in Trans-Pecos Texas and in northern Durango state. A convergent-margin magmatic arc was active in the present-day Gulf of California region between 24 and 11.5 Ma. The earliest extension in south-central Sonora was therefore occurring in a back-arc setting. Continued sedimentation and rotation of volcanic strata indicate that extension continued throughout the region after 10 Ma with little coeval volcanism.

Three major volcanic events have occurred in Nicaragua since middle Tertiary time. First, the Oligocene was dominated by rhyolite shield volcanism, producing the Highland ignimbrite. Second, creation of northeast- and later northwest-striking dilatational fracture zones allowed the extrusion of basaltic to andesitic magmas along the Pacific coast, constructing the northwest-oriented Coyol volcanic arc. This Miocene arc can be followed all along the Pacific coast of the Chortis block. Third, a Pliocene-Pleistocene southwestward shift of arc volcanism created the modern volcanic arc. This volcanic history is supported by a new stratigraphy based on volcanic centers and related facies. The old cyclic, layer-cake stratigraphy of the Coyol Group has been abandoned.

The Neogene Puertecitos Volcanic Province of northeastern Baja California records a transition from arc-related volcanic activity to rift volcanism associated with opening of the Gulf of California. The eastern Puertecitos Volcanic Province is divided into three volcanic sequences based on mapping, petrology, and ^(40)Ar/^(39)Ar geochronology. The lowest sequence comprises early to middle Miocene (20–16 Ma) arc-related andesitic lava flows, volcanic necks, and proximal pyroclastic and epiclastic deposits up to 400 m in thickness, with minor basaltic lava flows. Following the initiation of crustal extension in the region (11–6 Ma), synrift volcanism produced two rhyolitic sequences that discordantly overlie the arc-related rocks. The older synrift sequence (6.4–5.8 Ma) is composed of rhyolite domes and a series of pyroclastic flows up to 300 m thick. The upper sequence (3.2–2.7 Ma) consists of ash-flow tuffs and pumice-lapilli pyroclastic flows, collectively up to 200 m thick. Minor andesite eruptions followed each episode of silicic synrift volcanism. Synvolcanic faults produced topographic relief that controlled deposition of the pyroclastic flows and caused gentler dips upsection. Rhyolite domes are aligned parallel to the predominant north-northwest to north-northeast fault pattern. All three volcanic sequences are calc-alkaline. However, the synrift andesite is characterized by lower K_(2)O, lower incompatible element concentrations, and less fractionation of light rare earth elements than the arc-related basalt and andesite. This suggests that the primary melts were more primitive for synrift andesite than for the arc-related rocks.

This study reports major- and trace-element compositions of late Cenozoic (5 Ma and younger) mafic volcanic rocks from southern New Mexico. Stratigraphic sampling is reported for two of the volcanic fields, the Potrillo and the Jornado del Muerto volcanic fields; reconnaissance sampling, for an additional seven fields. These are among the first chemical analyses for some of these volcanic fields, and they provide insight into the dynamics of melt generation in this continental rift zone. The lavas fall into two groups: (1) an alkaline suite of basanite, alkali basalt, and trachybasalt with low SiO2 and high abundances of incompatible elements (TiO2, Nb, Rb, and Sr); (2) a subalkaline suite of subalkali basalts with high SiO2 and low abundances of the incompatible elements. Compositions of volcanic rocks from the Geronimo volcanic field, the Taos Plateau volcanic field, and the transition zone in central New Mexico are similar to those reported in this study, confirming that these are general characteristics of lavas throughout the region. For many of the large fields, the volcanic rocks belong exclusively to one compositional group or the other. Furthermore, there appear to be no patterns to the distribution of these compositional groups; for example, the Potrillo volcanic field, which is composed exclusively of alkaline lavas, and the Jornado del Muerto volcanic field, which is dominantly subalkaline, both occur in the axis of the Rio Grande rift. We see no evidence for evolution in melt chemistry during the past 5 m.y. In the Potrillo volcanic field, for which we have the best documentation of stratigraphic changes in composition, incompatible elements increase upsection and are correlated with decreasing Mg number, suggesting that magmas underwent differentiation before their final ascent. Phenocrystic olivine and plagioclase, which are liquidus phases at low pressure, imply a shallow depth for this crystallization event. Processes responsible for chemical divergence between the alkaline and subalkaline groups occurred at an early stage of magmatic evolution. Correlation between incompatible-element enrichment and silica undersaturation suggests that variable degrees of partial melting played a significant role in the generation of the two lava types.

Dacitic pyroclastic flow deposits within the Tecuya Formation of the San Emigdio Mountains, California, exhibit a clear transition from subaerial to submarine deposition across the southeastern margin of the San Joaquin basin. The dacitic pyroclastic deposits are part of an informal volcanic member within the Tecuya Formation, consisting of a lower dacitic unit and an upper basaltic unit. Volcanogenic facies transitions within each of these units can be traced westward, obliquely across paleoshoreline, for 15 km. Depositional environments of the dacitic unit are well constrained by underlying nonvolcanic siliciclastic and overlying basaltic deposits. The lowermost dacitic unit consists of pyroclastic surge and fallout deposits, which overlie nonmarine alluvial-fan deposits and were produced by subaerial, phreatomagmatic eruptions. A second, magmatic eruptive stage resulted in deposition of dacitic pyroclastic flows on top of the subaerial surge and fallout deposits, and within marine, nearshore to outer-shelf settings. These pyroclastic flows, generated from subaerial vents, were high-concentration, poorly fluidized flows which traveled down paleoslope into the marine part of the basin. Upon reaching water, the hot pyroclastic flows continued to flow subaqueously, partially mixed with water, and were quenched. Transitions to cool submarine pyroclastic debris flows may have occurred. Surface transformations of the submarine pyroclastic flows/pyroclastic debris flows generated dilute pyroclastic turbidity flows, which at times became detached and continued to flow independently downslope. Progradation of submarine pyroclastic flow deposits to the west into deeper marine settings likely accompanied the growth of subaerial volcanic vents to the east. A second, probably submarine, vent also produced submarine pyroclastic flow deposits in the western part of the basin. Deposition of reworked tuff and epiclastic debris flows within marine settings accompanied primary pyroclastic surge, fallout, and flow deposition. The Tecuya volcanic member is an important ancient example in which the depositional environments of subaerial to submarine pyroclastic deposits are well constrained by bounding units. The characteristics of the subaerial and submarine pyroclastic flow deposits within the Tecuya volcanic member thus may be useful in other ancient volcaniclastic successions which lack bounding units of obvious paleoenvironmental origin.

The Kerguelen Plateau is a large structure in the south Indian Ocean of which the nature (oceanic or continental) and origin are not known. In this work, multichannel seismic reflection surveys, particularly in the southern domain of the plateau, are used to study its origin. Internal basement reflections characterize most of the investigated area. Basement reflections, together with other geological and geophysical information, indicate that the entire plateau is underlain by a volcanic basement, suggesting that both the northern and the southern domains have an oceanic origin. Basement areas with no internal reflections are associated mostly with basement ridges; basement reflector wedges, associated with volcanic flows from the elevated volcanoes, appear on the flanks of these ridges. We suggest that the basement ridges are the extinct volcanic sources of the Southern Kerguelen Plateau. Typical volcanic morphology is absent in the southern domain, because it was removed during the erosional phase known to have occurred following the emplacement of the plateau and which is apparent in the seismic data. Two types of volcanic sources are identified in the southern domain: isolated volcanoes and linear sources. This pattern is somewhat similar to the volcanic pattern in Iceland, suggesting that the Kerguelen Plateau may have been a similar structure, related to excessive volcanism associated with a hot spot on a mid-ocean spreading ridge in the Early Cretaceous south Indian Ocean.

A spatio-temporal near-neighbor model is used to identify and map variations in the recurrence rate of volcanism in the Springerville volcanic field, Arizona, a large field on the Colorado Plateau boundary. Detailed mapping of individual lava flows and their associated vents, together with radiometric and paleomagnetic dating, demonstrates that 366 volcanic events have formed the Springerville volcanic field. These volcanic events consist of mapped units that erupted between 2.1 and 0.3 Ma over an area of 3000 km 2 . Cumulatively, the rate of vent formation in the Springerville field waxed prior to 1.5 Ma, was near steady state from 1.5 to 0.75 Ma, and has waned since 0.75 Ma. The increase in the rate of vent formation at ca. 1.5 Ma coincided with a shift in the locus of Springerville magmatism from west to east and an increase in the alkalic nature of the magma, including eruption of mugearites and benmoreites. The volume of flows, inferred from lava-flow areas, was steady state from 1.75 to 0.75 Ma. A near-neighbor spatio-temporal recurrence-rate model using seven near-neighbor volcanoes and a 0.5 m.y. time window reveals that (1) areas of waxing and waning magmatism in the Springerville volcanic field are much more localized and (2) volcanic activity within these areas is much more intense than implied by field-wide temporal trends. These volcano clusters are 10–20 km in diameter; they were commonly active for less than 0.25 m.y. Mugearites and benmoreites are limited to these areas of high recurrence rate. This clustered and petrologically distinctive, rather than distributed or random, volcanic activity suggests that individual source regions for the magma also are localized and short-lived compared with the area and longevity of the entire field. Because volcanic activity is spatially and temporally clustered, forecasting subsequent activity is more successful if the spatio-temporal recurrence-rate model is used, rather than the average recurrence rates. This success indicates that spatio-temporal recurrence-rate models are useful tools for the quantification of long-term volcanic hazards in basaltic volcanic fields.

Multichannel seismic reflection profiles show that most of the western Colombian Basin is underlain by a large oceanic plateau. The crust of this plateau has a smooth upper surface, an oceanic velocity structure, and a thickness at least twice that of typical oceanic crust. Locally well defined reflections within the plateau crust are related to layered basalt flows and interbedded sediments. A mounded reflection pattern within acoustic basement at the crest of the plateau (Mono Rise) may represent volcanic knolls. Smooth-surfaced basement of the western Colombian Basin is probably equivalent to the Upper Cretaceous basement complex below seismic reflection horizon B″ in the central Venezuelan Basin and resembles the basements of some Cretaceous oceanic plateaus and aseismic rises in the western Pacific. An area of apparently typical oceanic crust lies adjacent to the Colombian Basin plateau and may represent relatively unmodified oceanic crust upon which the plateau was constructed. Reflections within the flank of the Colombian Basin plateau appear to overlap this crust, indicating that the plateau is younger than the oceanic crust. Oceanic plateau crust extends to Panama and Costa Rica without apparent structural break and may be continuous with the basement complex of southern Central America.

INTRODUCTION Scores of volcanic necks, dikes, and lava-capped mesas rise from the high plateau of northeast Arizona and the adjacent parts of Utah and New Mexico. These are the remnants of a volcanic field that formerly covered many thousands of square miles. Erosion has so far dissected this field that the original cones have disappeared, the sheets of lava have been dismembered, and the old volcanic conduits now rise as giant towers, revealing their inner structure with singular clarity. The objects of this paper are to discuss the features of these denuded volcanoes, the character of their ejecta, and the age of their eruptions. To petrologists, the region is one of unusual interest, because of the similarity of its igneous rocks to certain of those in the classic regions of alkaline rocks in Montana and Wyoming. So extensive is the Navajo-Hopi volcanic province, that the present survey is to be . . .

Introduction Only the northern part of the Columbia Plateau is concerned in this study and only waters from the Cordilleran ice-sheet. Two glacial epochs are involved—one the Wisconsin, the other as yet undated. Besides contributing to the geological history of the region, this article endeavors to show that glacier-born streams, under proper conditions, are erosive agents of great vigor over large tracts far from the front of the melting ice. Topography and Drainage The Columbia basalt plateau is separated from several mountain ranges on the north and from the Cascade Range on the west by the trench in which flows Columbia River. On the east, the plateau abuts against and interfingers with the Coeur d’Alene Mountains of Idaho. When the Miocene basalt flows of this region ceased, the lava plain abutted in a similar fashion against the mountains on the north and west. The cutting of the Columbia Valley . . .

Volcanic rocks of the Davis and Barrilla Mountains constitute the largest contiguous segment of the mid-Cenozoic volcanic field of Trans-Pecos Texas. Salic volcanic rocks form a silica-oversaturated series ranging in composition from trachyte to peralkalic rhyolite. Mafic lavas are subordinate in volume to the salic, silica-oversaturated series. Stratigraphic relations and K-Ar age determinations allow a reconstruction of the evolution of the area. Major volcanic activity occurred in a period of about 3 m.y. in early Oligocene time (38 to 35 m.y. B.P.), during which several large lava shields were constructed and major ash-flow sheets emplaced. A late episode of mafic lava extrusion and intrusion of phonolite bodies remains undated, but may have occurred later than 31.5 m.y. ago. The silica-oversaturated volcanic rocks are the eruptive products of a group of highly evolved magmas emplaced into high crustal levels during early Oligocene time. In rapid succession, plutons vented to the surface, forming as many as five different eruptive centers, each of which was active for a period not exceeding 1 m.y. The eruptive centers are marked by concentrations of salic lava flows, dike swarms, intrusive masses, and at least one caldera.

The morphology of submarine volcanic terrain on the crest of the Puna Ridge, Hawaii, was observed during two manned submersible dives in water depths of as much as 2,000 m. Steep-walled linear ridges 30 m high, trending 060/sup 0/, and composed of lava pillows and narrow fissures with the same strike were the principal volcanic features observed on the ridge crest. Lava tunnels, tubes, and pillows frequently were observed to be partially broken through, thereby exposing water-filled voids, which are inferred to be interconnected to great depths within the extrusive submarine volcanic pile. Low compressional wave velocities reported by other authors for the crustal layer composed of submarine volcanic extrusives and the low effective density of this layer, as determined from published surface gravity observations, are attributed to the large intraflow and interflow porosity of submarine volcanic terrain. Areal variations in heat flow through the crust of submarine volcanic features are also attributed to the high porosity and consequent permeability of submarine volcanic terrain which is likely to persist to the bottom of the submarine extrusive pile.

The Sao Paulo Plateau is a prominent marginal plateau in the southeastern Brazilian margin. Its surface displays a rough morphology either because of diapiric structures that have pierced and/or domed the overlying sediments or because of outcrops of igneous basement rocks. The plateau is bounded to the north and south by fracture zones and appears to be situated on oceanic crust created between Neocomian and Aptian time. At the end of Aptian, the segment of the initial spreading center that was bounded by the fracture zones that now form the northern and southern boundaries of the plateau shifted to the east. The newly created segment of the spreading center formed the eastern boundary of the plateau. The stratigraphy of the plateau has been inferred through correlation of seismic-reflection profiles with the drilling data from Deep Sea Drilling Project site 356, located on the southeastern corner of the plateau. We have attempted a comparison of the sedimentary facies of the plateau with those of the Santos Basin, a shelf basin located adjacent to the plateau. A continuous evaporitic layer of Aptian age extends from the basin to the plateau. In Albian time, shallow-platform limestone was deposited in the basin, while open-marine limestone was deposited on the plateau. Coarse conglomerates were deposited in the basin during Turonian-Coniacian time, whereas distal turbidites were contemporaneously deposited on the plateau. A major transgression in Maestrichtian time trapped the terrigenous sediments within the Santos Basin and halted terrigenous sedimentation on the plateau. Cenozoic sediments in both areas are open marine. The sedimentary history of the Santos Basin and the Sao Paulo Plateau thus suggests that the two regions have been structurally continuous at least since Late Cretaceous time. The region occupied by the plateau has acted as a depocenter and has evolved as a marginal plateau since the termination of evaporitic deposition in the South Atlantic.

Volcaniclastic sediments and rocks are divided here into autoclastic, pyroclastic, and epiclastic types with grain-size limits the same as non-volcanic epiclastic rocks. Autoclastic rocks contain fragments that are produced within (but not usually extruded from) volcanic vents, during movement of lava flows, or by gas explosions within flows that have ceased to flow. Pyroclastic rocks contain fragments produced by volcanic explosion and extruded as discrete particles from volcanic vents. Epiclastic volcanic rocks contain fragments produced by weathering and erosion of solidified or lithified volcanic rocks of any type. Volcaniclastic types may be mixed in all proportions with each other or with nonvolcanic fragments, although these mixtures are not designated within this classification. A non-genetic category, based only upon particle size and the presence of volcanic material, is included for rocks with clasts of unknown origin.

Silica content of fine-grained volcanic rock may be estimated by means of a curve that correlates index of refraction of artificial glass beads made from the rock with chemically determined silica. Such a correlation curve for a suite of volcanic rocks from the Owyhee Plateau, Oregon, is presented. It is essentially indistinguishable from a curve based upon several other volcanic suites from western North America.

Volcanic breccias are grouped into three major categories based upon process of fragmentation: autoclastic, pyroclastic, and epiclastic. Autoclastic volcanic breccias result from internal processes acting during movement of semisolid or solid lava; they include flow breccia and intrusion breccia. Pyroclastic breccia is produced by volcanic explosion and includes vulcanian breccia, pyroclastic flow breccia, and hydrovolcanic breccia. Epiclastic volcanic breccias result from transportation of loose volcanic material by epigene geomorphic agents, or by gravity, and include laharic breccia, water-laid volcanic breccia, and volcanic talus breccia. Other volcanic breccia terms are discussed. The rock types mentioned heretofore may also be designated by prefixing compositional terms as in basaltic block breccia and andesitic vulcanian breccia. The term volcanic breccia is used as a general term applying to all coarse-grained rocks composed of angular volcanic fragments.