Columbia River Flood Basalts Research Articles

Ages of the Steens and Columbia River flood basalts and their relationship to extension-related calc-alkalic volcanism in eastern Oregon

Research Article| January 01, 2002 Ages of the Steens and Columbia River flood basalts and their relationship to extension-related calc-alkalic volcanism in eastern Oregon P.R. Hooper; P.R. Hooper 1Department of Geology, Washington State University, Pullman, Washington 99164, USA2Department of Earth Sciences, Open University, Milton Keynes MK7 6AA, UK Search for other works by this author on: GSW Google Scholar G.B. Binger; G.B. Binger 1Department of Geology, Washington State University, Pullman, Washington 99164, USA Search for other works by this author on: GSW Google Scholar K.R. Lees K.R. Lees 2Department of Earth Sciences, Open University, Milton Keynes MK7 6AA, UK Search for other works by this author on: GSW Google Scholar GSA Bulletin (2002) 114 (1): 43–50. https://doi.org/10.1130/0016-7606(2002)114<0043:AOTSAC>2.0.CO;2 Article history received: 16 Feb 2001 rev-recd: 25 Jun 2001 accepted: 29 Aug 2001 first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation P.R. Hooper, G.B. Binger, K.R. Lees; Ages of the Steens and Columbia River flood basalts and their relationship to extension-related calc-alkalic volcanism in eastern Oregon. GSA Bulletin 2002;; 114 (1): 43–50. doi: https://doi.org/10.1130/0016-7606(2002)114<0043:AOTSAC>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Stratigraphic and chemical correlations of Tertiary volcanic units in eastern Oregon confirm that the Steens Basalt represents the earliest eruptions of the Columbia River flood-basalt province. Field correlations are supported by major and trace element analyses and confirmed by 40Ar/39Ar dates. Within the basalt of Malheur Gorge, situated between Steens Mountain and the southernmost extent of the previously mapped Columbia River Basalt Group, the lowest unit correlates with the Steens Basalt, and the conformably overlying middle and upper units correlate with the Imnaha and Grande Ronde Basalt Formations of the Columbia River Basalt Group. New dates indicate that Imnaha and Grande Ronde Basalt Formations on the Columbia Plateau (>90% of the Columbia River Basalt Group) erupted between 16.1 and 15.0 Ma. These were immediately preceded by the Steens Basalt, a plagioclase-phyric tholeiite that erupted above the calculated position of the Yellowstone hotspot at 16.6 Ma. In eastern Oregon, the flood-basalt tholeiites of Steens Mountain and Malheur Gorge form a voluminous but brief interlude (16.6–15.3 Ma) superimposed on the low-volume, calc-alkalic to mildly alkalic, volcanism associated with continuing Eocene to present east-west extension. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Constraints on the preeruptive volatile concentrations in the Columbia River flood basalts

Many of the Columbia River flood basalts that erupted in the states of Oregon, Washington, and Idaho during the Miocene are remarkably iron rich (12–17 wt% total FeO) and under volatile-free conditions would have been too dense to erupt through continental crust. Their eruption through >20 km of sialic crust sets limits on their minimum, preeruptive total volatile (H2O + CO2) concentrations. These limits are obtained by quantitatively comparing the average density of the overlying crustal column with the calculated effect of dissolved H2O or exsolved CO2 on the density of various Columbia River tholeiitic basalts as a function of depth. Minimum total volatile (H2O + CO2) concentrations range to >4 wt% and are therefore considerably higher than those found in tholeiitic basalts at oceanic islands, like Hawaii. These conclusions are consistent with the strong enrichments in other incompatible elements (e.g., K, P, Ba) observed in the Columbia River flood basalts. Such high concentrations of H2O + CO2 would have had a marked effect on the solidus temperature of the source region, which may explain the remarkably large individual flow volumes (∼103 km3) that characterize this flood-basalt province.

The summer 1997 eruption at Pillan Patera on Io: Implications for ultrabasic lava flow emplacement

Galileo data and numerical modeling were used to investigate the summer 1997 eruption at Pillan Patera on Io. This event, now defined as “Pillanian” eruption style [Keszthelyi et al., this issue], included a high‐temperature (&gt;1600°C), possibly ultrabasic, 140‐km‐high plume eruption that deposited dark, orthopyroxene‐rich pyroclastic material over &gt;125,000 km2, followed by emplacement of dark flow‐like material over &gt;3100 km2 to the north of the caldera. We estimate that the high‐temperature, energetic episode of this eruption had a duration of 52–167 days between May and September 1997, with peak eruption temperatures around June 28, 1997. Galileo 20 m pixel−1 images of part of the Pillan flow field show a widespread, rough, pitted surface that is unlike any flow surface we have seen before. We suggest that this surface may have resulted from (1) a fractured lava crust formed during rapid, low‐viscosity lava surging, perhaps including turbulent flow emplacement; (2) disruption of the lava flow by explosive interaction with a volatile‐rich substrate; or (3) a combination of 1 and 2 with or without accumulation of pyroclastic materials on the surface. Well‐developed flow lobes are observed, suggesting that this is a relatively distal part of the flow field. Shadow measurements at flow margins indicate a thickness of ∼8–10 m. We have modeled the emplacement of putative ultrabasic flows from the summer 1997 Pillan eruption using constraints from new Galileo data. Results suggest that either laminar sheet flows or turbulent channelized flows could have traveled 50–150 km on a flat, unobstructed surface, which is consistent with the estimated length of the Pillan flow field (∼60 km). Our modeling suggests low thermal erosion rates (&lt;0.1 m d−1), and that the formation of deep (&gt;20 m) erosion channels was unlikely, especially distal to the source. We calculate a volumetric flow rate of ∼2–7×103 m3 s−1, which is greater than those for typical Mauna Loa/Kilauea flows but comparable to those for the (1783) Laki eruption and the inferred flow rates of the Roza flows in the Columbia River flood basalts. The differences in ultrabasic eruption styles on Earth and Io appear to be controlled by the different eruption environments: Plumes at sites of ultrabasic eruptions on Io suggest strong magma‐volatile interactions on a low‐gravity body lacking an atmosphere, whereas the geology at sites of komatiite eruptions on Earth suggest mostly submarine emplacement of thick flows with a pronounced lack of subaerial explosive activity.

Sharp boundary inversion of 2D magnetotelluric data

We consider 2D earth models consisting of laterally variable layers. Boundaries between layers are described by their depths at a set of nodes and interpolated laterally between nodes. Conductivity within each layer is described by values at a set of nodes fixed within each layer, and is interpolated laterally within each layer. Within the set of possible models of this sort, we iteratively invert magnetotelluric data for models minimizing the lateral roughness of the layer boundaries, and the lateral roughness of conductivities within layers, for a given level of data misfit. This stabilizes the inverse problem and avoids superfluous detail. This approach allows the determination of boundary positions between geological units with sharp discontinuities in properties across boundaries, while sharing the stability features of recent smooth conductivity distribution inversions.We compare sharp boundary inversion results with smooth conductivity distribution inversion results on a numerical example, and on inversion of field data from the Columbia River flood basalts of Washington State. In the synthetic example, where true positions and resistivities are known, sharp boundary inversion results determine both layer boundary locations and layer resistivities accurately. In inversion of Columbia flood basalt data, sharp boundary inversion recovers a model with substantially less internal variation within units, and less ambiguity in both the depth to base of the basalts and depth to resistive basement.

Emplacement of Columbia River flood basalt

Evidence is examined for the emplacement of the Umatilla, Wilbur Creek, and the Asotin Members of Columbia River Basalt Group. These flows erupted in the eastern part of the Columbia Plateau during the waning phases of volcanism. The Umatilla Member consists of two flows in the Lewiston basin area and southwestern Columbia Plateau. These flows mixed to form one flow in the central Columbia Plateau. The composition of the younger flow is preserved in the center and the composition of the older flow is at the top and bottom. There is a complete gradation between the two. Flows of the Wilbur Creek and Asotin Members erupted individually in the eastern Columbia Plateau and also mixed together in the central Columbia Plateau. Comparison of the emplacement patterns to intraflow structures and textures of the flows suggests that very little time elapsed between eruptions. In addition, the amount of crust that formed on the earlier flows prior to mixing also suggests rapid emplacement. Calculations of volumetric flow rates through constrictions in channels suggest emplacement times of weeks to months under fast laminar flow for all three members. A new model for the emplacement of Columbia River Basalt Group flows is proposed that suggests rapid eruption and emplacement for the main part of the flow and slower emplacement along the margins as the of the flow margin expands.

Crust–mantle interaction in large igneous provinces: Implications from the Re–Os isotope systematics of the Columbia River flood basalts

The source and evolution of magmas that form large igneous provinces is a controversial topic. At the center of the debate is whether the different reservoirs — continental crust, asthenosphere or sub-continental lithosphere — contribute to the formation and evolution of these provinces. The isotopic systematics of Re and Os offer significant new information to constrain this debate. Because Os is compatible and Re incompatible during mantle melting, the different possible melt reservoirs will develop distinct Os isotopic signatures over time. Therefore, the source of magmas and the different possible contaminants should be readily distinguishable using the Os isotopic system alone. We have focused our study on the Re and Os composition of the Columbia River basalt Group (CRBG), which is one of the youngest and best studied continental flood basalts provinces in the world. Samples from throughout the well documented stratigraphic column, that represent primitive and contaminated magmas, as well as samples emplaced in stable North American Craton and Mesozoic accreted terrane were analyzed. The initial 187Os/ 188Os ranges from the lowest value of ∼0.13 in the most primitive Imnaha Formation and increases to values as high as 0.4 in flows from formations emplaced in the accreted terrane. Samples located on the craton have initial 187Os/ 188Os isotopic ratios that vary from ∼1 to ∼3. The data conclusively demonstrate that continental crust, most likely mafic lower crust, played an important role in the evolution of this flood basalt province. These data also show that the sub-continental lithospheric mantle (SCLM) was not significantly involved in the formation or modification of these continental flood basalts.

Helium and neon isotopes in the Imnaha Basalt, Columbia River Basalt Group: Evidence for a Yellowstone plume source

Helium, neon, and argon isotopic compositions were measured in two flows of the Columbia River flood basalt. The Imnaha Basalt has a 3He/4He ratio of 11.4 times atmospheric and 20Ne/22Ne and 21Ne/22Ne ratios characteristic of a plume component. The measured 3He/4He is a lower limit, due to possible preferential 3He loss and/or addition of radiogenic 4He. A Wanapum Basalt flow, erupted approximately 2 Ma later in the waning stages of volcanism, has more MORB-like noble gases. The He, Nd and Sr isotopic compositions of these lavas suggest that the Columbia River basalts were derived from the Yellowstone plume head which contained both ‘high-helium’ plume material and entrained depleted mantle. As the eruptions progressed the plume component in the melting region was gradually diluted or replaced.

Assessing subcontinental lithospheric mantle sources for basalts: Neogene volcanism in the Pacific Northwest, USA as a test case

Many objections have been raised as to the ability of subcontinental lithospheric mantle to produce voluminous amounts of basalt, because this upper part of the mantle is thought to be refractory, and the geotherm is rarely above the peridotite solidus at these depths under continents. However, in the Pacific Northwest of the USA during the Neogene, the subcontinental lithospehric mantle has been proposed as a key source for basalts erupted within the northern Basin and Range, and for the Columbia River flood basalts erupted on the Columbia Plateau. An alternative explanation to melting in the subcontinental lithospheric mantle, which equally well explains the chemical compositions thought to originate there, is that these magmas were contaminated by crust of varying ages. Calc-alkaline lavas, which occupy the Blue Mountains in the center of this region, hold clues to the latter process. Their elevated trace element ratios (e.g., Ba/Zr, K2O/P2O5), coupled with differentiation indicators such as Mg? [molar Mg/(Mg + Fe)], and Sr, Nd, and Pb isotopic compositions, can most reasonably be explained by crustal contamination. Appraisal of continental peridotite xenolith data indicates that high trace element ratios such as Ba/Zr in continental basalts cannot result from melting in the subcontinental lithospheric mantle. Instead, as with the calc-alkaline lavas, these high ratios in the tholeiites most likely indicate crustal contamination. Furthermore, the peridotite xenoliths do not have a relative depletion in Nb and Ta that is observed in most of the lavas within the region. Relatively minor volumes of tholeiites erupted in late Neogene times in the northern Basin and Range (Hi-Mg olivine tholeiites) and Columbia Plateau (Saddle Mountains basalts), are the only lavas which have trace element and isotopic compositions consistent with being derived from, or largely interacting with a subcontinental lithospheric mantle in the Pacific Northwest. In contrast to the prior studies, we suggest that the mantle sources for most of the basalts in this region were ultimately beneath the lithospheric mantle.

Variations in the strontium isotopic ratio of seawater during the Miocene: Stratigraphic and geochemical implications

A composite strontium isotopic seawater curve was constructed for the Miocene between 24 and 6 Ma by combining 87Sr/86Sr measurements of planktonic foraminifera from Deep Sea Drilling Project sites 289 and 588. Site 289, with its virtually continuous sedimentary record and high sedimentation rates (26 m/m.y.), was used for constructing the Oligocene to mid‐Miocene part of the record, which included the calibration of 63 biostratigraphic datums to the Sr seawater curve using the timescale of Cande and Kent (1992). Across the Oligocene/Miocene boundary, a brief plateau occurred in the Sr seawater curve (87Sr/86Sr values averaged 0.70824) which is coincident with a carbon isotopic maximum (CM‐O/M) from 24.3 to 22.6 Ma. During the early Miocene, the strontium isotopic curve was marked by a steep rise in 87Sr/86Sr that included a break in slope near 19 Ma. The rate of growth was about 60 ppm/m.y. between 22.5 and 19.0 Ma and increased to over 80 ppm/m.y. between 19.0 and 16 Ma. Beginning at ∼16 Ma (between carbon isotopic maxima CM3 and CM4 of Woodruff and Savin (1991)), the rate of 87Sr/86Sr growth slowed and 87Sr/86Sr values were near constant from 15 to 13 Ma. After 13 Ma, growth in 87Sr/86Sr resumed and continued until ∼9 Ma, when the rate of 87Sr/86Sr growth decreased to zero once again. The entire Miocene seawater curve can be described by a high‐order function, and the first derivative (d87Sr/86Sr/dt) of this function reveals two periods of increased slope. The greatest rate of 87Sr/86Sr change occurred during the early Miocene between ∼20 and 16 Ma, and a smaller, but distinct, period of increased slope also occurred during the late Miocene between ∼12 and 9 Ma. These periods of steepened slope coincide with major phases of uplift and denudation of the Himalayan‐Tibetan Plateau region, supporting previous interpretations that the primary control on seawater 87Sr/86Sr during the Miocene was related to the collision of India and Asia. The rapid increase in 87Sr/86Sr values during the early Miocene from 20 to 16 Ma imply high rates of chemical weathering and dissolved riverine fluxes to the oceans. In the absence of another source of CO2, these high rates of chemical weathering should have quickly resulted in a drawdown of atmospheric CO2 and climatic cooling through a reversed greenhouse effect. The paleoclimatic record, however, indicates a warming trend during the early Miocene, culminating in a climatic optimum between 17 and 14.5 Ma. We suggest that the high rates of chemical erosion and warm temperatures during the climatic optimum were caused by an increase in the contribution of volcanic CO2 from the eruption of the Columbia River Flood Basalts (CRFB) between 17 and 15 Ma. The decrease in the rate of CRFB eruptions at 15 Ma and the removal of atmospheric carbon dioxide by increased organic carbon burial in Monterey deposits eventually led to cooling and increased glaciation between ∼14.5 and 13 Ma. The CRFB hypothesis helps to explain the significant time lag between the onset of increased rates of organic carbon burial in the Monterey at 17.5 Ma (as marked by increased δ13C values) and the climatic cooling and glaciation during the middle Miocene (as marked by the increase in δ18O values), which did not begin until ∼14.5 Ma.

The northern Nevada rift: Regional tectono-magmatic relations and middle Miocene stress direction

As defined by the most recent aeromagnetic surveys, the north-northwest-trending northern Nevada rift zone extends for at least 500 km from southern Nevada to the Oregon Nevada border. At several places along the rift, the magnetic anomaly is clearly related to north-northwest-trending dikes and flows that, based on new radiometric dating, erupted between 17 and 14 Ma and probably during an even shorter time interval. The tectonic significance of the rift is dramatized by its length, its coincidence in time and space (at its northern terminus) with the oldest silicic caldera complex along the Yellowstone hot-spot trend, and its parallelism with the subduction zone along the North American coast prior to the establishment of the San Andreas fault. The northern Nevada rift is also equivalent in age, trend, and composition to feeder dikes that fed the main eruptive pulse (∼95% volumetrically) off the Columbia River flood basalts in northern Oregon ∼15.5-16.5 Ma. Because of these similarities, both regions are considered to be part of an enormous lithospheric rift that propagated rapidly south-southeast and north-northwest, respectively, from a central mantle plume. The site of the initial breaching of the North America plate by this plume is probably the McDermitt volcanic center at the north end off the rift near the Oregon-Nevada border. The present north-northwest trend of the rift and its internal elements, such as dikes and lava-filled grabens, record the orientation of the arc-normal extensional stress in this back-arc region at the time of emplacement. Paleomagnetic evidence presented by others and interpreted to indicate block rotations at three sample localities is not consistent with either a rotation of dikes within the rift or with a regional rotation of the entire rift. The present north-northwest trend of the rift reflects the state of stress in the Basin and Range during middle Miocene time and is consistent with stress indicators of similar age throughout the Basin and Range and Rio Grande rift provinces.

Volcanism and continental break-up: a global compilation of large igneous provinces

Abstract Large igneous provinces (LIPs) include continental flood basalts and associated intrusive rocks, volcanic passive margins, oceanic plateaus, submarine ridges, seamount groups, and ocean basin flood basalts. In some cases transient episodes of voluminous magmatism are temporally and spatially related to continental break-up, e.g. North Atlantic Volcanic Province, Deccan Traps, Paraná-Etendeka basalts. In other cases, however, no relationships are apparent, e.g. Siberian flood basalts, Columbia River flood basalts. Herein we review LIPs worldwide in order to better understand their relationship to the break-up and separation of lithospheric plates. The two most voluminous episodes known of basaltic magmatism not associated with the creation of ‘normal’ oceanic crust, the emplacements of the Ontong Java and Kerguelen plateaus, do not appear to be linked to continental break-up. Volcanic passive margins have now been identified on the edges of many continents, and are clearly related to continental break-up and separation. They cannot always, however, be tied to continental flood basalts. Ocean basin flood basalts and seamount groups are not commonly related to continental break-up. In most instances submarine ridges show temporal and spatial relationships with continental flood basalt provinces or oceanic plateaus. Observational data suggest that existing models do not adequately explain all LIPs; we suggest that a thermally and chemically heterogeneous asthenosphere, occasionally penetrated by deep mantle plumes, can account for their origin.

Comparative geochemistry and petrology of Triassic basaltic rocks from the Taku terrane on the Chilkat Peninsula and Wrangellia

Upper Triassic metabasalt from the Chilkat Peninsula in southeastern Alaska is lithologically similar to the Middle and (or) Upper Triassic Nikolai Greenstone from the Wrangell Mountains, east-central Alaska. Both basaltic sequences show comparable petrologic and geochemical features. The informally designated Chilkat metabasalt, which forms part of a Triassic sequence between the Denali fault and the Chilkoot lineament, is included in the Taku terrane, whereas the Nikolai Greenstone characterizes the allochthonous Wrangellia terrane. The Chilkat metabasalt consists mostly of massive to inconspicuously layered flows with well developed pillow structures at the top of the section; the sequence may reach a total thickness of 3000 m. The metabasalt is predominantly amygdaloidal and contains secondary minerals typical of greenschist-facies metamorphism. The Nikolai Greenstone is a widespread, mostly subaerial but locally pillowed, amygdaloidal basaltic sequence that reaches a total thickness of more than 3500 m; it is commonly less metamorphosed than the Chilkat metabasalt and contains secondary minerals typical of zeolite-facies to very low greenschist-facies metamorphism. Both the Chilkat and Nikolai basaltic rocks are predominantly tholeiitic. The major-element compositions of both sequences are very similar and cluster within a narrow range. The trace-element data for both sequences suggest a composition similar to the high-magnesian Columbia River flood basalts. Sediments associated with the Chilkat and Nikolai basaltic rocks share gross lithologic features and a similar depositional environment, even though the upper part of the Chilkat sequence appears to be a more offshore facies. The petrologic and geochemical data are compatible with the inference that the Taku terrane may have been coextensive with Wrangellia during the Late Triassic but has subsequently been laterally offset by dextral movement on the Denali fault.

Isotopic constraints on Columbia River flood basalt genesis and the nature of the subcontinental mantle

Pb, O, Nd, and Sr isotopic data for the Columbia River basalts paint a complex picture for the origin of this flood basalt province. At least 3 distinct mantle sources appear to have been involved, superimposed upon which are the effects of crystal fractionation and mass exchange with evolved crustal wallrocks. To a large degree, the initiation of Columbia River volcanism and the geochemical characteristics of the basalts appear to have been influenced by subduction of the Juan de Fuca plate beneath the North American plate in a manner analogous to the origin of back-arc basins. The physical structure of the crust appears to have influenced the late stage evolution of the magmas by directing the locus of eruption to the border between the ancient continental interior and much younger crust to the south and west. This proximity to the continental interior also allowed old enriched subcontinental mantle to become involved in the very late stages of Columbia River volcanism. An important consequence of the existence of enriched mantle regions beneath continents is that the combination, crust plus enriched mantle, requires more incompatible elements to have been extracted from the remainder of the mantle than would be the case if no enriched mantle existed.

Columbia River volcanism: the question of mantle heterogeneity or crustal contamination

Basalts from the Columbia River flood basalt province of the northwestern U.S.A. show a large diversity in chemical and Nd and Sr isotopic compositions. 143Nd144Nd ranges from 0.51303 to 0.51208 and is strongly correlated with variations in 87Sr86Sr. This correlation suggests mixing between two end member compositions, one characterized by 143Nd144Nd > 0.51303 and 87Sr86Sr < 0.7035, and the other with 143Nd144Nd < 0.5120 and 87Sr86Sr > 0.715. The more radiogenic component could be mantle enriched in incompatible elements during the Precambrian, or Precambrian materials of the continental crust. A quartz-rich xenolith found in the Columbia lavas has Rb-Sr and Sm Nd model ages of ≈ 1.4Æ, implying the existence of old, isotopically evolved crustal basement which could serve as contaminant. Nevertheless, crustal contamination alone cannot explain the chemical variation of the samples studied, and other fractionation processes must have occurred simultaneously. A model involving combined assimilation and crystal fractionation reproduces the chemical and isotopic characteristics of the volumetrically dominant Grande Ronde unit for an assumed crystallizing component of plagioclase, low calcium pyroxene and minor olivine. The data are not consistent with the suggestion that a ‘primordial’ mantle is the source for this continental flood basalt province. Rather they suggest that the main volume of these lavas was originally derived from a mantle similar in isotopic composition to island arc and ocean island basalts of the north Pacific. The primary magma was modified chemically and isotopically by crystal fractionation and assimilation of sialic crustal materials during its transport through, or storage in the continental crust.

Ascent and eruption of basaltic magma on the Earth and Moon

Geological and physical observations and constraints are applied to the development of a model of the ascent and emplacement of basaltic magma on the earth and moon. Mathematical models of the nature and motion of gas/liquid mixtures are developed and show that gas exsolution from terrestrial and lunar magmas commonly only occurs at shallow depths (less than 2 km); thus the ascent of bubble‐free magma at depth can be treated separately from the complex motions caused by gas exsolution near the surface. Magma ascent is related to dike or conduit width; a lower limit to width is determined by the presence of a finite magma yield strength or by excessive magma cooling effects related to magma viscosity. For terrestrial basalts with negligible yield strengths and viscosities greater than 102 Pa s, widths in the range 0.2–0.6 m are needed to allow eruptions from between depths of 0.5–20 km. Fissure widths of about 4 m would be needed to account for the output rates estimated for the Columbia River flood basalt eruptions. As the magma nears the surface, bubble coalescence will tend to occur, leading to intermittent explosive strombolian‐style activity. For commonly occurring lunar and terrestrial basalts the magma rise speed must be greater than 0.5–1 m/s if strombolian activity is to be avoided and relatively steady fire fountaining is to take place. Terrestrial fire fountain heights are dictated by the vertical velocity of the magma/gas dispersion emerging through the vent, increasing with increasing magma gas content and mass eruption rate, and decreasing with increasing magma viscosity. Terrestrial fire fountain heights up to 500 m imply the release of up to 0.4 wt % water from the magma, corresponding to initial water contents up to 0.6 wt %. The presence of extremely long lava flows and sinuous rules on the moon has often been cited as evidence for very high extrusion rates and thus a basic difference between terrestrial and lunar magmas and crustal environments. However, the differences between terrestrial and lunar magma rheologies and crustal environments do not lead to gross differences between the effusion rates expected on the two planetary bodies, for similar‐sized conduits or fissures. Thus the presence of these features implies only that tectonic and other forces associated with the onset of some lunar eruptions were such as to allow wide fissures or conduits to form. The surface widths of elongate fissure vents need be no wider than 10 m to allow mass eruption rates up to 10 times larger than those proposed for terrestrial flood basalt eruptions; 25‐m widths would allow rates 100 times larger. It therefore appears unlikely that source vents on the moon with widths greater than a few tens of meters represent the true size of the unmodified vent. The main volatile released from lunar magmas was probably carbon monoxide, released in amounts proportionally less than terrestrial magmas by more than an order of magnitude. However, decompression to the near‐zero ambient lunar atmospheric pressure causes much greater energy release per unit mass and this, coupled with vertical and horizontal expansion of the gas, suggests a much more efficient use of the available gas on the moon than on the earth. Some amount of magma disruption must always have taken place in lunar eruptions unless the gas content was truly zero or the magma possessed an appreciable yield strength. Pyroclastic deposits, such as the extensive lunar dark mantling material, could be produced from a single source vent and extend to diameters of up to 200 km. Such deposits could result either from steady eruptions at high effusion rates (with less than 1% of the magma disrupted into submillimeter droplets) or low effusion rate eruptions in which strombolian activity occurred. The wide dispersal of pyroclastic debris is a result of small particles being locked into the expanding gas cloud. Finally, we consider the details of basaltic eruption processes on the moon and predict the nature and geometry of several types of volcanic landforms (flows, pyroclastic blankets and cones, cinder and spatter ridges, etc.) that should result from specific eruption conditions.

Tertiary spilites and quartz keratophyres of the Wagwater Belt, Jamaica, west Indies

Geochemical and petrographic characteristics are used to identify spilites, quartz keratophyres and potassic quartz keratophyres as the most abundant rock types among the Tertiary volcanics of Jamaica. A few low K2O dacites occur within the sequence. The rock suite is bimodal and was erupted in a nonorogenic environment, an Eocene rift. The evidence indicates that the keratophyres were formed by alteration of the original dacites during an episode of alkali metasomatism. The concentrations of TiO2, P2O3, Cr, Zr and Y suggest that the spilites were formed from basic rocks similar to the Columbia River or Deccan flood basalts. Their chemical variations indicate that they were formed by processes similar to those leading to the formation of the keratophyres.

The tectonic evolution of the western United States

The tectonic evolution of the western United States