Many modern linear volcanic chains on the Pacific plate can be backtracked to the general region of large Mesozoic seamount provinces in the far western Pacific, but those provinces differ in many ways from their younger counterparts in volcano arrangement, spacing, linearity, and age progression. Over the past 165 m.y., regions of volcanism have become narrower, more linear, and more consistently age-progressive. Rather than viewing this trend as a consequence of changes in the number, location, duration, and intensity of mantle plumes, we propose instead that five sets of stresses in the upper mantle combine to produce the patterns of great fissures and systems of fissures through the lithosphere along which seamount provinces and linear chains form. These are (1) lateral lithospheric contraction; (2) subduction-driven flow of the asthenosphere, which presently is antiparallel to Pacific plate motion, but which can be uneven in velocity and somewhat divergent in direction; (3) stresses in the plate produced by changes in patterns, relative orientations, and rates of subduction; (4) accumulation of low-density material and melt at places of lateral heterogeneity of density, viscosity, and composition in the asthenosphere; and (5) volcanic loading and redistribution of mass during differentiation at volcanic centers. Of these, the present direction of propagation of linear volcanic chains is most strongly controlled by contractive stresses and the pattern of subduction-driven asthenospheric counterflow, with volcanic spacing and volume arising from different combinations of the other stresses. Eruption of midplate seamounts results from buoyancy contrasts in the uppermost asthenosphere stemming from mantle heterogeneity, slightly variable temperature gradients, and the ponding of melt. Melt generally present in the low-velocity zone flows into shear gashes and then pools in traps beneath permeability barriers at the base of the lithosphere, with higher melt concentrations occurring above regions of greater mantle fertility and/or higher temperature. The lithospheric stress regime coupled with magma overpressure then induces lithospheric fracturing and eruption. Eruption, even above fertile mantle, may be stifled when the combination of lithospheric stresses is compressive, and fairly voluminous, even above refractory asthenosphere, when lithospheric stresses are extensional. Extremely voluminous volcanism occurs when a strongly extensional regime and/or a lithospheric thin spot coincides with asthenosphere having both high mantle fertility and higher temperature (e.g., oceanic plateaus at triple junctions). The relative importance of the five sets of stresses has varied in the past. Assuming that linear island chains propagate mainly in the local direction of least principal stress in the underlying lithosphere, the changes in fissure patterns through time are probably related to the increasing proportion and distribution of subduction boundaries on the northern and western margins of the plate as it grew in size to become the largest on Earth. This initiated counterflow in the asthenosphere and caused changes in the orientations of spreading ridges, the number and pattern of transform faults, the existence and distribution of microplates, and the magnitudes and orientations of the stress fields. Stress concentrations in the plate were far from uniform. Stress differences led to varieties of fracture patterns, which prompted the release of magma along volcanic ridges of varied morphology, size, and distribution of volcanic centers. The orientation and magnitude of counterflow in the asthenosphere and stress contrasts across the plate increased with the growth of subduction boundaries. Fissure patterns accordingly became more consistently oriented. Important events at the boundaries of the plate, such as the Australian-Asian collision and the demise of the Kula ridge in the northern Pacific changed the direction of asthenospheric counter-flow, midplate stress orientations, and the patterns of stress concentration, and these conditions—rather than shifts in plate motions over fixed hotspots—resulted in changes in directions of volcanic chains.

The fissure eruptions of Miocene basalt in the Columbia River region and the even more voluminous Eocene basalts of the Olympic Mountains—Oregon Coast Range occupy two different tectonic settings. Each of these great piles of “tholeiitic” lavas shows almost no differentiation, and rocks of intermediate (andesitic) composition are generally absent. In the adjacent “first cycle” Cascade Mountains, about 75 per cent of the volcanic rocks are andesites, though they range from olivine basalts to rhyolites. Violent steam explosions characterized their extrusion. The rocks are notably higher in CaO and Al 2 O 3 than those of the adjacent basaltic provinces. Rejuvenated or “second cycle” mountains,—the Rocky Mountains of Colorado—show a highly diversified volcanic sequence, including rhyolites, trachytes, latites, andesites, basalts, and also feldspathoidal lavas. Larsen’s studies of the San Juan area prove the coexistence and comingling of magmas of widely different compositions in spongy subterranean chambers. Tertiary igneous sequences occur in the elevated continental plates on both sides of the Rockies. The laccolithic complexes of the Colorado Plateau and the diorite-syenite-shonkinite (or lamprophyre) complexes found east of the Rockies resemble the San Juan sequence but are more alkalic. Still more alkalic are the highly feldspathoidal and ferromagnesian extrusives of the Navajo-Hopi country, which in their general features resemble the highly alkalic and ultramafic volcanic rocks of Uganda. Such alkalic mafic lavas commonly appear in small volume on the Precambrian shields. Common associates are intrusive complexes of alnoite, kimberlite, and mica peridotite. Also present on some shields are voluminous piles of tholeiitic basalt. The igneous cycle has ended; a new one begins. The widely held theory that all volcanic rocks originate by differentiation of basalt does not seem to fit this complex picture. The following hypothesis, admittedly oversimplified, is tentatively offered: Tholeiitic magma arises in a deep universal earth shell, but the fissure eruptions do not tap a great molten pool undergoing crystal differentiation. Differentiation occurs only after irruption to high levels. When a submarine trough is downbuckled into a tectogene, water plus other easily removable constituents are distilled from the metamorphosing geosynclinal sediments. These mix with the primitive tholeiite, modifying it to highly explosive steam-rich andesitic magma. Continued metamorphism of the tectogene root ultimately results in partial melting, causing the rise of trondjhemitic and granodioritic batholiths which may invade the slightly older extrusive andesites. Successive periods of anatexis will sweat upward more and more of the granitic fractions of the tectogene. Left behind in the dwindling root, the refractory greenstones, limestones, and serpentines have meanwhile been dehydrated by increasing metamorphism to amphibolites, skarns, and uralitized or biotized ultramafics. Locally these refractory hornblende- and biotite-rich rocks may be partially melted and erupted to produce the highly alkalic mafic lavas that characterize shields and continental plates.

More than 95% of the eastern Snake River Plain (ESRP) is covered by basaltic lava flows erupted in the Brunhes Normal-Polarity Chron; thus they are younger than 730 ka. About 13% of the area of the ESRP is covered by lava fields of latest Pleistocene and Holocene age <15 ka. More than 90% of the basalt volume of the ESRP is included in coalesced shield and lava-cone volcanoes made up dominantly of tube- and surface-fed pahoehoe flows. Deposits of fissure-type, tephra-cone, and hydrovolcanic eruptions constitute a minor part of the basalt volume of the ESRP. Eight latest Pleistocene and Holocene lava fields serve as models of volcanic processes that characterize the basaltic volcanism of the ESRP. The North Robbers, South Robbers, and Kings Bowl lava fields formed in short-duration (a few days), low-volume (each <0.1 km 3 ), fissure-controlled eruptions. The Hells Half Acre, Cerro Grande, Wapi, and Shoshone lava fields formed in long-duration (several months), high-volume (1 to 6 km 3 ), lava cone- and shield-forming eruptions. Each of these seven lava fields represents monogenetic eruptions that were neither preceded nor followed by eruptions at the same or nearby vents. The Craters of the Moon lava field is polygenetic; about 60 flows were erupted from closely spaced vents over a period of 15,000 years. Most of the basaltic volcanism of the ESRP is localized in volcanic rift zones, which are long, narrow belts of volcanic landforms and structures. Most volcanic rift zones are collinear continuations of basin-and-range-type, range-front faults bordering mountains that adjoin the ESRP. It is not clear whether the faults extend into the ESRP in bedrock beneath the basaltic lava flows. The great bulk of basaltic flows in the ESRP are olivine basalts of tholeiitic and alkaline affinities. The olivine basalts are remarkably similar in chemical, mineralogical, and textural characteristics. They were derived by partial melting of the lithospheric mantle at 45 to 60 km, and they have been little affected by fractionation or contamination. Evolved magmas having SiO 2 contents as high as 65% occur locally in and near the ESRP. The chemical and mineralogical variability of the evolved rocks is due to crystal fractionation in the crust and to contamination by crustal minerals and partial melts of crustal rocks. The trace-element compositions of the olivine basalts and the most primitive evolved basalts do not overlap, suggesting that the evolved rocks were derived from parent magmas that are fundamentally different from the parent magmas of the olivine basalts. The distribution and character of volcanic rift zones in the ESRP are partly controlled by underlying Neogene rhyolite calderas. Areas that lack basalt vents and have only poorly developed volcanic rift zones overlie calderas or parts of calderas filled by thick, low-density sediments and rocks, which served as density barriers to the buoyant rise of basaltic magma. Volcanic rift zones are locations of concentrated extensional strain; they define regional stress patterns in the ESRP.

ABSTRACT In recent models of earth-system crises, the correlation between the major Phanerozoic mass extinctions and large igneous provinces has been well established. Specifically, pulsed massive exhalations of large amounts of volcanogenic CO 2 transformed Earth’s atmosphere, leading to an excessive greenhouse effect and global warming, combined with slowed oceanic circulation, oxygen deficiency, and seawater acidification. In a historical context, however, the path leading to this neocatastrophic doctrine, traced by way of ever-more-convincing proofs (in recent years, via mercury anomalies), was convoluted for many objective and notional-personal reasons. From the late eighteenth century to the revolutionary 1980s, the reception of this conceptual route in the English-language mainstream science was determined principally by the rise and fall of the orthodox nonprogressive (steady-state) paradigm of the Lyellian uniformitarian. The main cognitive steps, pioneered frequently in continental Europe, included such principal conclusions as: (1) volcanic eruptions are a natural process, consisting of heat being vented from a central incandescent core, itself a relic of an initial nebular state; (2) cataclysmic phenomena were far more intense in the geologic past, both in orogenic and nonorogenic time intervals, with a dominant nonactualistic style of fissure-type effusive activity in intraplate settings, recorded in vast trap-type basalt successions (= large igneous provinces); (3) volcanogenic gaseous emanations, dominated by carbon dioxide and water vapor, had a strong impact on the global climate in the geological past toward the global warmth mode; and (4) this “volcanic greenhouse” was deleteriously augmented by several forms of immanent stress feedback (resulting in anoxia, acidification, hypercapnia, acid rains, ultraviolet radiation, etc.). Overall, diverse global ecosystem interactions, combined with the updated large igneous province scenario, can elucidate all major destructive factors in the biosphere, such as regressive versus transgressive sea-level changes and cooling versus warming climatic responses. Notwithstanding the particularity of each major biodiversity crisis in the Phanerozoic, however, a greenhouse/icehouse volcanism-driven catastrophe is a well-confirmed key toward better understanding these biotic turnovers over a variety of time scales and feedbacks. The holistic volcanic “press-pulse” model involves the joint action of two different types of stress factors: long-lived (“press”) large igneous provinces and a variety of critically sudden (“pulse”) disturbances. Therefore, the killing effectiveness of volcanic cataclysm should be viewed not only by the large igneous province size but also by their host geology, magma plumbing system, and eruption dynamics, determining the magnitude and composition of disastrous thermogenic outgassing. In search of possible pulse signals, emphasis has recently been placed on large igneous province–related, volatile-rich, mafic-ultramafic intrusions (owing to the great fluid-bearing capacity of their magmas) and sill-type intrusions (resulting in the most-effective devolatilization of sedimentary rocks). A simultaneous burst of arc magmatism and coeval impact of arc-continent collisions (especially in tropical domain) on global weatherability are additional cumulative cataclysmic stimuli awaiting more rigorous numerical simulations.