Style Of Volcanism Research Articles

Geology of Venus: Mapping of a global geotraverse at 30°N latitude

We mapped a swath of terrain on Venus that extends circumferentially completely around the globe at 30°N latitude representing about 11% of the surface of the planet in order to address the following questions: Do presently defined stratigraphic units prove useful for broad application to regional and global mapping? Do units described in one area occur in other regions of the planet and how laterally continuous are they? If similar sequences of units are found in different parts of the planet, do they correlate temporally (e.g., time‐correlative), or do they represent similar sequences formed at different times (e.g., time‐transgressive or independently locally repetitive)? We found that similar sequences of units are seen in different parts of the global geotraverse and that key stratigraphic units could be traced across almost the entire circumference of Venus at this latitude, providing evidence that these units are not only global in extent, but also laterally contiguous. This gives confidence that the stratigraphic sequence can be extended to other parts of Venus for further testing and refinement. On the basis of this mapping, we find that several areas, including tessera massifs and Beta Regio, show a concentration of older units and appear to have been topographically high over much of the visible history of Venus. Tessera terrain is not uniformly distributed in some areas where these older units are concentrated. Ridge belts and fracture belts are not randomly distributed throughout the area, suggesting that they represent regional patterns of deformation. The altitude distribution of the stratigraphic units and structures shows that the regional plains tend to be the lowest, with older units (tessera and earlier plains) at intermediate to high elevations, consistent with regional plains emplacement following the formation of the most significant regional topography. Sixty corona and corona‐like features were found in the geotraverse, and the vast majority show evidence of being initiated in the earliest posttessera time (early Guineverian) in an extensional tectonic environment; few show evidence of postregional plains volcanism, and no positive evidence was found for the initiation of coronae in the postregional plains period of the history of Venus. Regional mapping shows that shield plains represent a unit occurring primarily prior to the emplacement of regional plains with wrinkle ridges (early part of the middle Guineverian), perhaps overlapping somewhat with its lower member. Evidence is found for changes in styles of volcanism with time, and tectonic activity changed in time in terms of pervasiveness, style, and areal distribution. On the basis of mapping of this globally continuous swath representing about 11% of the planet, we find that our analysis supports the presence of changes in the style and relative importance of different types of geological activity as a function of time, rather than the “nondirectional” history proposed by Guest and Stofan [1999].

Sandstone petrofacies expressions of multiphase basinal tectonics and arc magmatism: Permian–Triassic north Bowen Basin, Australia

Abstract Modal analyses of 209 sandstone samples from the Permian–Triassic succession of the northern Bowen Basin, eastern Australia, identify two stratigraphically and compositionally distinct petrofacies. The Lower to Mid Upper Permian Back Creek Group is characterised by Petrofacies A which is quartz-rich (Q82F8L10). It was sourced primarily from cratonic basement terranes to the west, where relief was subdued, and the quartz content of the sandstones may also reflect some reworking in the marine realm with a consequent loss of labile grains. Petrofacies B is volcanolithic and characterises alluvial sediments of the Upper Permian Blackwater and Lower Triassic Rewan Groups. It was sourced from an undissected to transitional magmatic arc provenance located in the contemporary New England Orogen to the east, which supplied abundant pyroclastic debris to the depositional complex. Petrofacies B of the northern Bowen Basin was derived primarily from felsic volcanics, compared to an intermediate association recorded for the southern basinal sector, indicating significant along-arc variation in volcanic style. As a consequence, its framework grain population is enriched in quartz and is not readily accommodated in the schemes of provenance interpretation currently in use. The consistency of framework detrital modes for sandstones distributed throughout the Blackwater Group (Q24F10L66) allows the recognition of subpetrofacies B1 and shows that the volcanism associated with the magmatic arc system was remarkably uniform in character and activity during deposition of the entire group. The magmatic arc delivered almost identical sedimentary debris over a period of some 9 My in the Late Permian. Relative enrichment of quartz within the Early Triassic Rewan Group (Q49F6L45) relative to the Blackwater Group discriminates subpetrofacies B2 and is attributed to a climatic change and shift in palaeotemperature at the Permian–Triassic boundary. The contact between the marine Back Creek Group (Petrofacies A) and the coal-bearing Blackwater Group (Petrofacies B) represents an abrupt switch in provenance, with little or no evidence of source mixing. The boundary marks the onset of volcanism and associated tectonic uplift in the northern part of the New England Orogen to the east. Sediment from the new source prograded rapidly westwards across the basin, coincident with regression. Direct evidence of contemporaneous volcanism is widespread in the Blackwater Group as shown by tuffs and tonsteins which are distributed throughout the entire late Permian succession.

Plio-Quaternary volcanotectonic activity in the northern sector of the Main Ethiopian Rift: relationships with oblique rifting

Deformation and magmatism within the ∼90 km wide northern Ethiopian Rift system is concentrated along a narrow zone - the Wonji Fault Belt. Two key areas (the Nazret-Dera and Asela-Ziway areas), located along the eastern margin of the north-northeast to northeast trending Main Ethiopian Rift, have been investigated in order to reconstruct the recent tectonomagmatic evolution of the northern branch of the Main Ethiopian Rift. In these areas, Early Pleistocene volcanic products (Wonji Group) overlie Pliocene volcanic rocks (Eastern Margin Unit). Detailed stratigraphical reconstructions have revealed the presence of several tectonomagmatic units which can be correlated between the two study areas. The stratigraphical and petrological study of these units outlined (1) the bimodal composition (basalts-pantellerites) of the oldest and youngest units and the unimodal character (pantellerites) of the products erupted during the intervening period; (2) the mainly fissural origin of the ignimbrites and oldest basalts; and (3) a mafic/felsic volumetric ratio of 1:5. The geological data suggest that, around the Pliocene-Quaternary boundary, a change in the stress field occurred in this Main Ethiopian Rift sector, passing from a direction of extension roughly orthogonal to the rift shoulders, to oblique rifting related to an east-west trending extension. In this framework the change in the style of volcanism observed in the Nazret-Dera and Asela-Ziway areas can be related to the change of the stress field. A new geodynamic model is presented for the Late Pliocene to Recent evolution of this sector of the Main Ethiopian Rift. According to this model, a large volume of rhyolitic products was erupted during an oblique rifting phase, following a previous period of pure extension. The change in the tectonic regime favoured partial melting of the underplated basalts as a decrease in the pressure and an elevation of isotherms occurred.

Stratigraphic resolution of a multiphase intracratonic basin system: The McArthur Basin, northern Australia

The McArthur Basin is a regionally extensive multiphase Proterozoic basin system in northern Australia. Existing lithostratigraphic elements are amalgamated into five coherent basin phases or ‘packages’, to provide a basin‐wide informal and non‐genetic stratigraphic framework on which future genetic models may be based. Packages are basin‐scale depositional and magmatogenic rock units, equivalent in magnitude to 2nd‐order basin cycles. Each package comprises a set of lithostratigraphic units with similar age, stratigraphic position, lithofacies or lithofacies associations, style and composition of volcanism, basin‐fill geometry and depocentre. The Redbank package (ca 1815–1710 Ma) comprises up to ∼6 km of shallow‐marine to fluvial sandstone, and lesser lutite, conglomerate, dolostone and bimodal volcanic and shallow intrusive rocks. It forms a semi‐contiguous sheet throughout most of the McArthur Basin, but thins against the large‐scale basin‐compartmentalising basement ridges (e.g. Murphy and Urapunga Tectonic Ridges) due to depositional onlap and/or differential erosion. The conformably overlying Goyder package (ca 1710–?1670 Ma) comprises shallow‐marine to fluvial sandstone and minor lutite and carbonate. It is areally restricted with a distinct ∼6 km‐thick depocentre in the central Walker Fault Zone (the proto‐Walker Trough) and is flanked by attenuated successions onlapping the adjacent shelves suggesting deposition in a synsedimentary ‘rift’. The conformably or disconformably overlying Glyde package (?1670–1600 Ma) comprises ∼5 km of stromatolitic‐evaporitic dolostone, siliciclastics and minor tuffaceous rocks, deposited in shallow‐ to moderately deep‐water and locally emergent environments. It is restricted to the Walker, Batten and Urapunga Fault Zones, subsequent to deposition in the fault‐bounded Walker and Batten Troughs. The unconformably to disconformably overlying Favenc package (ca 1600–?1570 Ma) is a regionally extensive, ∼50–1600 m‐thick, stromatolitic and evaporitic dolostone‐sandstone succession with minor basaltic volcanics deposited in shallow‐water, marginal‐marine, peritidal‐shelf and/or continental sabkha settings. It was deposited as a series of broad interconnected sag basins that transgressed the pre‐existing tectonic framework of troughs and shelves. The Wilton package (?1500-?1400 Ma) is a widely distributed cyclic sequence of siliciclastics deposited in a shallow‐marine, nearshore to shelf environment. It unconformably overlies all older basin packages, transgressing the pre‐existing tectonic elements of the McArthur Basin. Distribution and depocentres have evolved significantly from the underlying packages, the Wilton package thickening to ∼5 km in the Beetaloo Sub‐basin.

Styles of volcanism and volcanic hazards on Furnas volcano, São Miguel, Azores

Explosive volcanic activity has been dominant throughout the history of Furnas volcano. Eruptions that switched between magmatic and phreatomagmatic activity have become typical in the more recent history of the volcano with 70% of the eruptions in the last 5000 years being of this style. It is thus likely that future eruptions will be of this type. Magmatic activity gives rise to lapilli-fall whereas phreatomagmatic activity produces tephra-fall, pyroclastic surges, and flows. Strong winds, rainstorms, and/or strongly heterogeneous fallout may have contributed to locally variable internal stratigraphy of tephra from single eruptions. Proximal exposures indicate reworking and erosion of lapilli layers by pyroclastic surges and possible contemporaneity of surge and lapilli fall. Evidence indicates that pulsatory magma supply may play an important role in the switching between magmatic and phreatomagmatic activity. Hazard mapping for two end member eruptions (1630 AD and Furnas `C' ∼2000 BP) using a technique with a moving vent location has provided worst case scenario tephra thicknesses in the region around Furnas volcano. Anywhere inside the caldera of Furnas volcano is under serious threat, in the event of an explosive eruption, from destruction by pyroclastic surges. The effect of pyroclastic surges outside the caldera, such as for the villages of Ponta Garça and Povoação, is likely to depend largely on vent location. The considerable topographic relief of the region will control areas affected by pyroclastic flows and also lahars. Towns located at the mouths of rivers, e.g., Ribeira Quente and Povoação, are particularly vulnerable. Lahars present a major hazard at Furnas Volcano.

Facies analysis of volcano-sedimentary successions hosting massive sulfide deposits in the Iberian pyrite belt, Spain

The Iberian pyrite belt volcanogenic sulfides are hosted in volcano-sedimentary successions of Upper Devonian to Lower Carboniferous age and represent the greatest concentration of large massive sulfide deposits on Earth. Most of the ore deposits are exposed at the surface, and little investment in geological research has been needed to mine them. As a consequence, fundamental aspects of the ore geology, such as the depositional environment, the eruptive style of volcanism, the chronostratigraphic relations between facies types, and the final mode of emplacement of volcanic rocks have not been studied previously in detail.A facies analysis has been systematically carried out on several volcano-sedimentary successions of the ore-host unit. Seventeen volcanic and sedimentary facies are defined. The main facies are silicic volcanic facies, mafic volcanic facies, reworked volcaniclastic facies, siliciclastic facies, slope instability facies, and hydrothermal facies. Facies analysis suggests that the Iberian pyrite belt volcanism took place in a submarine, below-wave-base, depositional environment and that water depth increased to the east and north in the ore-hosting basin. Fragmentation mechanisms suggest that the eruptive style of volcanism was mainly nonexplosive, though explosive volcanism may have occurred outside the basin. Transport and depositional processes suggest that most of the volcaniclastic rocks were emplaced from syneruptive, nonpyroclastic mass flows. Contact relationships of volcanic and sedimentary facies reveal that peperitic facies are very widespread, silicic and mafic magmas intruded simultaneously at shallow levels in the volcano-sedimentary pile, and low-temperature hydrothermal processes may have occurred prior to volcanism at many sites.

Gruithuisen domes region: A candidate for an extended nonmare volcanism unit on the Moon

Small lunar areas known as red spots are thought to represent good candidates for nonmare volcanic units predating or contemporaneous with the period of mare volcanism. In this study, we present imaging telescopic and orbital Clementine multispectral surveys of the Gruithuisen region, located in the northwestern border of Mare Imbrium, which contains Imbrian age domical features of likely viscous volcanic origin with spectral characteristics of red spot areas. A rather spectrally homogeneous unit having typical characteristics of red spots is found to coincide with the material within the morphological boundaries of the Gruithuisen Delta, Gamma and Northwest domes. An extended “dome‐like” unit is identified spectrally and surrounds and extends to the west and the north of the domes. This unit shows spectral characteristics close to those of the domes, suggesting the presence of a significant amount of dome material extending many kilometers away from the domes themselves. Both the spectral characteristics of the dome and the dome‐like unit are clearly different from those of highlands and the surrounding mare basalts. The spectral identification of a widespread dome‐like unit suggests that the specific style of eruption that is inferred for the formation of the domes (i.e., viscous flows of possible more silicic composition) might have occurred on a regional scale in this part of the Moon, prior to the Iridum event about 3.8 Gyr ago. This volcanic style appears to be more widespread in the early part of lunar history than previously thought.

Shallow-seated controls on styles of explosive basaltic volcanism: a case study from New Zealand

The pyroclastic deposits of many basaltic volcanic centres show abrupt transitions between contrasting eruptive styles, e.g., Hawaiian versus Strombolian, or `dry' magmatic versus `wet' phreatomagmatic. These transitions are controlled dominantly by variations in degassing patterns, magma ascent rates and degrees of interaction with external water. We use Crater Hill, a 29 ka explosive/effusive monogenetic centre in the Auckland volcanic field, New Zealand, as a case study of the transitions between these end-member eruptive styles. The Crater Hill eruption took place from at least 4 vents spaced along a NNE-trending, 600-m-long fissure that is contained entirely within a tuff ring generated during the earliest eruption phases. Early explosive phases at Crater Hill were characterised by eruption from multiple unstable and short-lived vents; later, dominantly extrusive, volcanism took place from a more stable point source. Most of the Crater Hill pyroclastic deposits were formed in 3 phreatomagmatic (P) and 4 `dry' magmatic (M) episodes, forming in turn the outer tuff ring and maar crater (P1, M1, P2) and scoria cone 1 (M2–M4). This activity was followed by formation of a lava shield and scoria cone 2. Purely `wet' activity is represented by the bulk of P1 and P2, and purely `dry' activity by much of M2–M4. However, M1 and parts of M2 and M4 show evidence for simultaneous eruptions of differing style from adjacent vents and rapid variations in the extent and timing of magma:water interaction at each vent. The nature of the wall-rock lithics, and these rapid variations in inferred water/magma ratios imply interaction was occurring mostly at depths of ≤80 m, and the vesicularity patterns in juvenile clasts from these and the P beds imply that rapid degassing occurred at these shallow levels. We suggest that abrupt transitions between eruptive styles, in time and space, at Crater Hill were linked to changes in the local magma supply rate and patterns and vigour of degassing during the final metres of ascent.

Lunar And Planetary Perspectives On The Geological History Of The Earth

During the latter part of the last century, a profound change took place in our perception of the Earth. First, this change was holistic: Plate tectonic theory provided a unifying theme that seems to explain disparate observations about the Earth and how it works, and lets us see the Earth as a planet. Secondly, actually seeing the Earth from the Moon, and exploring the other planets provided additional perspectives on our own home planet and hastened the decline of scientific terracentrism. Thirdly, learning that the uniqueness of the Moon in terms of size and aspects of its chemistry may be due to its derivation from the Earth as the result of a giant impact, provided a concrete filial link. Finally, the geological record revealed by exploration of the Moon and planets has provided us with the missing chapters in the dynamic history of the Earth. We now know that gargantuan impact basins formed in Earth's formative years and that impact events are likely to be the cause of many punctuations in Earth's biological evolution. Perspectives on ancient tectonic activity are provided by Mercury, Venus, Mars, and the Moon, and show that the Earth has changed considerably since its youth. Widely varying volcanic eruption styles are seen on the planets, providing insight into how puzzling rocks from early Earth history formed. The composition of planetary atmospheres has revealed the unusual nature of Earth's, and its link to the evolution of life. The atmospheres of the planets have undergone radical changes with time, providing clues to Earth's history and destiny. Fundamentally different hydrological cycles on Earth, Venus, Europa and Mars, and evidence for significant changes with time, have provided insight into Earth's history. The probable presence of oceans on Europa and Mars has changed our thinking about the origin and evolution of life on Earth. We no longer think of the Earth in isolation. Instead, Earth is now perceived of as a member of a family of planets, each of which provides important missing information and perspective on the other, and together reveal the fabric of the history of the Solar System. Future exploration and perspectives will place our Solar System in the context of all of the others.

Change in the quaternary volcanic style for the east part of Izu Peninsula-using K-Ar AGE dating-

Change in the quaternary volcanic style for the east part of Izu Peninsula-using K-Ar AGE dating-

Tectonic evolution of Bell Regio, Venus: Regional stress, lithospheric flexure, and edifice stresses

In order to understand the relationship between volcanic and tectonic processes and the stress state in the lithosphere of Venus, we analyzed the stress environments and resulting tectonic features associated with the major volcanic edifices in Bell Regio, using Magellan synthetic aperture radar (SAR) images and altimeter measurements of topography. The major volcanoes of Bell Regio, Tepev Mons and Nyx Mons, exhibit tectonic characteristics that are unique relative to other volcanic edifices on Venus. The most prominent distinction is the lack of large rift zones within the overall highland uplift, which characterize many other highland rises on Venus. Also, previous studies have determined that many large Venus volcanoes exhibit radial tectonic structures on their flanks but generally lack the circumferential graben which surround volcanoes on Earth and Mars. Tepev and Nyx Montes exhibit both the radial tectonic features associated with other Venusian edifices and numerous concentric graben. Nyx Mons implies a more distributed magmatic system by its broad shape, radial chains of pit craters, and expansive flow fields, whereas Tepev Mons is a more centralized volcanic system, with limited associated long flows. We investigate the regional stresses associated with Bell Regio and structural features believed to be a consequence of lithospheric flexure due to volcanic loading, modeling both Nyx Mons and Tepev Mons as axisymmetric loads with Gaussian mass distributions on an elastic plate. The relationship between the tectonic features surrounding Tepev Mons and stresses associated with magma chamber inflation are also examined through finite element analysis. Using topography data to model the shape of the volcano, we determine that a horizontally ellipsoidal or tabular reservoir at a range of depths from approximately 20 to 40 km can satisfy the locations of graben formation observed in Magellan images. These results imply a shift in volcanic style within Bell Regio from an early phase of broad, low shield formation to later steep‐sided, more centralized edifice development. Such changes are consistent with an increase in the thickness of the lithosphere over time.

Emplacement and reworking of Cretaceous, diamond-bearing, crater facies kimberlite of central Saskatchewan, Canada

In central Saskatchewan, Canada, kimberlites were emplaced into Cretaceous marine and nonmarine clastic sediments. Core recovered from one drill hole that intersects kimberlite (Smeaton FAC/UK core 169/8) was selected for an integrated study involving sedimentology, volcanology, mineralogy, geochemistry, palynology, micropaleontology, organic petrology, and radiometric age determination. Only crater facies kimberlite has been observed; there is no indication of the locations of feeder dikes. Four varieties of kimberlite occur, all originating from subaerial volcanism: (1) fluvial-reworked kimberlite; (2) diamondiferous kimberlite lapillistone air-fall deposits; (3) kimberlite olivine crystal-tuff air-fall deposits; and (4) diamondiferous marine wave-reworked kimberlite. Within the multiple primary eruptive phases of the kimberlite air-fall deposits, the volcanic style changed upward with time, from violent Strombolian to more explosive volcanism. The bulk of the volcanism formed conformable, air-fall deposits on terrestrial sediments of the Cantuar Formation, resulting in the development of positive-relief tephra cones. Subsequent marine transgression associated with the Westgate Formation partially beveled the top of the cone. The kimberlite air-fall deposits contain microdiamonds, 5 to 25 μm in diameter. The maximum temperature and vitrinite reflectance values of coaly matter in the kimberlites indicate that these deposits, although originally derived from magma at high temperatures, did not thermally affect entrained surficially derived clasts or the country rock during emplacement. The chemical content of intrakimberlite shale clasts is markedly different from the marine and nonmarine shales and indicates significant synemplacement and postemplacement fluid movement through the volcanic pile. At least two episodes of kimberlite volcanism occurred in the middle and late Albian (paleontologically assigned). A U-Pb perovskite radiometric age of 101.1 ± 2.2 Ma from a kimberlite lapillistone from the younger episode of volcanism is internally consistent with biostratigraphic studies that constrain the kimberlite volcanism as post–middle Albian and pre–late Albian to late Albian.

Tectonic development of the northern Taiizaiiian sector of the East African Rift System

The Eastern Branch of the East African Rift System diverges from a single, c. 50 km wide rift in southern Kenya to a c. 200 km wide zone in northern Tanzania, where it is comprised of three distinct rifts with different orientations. The western part of this zone contains two rift branches: the Natron-Man yara-Balangida and Eyasi-Wembere rifts. Each rift contains individual basins that are defined here on the basis of structural and geophysical interpretations. These basins are shallow (<3km) and total extension across the bounding faults is small. New K/Ar age determinations on basalts from the western rift basins show that volcanism and sedimentation began in the area at c. 5 Ma. Major fault escarpments were present by c. 3 Ma and the present-day rift escarpments developed later than c. 1.2 Ma. Pre-rift volcanism produced large shield volcanoes of a basalt-trachyte-phonolite association that now lie on the rift flanks. Volcanism after the main phase of rift faulting produced volatile- and alkali-rich explosive centres which are active today, and have no equivalent in southern Kenya. The change in morphology of the Eastern Branch of the East African Rift System, and the style of volcanism in northern Tanzania, may be the result of the transition from the rifting of Proterozoic Mozambique Belt lithosphere to the rifting of cratonic Archaean lithosphere.

Side-Scan Processing of GLORI-B and SeaBeam 2000

A new side-scan processing software package has been developed to postprocess data from the new GLORI-B system and SeaBeam 2000 side-scan data using preexisting techniques. We describe the procedures used to process the side-scan data collected during Legs 5, 6 and 7 of the Gloria Expedition in the spring of 1993. The procedures included minimizing artifacts and merging the two types of side-scan data together. The final mosaic shows recent widespread volcanic flows over large areas of the seafloor, unlike any other volcanic chain surveyed on the seafloor. The side-scan data were also very useful in dredging young rocks which are described in recently submitted geochemical manuscripts. The unique style of widespread low-lying young volcanism is attributed to the combination of superfast seafloor spreading and an underlying hotspot channel.

Quaternary volcanism of the Japanese Islands

Abstract Quaternary volcanism of the Japanese Islands is examined from the perspective of experimental petrology, geographic distribution of volcanoes and spatial geochemical variations. The dehydration of amphibole and chlorite at a 110 km depth and of phlogopite at ∼180 km in the downdragged hydrous mantle layer would result in the occurrence of two volcanic chains parallel to the trench axis. Long‐term subduction of the old Pacific plate and recent subduction of the young Philippine Sea plate beneath East Japan and West Japan volcanic belts respectively, would be critical for the significant difference in intensity, style and geochemistry of Quaternary volcanism between the two volcanic belts. The geochemistry of volcanic rocks in Northeast Japan and those in the Ryukyu arc is typical of ‘island‐arcs’ having low LIL/HFS element ratios, while alkalic basalts along the Japan Sea coast side in Southwest Japan have high LIL/HFS ratios similar to intra‐continental or oceanic island basalts. Across‐arc variations in eruptive volume and distributional density of volcanoes and in geochemistry are documented in Northeast Japan and are well explained by the decreasing degrees of partial melting toward back‐arc side, and the difference in geochemistry of fluids supplied by the downdragged hydrous layer.

Magmatism in the Garrett transform fault (East Pacific Rise near 13°27′S)

The Garrett transform is characterized by recent (zero age) volcanic activity located within the active tectonic domain of the transform valley at depths greater than the 3500 m. This intratransform volcanic activity contributed to the formation of constructional edifices forming ridges (>300 m in height) and small mounds (<20 m in height) built near slivers of serpentinized peridotites. The erupted lavas are depleted mid ocean ridge basalts (MORBs) with low ratios of K/Ti (0.02–0.11), Zr (30–100 ppm), Y (18–50 ppm), and (La/Sm)N (0.25–0.60). Their more depleted nature and smaller range of variability for the compatible elements (Ni = 70–180 ppm, Mg# = 0.58–0.71, where Mg# is the magnesium number (= Mg+2/Mg+2 + Fe+2)) are the main points of difference between the Garrett intratransform volcanics (GITV) and those from the ultrafast south East Pacific Rise (SEPR). However, ferrobasalts (Mg# = 0.41–0.55) were collected from the intratransform walls as well as at the EPR‐transform intersection. The GITV are even more depleted in incompatible elements than lava from the north East Pacific Rise (21°N‐11°26′N). The Garrett recent lava is believed to have erupted after the successive, incremental partial melting and discontinuous melt extraction of a composite lherzolitic mantle similar to that of the SEPR. The limited range of incompatible element ratios (Zr/Y = 1–2.5, (Ce/Yb)N = 0.4–1) and K/Ti ratios (<0.14) and the samples more porphyritic nature with respect to other SEPR rocks suggest that the intratransform volcanics from the Garrett are extracted from their source and channeled directly toward the surface without extensive mixing in magma chambers. In order to explain the restricted range of compositional variabilities and the absence of the enriched basalts produced by prior melting, we postulate that even though they were produced, these most enriched end‐member lavas did not reach the surface; instead, we propose these melts contribute to the formation of impregnated mantle material in the lithosphere. We suggest that this same petrogenetic style of intratransform volcanism might also characterize other oceanic provinces associated with low magmatic to quasi‐amagmatic regimes.

Seamount volcanism at the Reykjanes Ridge: Relationship to the Iceland hot spot

The axial zone of the Reykjanes Ridge is covered with small (0.5–3 km in diameter) volcanoes that pile together to form larger axial volcanic ridges. This style of volcanism is similar to that at the Mid‐Atlantic Ridge (MAR) and may be common to slow spreading ridges despite proximity of the Reykjanes Ridge to the Iceland hot spot. In this study we quantitatively investigate the population of seamounts in three study areas at the Reykjanes Ridge. Areas A and B are centered at 62°N and 60°N, respectively. Area C is centered at 58°N and is located south of the transition in ridge morphology from an axial high to an axial graben. Using multibeam bathymetry data, 541 seamounts (summit height H > 50 m) were identified in the three areas, and their size and shape statistics were compiled. Additionally, 105 seamounts in areas B and C were recognized in deep‐towed side scan images, and their surface morphologies (hummocky or smooth) were recorded. On the basis of estimated population parameters, we find that seamounts at the Reykjanes Ridge are more abundant (310±20 per 103 km2), on average, than at the MAR between 24° and 30°N (200±10 per 103 km2). Significant along‐axis variations exist at the Reykjanes Ridge, however, which are not simply related to distance from the hot spot: area B has nearly twice the abundance of seamounts as either area A or area C. Variation in the characteristic height of the seamount population is also observed between the Reykjanes Ridge (68±2 m) and the MAR (58±2 m), but no significant variation is found between our three study areas. A dramatic change in seamount surface morphology occurs between areas B and C (there are no side scan data from area A). Area C has 78% hummocky seamounts (similar to the proportion observed at the MAR), while area B has 83% smooth seamounts. On the basis of these results, we present a conceptual model for building the shallow crust at the slow spreading Reykjanes Ridge that takes into account the possible influence of the Iceland hot spot on the crustal melt delivery system and its influence on variables that control seamount abundances, sizes, shapes, and surface morphologies. In this model we suggest that the increased seamount production and proliferation of smooth seamounts in area B may be associated with a pulse of hot spot material, in the form of asthenosphere of higher temperature, that has recently affected area B.

Volcanic Expression of Bimodal Magmatism: The Cranberry Island-Cadillac Mountain Complex, Coastal Maine

The Cranberry Island series is a thick succession of Silurian volcanic breccias, ash flow tuffs, lava flows, and hypabyssal intrusions comprising the southern margin of Mount Desert Island and the Cranberry Islands of southeastern Maine. The rocks of the series preserve remarkable textures indicative of the simultaneous eruption of mafic and felsic magmas, during both explosive and effusive events. The Cadillac Mountain intrusive complex is an approximately 20-km-wide composite layered mafic to felsic plutonic body that dominates Mount Desert Island immediately north of the outcrop of the Cranberry Island series. The rocks of the Cranberry Island series and those of the Cadillac Mountain intrusive complex are correlative in age and similar in composition. The age of the Cranberry Island series, based on new U-Pb geochronology of zircon, is . The Cadillac Mountain granite, the Somesville granite, and the Southwest Harbor granite, all parts of the Cadillac Mountain intrusive complex, are similar in composition to parts of the Cranberry Island series. The Somesville granite has been dated at and the Cadillac Mountain granite at . These relations suggest that the Cadillac Mountain intrusive complex and the Cranberry Island series may be a deeply eroded, shallow-level pluton and its erupted equivalent, respectively. The volumes of the pluton and of the volcanic succession, the style of volcanism, and the compositional bimodality of the complex are reminiscent of caldera-style volcanism in terranes of crustal extension and thinning.

Physico-chemical processes in planetary evolution

Data returned from exploration of the planets teach us that planets are highly individualistic despite the assumed underlying universality of the processes. It is argued that the diversity of evolutionary outcomes arises mainly because of the interplay of thermal and compositional effects, both in the assembly of planets and in their subsequent convective evolution. The role of compositional differences and phase transitions invalidates any view of planets that relies heavily on models consisting of a small number of homogeneous layers. Four illustrative examples of real planetary behaviour are discussed: sold-solid phase transitions in terrestrial mantles, volcanic styles and recycling in Venus, heat flows of the giant planets and the role of the Lorentz force in the dynamics of the non-metallic portions of giant planets.

The influence of regional stress and magmatic input on styles of monogenetic and polygenetic volcanism

A new model is proposed to relate the development of monogenetic and polygenetic volcanoes to magmatic input and regional stress in the lithosphere. Output‐stress diagrams relate magmatic output rate and crustal deformation rate (or strain rate). The magmatic output rates of polygenetic volcanoes and monogenetic volcano fields, including lava fields and central (axial) volcanoes on the mid‐oceanic ridge systems, are estimated. Crustal deformation rates obtained from the literature are used as indicators of differential stress (Δσ). These data are normalized over time periods of 104 years, and over areas of 103 km2. The ratio of output rate to input rate, inferred from ophiolite sections, and from crustal deformation around volcanoes, is about 0.1 to 0.3. Based on the average ratio, an input‐stress diagram may be obtained at some volcanoes. Polygenetic volcanoes are plotted in the region of both larger output rate and smaller Δσ. Monogenetic volcano fields are plotted on a rift trend, or in the region of a small output (≤1 km3/104 yr/103 km2). In some regions, polygenetic volcanoes and monogenetic volcanoes coexist, for example, during 0.1 m.y. periods, and within the area of 30 km × 30 km. The output‐stress diagram indicates that coexistence of these volcano types results from variation in differential stress, or variation in production rate of magma in the mantle at different levels over periods of time less than 0.1 m.y. This is supported using extrusive volumes and crustal deformation data from the eastern volcanic zone, Iceland, Taupo Volcanic Zone, New Zealand, TransMexican Volcanic Belt, Mexico, and the volcanic field on and around the Izu peninsula, Japan. The observed relationship between deformation rate, magmatic output rate, and volcano type supports the crack interaction theory of magma‐filled cracks. The output‐stress diagram also indicates that the balance between local stress induced by magma accumulation and regional stress, and stress relaxation, govern the structure of a volcano.