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A common pyroclastic facies in subaqueous volcanic successions comprises massive to graded, very thick (several to tens of meters), laterally extensive (several kilometers) beds of nonwelded pumice lapilli with volumes ranging to tens of cubic kilometers. This facies may be overlain by laminated ash or bimodal ash and giant (>1 m) pumice clasts, and underlain by coarse lithic breccia. The association is inferred to be the typical product of sustained magmatic volatile--driven explosive eruptions from vents at water depths of ~1300-200 m. We propose the term 'neptunian' for such eruptions and their products. The eruption column rapidly mixes with the surrounding water, cools, increases in density, and collapses, while remaining under water. Lithic clasts that are too heavy to be entrained in the column are deposited close to the source, forming a neptunian lithic breccia. Pumice lapilli are rapidly waterlogged and form the dominant component in the collapsing column and in eruption-fed, water-supported density currents (neptunian density currents). Hot, buoyant, giant pumice clasts continue to rise and may reach the water surface before being waterlogged and settling, along with temporarily suspended ash, forming neptunian suspension deposits. Eruption magnitude, fragmentation mechanisms, and juvenile pyroclast characteristics, especially vesicularity, are very similar in neptunian and Plinian-style eruptions, but column behavior differs primarily because of the contrasting physical properties of the ambient fluid (water versus air).

Hawaiian volcanoes evolve through stages that have been delimited by the compositions of their erupted lavas. New in situ [sup.238]U-[sup.230]Th and U-Pb dating of single zircons from leucocratic plutonic xenoliths erupted at Mauna Kea and Hnalalai volcanoes, Hawaii, reveals that important episodes of magmatic evolution are not necessarily reflected in the stratigraphy or compositions of erupted lavas. Zircons from Mauna Kea diorites form a heterogeneous population with apparently bimodal ages of ca. 125 ka and ca. 65 ka, suggesting fractionation, intrusion, and crystal recycling about the time of transition of post shield volcanism from basaltic to hawaiitic compositions. Hualalai syenogabbros and diorites record extreme fractionation and generation of alkalic magma at 41 [+ or -] 9 ka and 261 [+ or -] 28 ka, indicating that alkalic magma was generated ~130,000 yr before the shield to postshield transition inferred from lava stratigraphy, and that coeval evolution of chemically distinct magma reservoirs at shallow and deep levels may have characterized the shield stage. These episodes at Hualalai did not cause eruption of evolved lavas, indicating that extreme differentiation in Hawaiian volcanoes is not necessarily followed by eruption of highly evolved magma. Keywords: uranium-series method, magma chambers, magmatic differentiation, cumulates, volcanism.

The early volcanic history and structural development of Olympus Mons, Mars, has been obscured by the formation of the large circum-Olympus aureole deposits in the Early Amazonian. Mars Orbiter Laser Altimeter data reveal an enormous preaureole extrusive flow unit in Amazonis Planitia that is more than 300 km wide and extends ~1800 km radially away from the Olympus Mons aureole. We interpret this volcanic unit to represent proto-Olympus Mons lava flows emplaced during the latest Hesperian or earliest Amazonian. The orientation and localization of the deposits appear to be due to channeling of the flows into a broad depression between the generally north dipping slope of Amazonis Planitia and the southwestward-dipping slope of the Alba Patera flanks. Formation of the aureole blocked this depression and caused subsequent summit flows to pond in the circum-Olympus trough and flow around the aureole lobes. Emplacement of this proto-Olympus flow unit formed a barrier along the northern margin of Amazonis Planitia, causing ponding of later lava flows and outflow channel events debouching there. The large volume and great lateral extent of the unit imply abundant magma supply and high effusion rates during the initial stages of Olympus Mons construction; this unit may be the result of the initial impingement of a melt-rich mantle plume head. Keywords: volcanism, Olympus Mons, Mars, plumes, lava flows.

Structural and stratigraphic relations in the Great Basin indicate widespread pre-middle Miocene crustal extension that appears to define two north-trending belts. Most extension in these belts was Oligocene age, but locally it began earlier or lasted into early Miocene time. The eastern belt straddles the Nevada-Utah border and includes the Snake Range, Nevada, area, with its southern end near 37.5 degrees N and its western edge at the Seaman-Butte Mountains breakaway. The southern boundary of the eastern belt is occupied by the 26-15 Ma Caliente caldera complex and approximately coincides with the present east-west-trending margin of the Great Basin north of Saint George, Utah. Crust north of this boundary extended approximately east-west before volcanism began at 30-32 Ma, but to the south, extension began after about 15 Ma. This boundary may have been a rooted zone of left-slip faults that allowed the footwall of the Stampede detachment to move west relative to unextended terrain to the south. The eastern margin of the eastern belt is probably located near the present eastern edge of the Great Basin, but its northern end is poorly defined. The western belt runs from the Funeral and Grapevine Mountains, California, to the Ruby Mountains, Nevada, and north-northeast to the Albion Range, Idaho. Tens of kilometers of crustal extension occurred at least locally in both belts, but magnitude of extension is poorly known for large areas of each. Tertiary volcanism in the Great Basin began in the north in Eocene time with predominantly effusive volcanism and swept southward, ending with voluminous Oligocene-Miocene ignimbrite eruptions from calderas in an irregular, discontinuous belt between Marysvale, Utah, and Reno, Nevada. A result of the southward migration of volcanism is that the onset of extension in both belts was syn- or post-volcanic in the north but was pre-volcanic in the south. Late Paleogene extension and crustal magmatism coincided in both time and space only locally, where south-migrating magmatism overlapped active north-south-trending extensional belts. Most calderas in the southern Great Basin formed in previously extended belts or on their margins. Southward migration of ignimbrite sources was apparently blocked by unextended crust to the south. In contrast, volcanism north of the ignimbrite province was dominated by nonexplosive effusion of lava prior to, or during, crustal extension. This is consistent with observations in the southern Basin and Range, where volcanism and crustal extension were generally synchronous, and volcanism was dominantly effusive. Thus, caldera formation may be controlled by the distribution of upper-crustal extension, although the physical mechanism for this control remains speculative. The space-time patterns of late Paleogene extension in the Great Basin are consistent with extension being triggered by thermal weakening of subducted oceanic lithosphere rather than by effects transmitted from the plate margin, but being driven by gravitational collapse of thick crust. Space-time patterns of Tertiary volcanism in the Basin and Range also appear to conform to patterns of thermal weakening or destruction of the subducted slab. Both active and passive rifting mechanisms are inapplicable on the scale of the extensional belts, because both predict close spatial and temporal association of extension and magmatism, which is not generally observed.