ABSTRACT The Providencia island group comprises an extinct Miocene stratovolcano located on a shallow submarine bank astride the Lower Nicaraguan Rise in the western Caribbean. We report here on the geology, geochemistry, petrology, and isotopic ages of the rocks within the Providencia island group, using newly collected as well as previously published results to unravel the complex history of Providencia. The volcano is made up of eight stratigraphic units, including three major units: (1) the Mafic unit, (2) the Breccia unit, (3) the Felsic unit, and five minor units: (4) the Trachyandesite unit, (5) the Conglomerate unit, (6) the Pumice unit, (7) the Intrusive unit, and (8) the Limestone unit. The Mafic unit is the oldest and forms the foundation of the island, consisting of both subaerial and subaqueous lava flows and pyroclastic deposits of alkali basalt and trachybasalt. Overlying the Mafic unit, there is a thin, minor unit of trachyandesite lava flows (Trachyandesite unit). The Breccia unit unconformably overlies the older rocks and consists of crudely stratified breccias (block flows/block-and-ash flows) of vitrophyric dacite, which represent subaerial near-vent facies formed by gravitational and/or explosive dome collapse. The breccias commonly contain clasts of alkali basalt, indicating the nature of the underlying substrate. The Felsic unit comprises the central part of the island, composed of rhyolite lava flows and domes, separated from the rocks of the Breccia unit by a flat-lying unconformity. Following a quiescent period, limited felsic pyroclastic activity produced minor valley-fill ignimbrites (Pumice unit). The rocks of Providencia can be geochemically and stratigraphically subdivided into an older alkaline suite of alkali basalts, trachybasalts, and trachyandesites, and a younger subalkaline suite composed dominantly of dacites and rhyolites. Isotopically, the alkali basalts together with the proposed tholeiitic parent magmas for the dacites and rhyolites indicate an origin by varying degrees of partial melting of a metasomatized ocean-island basalt–type mantle that had been modified by interaction with the Galapagos plume. The dacites are the only phenocryst-rich rocks on the island and have a very small compositional range. We infer that they formed by the mixing of basalt and rhyolite magmas in a lower oceanic crustal “hot zone.” The rhyolites of the Felsic unit, as well as the rhyolitic magmas contributing to dacite formation, are interpreted as being the products of partial melting of the thickened lower oceanic crust beneath Providencia. U-Pb dating of zircons in the Providencia volcanic rocks has yielded Oligocene and Miocene ages, corresponding to the ages of the volcanism. In addition, some zircon crystals in the same rocks have yielded both Proterozoic and Paleozoic ages ranging between 1661 and 454 Ma. The lack of any evidence of continental crust beneath Providencia suggests that these old zircons are xenocrysts from the upper mantle beneath the Lower Nicaraguan Rise. A comparison of the volcanic rocks from Providencia with similar rocks that comprise the Western Caribbean alkaline province indicates that while the Providencia alkaline suite is similar to other alkaline suites previously defined within this province, the Providencia subalkaline suite is unique, having no equivalent rocks within the Western Caribbean alkaline province.

Providencia is the only example of subaerial volcanism on the Lower Nicaraguan Rise. In this volume, the authors examine this volcanism and the geological history of the western Caribbean and the Lower Nicaraguan Rise, whose origin and role in the development of the Caribbean plate has been described as enigmatic and poorly understood. While the Providencia alkaline suite is similar to others within the Western Caribbean Alkaline Province, its subalkaline suite is unique, having no equivalent within the province. In order to unravel its complex history and evolution, this volume presents new and previously published results for the geology, geochemistry, petrology, and isotopic ages from the Providencia island group.

Abstract Erebus volcano, Antarctica, is the southernmost active volcano on the globe. Despite its remoteness and harsh conditions, Erebus volcano provides an unprecedented and unique opportunity to study the petrogenesis and evolution, as well as the passive and explosive degassing, of an alkaline magmatic system with a persistently open and magma-filled conduit. In this chapter, we review nearly five decades of scientific research related to Erebus volcano, including geological, geophysical, geochemical and microbiological observations and interpretations. Mount Erebus is truly one of the world's most significant natural volcano laboratories where the lofty scientific goal of studying a volcanic system from mantle to microbe is being realized.

Abstract Geodetic surveying is a core volcano monitoring technique. Measurements of how the crust deforms can give valuable insight into the mechanisms and processes that drive an eruption, and the way in which they change. Various geodetic observables, including ground deformation and gravity changes, have been recorded on Montserrat throughout the eruption. Instrumentation and surveying networks used to make such measurements have evolved significantly since 1995, providing increasingly accurate and robust observations. The detailed research that has been facilitated by these rich geodetic datasets has illuminated many aspects of the Soufrière Hills Volcano (SHV) and demonstrated eruptive mechanisms that are relevant to the study of other volcanoes. We have compiled a history of the geodetic study of the eruption on Montserrat, detailing the development of surveying techniques, network design and data processing since 1995. We then underline some of the key geodetic observations and review some of the most significant research that has contributed to our understanding of this volcanic system. Finally, we apply a series of typical deformation inversion models to deformation observations, and discuss the parameter sensitivity of such modelling approaches and how confidently they can be applied to identify the characteristics of the mechanisms feeding the eruption.

Abstract We use histories of magma efflux and surface deformation from a continuously operating global positioning system (cGPS) to quantitatively constrain magma transfer within the deep crustal plumbing of the Soufrière Hills Volcano (SHV). Displacement records reach a surface aperture of approximately 11 km and are continuous over three successive cycles of eruption followed by a pause spanning 1995–2008, and we focus on data of this time period. The assumed geometry and flow topology is for twin vertically stacked spherical chambers pierced by a vertical conduit that transmits magma from the deep crust to the surface. For a compressible magma column within an elastic crust we use mean deformation rates measured at between 6 and 13 cGPS stations for periods of effusion then repose and the time-history of magma efflux to define optimal chamber depths and basal magma input. The best fit for a constrained constant basal input to the system is obtained for chambers at 5 and 19 km, and a constant magma input rate of approximately 1.2 m 3 s −1 . Eruptive then pause episodes are, respectively, characterized by synchronous deflation then inflation of both shallow and deep chambers. Throughout this period of three repeated episodes of effusion then repose, the total effusive volume ( c. 0.95 km 3 dense rock equivalent, DRE) has been sourced half from the lower chamber ( c. 0.5 km 3 ) and half from below this chamber ( c. 0.45 km 3 ). A consistent observation, repeated through three episodes, is that the eruption restarts as the shallow chamber regains its original volume following the pause and that eruption rearrests when the shallow chamber has deflated by a small but constant volume change ( c. 16–22 Mm 3 ). This magmatic metering is consistent with a control on eruption periodicity that involves overpressured breaching of the shallow chamber followed by underpressured sealing. We contrast these observations with other contemporary models that have consistently placed an upper chamber at a depth of approximately 5–6 km, and deeper chambers at 12 km and deeper.

Puerto Rico and the northern Virgin Islands define the eastern terminus of the Greater Antilles, which extend eastward from offshore eastern Central America to the Lesser Antilles volcanic arc and mark the boundary between the Caribbean and North America plates. In Hispaniola, Puerto Rico, and the northern Virgin Islands, the Puerto Rico trench and the Muertos trough define the northern and southern limits of the plate boundary zone, respectively. Three microplates lie within the boundary zone: (1) the Gonave in the west; (2) the Hispaniola in the center; and (3) the Puerto Rico–northern Virgin Islands in the east. Results from Global Positioning System (GPS) geodesy conducted in the region since 1994 confirm the presence of an independently translating Puerto Rico–northern Virgin Islands microplate whose motion is 2.6 ± 2.0 mm/yr toward N82.5°W ± 34° (95%) with respect to the Caribbean. Geodetic data are consistent with east-west extension of several mm/yr from eastern Hispaniola to the eastern Virgin Islands. Extension increases westward with the most, 5 ± 3 mm/yr, accommodated in the Mona rift, confirming earlier GPS geodetic results. East-west extension of 3 ± 2 mm/yr also is observed across the island of Puerto Rico, consistent with composite focal mechanisms and regional epicentral distributions. Although the loci of extension are not known, similarity of GPS-derived velocities among sites in eastern Puerto Rico suggests the active structures lie west of the San Juan metropolitan area. Reactivation of the Great Northern and Southern Puerto Rico fault zones as oblique normal faults with right-lateral slip is a possibility. East-west extension of 2 ± 1 mm/yr also must exist between eastern Puerto Rico and Virgin Gorda, which likely is attached to the Caribbean plate. These extensional belts allow eastward transfer of slip between North America and the Caribbean from the southern part of the plate boundary zone in the west to the northern segment in the east. Motions along or across any of the individual subaerial structures of Puerto Rico are ≤2 mm/yr. The Lajas Valley in the southwest, where microseismicity is greatest, is the locus of highest permissible on-land deformation. Northwest-southeast to east-west extension of 2 ± 1 mm/yr is also observed across the Anegada Passage.