ABSTRACT Two spatially separated areas of Neogene volcanic rocks are located on the northeast limb of the Mount Diablo anticline. The southernmost outcrops of volcanics are 6 km east of the summit of Mount Diablo in the Marsh Creek area and consist of ~12 hypabyssal dacite intrusions dated at ca. 7.8–7.5 Ma, which were intruded into the Great Valley Group of Late Cretaceous age. The intrusions occur in the vicinity of the Clayton and Diablo faults. The rocks are predominantly calc-alkaline plagioclase biotite dacites, but one is a tholeiitic plagioclase andesite. Mercury mineralization was likely concomitant with emplacement of these late Miocene intrusions. The northernmost outcrops of Neogene volcanic rocks occur ~15 km to the north of Mount Diablo in the Concord Naval Weapons Station and the Los Medanos Hills and are probably parts of a single andesite flow. A magnetometer survey indicates that the flow originated from a feeder dike along the Clayton fault. The lava flow is flat-lying and occupies ancient stream channels across an erosional surface of tilted Markley Sandstone of middle Eocene age. New radiometric dates of the flow yield an age of 5.8–5.5 Ma, but due to alteration the age should be used with caution. The flow is a calc-alkaline andesite rich in clinopyroxene and plagioclase. What appear to be uplifted erosional remnants of the flow can be traced northeastward in the Los Medanos Hills across a surface of tilted Cenozoic rocks that eventually rest on formations as young as the Lawlor Tuff dated at 4.865 ± 0.011 Ma. This stratigraphic relationship suggests that the andesite flow is probably late Pliocene in age and was impacted by the more recent uplift of the Los Medanos Hills but postdates the regional folding and faulting of the rocks of Mount Diablo. In terms of timing, location, and composition, the evidence suggests these two areas of dacitic and andesitic volcanics fit into a series of migrating volcanic centers in the California Coast Ranges that erupted following the northward passage of the Mendocino Triple Junction.

ABSTRACT Over the past 150 years, Mount Diablo has served as a window into the evolving understanding of California geology. In the 1800s, geologists mapped this easily accessible peak located less than 100 km (62 miles) from the rapidly growing city of San Francisco and the geology departments at the University of California at Berkeley and Stanford University. Later, the mountain served as a focal point for investigating San Francisco Bay area tectonics. The structural interpretation of the up-thrusting mechanisms has evolved from a simple compressional system involving a few local faults to a more complex multifault and multiphase mountain-building theory. The stratigraphic interpretation and understanding have been advanced from a general description of the lithologies and fossils to a detailed description using sequence stratigraphy to define paleogeographic settings and depositional regimes.

Abstract We divide the narrative history of legends on volcanoes, descriptions of volcanic landscape, eruptions and products, and the gradual development of volcanology in Costa Rica as a formal science, into several periods or stages. Each stage is discussed in detail in the text: (a) pre-Columbian period (11 500 BC–AD 1502); (b) colonization period (1502–1718); (c) first period (1719–1887); (d) second period (1888–1925); (e) third period (1926–62), and (f) fourth period (1963–present). The social and economic contribution of volcanology research is also discussed.

Abstract Poás Volcano (Costa Rica) resumed phreatic activity on 24 March 2006 after twelve years of quiescence. From March 2006 to June 2010, the initial phase of the ongoing eruption cycle, 110 phreatic eruptions were reported. This study presents the temporal variations in the chemical and isotopic (δD and δ 18 O) compositions of Laguna Caliente crater lake for the period prior to the eruption until June 2010. No systematic relationship with the phreatic eruptive activity exists. A combined mass and Cl budget analysis enables quantification of the seepage rate (7 kg s −1 ), the input rate of the ‘volcanic fluid’ ( Q f ), and the Cl concentration (Cl e ) in the evaporation plume (13 300 mg l −1 ). A modelling procedure for variable seepage rates leads to more realistic estimates of 50–100 kg s −1 , which better represent the observed lake water chemistry, hence suggesting dynamic fluid recycling between the lake and the underlying magmatic–hydrothermal system. The high Cl concentration in the evaporation plume and the dynamic fluid recycling at the lake bottom characterize Laguna Caliente as an ‘open-air’ fumarole, discrediting water chemistry as an efficient monitoring tool at the classic monitoring frequency. A conceptual model of phreatic eruptions is linked to our observations.

Arenal Volcano is a small (~1750 m above sea level, ~10 km 3 ) stratovolcano that continuously erupted between July 1968 and October 2010. During this long-lasting eruption (over 42 yr), a large volume of material—~5.6 × 10 8 m 3 of dense rock equivalent—has been extruded and has produced a thick and extended lava field, mainly on the western flank of the edifice. Measurements of ground deformation obtained using a network of dry-tilt stations are presented for the period 1986–2000. They show a continuous subsidence of the volcano with maximal amplitude on the western side. The load effect of the lava field is calculated and explains the largest part of the observed tilts. Once the data are corrected by this load effect, pressure source models are not supported by the observations and by quality criteria on the models. Although the dry-tilt data from Arenal Volcano give limited constraints on the deformation models, they are representative of a long period of activity that cannot be recovered by other means. Moreover, the corresponding interpretative model is consistent with results obtained by geotechnical studies and modern ground deformation methods like interferometric synthetic aperture radar (InSAR).