After 200 yr of repose, Pacaya Volcano resumed Strombolian activity in 1961 and has remained active until the time of this writing (2013). A three-dimensional map of 50 yr of nearly continuous activity of Pacaya depicts an accumulation of homogeneous, crystal-rich high-Al basalt on the west side of a preexisting cone. The material erupted is loose and welded spatter, volcanic ash, and 249 pahoehoe and a‘a lava flows, most of which were extruded in a few days, and most have extended less than 2 km in length from vents near the 2500-m-high summit down slopes of 20°–33°. The configuration of lava flows makes up a rigid, web-like network that welds the asymmetrical, steep western slope of an expanded Pacaya cone. The vents have fed the lava flows, forming a sieve-like pattern where lava leaks out. The cone contains a complex network of intrusive feeders, which fill and empty with lava, degas, and drain back. The volcano has shown explosive lava fountaining and effusive periods of activity and often exhibits both, as summit eruptions occur while lava drains from the cone. Lava flows and pyroclastic units from collapse-related avalanches and tephra fall tend to alternate. The overall length of lavas is limited, so that inhabited areas below the cone on most flanks are unlikely to be reached by flows, although topographic barriers, which blocked the flow of lava to the closest villages north of Pacaya, are now filled, so that lavas of moderate length (~2 km) could reach towns to the north under some conditions. The volcano is known to have experienced catastrophic explosive collapse in the last few thousand years. The current cone itself may be unstable because the new material has mostly asymmetrically loaded the west side of an old cone, and collapse to the west may be more likely because of mass imbalance and because of persistent activity that opens paths and accumulates on that side. Collapse to the west would threaten significant populations. Pacaya's past eruptions lasted centuries, with repose intervals of similar length, so the current activity may continue for another century or more. Overall, Pacaya is a complex of overlapping basaltic cones, and its pattern of activity provides insight into the early stages of composite cones such as nearby Agua, Fuego, Atitlán, and Santa María, all larger and older cones on the volcanic front of Guatemala with Pacaya.

The Pinacate volcanic field is ~330 km SSW of Phoenix, and it is a popular destination for volcanology and planetary geology field trips. The volcanic field, located on the Pinacate Biosphere Reserve in Sonora, Mexico, is a 1500 km 2 basaltic field including a shield volcano, lava tubes, maars, a tuff cone, cinder cones, pāhoehoe and ‘a‘ā lava flows as young as 12 ka, and phreatomagmatic constructs as young as 32 ka. We developed an image-based set of exercises for a 2 day field trip focusing on (1) Crater Elegante, a maar crater, (2) pāhoehoe and ‘a‘ā flows near Tecolote Cone campground, (3) the complex eruptive history of Mayo (cinder) Cone, and (4) Cerro Colorado tuff cone. This paper discusses exercises to teach concepts in visible and radar image interpretation and planetary volcanology, and provides an overview of the field trip.

The Mesoarchean Pongola Supergroup is a thick volcano-sedimentary succession that is unique in the early record of Earth's history because of the nature and composition of the lava flows that, in addition to basalts, have a high proportion of intermediate and acid compositions. The succession contrasts with other contemporaneous volcanic deposits in southern Africa and in other parts of the world that gave rise to greenstone belts by having a low state of deformation and a uniformly low grade of metamorphism. The White Mfolozi inlier is a deeply eroded river section that exposes spectacular structures and textures of two sequences of volcanic rocks, the lower Nhlebela Formation and the upper Agatha Formation. The Nhlebela Formation contains pillowed successions, peperite-like breccias, and extensive fragmental deposits formed as a result of hydroclastic eruption. The shallow water hydromagmatic deposits developed into amygdaloidal pahoehoe units higher in the succession as the lava eruptions raised the ground level. The Agatha Formation, studied in two traverses, commenced with the formation of a hydroclastic tuff formed by the emplacement of a high-Mg basalt magma interacting with unconsolidated sediment. Subaqueous eruptions rapidly progressed into subaerial deposits consisting initially of aa lavas that then developed into pahoehoe lavas higher in the succession. The pahoehoe lavas in the Agatha Formation preserve spectacular ropy surfaces. The internal structures of the flow lobes and distribution of vesicles indicate that lava inflation was the dominant mechanism of emplacement. Therefore important processes that are observed in modern and recent subaerial and continental basalts also operated in the mid-Archean. Changes in structures and styles of the pahoehoe lava flow lobes can be linked to magma compositions. Compositions include basalts, Fe-rich basalts, siliceous Mg basalts, andesites, and dacites. Trace elements identify at least four flows made up of 75 flow lobes that make up the 210-m-thick Agatha Formation. Geochemical tectonic discrimination diagrams fail to resolve the tectonic setting because of crustal contamination, but field observations indicate it is likely that the depositional setting in this area was a subsiding continental margin.

The effects of primary porosity on fluid flow during contact metamorphism were studied in basalts from central East Greenland. The gabbroic Skaergaard magma intruded interbedded massive and aa basalts with mean macroscopic primary porosities of 4% and 11%, respectively. Heat transport from the cooling gabbros led to three metamorphic mineral zones within 1 km of the contact: the actinolite + chlorite zone beyond 250 m, where the mineral assemblage records peak temperatures (7) of ≤550 °C; the pyroxene zone ( T = 700–850 °C); and the olivine zone, within 10 m ( T > 850 °C). In the actinolite + chlorite zone, aa clasts record more extensive mineralogic alteration of igneous minerals than do massive samples. Extents of prograde recrystallization in the olivine and pyroxene zones are 100% in both flow morphologies, but modal volumes of retrograde minerals in the pyroxene and olivine zones are higher in aa units. Extents of prograde reactions do not correlate with primary porosity because they were solid-solid reactions that occurred at high temperatures, whereas retrograde alteration involved low-temperature hydration reactions in which the availability of H 2 O as a reactant, as controlled by porosity, probably influenced reaction extent. In the pyroxene zone, where mineralogic and textural evidence suggests oxygen isotope exchange equilibrium, whole-rock δ 18 O compositions are 1.7‰ to 3.0‰ and are similar or lower in aa units than in massive units at any given distance from the contact. The isotopic ratios suggest average time-integrated fluid fluxes of 3.6 and 4.0 × 10 3 mol cm −2 in massive and aa units, respectively, if fluid infiltration occurred during prograde metamorphism. Similar values were computed assuming that part of the isotopic exchange was retrograde. These differences imply that time-averaged matrix permeability was ~10% higher in aa flow breccias.