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Active Haleakala volcano on the island of Maui is the second largest volcano in the Hawaiian Island chain. Prominently incised in Haleakala's slopes are four large (great) valleys. Haleakala Crater, a prominent summit depression, formed by coalescence of two of the great valleys. The great valleys and summit crater have long been attributed solely to fluvial erosion, but two significant enigmas exist in the theory. First, the great valleys of upper Keanae/Koolau Gap, Haleakala Crater, and Kaupo Gap are located in areas of relatively low annual rainfall. Second, the axes of some valley segments are oblique for long distances across the volcanic slopes. This study tested the prevailing erosional theory by reconstructing the volcano's topography just prior to valley incision. The reconstruction produces a belt along the volcano's east rift zone with a morphology that is inconsistent with volcanic aggradation alone, but it is readily explained if it is assumed the surface was displaced along scarps formed by a giant landslide on Haleakala's northeastern flank. Although the landslide head location is well defined, topographic evidence is lacking for the toe and lateral margins. Consequently, the slope failure is interpreted as a sackung-style landslide with a zone of deep-seated distributed shear and broad surface warping downslope of the failure head. Maximum downslope displacement was likely in the range of 400-800 m. Capture of runoff at the headscarps formed atypically large streams that carved Haleakala's great valleys and explains their existence in low-rainfall areas and their slopeoblique orientations. Sackung-style landslides may be more prevalent on Hawaiian volcanoes than previously recognized. [ABSTRACT FROM AUTHOR]

Sunset Crater volcano, located ~25 km northeast of downtown Flagstaff (Arizona, USA) in the San Francisco volcanic field, represents an interesting case of highly explosive monogenetic volcanism. Sunset Crater is a young (ca. 1085 CE) scoria cone, ~290 m high, that produced 0.52 km³ dense rock equivalent (DRE) of basaltic magma (0.16, 0.12, and 0.24 km³ DRE for the scoria cone, lava flows, and tephra deposit, respectively). The activity developed in three distinct phases: an initial fissure phase, followed by a highly explosive phase, and a final low-explosivity waning phase. The first phase was characterized by the opening of a 10-kmlong fissure, which produced the Gyp Crater spatter mounds and the Vent 512 lava flow ~10 km SE of the main scoria cone, followed by lava fountaining activity that produced the first tephra layer (unit 1, 0.01 km³ DRE). During the second highly explosive phase, the activity migrated to the northwest to evolve into a single-vent eruption, which formed the main scoria cone. The central vent activity was initially characterized by variable eruptive styles, which started the process of cone building, including deposition of a second tephra layer (unit 2, 0.01 km³ DRE), and the effusion of the two main lava flows (Bonito and Kana'a). Following the initiation of effusive activity, the eruption increased in explosivity to produce three subplinian tephra layers from the central vent (units 3, 4, and 5), which emitted more than 0.22 km³ DRE of basalt, with associated eruption columns >20 km high. By the end of this phase, the Kana'a lava flow was almost completely emplaced, but the Bonito flow continued to grow, forming a 3-km-diameter flow field. A final waning phase produced several tephra units, commonly discontinuous and reworked, with <0.01 km3 DRE cumulative volume. The comparatively large volume of tephra, the high eruptive columns, and high mass eruption rates make Sunset Crater the most explosive monogenetic eruption studied to date (volcano explosivity index [VEI] 3-4). Sunset Crater volcano represents an interesting case of monogenetic volcanism in that its activity was characterized by highly explosive eruptive phases up to subplinian in scale and intermittent episodes of lava effusion that heavily affected prehistoric communities in the area. Explosive basaltic volcanism should be considered when assessing volcanic hazards in continental settings, such as in the western United States. [ABSTRACT FROM AUTHOR]

Catastrophic caldera collapse following a large volcanic eruption or subsurface magmatic outflow is associated with very large displacements on ring faults. The slip resistance or friction on the ring faults is, thus, a key mechanical property that controls the collapse. However, the dynamic friction on the ring faults during caldera collapse remains unknown. Here, we show that dynamic weakening of ring faults caused by frictional melting can play a critical role in the catastrophic collapse of volcanoes. From direct observations of the ring fault of the ancient Jangsan caldera in southeastern Korea, we identified a layer of solidified frictional melt (or pseudotachylyte) measuring ~0.1 m thick. The dynamic friction (expressed as the ratio of shear to normal stress) estimated from the layer, based on high-velocity shear tests and analyses of microstructures and materials, was lower than 0.1. Given the low resistance, it follows that an extraordinarily large fault slip (>100 m) causing a large earthquake was possible during the formation of the ancient caldera. We conclude that the dynamic weakness of ring faults should be incorporated in mechanical models of caldera collapses. [ABSTRACT FROM AUTHOR]

We present the geologic history of the High Rock caldera complex (HRCC; Nevada, USA), a major mid-Miocene silicic center associated with flood basalt volcanism. Based on 70 40Ar/39Ar ages and new 1:24,000- and 1:100,000-scale geologic mapping, we document that between ca. 16.5 and 15.5 Ma a minimum of ~700 km³ of rhyolitic magma erupted from the HRCC, covering an area of ~8300 km² in northwestern Nevada and southern Oregon (USA). The volcanism immediately followed eruption of the Steens flood basalt in the region, and was contemporaneous with eruption of compositionally similar rhyolites from the McDermitt volcanic field (MVF) to the east. The HRCC and MVF together mark the starting point for the eastward- younging trend of voluminous rhyolitic calderas of the Snake River Plain-Yellowstone trend. The HRCC comprises 4 major calderas, 24-40 km in diameter, that young from north-northeast to south-southwest: the Virgin Valley caldera formed on eruption of the 16.38 Ma high-silica alkali rhyolite Idaho Canyon Tuff; the overlapping Badger Mountain caldera collapsed due to eruption of the 16.34 Ma crystal-rich, low-silica rhyolite Summit Lake Tuff; eruption of the moderately peralkaline high-silica alkali rhyolite Soldier Meadow Tuff at 16 Ma resulted in formation of the Hanging Rock caldera; and the Cottonwood Creek caldera formed on eruption of the newly recognized, 15.70 Ma tuff of Yellow Rock Canyon, which is zoned from high-silica alkali rhyolite to trachyte. The four calderas contain caldera fill deposits, including pumice and ash falls, lahars, phreatomagmatic deposits, and well-bedded lacustrine deposits, which preserve diverse mid-Miocene fossil fauna and flora. Au and U mineralization developed along ring fractures of the Virgin Valley and Cottonwood Creek calderas. After silicic volcanism largely ceased, trachyte, trachyandesite, and alkalic basaltic lavas erupted through the caldera lakes. Intense silicic volcanism at the HRCC during the interval 16.4-15.5 Ma overlapped the eruption of the Steens and Columbia River Basalts, strongly suggesting a petrogenetic link. We propose that the HRCC and MVF caldera centers are localized where dikes of Steens flood basalt encountered transitional crust west of the craton with a composition and thickness that allowed significant partial melting, based on the O and Nd isotopic values of the rhyolites, which require involvement of crustal melts in their origin. Steens Basalt eruptions largely ceased in the area by the time the oldest caldera-forming ignimbrites erupted at both the HRCC and MVF, indicating that once large silicic magma bodies aggregated in the crust, they intercepted flood basalt dikes. We suggest that the roots of the HRCC and MVF are composed of large volumes of gabbroic intrusions and cumulates formed by fractional crystallization of HRCC magmas, which strengthened the middle crust beneath the calderas; major basin-bounding normal faults are diverted around them, but outflow ignimbrites are prominently offset by Basin and Range faults. [ABSTRACT FROM AUTHOR]