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Sierra Negra volcano began erupting on 22 October 2005, after a repose of 26 years. A plume of ash and steam more than 13 km high accompanied the initial phase of the eruption and was quickly followed by a~2-kmlong curtain of lava fountains. The eruptive fissure opened inside the north rim of the caldera, on the opposite side of the caldera from an active fault system that experienced an m b 4.6 earthquake and~84 cm of uplift on 16 April 2005. The main products of the eruption were an`a`a flow that ponded in the caldera and clastigenic lavas that flowed down the north flank. The`a`a flow grew in an unusual way. Once it had established most of its aerial extent, the interior of the flow was fed via a perched lava pond, causing inflation of the`a`a. This pressurized fluid interior then fed pahoehoe breakouts along the margins of the flow, many of which were subsequently overridden by`a`a, as the crust slowly spread from the center of the pond and tumbled over the pahoehoe. The curtain of lava fountains coalesced with time, and by day 4, only one vent was erupting. The effusion rate slowed from day 7 until the eruption's end two days later on 30 October. Although the caldera floor had inflated by~5 m since 1992, and the rate of inflation had accelerated since 2003, there was no transient deformation in the hours or days before the eruption. During the 8 days of the eruption, GPS and InSAR data show that the caldera floor deflated~5 m, and the volcano contracted horizontally~6 m. The total eruptive volume is estimated as being~150×10 6 m 3 . The opening-phase tephra is more evolved than the eruptive products that followed. The compositional variation of tephra and lava sampled over the course of the eruption is attributed to eruption from a zoned sill that lies 2.1 km beneath the caldera floor.

At Cotopaxi volcano, Ecuador, rhyolitic and andesitic bimodal magmatism has occurred periodically during the past 0.5 Ma. The sequential eruption of rhyolitic (70-75% SiO 2 ) and andesitic (56-62% SiO 2 ) magmas from the same volcanic vent over short time spans and without significant intermingling is characteristic of Cotopaxi's Holocene behavior. This study documents the eruptive history of Cotopaxi volcano, presenting its stratigraphy and geologic field relations, along with the relevant mineralogical and chemical nature of the eruptive products, in order to determine the temporal and spatial relations of this bimodal alternation. Cotopaxi's history begins with the Barrancas rhyolite series, dominated by pumiceous ash flows and regional ash falls between 0.4 and 0.5 Ma, which was followed by occasional andesitic activity, the most important being the ample andesitic lava flows (∼4.

Tungurahua, located in the Eastern Cordillera of the Ecuadorian Andes, is a 5023 m-high active volcano, notable for its extreme relief (3200 m), steep sides, and frequent eruptive cycles. From 1999 until 2006 Tungurahua experienced short periods of low to moderate strombolian activity, characterized by fire fountaining, explosions, frequent ash falls and debris flows, and no PDC events. Without warning, Tungurahua initiated PDC activity on 15-16 July 2006, which became more intense on the night of 16-17 August 2006, which is the focus of this study. Continuous monitoring of Tungurahua has employed seismic (both short period and broadband (BB) instruments), SO 2 gas emission (COSPEC and DOAS), and geodetic methods (EDM, tilt meters, and GPS), in addition to thermal imagery (airborne and ground-based). Acoustic flow monitors (AFM) installed to monitor lahar activity were important for detecting PDC events. Acoustic signals were monitored at Riobamba, 40 km to the SW, as well as by infrasound sensors at Tungurahua's BB seismic stations. Based on geophysical parameters, visual observations, and PDC deposit characteristics, four phases of distinct eruptive activity are recognized during the 16-17 August episode. Phase I (08H37 to 21H13 of 16 Aug.) (local time) experienced low to moderate strombolian activity with occasional high energy impulsive bursts and small PDCs. Phase II (21H13-16 Aug.) to (00H12-17 Aug.) was characterized by a number of discrete events with high amplitude seismoacoustic signals, followed by the generation of larger PDCs that overran monitoring stations and had velocities of 30-33 m/s. After midnight, Phase III (00H12 to 01H14) saw an intense period of unrelenting eruptive activity corresponding to the episode's greatest energy release. It was characterized by subplinian activity accompanied by a series of high energy outbursts and constant low frequency