Reventador volcano is a young andesitic cone situated in the western corner of an older caldera, one of many volcanic edifices that comprise the Eastern Cordillera of Ecuador's volcanic arc. The horseshoe-shaped avalanche caldera has diameters of 4 km (north-south) and 6 km (northwest-southeast) and is breached on its east-southeast side. ... The interior walls of the caldera are very steep and have heights that range from 50 to 400 m above the southeast-sloping floor of the amphitheater.
The young cone has a symmetrical shape with steep slopes of up to 34 degrees. Eruptive products from the volcano are slowly filling the caldera, especially the laharic deposits along the caldera's northern and southern limits. Older lava flows that originated in the summit crater or from flank vents are observed everywhere within the caldera and down to the Coca river. Prior to the 2002 eruption, the cone had a height of 3,560 m, 1,500 m above the lowest eastern part of the caldera floor (2000 m); as such, it was slightly higher than the nearby western caldera rim. An elongate crater, approximately 200 m long, adorned the summit and was characterized by mild, but persistent fumarolic activity during the past 26 years.
The detailed history of this volcano is not well known, due to its remoteness, inaccessibility, and persistent cloudy weather. It is estimated that Reventador volcano has had, at least, 16 eruptions between 1541 and 2002. During the XX century, eruptions were reported in the period 1898-1912, 1926-1929, 1944, 1958-1960, 1972, 1973-1974, and 1976. These eruptions were characterized by small pyroclastic flows, blocky lava flows, moderate-size debris flows (lahars), and ash falls that deposited only a few millimeters of ash in the Interandean Valley.
On the day of the 2002 eruption workers at a camp only 8 km away had a good view of the cone, and had not noticed a steam column on the cone in the days preceding the eruption. However, they did see an energetic steam column at daybreak on November 3. With the onset of the eruption, steam generation became continuous, which suggests that the cone had not dried out beforehand. Consequently, it would seem that the magma must have ascended the conduit very rapidly, probably from depths of <10 km, and only came in contact with groundwater in the early morning hours of November 3.
The climactic eruption began at 0912 h and lasted about 45 minutes. It largely destroyed the summit and crater of the cone, leaving deep notches on the NNW and SSW sides of the summit where the crater had been. During the first minutes of this stage, at least, 5 significant andesitic pyroclastic flows (PF) were produced.
The initial pyroclastic flows were pumice-poor, lithic-rich flows that traveled ESE out of the breached caldera and down steep slopes to reach the Coca river. In doing so, they crossed principal oil and gas pipelines, destroying a new, empty oil pipeline and carrying away small bridges on the main gravel highway leading to the oilfields. Another pipeline filled with crude oil under pressure did not break, but was displaced ~100 m downslope from its path. This was the closest that the pyroclastic flows came to human activities. No casualties were reported. One small house and 20 head of cattle were buried by the pyroclastic flow. These flows covered approximately 12 km2, chiefly in the caldera, and had a total bulk volume of about 55 million m3.
The first lava flow surfaced on November 6 which was confirmed by distant photographs taken on that day and later by NOAA thermal images taken at 1900 h on November 7. The blocky lava flow originated in the newly formed crater, descended the southeast side of the cone, and attained a width of several hundred meters and a thickness of ~15 m. From there, it traveled southeastward down the caldera floor near the southern caldera wall. A November 8 overflight visually confirmed a 4 km long lava flow, meaning the lava flow was initially very fast -- in excess of 80 meters per hour. By December 3 it had traveled 5 km but was advancing at only ~1-3 meters per day. In late December, observers at a distance thought that the lava had all but stopped.
The total bulk volume of erupted material in November 2002 (tephra, lava, and pyroclastic flows) was approximately 0.37 km3. Based upon the bulk volume of tephra ejected, the eruption qualifies as a small VEI 4 event. It is one of the largest explosive eruptions of an Ecuadorian volcano in the past two centuries.
Additional lava flows occured late 2004 (2.5 km), March 2005 (4.5 km), June 2005 (? km), September 2005 (1 km), and November 2005 (1 km). The explosive activity since mid 2005 has been accompanied by strong long-period seismic and acoustic transients and has alternatively manifested both Strombolian type explosions and ash rich Vulcanian blasts with columns reaching 8.5 km above the vent. Reprives between explosive events have generally lasted hours to days.
While violent eruptions like November 2002 are less likely to occur now that the Reventador conduit is more open, as of early 2006 relatively large Vulcanian eruptions may be expected to occur with little or no warning. These could create considerable objective hazard for individuals visiting the caldera and especially on the cone.
