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Report on Guagua Pichincha (Ecuador) — August 1999

Guagua Pichincha

Bulletin of the Global Volcanism Network, vol. 24, no. 8 (August 1999)
Managing Editor: Richard Wunderman.

Guagua Pichincha (Ecuador) Phreatic explosions, seismic increases, and elevated hazard status

Please cite this report as:

Global Volcanism Program, 1999. Report on Guagua Pichincha (Ecuador) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 24:8. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199908-352020

Guagua Pichincha


0.171°S, 78.598°W; summit elev. 4784 m

All times are local (unless otherwise noted)

The Instituto de Geofísico reported that during the interval July through August, the 2-km-diameter, horse-shoe-shaped caldera at the summit of Guagua Pichincha continued to emit frequent steam-and-ash eruptions. The caldera contains an active dome, and fumaroles there typically sent visible steam plumes to heights of tens to hundreds of meters, and occasionally to about a kilometer. The SO2 flux on 1 July measured under 10 tons/day.

Near the end of the previous reporting period, on 26 June, a sequence of earthquakes was felt in the volcano's NE sector by residents of the N part of the Capital and in the town of San Antonio de Pichincha. Earthquakes with magnitudes over 3.9 caused alarm and concern in local populations.

On 8 July a park guard observing the crater area reported a new crater located in the NW sector of the 1981 explosion crater. Surrounding the crater for 20 m lay a deposit of reddish- yellow ejecta. The crater's initial diameter was 8 m but the next day it reached ~20 m. A few days later, on 11 July, observers saw into the crater and viewed 3 plumes escaping from new orifices arranged along a line in the NW part of the crater. The daily update on 15 July noted that reduced fumarolic activity enabled observers to see both a recent ash layer extending out to 200 m around the explosion crater and an accumulation of mud agglomerates on a terraced portion of the caldera. Similar observations of new intracrater deposits were also noted on 30 July. The headwaters of the westward-flowing Cristal river, which is monitored by both instruments and observers, acquired a coating of fine ash in early August. On 17 August the ash became mobile and contributed to mud flows of undisclosed size that proceeded down the river.

Daily reports mentioned occasional phreatic events. For example, two phreatic explosions took place on 3 July. The smaller had a reduced displacement (RD) of 5.3 cm2; the larger, 14.1 cm2 and sustained tremor for 40 minutes. The seismic sources for these explosions resided at 1-3 km depth below the caldera's E border. A phreatic explosion on 19 July was relatively small (RD of 4.3 cm2) but it was preceded by an hour of long-period earthquakes located beneath the town of Lloa (~12 km SSE of the caldera); 10 minutes prior to the explosion there were 7 volcano-tectonic earthquakes and one hybrid earthquake. The seismic signal associated with the event saturated the near-source seismic stations for ~40 minutes and tremor continued afterwards. Another small phreatic explosion took place on 26 July (RD, 3.2 cm2) and associated tremor prevailed for 10 hours. A large phreatic explosion on 2 August (RD, 21 cm2) vented at the recently formed intra-caldera crater E of the dome. The explosion sent a column up to ~ 2 km high despite strong winds. It was seen by residents of Quito.

Similar small phreatic explosions took place frequently during the rest of August. For example they occurred on 5 August, 7 August (RD, 2.3 cm2), twice on 10 August (RDs, 1.3 and 1.1 cm2, column reaching 1.5 km), 12 August (RD, 5.2 cm2), twice on 14 August (RDs, < 2 cm2), 16 August (RD, < 1 cm2), 19 August (RD, 3.7 cm2). Other small (and occasionally moderate) phreatic eruptions occurred on 24, 27, 28, and 29 August.

An unusually large phreatic eruption took place at 0807 on 21 August (RD, 21 cm2). The resulting steam plume rose to a height of 6 km and ash fell to the volcano's S. After the main explosion, smaller exhalations yielded a plume to a height of 3 km.

A comparatively large, violent, cinder-bearing emission occurred on 31 August; it rose to 3.5 km and blew W. The associated seismic source was in the W caldera at 1.7 km depth. The emission marked the end of an 8-day interval that included three relatively large expulsions.

During the July-August interval authorities maintained a yellow alert hazard status. Although this report does not otherwise discuss September events, it was learned at press time that on 27 September the hazard status was raised to orange (their second-to-highest rank, table 4). Residents began to evacuate the S-flank town of Lloa and its vicinity. On 28 September the newspaper Diario Hoy reported that from 0345 the previous morning seismicity intensified, reaching 2-3 earthquakes a minute. Press reports suggested that local scientists interpreted this ongoing seismic swarm (several thousand earthquakes) as a manifestation of rising magma. More details will be provided in a subsequent Bulletin.

Table 4. Hazard status at Guagua Pichincha (translated from a more detailed one posted on the Instituto's web site [26 October 1998 revision]). The status during July to late September remained at yellow; on 27 September authorities raised the hazard status to orange and began some evacuations.

Alert Level Observations Duration Interpretation
White 0 Activity at background levels. Centuries, decades, or years. Non-eruptive repose.
White 1 Small increase over background in measured parameters. Years, months. Hydrothermal, magmatic, or tectonic disturbance(s) - No eruption imminent.
Yellow 2 Moderate increase in significant measured parameter(s) (seismic, temperature, phreatic eruptions, gases, deformation, etc.). Months, weeks. Possibly indicative of a magmatic intrusion that could lead to an eruption.
Yellow 3 Upscaling of earlier parameters; exit of gases, observed deformation. Weeks, days. Confirmed magmatic intrusion-possible eruption.
Orange 4 Intensive precursory activity that includes some combination of harmonic tremor, long-period earthquakes, accelerated deformation, dome growth, and explosions. Days, hours. Near-surface magma-probable explosive eruption.
Red 5 Eruption underway. In progress. Explosive eruption with the possibility of still more vigorous eruption(s) to follow.

Geological Summary. Guagua Pichincha and the older Pleistocene Rucu Pichincha stratovolcanoes form a broad volcanic massif that rises immediately W of Ecuador's capital city, Quito. A lava dome grew at the head of a 6-km-wide scarp formed during a late-Pleistocene slope failure ~50,000 years ago. Subsequent late-Pleistocene and Holocene eruptions from the central vent consisted of explosive activity with pyroclastic flows accompanied by periodic growth and destruction of the lava dome. Many minor eruptions have been recorded since the mid-1500's; the largest took place in 1660, when ash fell over a 1,000 km radius and accumulated to 30 cm depth in Quito. Pyroclastic flows and surges also occurred, primarily to then W, and affected agricultural activity.

Information Contacts: Instituto Geofísico, Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador; and Diario Hoy ("Hoy Digital,", URL: http://www.hoy.com.ec/).