Northern Central Andes, Peru
Structure and Evolution
Volcan Sabancaya (S) is the youngest component of a complex consisting of three centers; Nevado Hualca Hualca to the north (HH), Sabancaya, and Nevado Ambato (NA) to the south. The edifice is built in the saddle between the two older centers, and consists of extensive flank lava flows (L) which form an apron around the simple cone. Although the upper reaches of the cone are covered by a permanent ice-cap, several short stubby lava flows distributed radially around a youthful summit crater can be discerned. One of these flows has been dated with 14C and yielding an age of ca. 5400 yr BP
The eruptive histories of individual volcanoes in this complex are difficult to determine. A recent set of cores drilled in a peat-bog near the volcano reveal an ashfall from Sabancaya/Ambato between 1200 and the 1400's AD and one from ca. 8,550 yr BP. Historical reports record moderate eruptions in 1752 and 1784, and the subsequent 200 years of dormancy was broken by an almost continuos eruption from May 1990 until 1998.
Between December 1986 and June 1988, a lava dome had grown in the crater and Sabancaya semi-continuously exhibited strong fumarolic activity during which glowing tephra ejections were visible at night. Analysis of TM images acquired in March 1987 and April 1988, showed a change in crater area and shape, but no thermal anomaly. Between the 22nd and 25th June 1988, voluminous steam pulses were emitted every few minutes. Shuttle photography (Mission STS-26 Sept, 1988) showed that the activity of June 1988 had abated and there was no visible sign of activity in the summit crater.
On May 29th 1990, the low level activity (fumarolic) at Sabancaya was interupted by an explosive phase of activity (Norman Banks, Cindy Stein: USGS Vancouver, pers. comm. June, 1990). Periodic loud sonic booms accompanied eruptions throwing ash and "glowing blocks" into the air suggest vulcanian activity. Initially activity was only 2 or 3 times a day but later reports (June 3rd, 1990) indicated explosions every 20 minutes or so, with a strong smell of sulfur, and an increased noise level. The resulting plume streamed NE and ENE with the prevailing winds and was dense to 5 km and ash fall was recorded up to 10 km away. Further explosive eruptions of ash followed this one, lasting until 1998.
Most recently, small explosive eruptions of gas and ash occurred in 2000 and 2003.
Sabancaya presents similar, but less severe hazard to Coropuna. About 30,000 people leave along the Rio Colca and Sihuas valleys near the complex. Mud and lava flow hazard is obvious, and pyroclastic hazard cannot be ruled out. Mudflows from the north-eastern flank could enter the Majes drainages (which hosts a major canal network resulting from an ogoing irrigation project in the area), while mudflows from the other flanks could enter the south flowing Rio Siguasi. During the 1988 activity cattle died in the areas adjacent to Sabancaya, either directly from poisonous fumes or from eating contaminated foliage (SEAN June 1988). Lavas erupted from vents in the erosional amphitheater on the northern flanks of Hualca Hualca have entered the Rio Majes canyon in the past and dammed it completely - the satellite images show flat-lying lake sediments deposited behind a breached dam, which also forms a prominent nick-point in the canyon profile. Breach of the dam may have caused serious flooding downstream at the time.
de Silva S.L., & Francis, P.W., 1990. Potentially active volcanoes of southern Peru - observations using Landsat Thematic Mapper and Space Shuttle imagery. Bull. Volcanol. 52, 286-301.
Gerbe, M.-C., & Thouret, J.-C. 2004. Role of magma mixing in the petrogenesis of tephra erupted during the 1990Ð98 explosive activity of Nevado Sabancaya, southern Peru. Bulletin of Volcanology, 66, 541-561.
Scientific Event Alert Network Bulletin, 1988. V 13, No. 6 p.10-11.
Thouret, J.C., Juvigne, E., Marino, J., Moscol, M., Legeley-Padovani, A., Loutsch, L., Davila, J., Lamadon, S., & Rivera, M. 2001. Late Pleistocene and Holocene tephro-stratigraphy and chronology in southern Peru. Proyecto Arqueologico Condesuyos, Boletin de la Mision Arqueologica Andina, Vol. 1 (ed. by M. Ziolkowski and L. Belan Franco), pp. 215Ð239. Ed. by Mision Arqueologica Andina, University of Warsaw, Warsaw.