The following summary describes Copahue's crater lake activity during 1992-2000 as presented by Varekamp and others (2001). Copahue erupted sporadically in the 1990's with a series of explosions, and later in July 2000. The 1992 and 1995 eruptions were phreatic and ejected fragmented, hydrothermally altered rocks, and copious amounts of liquid sulfur. In contrast, the July 2000 eruption (BGVN 25:06, 25:09) was phreato-magmatic and was characterized by explosions that ejected incandescent juvenile material and chilled sulfur fragments; dark ash covered an area up to 50 km from the source.

Data gathered during 1997-2000 by Varekamp and others (2001) suggested that Copahue acts as a "beehive" volcano, meaning that void spaces were generated by near-congruent rock dissolution, and these spaces were subsequently filled by accumulating liquid sulfur and precipitated silica. During the phreatic eruptions of 1992 and 1995, much of that liquid sulfur was ejected; in contrast the phreato-magmatic eruptions of 2000 left the hydrothermal system largely intact. The RFE/Cl ratio (RFE = rock-forming elements = Ca + Na + Mg + K + Al + Fe) measured in the crater lake water increased dramatically in July 2000. This spike was interpreted by Varekamp and others (2001) to be a result of a recent intrusion of fresh magma and therefore more interaction between water and newly solidified rock.

Varekamp and others (2001) concluded that "Integration of [data from the Upper Rio Agrio river] and the modeled volcanic fluxes into the crater lake provide the following parameters for the whole Copahue system in November 1999: an energy flux of ~32 MW, an equivalent sulfur gas input of ~344 t SO₂/d, a net rock removal rate of about 12,000 m³/yr, and an elemental sulfur accumulation rate of about 11,600 m³/yr." The eruption of large amounts of liquid sulfur in 1992 and 1995 strongly supported inferences about gradual elemental sulfur accumulation inside the volcano. Analyses also suggested that the eruptions of Copahue during 2000 were preceded by a dramatic increase of volcanically-derived elements into the crater lake. The authors further concluded that "The evolution of the crater lake fluids during the twentieth century suggest a gradual awakening of Copahue volcano". Continued monitoring of crater lake temperature and composition would be useful to assess future volcanic activity.

Other studies. Varekamp and others (2004) concluded that Copahue represented "a rare example of the occurrence of hydrothermal fluids on the surface of the earth from a deep source with a strong magmatic signal." Varekamp (2004) noted that Copahue "has a volcanic spring-fed acid river with deposits of red hematite in the streambed and abundant gypsum mineralization, and these fluids are saturated with K-jarosite. This riverbed may be a modern terrestrial analog of the Opportunity Rover landing site on the planet Mars." Another paper (Varekamp and others, 2006) described the geochemistry and isotopic characteristics of the Caviahue-Copahue volcanic complex; they discuss the roles of crust and sediments from the subducting slab as contributions to the magma systems.

According to the Chilean National Service of Geology and Mining (SERNAGEOMIN), Copahue had been quiet since its last major eruption in 2000 until recently. A new eruption took place in mid-2012; subsequent reports will discuss this event.

References. Varekamp, J.C., 2004, Copahue Volcano: A Modern Terrestrial Analog for the Opportunity Landing Site?, Eos, v. 85, no. 41, p. 401 and 407.

Varekamp, J.C., Ouimette, A.P., Hermán, S.W., Bermúdez, A., and Delpino, D., 2001, Hydrothermal element fluxes from Copahue, Argentina: A "beehive" volcano in turmoil, Geology, v. 29, no. 11, p. 1059-1062.

Varekamp, J.C., Ouimette, A.P., and Kreulen, R., 2004, The magmato-hydrothermal system of Copahue volcano, Argentina, in Wanty, R.B., and Seal, R.R., (eds), Water Rock Interaction: Proceedings of the Eleventh International Symposium on Water Rock Interaction, WRI-11, 27 June-2 July 2004, Saratoga Springs, NY, v. 1, p. 215-218, Taylor and Francis Group, London.

Varekamp, J.C., deMoor, J.M., Merrill, M.D., Colvin, A.S., Goss, A.R., Vroon, P.Z., and Hilton, D.R., 2006, Geochemistry and isotopic characteristics ofthe Caviahue-Copahue volcanic complex, Province of Neuquen, Argentina, Geological Society of America Special Papers 2006, v. 407, pp. 317-342.

Geological Summary. Volcán Copahue is an elongated composite cone constructed along the Chile-Argentina border within the 6.5 x 8.5 km wide Trapa-Trapa caldera that formed between 0.6 and 0.4 million years ago near the NW margin of the 20 x 15 km Pliocene Caviahue (Del Agrio) caldera. The eastern summit crater, part of a 2-km-long, ENE-WSW line of nine craters, contains a briny, acidic 300-m-wide crater lake (also referred to as El Agrio or Del Agrio) and displays intense fumarolic activity. Acidic hot springs occur below the eastern outlet of the crater lake, contributing to the acidity of the Río Agrio, and another geothermal zone is located within Caviahue caldera about 7 km NE of the summit. Infrequent mild-to-moderate explosive eruptions have been recorded since the 18th century. Twentieth-century eruptions from the crater lake have ejected pyroclastic rocks and chilled liquid sulfur fragments.

Information Contacts: J.C. Varekamp, Department of Earth & Environmental Sciences, 265 Church St., Wesleyan University, Middletown CT 06459-0139 USA; Servicio Nacional de Geología e Minería (SERNAGEOMIN), Av. Santa María 0104, Casilla 10465, Santiago, Chile (URL: http://www.sernageomin.cl/).