The volcano remained in a non-eruptive state. During May and June 1999, low-intensity seismic activity has continued, similar to that reported in previous months. Instruments registered 90 volcano-tectonic earthquakes; their depths were between 0.30 and 19 km in relation to the top of Galeras; the energy released was 8.35 x 10¹⁴ ergs. They were spread broadly over the region.

Three small earthquake swarms were recorded. The first on 8 May consisted of five recorded events located to the NW of Galeras, and released 1.57 x 10¹³ ergs. The second on 14 May consisted of 15 events again located to the volcano's NW, and released 7.24 x 10¹⁴ ergs of energy. The third swarm on 29-30 June consisted of 15 events located NE of Galeras, and released 3.78 x 10¹³ ergs. The greatest magnitude event occurred during the second swarm on 14 May; it had a coda magnitude of 2.5 and a depth of 8 km.

In addition, 27 long-period earthquakes and six tremor episodes were registered, with a released energy value of 7.00 x 10¹² ergs. Tremor episodes had dominant frequencies of 2.4-2.6 Hertz. Periodic temperature measurements were taken in diverse fumaroles of the active crater; in May and June they registered highs of 122-390°C.

During May and June, Radon-222 emissions collected in the soil at about six stations arcing around the W, N, and E sides of the volcano in a 5-10 km radius from it showed no significant changes with respect to previous months. The highest Radon-222 emissions during the interval were recorded in late May at a station ~5 km NE of the summit (station "Sismo2"), where the maximum value was 3,369 pCi/l. For comparison, during May-June many stations yielded values in the range 750-1,200 pC/l.

Geological Summary. Galeras, a stratovolcano with a large breached caldera located immediately west of the city of Pasto, is one of Colombia's most frequently active volcanoes. The dominantly andesitic complex has been active for more than 1 million years, and two major caldera collapse eruptions took place during the late Pleistocene. Long-term extensive hydrothermal alteration has contributed to large-scale edifice collapse on at least three occasions, producing debris avalanches that swept to the west and left a large open caldera inside which the modern cone has been constructed. Major explosive eruptions since the mid-Holocene have produced widespread tephra deposits and pyroclastic flows that swept all but the southern flanks. A central cone slightly lower than the caldera rim has been the site of numerous small-to-moderate eruptions since the time of the Spanish conquistadors.

Information Contacts: Observatorio Vulcanológico y Sismológico de Pasto (OVSP), Carrera 31, 18-07 Parque Infantil, PO Box 1795, Pasto, Colombia (URL: https://www2.sgc.gov.co/volcanes/index.html).