During July, the main crater continued to weakly emit fumarolic gases with temperatures of 90°C. These escaped along the crater's NE, N, W, and S walls, and small landslides along the crater's N and S walls have partly covered the crater floor. Also, in the central crater, new points appeared on the N side where sulfur-rich gases gently escaped; gas temperatures measured 88°C.

Condensate chemistry and fumarole temperatures taken during 1992 through July 1998 appear on figure 4. Elevated SO₄ was measured in condensate sampled on 22 April 1998 (figure 4). This coincided with the appearance of high-frequency earthquakes. Although the SO₄ concentration declined in the next condensate sample (22 July), there were 68 high-frequency earthquakes from April through July (table 3).

Figure 4. Turrialba fumarolic temperature (top) and condensate chemistry (bottom) shown for the interval 14 January 1992 to 22 July 1998. The measured pH appears as a series of points; Cl and SO4 concentrations, as shaded and unshaded bars, respectively. Note that the horizontal scale ("Sample date") is non-linear and that the lower right-hand vertical axis (Cl and SO4 concentration) is broken into segments of dissimilar scale. Courtesy of OVSICORI-LAQAT (National University).

Table 3. Seismicity registered at Turrialba's seismic station VTU, January-July 1998. Courtesy of OVSICORI-UNA.

During 1998 the seismic system (station VTU, 0.5 km NE of the active crater) registered fewer than either 100 microseisms a month or five low-frequency earthquakes a month (table 3). The growth in the number of high-frequency earthquakes was thought to be related to the above-mentioned appearance of the new fumaroles in the central crater. Microseisms were generally weak, with amplitudes below 10 mm.

Geological Summary. Turrialba, the easternmost of Costa Rica's Holocene volcanoes, is a large vegetated basaltic-to-dacitic stratovolcano located across a broad saddle NE of Irazú volcano overlooking the city of Cartago. The massive edifice covers an area of 500 km2. Three well-defined craters occur at the upper SW end of a broad 800 x 2200 m summit depression that is breached to the NE. Most activity originated from the summit vent complex, but two pyroclastic cones are located on the SW flank. Five major explosive eruptions have occurred during the past 3500 years. A series of explosive eruptions during the 19th century were sometimes accompanied by pyroclastic flows. Fumarolic activity continues at the central and SW summit craters.

Information Contacts: E. Fernández, V. Barboza, M. Martinez, E. Duarte, R. Van der Laat, E. Hernández, and T. Marino, Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica.