During January-July 2000 Tungurahua volcano experienced continuous but relatively mild activity with occasional lava fountaining. There were periods (hours to days) of relative calm during June and July.

The volcano continues to generate a variety of seismic events, most events being the long-period (LP) type. Two episodes of volcano-tectonic (VT) events were observed; one between late January and early March, and one less intense event between early May and mid-June. Epicenters for these events were across the top of the volcano's cone with focal depths at 3-13 km. Hybrid events, whose waveforms consist of a short, higher-frequency onset followed by lower-frequency, larger-amplitude signals, were most abundant in January and February (~50 events/week), partially coinciding with the greater VT activity. Subsequently these events diminished to 1-2 events/week, except for a brief swarm in early April.

Events of classical LP waveform were frequent, varying from ~400 events/week in January, ~600 in February, ~400 in March, ~600 in April, ~500 in May, and ~400 in June. A sharp increase to ~950 events/week was observed in July. Some of the LP events (3.7-4.0 Hz) were located tentatively at depths of 7-10 km below the crater. However, the great majority of LP events (1.5-3.3 Hz) were 3-7 km deep. They were often associated with explosion clouds or forceful emissions of ash-and-steam within 1-3 seconds of the seismic onset, suggesting a high-level origin.

Explosions, recognized principally by their impulsive onset, were more frequent during January and February (~80-90 events/week), but in subsequent months dropped to ~20-30 events/week, with many accompanied by a sonic boom. Reduced displacement values for the explosions typically were 5-10 cm², and occasionally 12-18 cm².

Low-frequency tremor with spectral frequencies between 0.5-1.6 Hz, but monochromatic at times, were observed in April and May, but only sporadically in June and July. During the period from the 2nd week of April through the 2nd week of May, the low-frequency episode coincided with lava fountaining in the summit crater. The fountains, comprised of the continuous ejection of incandescent material 100-500 m into the air, lasted hours; sustained roaring and surf-like noises heard 12 km away.

The constant glow of incandescent material in the crater, which was observed frequently in late 1999, was seen only occasionally during August, possibly due to unfavorable weather conditions. Better viewing conditions in late June and July confirmed that incandescent lava still remained in the crater or immediately below it.

The emissions have consisted of a permanent, grayish-white to light-gray column of steam with varying amounts of fine-grained ash that commonly rise less than 1 km above the crater. Explosions or strong emissions have consisted of blocks being thrown hundreds of meters into the air and by the formation of Vulcanian-like eruption clouds that are medium-to-dark gray in color and sometimes with a mushroom shape. The clouds have reached as high as 5 km above the summit. Primarily, easterly winds have carried the very fine ash to the W and WSW, but occasionally anywhere in the azimuthal arc between NW and SW. Both national and international flights reported the ash plume. The ash deposits were several centimeters thick on the lower W flank of the cone, but only several millimeters in the agriculturally important lands farther W.

Ballistic blocks were vesicular, black, glassy andesite containing phenocrysts of olivine, plagioclase, augite, and hypersthene, in a glassy matrix with 10-20% microlites. More recent samples had fewer olivines and larger augites. Chemical analyses of these blocks as well as collected ash gave the following typical values: SiO₂ ~58.5%, K₂O ~1.72%, MgO ~3.9%, Ni ~33 ppm, and Cr ~65 ppm.

COSPEC monitoring since November was hindered by heavy cloud cover. Following the consistently high SO₂ flux values of 6,000-8,000 metric tons/day (t/d) during September-October 1999, values decreased to an average of 3,000-4,000 t/d in November-December 1999. Values then rose to ~8,000 t/d in January and subsequently dropped to an average of ~1,000-2,000 t/d in June and July 2000. An exception to this trend was an increase to ~4,000 t/d observed in April-May, 2000, which coincided with the lava fountaining episode. In general, higher SO₂ values seem to be associated with greater tremor activity.

Monthly water analyses of hot springs at both the N and S bases of the edifice have not shown any variation in temperature, pH, conductivity, nor in the concentrations of SO₄, Cl^-, Na+, CO₃^--, Ca⁺⁺, Mg⁺⁺, and K⁺, since chemical monitoring began in 1992 and since the activity on Tungurahua began in July 1999.

Lahars coincided with the rainy season and became frequent in October and November 1999; they rapidly cut the main highway at every stream crossing along the western half of the cone (the area of greatest ash fall). Occasional rains from December to June generated flows of debris. The main highway to Baños and to the Amazon Basin was frequently blocked for hours due to lahar deposits.

In general, the activity appeared to be subsiding. However, during the 1916-18 eruptive period the volcano experienced 1.5 years of little activity between major eruptions. An orange alert is still in effect. In the past, Tungurahua typically generated both Merapi- and St. Vincent-like nuées ardentes. The W sector of Baños (17,000 inhabitants) lies at the mouth of a canyon that starts near the summit of the volcano, 9 km away and 3,000 m above the town.

Following the evacuation of Baños on 17 October 1999, the town remained abandoned until late December (BGVN 25:01). As of August 2000, about 80% of the population had returned and tourism has re-established itself.

Geological Summary. Tungurahua, a steep-sided andesitic-dacitic stratovolcano that towers more than 3 km above its northern base, is one of Ecuador's most active volcanoes. Three major edifices have been sequentially constructed since the mid-Pleistocene over a basement of metamorphic rocks. Tungurahua II was built within the past 14,000 years following the collapse of the initial edifice. Tungurahua II collapsed about 3,000 years ago and produced a large debris-avalanche deposit to the west. The modern glacier-capped stratovolcano (Tungurahua III) was constructed within the landslide scarp. Historical eruptions have all originated from the summit crater, accompanied by strong explosions and sometimes by pyroclastic flows and lava flows that reached populated areas at the volcano's base. Prior to a long-term eruption beginning in 1999 that caused the temporary evacuation of the city of Baños at the foot of the volcano, the last major eruption had occurred from 1916 to 1918, although minor activity continued until 1925.

Information Contacts: Geophysical Institute (Instituto Geofísico), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador.