PHIVOLCS reported that field observations of Taal conducted at the E sector inside the main crater lake on 5 April 2011 showed that steaming at the thermal area was weak. The water level had receded 3 mm and the water temperature slightly increased from 30 to 30.5 degrees Celsius. Since the previous measurement on 29 March, the pH value increased indicating that the water had become slightly less acidic. Gas measurements conducted last January, February, and March yielded carbon dioxide emission values (in tonnes per day) of 2,250, 1,875, and 4,670, respectively.

On 9 April PHIVOLCS noted that after 31 March the number of earthquakes gradually rose and the depths become more shallow (1-4 km). Steaming at the N and NE sides of the main crater occasionally intensified and was occasionally accompanied by hissing sounds. The Alert Level was raised to 2 (on a scale of 0-5) because of increased seismicity and carbon dioxide emissions. PHIVOLCS warned tourists and residents to avoid Volcano Island. According to news articles, about 100 families had volunteered to evacuate; about 7,000 people remained.

Geological Summary. Taal is one of the most active volcanoes in the Philippines and has produced some powerful eruptions. The 15 x 20 km Talisay (Taal) caldera is largely filled by Lake Taal, whose 267 km2 surface lies only 3 m above sea level. The maximum depth of the lake is 160 m, with several submerged eruptive centers. The 5-km-wide Volcano Island in north-central Lake Taal is the location of all observed eruptions. The island is composed of coalescing small stratovolcanoes, tuff rings, and scoria cones. Powerful pyroclastic flows and surges have caused many fatalities.

Sources: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Philippine Daily Inquirer