PHIVOLCS reported that steam emissions were seen rising from Mayon's crater from 5 to 12 January during periods of clearer weather; meteorological clouds often prevented observations of the summit. Weak incandescence from the crater was occasionally seen at night. The majority of the seismic signals originated from rockfalls and detached lava fragments rolling down the flanks. Sulfur dioxide emissions fluctuated between 670 and 1,900 tonnes per day. On 12 January, PHIVOLCS noted that ground deformation measurements showed a deflationary trend compared to a 2 December 2009 survey. These measurements, along with decreased seismicity and sulfur dioxide output, prompted PHIVOLCS to lower the Alert Level to 2 on 13 January.

Geological Summary. Symmetrical Mayon, which rises above the Albay Gulf NW of Legazpi City, is the most active volcano of the Philippines. The steep upper slopes are capped by a small summit crater. Recorded eruptions since 1616 CE range from Strombolian to basaltic Plinian, with cyclical activity beginning with basaltic eruptions, followed by longer periods of andesitic lava flows. Eruptions occur predominately from the central conduit and have also produced lava flows that travel far down the flanks. Pyroclastic density currents and mudflows have commonly swept down many of the approximately 40 ravines that radiate from the summit and have often damaged populated lowland areas. A violent eruption in 1814 killed more than 1,200 people and devastated several towns.

Source: Philippine Institute of Volcanology and Seismology (PHIVOLCS)