Tambora volcano (Sumbawa Island, Indonesia) - Smithsonian / USGS Weekly Volcanic Activity Report for 24 August-30 August 2011 (New Activity / Unrest)

Based on visual observation and seismic data, CVGHM reported an increase in activity at Tambora during the previous five months. Ground-based observers at an observation post in Tambora village noted dense white plumes rising 50-75 m above the caldera rim during April and June, but no plumes during May or July. In August dense white plumes rose 20 m above the caldera rim.
Seismicity started to increase in April and continued to increase through August. On 30 August the Alert Level was raised to 2 (on a scale of 1-4).

Source: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM)
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From: Global Volcanism Program, 2011. Report on Tambora. In: Mayberry, G (ed.), Weekly Volcanic Activity Report 24 August-30 August 2011. Smithsonian Institution and US Geological Survey.