Volcanic Ash the unconsolidated pyroclastic material consisting of finely (under 2.0 mm diameter) comminuted fragments of rock and lava which are ejected explosively from a volcano. The term ash, which dates from the time when a volcano was thought to be a burning mountain, is a misnomer. Reworked volcanic ash of Quaternary age has been observed at Kumarpur in Rajshahi, Aliarhat river section in Bogra, within upper Madhupur clay residuum at Gazipur in Dhaka, western flank of Lalmai anticline, Comilla, Shahbazpur at Habiganj structure and Niamatpur upazila of Naogaon district.
The ash bed is white, brownish yellow and yellowish brown, slightly to moderately compact, slake in water with bubble effusion, silty to sandy silt, and are easily crushable into powdery mass by thumb pressure. It resembles white clay. In Rajshahi and Bogra, this unit is covered by a 20 to 30 cm alluvium with a very little weathering while in Naogaon it is found within the Barind clay residuum at an average depth of 60 cm. Its mineralogy is complex containing quartz, feldspar, calcite, biotite, rutile, zircon epidote, smectite, illite, kaolinite and most significantly Opal-CT. Opal-CT can be derived from amorphous Opal-A, which is usually formed and precipitated from the solution of volcanic sources. The percentages of silica of these powdery mass sediments also range from 84 to 90. A number of localities in Eastern India as well as in the bay of bengal, have similar ash beds, which are related to the Toba eruption of Sumatra, Indonesia, 75,000 years ago. Being a neighbouring country, it is possible Bangladesh received and preserved this ash bed. Volcanic ash layers of different localities could play a dominant role as a marker horizon to correlate the Quaternary sediments and establish the Quaternary stratigraphy of this region. [Sifatul Quader Chowdhury]