Rounded [[tephra]] particles in this size range are called accretionary lapilli if they consist of tiny ash grains stuck together. Ash sometimes form such rounded particles in an eruption column or cloud, owing to moisture or electrostatic forces. | Rounded [[tephra]] particles in this size range are called accretionary lapilli if they consist of tiny ash grains stuck together. Ash sometimes form such rounded particles in an eruption column or cloud, owing to moisture or electrostatic forces. |