Volcanic ash is finely divided magma, with a diameter of <2 mm. It forms in eruptions from at least three distinct mechanisms

Chaiten 2008 ash

Augustine ash P Izbekov

Spreadsheets from Chuck Conner that are very useful for learning (download):

Estimating Magma Viscosity

Conduit conditions

Software for Numerical Models of Phreatomagmatic Explosions from K Wohletz