Characteristics of some basaltic pyroclastics

The pyroclastic deposits of strombolian/hawaiian-type basaltic activity are readily distinguished from those ofsurtseyan type (i.e., the type exemplified by the opening stages of the Surtsey eruption of 1963–4) by being coarsergrained and better sorted. Moreover the fragments in the former have an external form in part controlled by surface tension — the termachnelith is proposed for such particles — whereas in the latter the fragments are clasts bounded by fracture surfaces. Other differences include the common occurrence of accretionary lapilli and « bomb sags » and the generally more thin-bedded character of the surtseyan deposits. The distinction between these two types of basaltic activity is important in palaeogeographic reconstructions.