EarthRef.org Digital Archive (ERDA) -- Evolution of a Seamount based on La Palma Extusive Series

Description
In this cartoon the evolution of a seamount is depicted based on a field study of the extrusive series on La Palma, Canary Islands. Four stages are distinguished: [A] Most seamounts on mature oceanic crust are formed in deep water and on top of the already deposited marine sediments. In the beginning building phases of a seamount, sills intrude into the soft sediments of lower density. This interaction of extruding lavas with sediments may produce peperites. [B] The bulk of the deep water stage deposits are characterized by pillow lavas and sills in a 1:1 ratio. Pillow fragment breccias with non-vesicular matrix material are deposited by grain flows on the flanks and in the aprons surrounding the seamount. The deep water stage ends with the beginning of explosive volcanism when the seamount has build-up sufficiently and reaches shallower water depths. [C] During the explosive stage that happens at intermediate and shallow water depths, more than 70% clastic materials are formed. The summit of the seamount is characterized by scoriaceous and often amoeboidal pillows, by poorly bedded and unsorted breccias with scoriaceous bomb and lapilli fragments and by minor volcanic ash deposits, normally in the matrix. The matrix is normally very much vesicular. Most materials are also redeposited on the flanks and aprons. Close to the shelf breaks on the seamount tops, the deposits are relatively thin units of pillow lavas, breccias and hyaloclastites. [D] The final stage is reached when the first subaerial eruption columns develop, as characterized by the deposition of spindle bombs, the oxidation of the fragments, and the appearance of relatively dense pillow basalts that have been outgassed almost completely subaerially before they entered the sea.