Relationships between pre-eruptive conditions and eruptive styles of phonolite–trachyte magmas

Phonolitic eruptions can erupt either effusively or explosively, and in some cases develop highly energetic events such as caldera-forming eruptions. However, the mechanisms that control the eruptive behaviour of such compositions are not well understood. By combining pre-eruptive data of well studied phonolitic eruptions we show that the explosive–effusive style of the phonolitic magma is controlled by the amount of volatiles, the degree of water-undersaturation and the depth of magma storage, the explosive character generally increasing with pressure depth and water contents. However, external factors, such as ingestion of external water, or latter processes occurring in the conduit, can modify the starting eruptive dynamic acquired at the levels of magma ponding.

► We have determined the viscosities of phonolite–trachyte magmas at pre-eruptive conditions.
► Melt viscosities of these compositions are 103.8 ± 0.4 Pa·s which are close magma viscosities.
► The explosive or non-explosive aspect of phonolite–trachyte magmas is acquired at storage depth.
► The eruptive dynamic can be changed during the time of the eruption by external factors.