Volcanic and sedimentary processes in phreatomagmatic volcanoes

• Main Author: Leys, Clyde Andrew
• Format: Thesis
• Language: English
• Published: University of Leeds 1982
• Subjects: Inner Crater; Iceland
• Online Access: https://etheses.whiterose.ac.uk/292/; https://etheses.whiterose.ac.uk/292/1/uk_bl_ethos_344386.pdf

Phreatomagmatic volcanoes form when ascending magma explosively interacts with surface or groundwater at shallow depths. Three types of phreatomagmatic activity are recognised- phreaticp phreatomagmatic (s. s. ) and surtseyan - based on the degree of involvement of magma with water and the depth of the interaction. Phreatic maars and phreatomagmatic tuff-rings are underlain by pipe-like diatremes but these structures are poorly developed or absent in surtseyan tuff-rings. Comparisons of phreatomagmatic volcanoes with their eroded diatreme equivalents, which contain subsided subaerially-deposited material, allow a model for activity of this type to be constructed. The Saefell tuff-ring SW Iceland is a surtseyantype structure whose crater remained open to the sea during most of its activity, allowing easy access of water to the magma. Base-surges sourced partly from directed blasts, formed large dunes with internal structures indicating deposition by density currents whose flowpower decreased with time and with distance from the vent. Syndepositional slumping and minor en masse collapse of crater deposits formed a pile of massive tuffs above which subsequent surge and airfall activity deposited a nestedg inner crater rim. The Medano tuff-ring Tenerife, is a phreatomagmatictype structure whose crater contains reworked tuffs deposited during subsidence into the underlying diatreme. Initial activity ejected much country rock material as magma contacted groundwater at depth but with time eruptions became more strombolian, as water was used up or failed to gain access to the vent. Surges were less common than in the Saefell eruption because the Medano water: magma ratio and explosion depth less often fulfilled the optimum conditions for surge production. The East Lothian diatremes in Scotland are subdivided into two groups on the basis of their infilling. The Red group diatremes contain high proportions of sediment and represent the subsided products of phreatic maars which erupted into a pile of water-rich ...