Time and space variations in magmatic and phreatomagmatic eruptive processes at Easy Chair (Lunar Crater Volcanic Field, Nevada, USA)

摘要

The products of monogenetic volcanoes often record complex sequences of eruptive processes. Easy Chair volcano (Lunar Crater Volcanic Field, Nevada, USA) was formed by a monogenetic eruption along a ∼2.5-km-long series of en echelon fissure vents. Hawaiian to Strombolian fountains along the fissures dominated initial activity, producing a series of agglomerate ramparts. Focusing of eruptive activity to two central vents and the formation of two overlapping scoria cones followed the early phase. Fountain-fed lavas from those cones merged to form a channel that fed lava onto a flow field at the foot of the cones. Focusing of subsurface magma flow toward the central conduits may have reduced magma flux in the remaining fissures, and the southern segment(s) entered a phase of phreatomagmatic explosions that destroyed the early agglomerate rampart and formed a maar and tephra ring composed of lapilli tuff rich in clasts derived from pre-Easy Chair lavas and early agglomerates. The eruption closed with a minor phase of magmatic activity that deposited scoria lapilli and bombs on top of the phreatomagmatic deposits. The eruptive sequence indicates that relatively low hazard Strombolian to Hawaiian activity can be replaced by more hazardous phreatomagmatic explosions well into a monogenetic eruption.