Eruption mechanisms in the back-arc Crater Basalt Volcanic Field, Northern Patagonia

HALLER MJ

Pucón

Congreso; International Association of Volcanology and Chemistry of the Earth's Interior, General Assembly; 2004

IAVCEI

The Crater Basalt Volcanic Field in Northern Patagonia (42º S) developed on glacial outwash terraces and poured out in Holocene stream valleys. It has an elongated form of NW-SE direction, 60 km length, and an extension of 700 km2. Nine main effusive centers erupted about 2.27 km3 of lava (2.05 km3 equivalent of solid rock, presuming 10% of vesicularity).The volcanic morphology is in general terms well preserved. Most of the vents are spatter or cinder cones, the flows are pahoehoe type and the thickness ranges between 1 to 10 meters. An eruptive collapse mechanism has been recognized in one of the eruptive centers. A lateral slump with developing of a well defined scar on the cone side and extensional translational sliding of abundant rafted blocks affected a scoria spatter cone. Large (> 3 m) block were rafted out more than 400 meters, carried away on the surface of an agglutinated basal lava flow. An almost equidimensional star shaped crown-like over-steepened lava with rear concentric lava flows encircles a gently sloping spatter cone. These features may suggest a squeeze-out lateral extrusion with basal coalescence mechanism for another scoria cane. The extrusion would have been generated in response to plastic deformation of an unstable hot scoria cone, with outward and upward lava displacement. Alignment of spatter cones, traceable for 7.6 km indicates initial fissure-fed eruption followed by sealing and developing of individual Lava fountains. All the described eruptive mechanisms support rapid building of scoria cones owing to high magma production rate.