MORPHOLOGICAL, STRUCTURAL AND TEXTURAL VARIATIONS IN THE 1988-1990 ANDESITE LAVA OF LONQUIMAY VOLCANO, CHILE

The 1988-1990 eruption of Lonquimay Volcano, Chile (38-degrees-S) formed a 10.2 km long andesite lava with a volume of 0.23 km3 over a period of 13 months. The lava extrusion rate decreased with time as chamber pressure and vent dimensions decreased. The velocity of the flow front decreased exponentially with distance from vent as a consequence of cooling and the increase of apparent viscosity at the flow front. The lava developed a central channel which decreased in width and depth with time. Three prominent lava levees were formed on each margin and resulted from abandonment as the channel decreased in width as a result of a rapid decrease of flow rate over the first 100 days of activity. A fourth major levee developed in February, during a brief period of flow rate increase down the main channel, but its walls were gradually exposed as the lava depth again decreased due to declining flow rate. The structure of lava levees depended on their age and longevity of the flow in the adjacent channel. Initial levees were formed in the first few days as the lava spread laterally and then retreated, leaving levees of massive lava. More mature rubble levees were formed during the next month by the lava pushing and then shearing aa and blocky breccia which formed on the cooling flow margin. Fragmentation and abrasion formed a characteristic zonation in the levees. A basal zone consists of very poorly sorted matrix-rich breccia with very rounded vesicular clasts and bimodal grain size distribution. The basal breccia zone strongly resembles block and ash flow deposits. This zone passes up into a zone of clast-supported clinker breccia which becomes increasingly matrix-poor and coarser with clasts becoming more angular upwards. The crest of the levee is composed of large (10-100 cm) angular to subangular blocks with no matrix. The zoned levees form after the active lava channel suddenly narrows. Lava depth initially increases and breccias are deposited on the channel margins and acquire the zoned structure by progressive shearing and accretion of clinkery aa breccia. The lava level then drops exposing the steep inner scarp of a levee. The most mature levee type formed in a long-lived channel over several months. The outer wall of the levee consists of zoned breccia, but the inner wall consists of a massive curving wall of strongly foliated lava with well-developed horizontal striations and ductile Reidel shears. The massive foliated facies is a consequence of prolonged flow which coats strongly sheared lava onto the inner levee wall. Scanning electron microscopy shows that the aa clinker clasts and foliated lava from the levee walls form at low melt fractions (much less than 15 %). In the last three months of the eruption the flow front ceased to advance but thickened as lava drained from proximal regions and intruded into the interior of the distal lava. The last stages of lava movement were characterized by updoming in the central channel. A lava surface feature, named here 'Armadillo structure', was formed by deformation of the cooler but still ductile lava crust. The deformation caused by underflow produced Reidel shears dipping upstream and doming of the lava due to rotation of the shear planes. The study demonstrates that lava morphology, structure and texture are strongly influenced by variations of effusion rate, local flow rate, channel topography and thermal maturity of the lava, which is reflected in downstream changes in viscosity.