The Llullaillaco volcano, northwest Argentina: construction by Pleistocene volcanism and destruction by sector collapse

Llullaillaco is one of a chain of Quaternary stratovolcanoes that defines the present Andean Central Volcanic Zone (CVZ), and marks the herder between Chile and Argentina/Bolivia. The current edifice is constructed from a series of thick dacitic lava flows, forming the second tallest active volcano in the world (6739 m). K-Ar and new biotite laser 40Ar/39Ar step-heating dates indicate that the volcano was constructed during the Pleistocene (less than or equal to1.5 Ma), with a youngest date of 0.048 +/- 0.012 Ma being recorded for a fresh dacite flow that descends the southern flank. Additional 40Ar/39Ar measurements for andesitic and dacitic lava flows from the surrounding volcanic terrain yield dates of between 11.91 +/- 0.13 Ma and 5.48 +/- 0.07 Ma, corresponding to an extended period of Miocene volcanism which defines much of the landscape in this region. Major- and trace-element compositions of lavas from Llullaillaco are typical of Miocene-Pleistocene volcanic rocks from the western margin of the CVZ, and are related to relatively shallow-dipping subduction of the Nazca plate beneath northern Chile and Argentina. Oversteepening of the edifice by stacking of thick, viscous, dacitic lava Rows resulted in collapse of its southeastern Rank to form a large volcanic debris avalanche. Biotite 40Ar/39Ar dating of lava blocks from the avalanche deposit indicate that collapse occurred at or after similar to0.15 Ma, and may have been triggered by extrusion of a dacitic How similar to the one dated at 0.048 +/- 0.012 Ma. The avalanche deposits are exceptionally well preserved due to the arid climate, and prominent levees, longitudinal ridges, and megablocks up to 20-m diameter are observed. The avalanche descended similar to2.8 km vertically, and bifurcated around an older volcano, Cerro Rosado, before debouching onto the salt Rats of Salina de Llullaillaco. The north and south limbs of the avalanche traveled 25 and 23 km, respectively, and together cover an area of approximately 165 km(2). Estimates of deposit volume are hampered by a lack of thickness information except at the edges, but it is likely to be between 1 and 2 km(3). Equivalent coefficients of friction of 0.11 and 0.12, and excess travel distances of 20.5 and 18.5 km, are calculated for the north and south limbs, respectively. The avalanche ascended similar to 400 m where it broke against the western Rank of Cerro Rosado, and a minimum now velocity of similar to 90 m s(-1) can be calculated at this point; lower velocities of similar to 45 m s(-1) are calculated where distal toes ascend similar to 200 m slopes. It is suggested that the remaining precipitous edifice has a high probability Tot further avalanche collapse in the event of renewed volcanism. (C) 2001 Elsevier Science B.V. All rights reserved. Llullaillaco is one of a chain of Quaternary stratovolcanoes that defines the present Andean Central Volcanic Zone (CVZ), and marks the herder between Chile and Argentina/Bolivia. The current edifice is constructed from a series of thick dacitic lava flows, forming the second tallest active volcano in the world (6739 m). K-Ar and new biotite laser 40Ar/39Ar step-heating dates indicate that the volcano was constructed during the Pleistocene (less than or equal to1.5 Ma), with a youngest date of 0.048 +/- 0.012 Ma being recorded for a fresh dacite flow that descends the southern flank. Additional 40Ar/39Ar measurements for andesitic and dacitic lava flows from the surrounding volcanic terrain yield dates of between 11.91 +/- 0.13 Ma and 5.48 +/- 0.07 Ma, corresponding to an extended period of Miocene volcanism which defines much of the landscape in this region. Major- and trace-element compositions of lavas from Llullaillaco are typical of Miocene-Pleistocene volcanic rocks from the western margin of the CVZ, and are related to relatively shallow-dipping subduction of the Nazca plate beneath northern Chile and Argentina. Oversteepening of the edifice by stacking of thick, viscous, dacitic lava Rows resulted in collapse of its southeastern Rank to form a large volcanic debris avalanche. Biotite 40Ar/39Ar dating of lava blocks from the avalanche deposit indicate that collapse occurred at or after similar to0.15 Ma, and may have been triggered by extrusion of a dacitic How similar to the one dated at 0.048 +/- 0.012 Ma. The avalanche deposits are exceptionally well preserved due to the arid climate, and prominent levees, longitudinal ridges, and megablocks up to 20-m diameter are observed. The avalanche descended similar to2.8 km vertically, and bifurcated around an older volcano, Cerro Rosado, before debouching onto the salt Rats of Salina de Llullaillaco. The north and south limbs of the avalanche traveled 25 and 23 km, respectively, and together cover an area of approximately 165 km(2). Estimates of deposit volume are hampered by a lack of thickness information except at the edges, but it is likely to be between 1 and 2 km(3). Equivalent coefficients of friction of 0.11 and 0.12, and excess travel distances of 20.5 and 18.5 km, are calculated for the north and south limbs, respectively. The avalanche ascended similar to 400 m where it broke against the western Rank of Cerro Rosado, and a minimum now velocity of similar to 90 m s(-1) can be calculated at this point; lower velocities of similar to 45 m s(-1) are calculated where distal toes ascend similar to 200 m slopes. It is suggested that the remaining precipitous edifice has a high probability Tot further avalanche collapse in the event of renewed volcanism. (C) 2001 Elsevier Science B.V. All rights reserved. Llullaillaco is one of a chain of Quaternary stratovolcanoes that defines the present Andean Central Volcanic Zone (CVZ), and marks the herder between Chile and Argentina/Bolivia. The current edifice is constructed from a series of thick dacitic lava flows, forming the second tallest active volcano in the world (6739 m). K-Ar and new biotite laser 40Ar/39Ar step-heating dates indicate that the volcano was constructed during the Pleistocene (less than or equal to1.5 Ma), with a youngest date of 0.048 +/- 0.012 Ma being recorded for a fresh dacite flow that descends the southern flank. Additional 40Ar/39Ar measurements for andesitic and dacitic lava flows from the surrounding volcanic terrain yield dates of between 11.91 +/- 0.13 Ma and 5.48 +/- 0.07 Ma, corresponding to an extended period of Miocene volcanism which defines much of the landscape in this region. Major- and trace-element compositions of lavas from Llullaillaco are typical of Miocene-Pleistocene volcanic rocks from the western margin of the CVZ, and are related to relatively shallow-dipping subduction of the Nazca plate beneath northern Chile and Argentina. Oversteepening of the edifice by stacking of thick, viscous, dacitic lava Rows resulted in collapse of its southeastern Rank to form a large volcanic debris avalanche. Biotite 40Ar/39Ar dating of lava blocks from the avalanche deposit indicate that collapse occurred at or after similar to0.15 Ma, and may have been triggered by extrusion of a dacitic How similar to the one dated at 0.048 +/- 0.012 Ma. The avalanche deposits are exceptionally well preserved due to the arid climate, and prominent levees, longitudinal ridges, and megablocks up to 20-m diameter are observed. The avalanche descended similar to2.8 km vertically, and bifurcated around an older volcano, Cerro Rosado, before debouching onto the salt Rats of Salina de Llullaillaco. The north and south limbs of the avalanche traveled 25 and 23 km, respectively, and together cover an area of approximately 165 km(2). Estimates of deposit volume are hampered by a lack of thickness information except at the edges, but it is likely to be between 1 and 2 km(3). Equivalent coefficients of friction of 0.11 and 0.12, and excess travel distances of 20.5 and 18.5 km, are calculated for the north and south limbs, respectively. The avalanche ascended similar to 400 m where it broke against the western Rank of Cerro Rosado, and a minimum now velocity of similar to 90 m s(-1) can be calculated at this point; lower velocities of similar to 45 m s(-1) are calculated where distal toes ascend similar to 200 m slopes. It is suggested that the remaining precipitous edifice has a high probability Tot further avalanche collapse in the event of renewed volcanism. (C) 2001 Elsevier Science B.V. All rights reserved. Llullaillaco is one of a chain of Quaternary stratovolcanoes that defines the present Andean Central Volcanic Zone (CVZ), and marks the herder between Chile and Argentina/Bolivia. The current edifice is constructed from a series of thick dacitic lava flows, forming the second tallest active volcano in the world (6739 m). K-Ar and new biotite laser 40Ar/39Ar step-heating dates indicate that the volcano was constructed during the Pleistocene (less than or equal to1.5 Ma), with a youngest date of 0.048 +/- 0.012 Ma being recorded for a fresh dacite flow that descends the southern flank. Additional 40Ar/39Ar measurements for andesitic and dacitic lava flows from the surrounding volcanic terrain yield dates of between 11.91 +/- 0.13 Ma and 5.48 +/- 0.07 Ma, corresponding to an extended period of Miocene volcanism which defines much of the landscape in this region. Major- and trace-element compositions of lavas from Llullaillaco are typical of Miocene-Pleistocene volcanic rocks from the western margin of the CVZ, and are related to relatively shallow-dipping subduction of the Nazca plate beneath northern Chile and Argentina. Oversteepening of the edifice by stacking of thick, viscous, dacitic lava Rows resulted in collapse of its southeastern Rank to form a large volcanic debris avalanche. Biotite 40Ar/39Ar dating of lava blocks from the avalanche deposit indicate that collapse occurred at or after similar to0.15 Ma, and may have been triggered by extrusion of a dacitic How similar to the one dated at 0.048 +/- 0.012 Ma. The avalanche deposits are exceptionally well preserved due to the arid climate, and prominent levees, longitudinal ridges, and megablocks up to 20-m diameter are observed. The avalanche descended similar to2.8 km vertically, and bifurcated around an older volcano, Cerro Rosado, before debouching onto the salt Rats of Salina de Llullaillaco. The north and south limbs of the avalanche traveled 25 and 23 km, respectively, and together cover an area of approximately 165 km(2). Estimates of deposit volume are hampered by a lack of thickness information except at the edges, but it is likely to be between 1 and 2 km(3). Equivalent coefficients of friction of 0.11 and 0.12, and excess travel distances of 20.5 and 18.5 km, are calculated for the north and south limbs, respectively. The avalanche ascended similar to 400 m where it broke against the western Rank of Cerro Rosado, and a minimum now velocity of similar to 90 m s(-1) can be calculated at this point; lower velocities of similar to 45 m s(-1) are calculated where distal toes ascend similar to 200 m slopes. It is suggested that the remaining precipitous edifice has a high probability Tot further avalanche collapse in the event of renewed volcanism. (C) 2001 Elsevier Science B.V. All rights reserved.