OREGON STATE UNIVERSITY

Stop 29: Cerro Bola

Cerro Bola

23°13.213'S, 67°27.889'W

 

Taking a break from the Purico complex, we travel into the La Pacana Caldera. Build on top of the resurgent dome of the caldera, Cerro Bola is the last large silicic dome we will see on our tour. Like most of the previous lava domes we have seen in our time on the APVC, Cerro Bola takes on the class “torta” shape. Around the flanks of the dome the fallen blocks seem to take quite a beating from the intense winds that dominate the area. Incredible yardangs and hoodoos can be seen all over the area, many taking on amazing and complex shapes, be sure to check it out.

 

Amazing wind erosion

 

Cerro Bola seems to have undergone a few different stages in its formation. The first stage was a series of pyroclastic flows that can now be seen at the base of the dome and in some places around its flanks. These flows are fine grained, ash rich and matrix supported. Pinkish in color, abundant lithics are commonly found with phenocrysts on quartz, plagioclase and hornblende. Obsidian can be found weathering out of this deposit, some pieces quite remarkable.

 

 

 

Petrographic and mineralogic details

Quartz from Bola

 

 

The second stage is the effusive dome building stage, where lavas of dacitic composition we erupted onto the floor of the La Pacana caldera. Like all the other lava domes of the area, Bola is extremely crystal rich (generally over 60% crystals). Mineralogically, the lava displays phenocrysts of plagioclase, quartz, and hornblende is a glassy, microvesicular matrix, which also leads to a somewhat friable texture.

 

 

 

 

 

 

 

Casey on top of Bola Dome Bob looking down from Bola Dome
Bob on Bola

 

 

After all that climbing... we were a bit tired!

Domed out?