OREGON STATE UNIVERSITY
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Continuing our voyage through the APVC we traveled south from the San Pedro/San Pablo area, through the wonderful village of Turi, to the slopes of mighty Chao. Located between the composite cones of Leon and Paniri volcanoes, Chao is the second major lava dome we have come across, and the largest we will see in our time in the APVC. At over 14 km long and 53 km2 in total area Chao is the largest silicic lava flow known on earth - roughly 26 km3 of erupted volume.
Dating of Chao has proven difficult, but due to deposits from recent glaciations it is certain that Chao is at least older than 11,000 years. Ar-Ar dating of Chao has put its age at 420,000 years +/- 100,000, however this is probably an overestimate. What is known is that the formation of Chao was not one discrete event but rather Chao was constructed in at least three stages.
Stage 1 was a mildly explosive stage, which produced coarse non-welded pumice and some block and ash flows later. Produced by this activity were also two overlapping cones on the northern flanks of Chao. Overall this activity resulted in roughly 1 km3 in erupted material.
Built on top of stage I pyroclastics, stage II was the major phase in the construction of Chao. In contrast to stage I, stage II was primarily effusive and resulted in over 22.5 km3 of erupted material. This enormous volume and the steep slope that Chao is formed on resulted in stage II stretching 14 km with flow fronts of over 400 m. As the lava flowed, it also formed impressive ogives, reaching heights of 30 m, seen clearly in the satellite image.
Stage III was the final stage in the formation of Chao. Built on top of the stage II flow, it's 6 km long and 3 km wide. The total volume is roughly 3 km3.
All three stages of Chao are mineralogically similar, with minor variations in mafic enclave content. Phenocrysts of Plagioclase, Quartz, hornblende, biotite, and sphene are seen in a friable glassy matrix. The friable nature of the matrix is due to microvesiculation that took place in the lava.
A few things to note, getting to Chao can be difficult. To get to the base of Chao, as with everything on the APVC, 4WD is required. The road that runs to the south of Chao, between Chao and Leon, is washed out in a few places. However, great camping does exist along this road with some amazing views.
Beware of Vescatchas!... Ok, not really.
