Stratovolcano flank vents and the origin of Black Butte
You might envision a stratovolcano as one massive cone with a single summit crater. But in reality, volcanoes are complex constructions that may have multiple vents, even on their slopes.
Scientists may refer to the smaller cones and domes on the flanks of a larger stratovolcano as 'subsidiary' or 'parasitic'. The vents which create these features may be related to faults below the volcanic edifice, or they could be the result of structural breaks or weaknesses within the volcano itself.
Black Butte at Mount Shasta is an example of a lava dome complex built over a flank vent on a stratovolcano. This group of dacite lava domes ~13 km (8 mi) west of the summit of Mount Shasta erupted about 10,700 years ago, and have distinctly different lavas from rocks erupted just before on Shasta's summit. In particular, Black Butte lavas have lots of large, black amphibole crystals (phenocrysts) and lack pyroxenes. The eruptions of sticky dacite lava also created pyroclastic flows, which covered the nearby valley floor with 45 km2 (17.4 mi2) of deposits reaching as far as 10 km (6.2 mi) from the cone.
To read more about the history of eruptions on Mount Shasta, visit https://www.usgs.gov/volcanoes/mount-shasta/eruption-history or check out https://pubs.er.usgs.gov/publication/sir20175022K3
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