Glassy and volcanic. Although, the base of large pyroclastic sheets will often be glassy where they've been quenched / compacted while still hot: a "welded tuff".

I've never been to this area but would love to go. Winter or summer.

And if there is a small eruption there...I'm loading the van and heading west!

I remember following the story of Mammoth Lakes during the 1980's when they detected alot of seismic activity and surface deformation. Talked later to a USGS guy whose life was threatened when he urged the town to have a better evacuation plan.

The USGS site you link to is very good (aren't they all?). I encourage others interested in this topic to go to the home page via your links.