This field trip offers the opportunity to explore the spectacular, well-exposed, hydrovolcanic tuff beds at Sinker Butte, a Pleistocene volcano that erupted underneath a freshwater lake in the western Snake River Plain (WSRP). The products of basaltic hydrovolcanism (maars, tuff rings, and tuff cones) are second only to scoria cones as the most abundant volcanic landforms on Earth (Cas and Wright, 1988); however, the mechanics of emplacement of these deposits remain poorly understood.