Joint width and mineral infillings are examined in approximately 3,200 cooling joints in basalt drill core. The core is from 4 Grande Ronde Basalt flows in the central Columbia Plateau of Washington. Statistical analysis shows that basalt flows, intraflow structural units, and core holes cannot be differentiated consistently on the basis of joint width. The joints appear to be members of only one, log normally distributed, width population with a median width of 0.14 mm.

Clay is the predominant infilling mineral in the basalt joints, followed by silica and zeolite. Only 0.6 percent of the joints have observable void space. Basalt flows and intraflow structural units generally cannot be differentiated on the basis of joint infilling minerals. The one exception is the Umtanum entablature. A relationship can be observed between joint width and predominant infilling minerals. As joint width increases, fewer of the joints are observed to be filled predominantly with clay and zeolite, and a greater percentage with silica. The narrowest joints are filled almost exclusively with clay.

A conceptual model of the mineral infilling sequence indicates that clay precipitated first in the joints and dominated throughout the entire infilling period. Silica precipitation was less in early stages of infilling and increased in later stages. Zeolite precipitation was also less in early stages, remained fairly constant throughout the middle stages, and dropped off dramatically in the latest stages.