Columnar dacite showing flow banding, Eocene volcanic rock: formed 33.9 to 56 million years old; Nipple Mountain, Kelowna, British Columbia; Donated by Don Sanberg.
Dacite is a fine-grained volcanic rock which forms from lava that has been extruded above or near the surface and is fluid enough to flow. When a thick lava flow cools it shrinks, sometimes causing it to fracture in a regular pattern, creating the columns seen in this specimen. The process is analogous to the shrinkage cracks, which form in mud as it dries. Dacite’s composition is different from basalt (seen in the other Rock Garden columns), causing it to be lighter in colour.
Flow banding is produced when structures, produced by the movement of the lava when it is still fluid, are frozen when it solidifies. This flow banding occurs in much the same way that folds in home-made toffee are hardened into the candy when it cools.