Mini-columns and ghost columns in Columbia River lava

The master joints bounding the columns that make up the basal colonnade of large lava flows of the Columbia Plateau are, in places, flanked by sub-horizontal mini-columns that have grown normal to the master joints. The secondary mini-columns grow into the main columns and are clearly younger than them. They are small adjacent to the master joint, but merge together and thicken away from the fracture toward the master column interior. Commonly the mini-columns are one-half meter in length and 2–12 cm in diameter. Where the horizontal mini-columns grow longer they intersect toward the middle of the master joints. This plexus of joints changes the aspect of the original master columns making them almost unrecognizable producing ghost columns. The basalt flow may acquire an entablature-like appearance where the ghost column outlines disappear due to extensive secondary fracturing. At the time that the hot flow center had cooled sufficiently below the brittle-plastic transformation, the primary vertical basal colonnade joints growing up from the bottom connected with those in the upper colonnade growing down. This allowed steam trapped beneath the flow to be released to the surface and ushered in a change from a conduction-cooling regime to a convection-cooling regime. The steam beneath the flow was formed and sustained by heat from the lava that boiled the groundwater in the underlying substrate. Large volumes of the rising steam was on average much cooler than the hot fractures in the flow interior through which it passed, causing contraction of the master column walls to produce the secondary horizontal mini-columns. The presence of mini-columns indicates emplacement of lava over moist ground and are absent where the lavas advanced across arid areas or flowed over recently-erupted lava. The extreme shattering that forms ghost columns by late stage convective cooling can produce a flow layer of considerable thickness, a layer that can later serve as an aquifer with high porosity and permeability.