The Cool(ing) Structures of Columbia River Flood Basalts

Walking through the Columbia Plateau, you might find a similarity to the Great Plains throughout the American Midwest, though under the surface a very different setting exists. An enormous network of solidified lava flows holds record of one of the most famous continental outpourings of basaltic lava on Earth. Though difficult to see from the surface, these lava flows may be seen at Palouse Falls.

The enormous rock walls at Palouse Falls that corral the Palouse River illustrate the dynamics of Columbia River flood basalts, both structurally and in relation to their adjacent flows. Three basic parts define the structure of flood basalt: the lower crust, the core, and the upper crust all of them related to the flow regime and cooling rates of the overall lava flow (Vye-Brown et al, 2013). The extruded lava encompassed an estimated area of ~210,000 km2 during the middle of the Miocene Era, 16-6 million years ago (mya); the bulk of the eruptions took place between 15-15.5mya (Reidel et al, 2013). The basic mechanism of transportation for such voluminous eruptions was via insulated sheets, or what is referred to as "Pahoehoe" lava, based on lavas from Hawaii..

The process began in southeastern Oregon with thin sheets 15 to 50 cm thick, which would cool and solidify on the margins, i.e., form a crust. The solidified marginal crusts then acted as insulation that minimized heat loss and enabled the lava to travel long distances. Once a lobe began advancing, additional lava was injected and caused inflation or ‘lava rising’ that increased the height of the flow sheets beginning at the source of the lava and extended to the northwest in a radial manner that tapered towards the extent of the lobe (Vye-Brown et al 2013).