“Terraced” Flood Basalt Canyons

Dylan J. Caldwell

Flood basalts belonging to the Colombia River Basalt Group (CRBG) characterize much of the Grande Ronde watershed. 1000 foot canyon walls, composed of the CRBG, shadow the deeply incised meanders of the Grande Ronde. For any boater who paddles the Grande Ronde, it is not difficult to recognize the “terraced” appearance of the canyon walls. These terraces are products of the processes by which they formed and the subsequent erosion that exposed them. The CRBG consists of many different individual flows that erupted over millions of years; sometimes with thousands of years in between consecutive eruptions. It is common for individual eruptions to have varying compositions and to cool under different conditions. For example, a small eruption will not create a very thick layer and it will cool quickly. Conversely, a large eruption will create a thick layer that takes longer to cool and therefore influencing the size of the grains that crystallize. Varying conditions like these can control the induration of a given layer. Induration can be thought of as the resistance to erosion, or the hardness/density of a layer. Layers that are better indurated will resist erosion and protrude out of the canyon wall and create a “terrace”.


Figure 1. Terraced canyon walls along the Grande Ronde River.