Fairy Falls of Wahkeena Creek in the Columbia River Gorge in Oregon cascades over mossy basalt, lined with maidenhair ferns that conceal the cataclysmic geological events of this landscape.
About 40 million years ago, the Columbia River flowed through a wide valley where Mt. Hood Volcano is now located. Violent volcanic eruptions with lava floods, ash falls and mudflows from long-eroded volcanoes dominated the landscape for the next 20 million years. This was followed by even more violent flood basalt flows 12 to 17 million years ago, which caused the river to change course again and again.
When thick lava flows slowly cool, fractures form and propagate inward into the cooling lava body. The reason is that solid, crystallized basalt is denser and therefore occupies a smaller volume than its melt. Such flood basalts often show a tripartite subdivision in cross-section, with thick hexagonal columns at the base, thinner columns in the central zone and more irregular columns at the top of the flow. This pattern represents the temperature zones within the cooling lava, which is uniformly hottest at the base and systematically cooler toward the top.
Today, these basalt columns are covered by mosses and ferns. The abundant rainfall in the temperate rainforest and the unlimited mineral supply from the basalt allow lush vegetation to thrive.
