How basalt forms into columns

Fairy Falls of Wahkeena Creek in the Columbia River Gorge in Oregon cascades over mossy basalt, lined with maidenhair ferns concealing 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 mud flows from long eroded volcanoes dominated the landscape for the next 20 million years. This was followed by even more violent flooding lava flows 12 to 17 million years ago that caused the river to change course again and again.
When thick lava flows slowly cool, fractures form that propagate inward into the cooling lava body. The reason is that solid, crystallized basalt is denser and hence occupies a smaller volume compared to its melt. Such flood basalts often show a tripartite subdivision 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, being uniformly hottest at the base and systematically cooler on top.
Today, these basalt columns are covered by mosses and ferns. The abundant rainfall in the temperate rain forest and the unlimited mineral supply from the basalt allow for lush vegetation.