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Typical joint features in the Roza Member of the Columbia River Flood Basalt based on the exposure at Banks Lake, Washington. From Self and others (1997).
Perhaps the most characteristic feature of the Columbia River Flood Basalt Province is the similarity of individual lava flows. Most flows consist of colonnade (base), entablature (middle), and a vesicular and scoracious top. Colonnade is caused by slow cooling of ponded lava. Entablature is probably the result of cooling caused by fresh lava being covered by water. The flood basalts probably damned rivers. When the rivers returned, the water seeped down the cracks in the cooling lava and caused rapid cooling from the surface downward (Long and Wood, 1986). The division of colonnade and entablature is the result of slow cooling from the base upward and rapid cooling from the top downward. Columnar jointing is also found in other igneous rocks.