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| Why does basalt form columns? When lava erupts, it flows and starts to cool, with the outside margin cooling faster than inside the flow. As lava cools, it shrinks, and polygonal cracks begin to form (similar to mudcracks). But unlike mudcracks, as the lava continues to cool, the cracks (joints) grow longer and penetrate into the flow unit forming columns, called "colonnade". These columns are typically hexagonal in shape. The interior part of the lava flow is the last to cool. It is very rubbley and chaotic, and is called the "entablature". /. |
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 Cooling features of a basalt flow. /. |
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| /. Notice anything unusual about the orientation of the basalt flows near Lewiston, Idaho? The basalts on the south side of the Snake River (right-hand side of photo below) are approximately horizontal, while the basalts on the north side of the river (left-hand side of photo) are dipping steeply towards the river. /. |
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 Lewiston fold. |
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 Schematic diagram of the Lewiston fold. |
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What type of fold is this? If the CRBs are ~16 million years old, is the Lewiston fold relatively older or younger than 16 million years? This fold is a result of localized tectonic activity and is the reason why Lewiston sits topographically lower than Moscow. |
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