31 May 2016

A baked horizon in the Columbia River flood basalts

Posted by Callan

Near Wilson Creek, Washington, we beheld a bright orange stripe on the cliff:

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It’s a bold color, and it begs the question: What’s going on here?

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Laterite, a tropical soil, has a similar color scheme, and I’ve seen laterite horizons sandwiched between lava flows before (e.g., at the Giant’s Causeway in northern Ireland). But when I zoomed in on this orange layer, it appeared to be vesicular basalt:

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Can a basalt flow contact metamorphose an older basalt flow?

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Maybe weathering is the key: So the story recorded here could therefore be: lava erupts, lava cools to basalt, basalt weathers from the top down, producing clay and rust-rich saprolite, new lava erupts, cooks weathered basalt beneath, baking it orange.

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Other thoughts? Chime in.