East of Shoshone, at a roadcut in the Resting Spring Range, we came across this dark band of rock. It is an amygdaloidal obsidian (volcanic glass), and all the amygdules (gas bubbles with mineral deposits inside) are flattened. It occurs in the middle of a thick, volcanic ash deposit (tuff) of rhyolitic composition. The explanation is that the ashes were still very hot when they fell and that heat was retained longest in the center of the deposit where it was hot enough to remelt the rhyolitic ashes and convert them to obsidian. Gas bubbles created in the ash fall collapsed as the weight of the overlying ash increased.
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