West Thumb Geyser Basin, including Potts Basin to the north, is the largest geyser basin on the shores of Yellowstone Lake in Yellowstone National Park. The West Thumb of Yellowstone Lake was formed by a large volcanic explosion that occurred approximately 150,000 years ago (125,000–200,000). The resulting collapsed volcano later filled with water forming an extension of Yellowstone Lake. The West Thumb is about the same size as another famous volcanic caldera, Crater Lake in Oregon, but much smaller than the great Yellowstone Caldera which formed 600,000 years ago. It is interesting to note that West Thumb is a caldera within a caldera. During the winter, the underwater thermal features are visible as melt holes in the icy surface of the lake. The ice averages about three feet thick during the winter. National Park Service
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