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Because of the abundance of silicic volcanic activity near Yellowstone, the rocks are typically quite high in silica. Because the hot water at thermal features dissolves the host rock, most of the geysers are filled with silica-rich water. When that water reaches the surface, the interesting features that form in the Geyser basins tend to be made from silica. The Mammoth Hot Springs area is different, however. Here, the water is carbonate rich because of nearby limestone in the subsurface from ancient seas covering the area. The thermal feature formations here are therefore carbonate rich, which is why some of the features at Mammoth look a lot like they come from a coral reef!