Sinter mounds are one of the impressive forms in which sinter— deposits of minerals precipitated out of water—accumulates around geysers and hot springs. Sinter mounds develop slowly, increasing by only about an inch per century, but can grow very large; the vast sinter mound surrounding White Dome Geyser at Yellowstone National Park, Wyoming, is an example. (Such a large deposit might eventually close off a geyser vent entirely.) In addition to mounds, sinter also forms as scalloped edges on hot pools, as ornate cones around geysers, and as sheets or terraces. Sheets of sinter, such as the broad sinter plain around Old Faithful Geyser at Yellowstone, occur when mineral deposits are carried across a geyser basin by ﬂowing water. Silica-rich, or siliceous, sinter, the kind that emerges around geysers, is more common than calcium rich, or calcareous, sinter. The former originates underground as the hot alkaline waters that feed a geyser dissolve silica from the surrounding volcanic rock. When the silica-enriched water gushes to the surface and begins to cool, silica precipitates out as siliceous sinter, a porous material commonly referred to as geyserite. (Siliceous sinter—technically, hydrous silicon dioxide—is chemically similar to, and sometimes resembles, opal.) Chemically different, calcareous sinter precipitates from calcium-rich waters, most often in limestone caves, where it accumulates in the form of calcite formations, rising from the cave’s ﬂoor (as stalagmites) or hanging from its ceiling (stalactites). Calcareous sinter also forms the beautiful sedimentary limestone called travertine. Sinter is the umbrella word that encompasses both the several types of material (travertine, geyserite, calcite) and the variety of forms these materials take (mounds, terraces, spikes, ribbons, tubes, drip curtains). The word comes from the Old German, and is an etymological cousin to the English cinder.