Geyserite

Siliceous sinter

Entrained within this man-made blob of glass are pieces of geyserite. The little white specks are siliceous sinter, or geyserite, from the Great Geysir in Iceland – the geyser that gave its name to all other geysers on earth. The souvenir was a gift from Patricia W. Dickerson. The blue colors and bubbles are part of the glass manufacture.

Siliceous sinter is a hot- or cold-water precipitate of silica, silicon dioxide (the mineral quartz) that may often (maybe usually) be hydrated and amorphous, in which case it is called opal (or opaline) and it is not strictly a mineral since it has no regular crystalline structure.

Eruptions of hot water – geysers – have been around for billions of years. A geyserite deposit in the Pilbara Craton of Western Australia may hold the oldest known evidence for life on land. The 3.48-billion-year-old rocks contain features interpreted to be stromatolites, layered structures that represent microbial (cyanobacterial) mats. The geyserite is inferred to indicate deposition on land rather than in the sea, and if correct it would push the presence of life on land back by more than half a billion years. There was no life larger than these algae and bacteria on land until the Ordovician or Silurian Period, 400 million years or so ago. The studies were done by Australian geologists.

The word geyser, from Icelandic geysir, ultimately derives from an Old Norse word meaning “to gush.” Since the Great Geysir wasn’t being great when I was there with a Smithsonian Journey in 2004, here’s a photo (above) of Castle Geyser erupting in Yellowstone in 1984. The siliceous sinter around Castle is probably almost indistinguishable visually from that at the Great Geysir.

The existence of the Great Geysir and all the hydrothermal areas of Iceland owe to the fact that Iceland sits on both a deep mantle hot spot as well as the mid-Atlantic Ridge, the location where upwelling heat brings magma to the surface. The convection currents that drive that heat upward split to pull the crust there apart, with new oceanic crust forming at the central rift. This zone is the boundary between the North American and Eurasian tectonic plates.

Faulted basalt along the mid-Atlantic rift in Iceland.

Sinter is a German word that essentially means the same as English cinder, and in siliceous sinter it specifically refers to the cinder-like deposits around geysers and mineral springs. A botryoidal form of geyserite is sometimes called fiorite, for occurrences at Santa Fiora, Italy.