English: Echinus Geyser, eastern Back Basin, southeastern Norris Geyser Basin, Yellowstone Hotspot, northwestern Wyoming, USA on 9 August 2011. (looking ~SE)

Echinus Geyser is now mostly dormant but was formerly a frequently erupting geyser with high spouts, sometimes exceeding 100 feet. The water at Echinus Geyser is acidic, hot, chloride-rich, and sulfate-rich, as are many hydrothermal features in Norris Geyser Basin. Variations in water chemistry measurements indicate that this geyser erupts and discharges a mix of deep groundwater and shallow groundwater.

Spinose geyserite (siliceous sinter) is common at and immediately around Echinus Geyser, inspiring the name (“echinus” means “spiny”). Chemical analyses of geyserite from here show that opaline silica (SiO2·nH2O - hydrous silica) is not the dominant material. About 20% of the geyserite here is silica and about 57% is hematite (Fe2O3 - iron oxide), which is typically brick red to reddish-brown in color (note the reddish-brown color of the geyserite at the geyser and in the runoff channels in the above & below photos). Other significant impurities in Echinus Geyser geyserite include arsenic, aluminum, sulfur, and calcium. Whitish gray, low-impurity geyserite occurs below the surface, usually as irregular-shaped blocks. Subfossil logs occur below that, dating to the early 1400s A.D.

At the surface, some geyserite rubble and blocks occur at Echinus Geyser. These fragments were formed and deposited during violent geyser eruptions. In the late 1970s, Echinus Geyser eruptions spewed small rock fragments composed of banded pyrite-marcasite (both are FeS2 - iron sulfide). These sulfides have been observed in thin hydrothermal veins intruding geyserite in Echinus Geyser’s crater.

Pebbles of lithified rhyolitic ash-flow tuff litter the Echinus Geyser area, derived from adjacent hillside outcrops of Lava Creek Tuff (lower Middle Pleistocene, ~640 ka). The geyserite rubble blocks and volcanic tuff pebbles have coatings of recent geyserite, often covered with spinose projections.