Old Faithful Geyser eruption (6:26 PM-onward, 10 August 2016) 1
Old Faithful Geyser is the # 1 most famous geyser in the world (except maybe Geysir in Icleand, the type locality & namesake for all episodically erupting hot springs). Full-force eruption columns range from about 100 feet to less than 200 feet tall. Most Old Faithful eruptions are "long" and last about 3.5 to a little over 4 minutes. Some eruptions are "short" and last about 2 minutes. Intervals between long eruptions are currently about 1.5 to 2 hours. Intervals after short eruptions are about one hour.
The mottled whitish-grayish-brownish colored rocks at & around the Old Faithful Geyser vent are geyserite, also called siliceous sinter. Geyserite is a friable to solid chemical sedimentary rock composed of opal (hydrous silica, a.k.a. opaline silica: SiO2•nH2O). It forms by precipitation of hydrous silica from hot spring water. Geyserite is the dominant material at & around Yellowstone hot springs and geysers (the Mammoth Hot Springs area is a major exception to this). The silica in the geyserite is ultimately derived from leaching of subsurface, late Cenozoic-aged rhyolitic rocks by superheated groundwater. Rhyolite is an abundant rock at Yellowstone.