Old Faithful Geyser eruption (7:34-7:38 PM, 9 April 2016) 1
(via Old Faithful webcam)
Geysers are hot springs that episodically erupt columns of water. They occur in few places on Earth. The highest concentration of geysers anywhere is in Yellowstone’s Upper Geyser Basin.
Old Faithful Geyser is the # 1 most famous geyser on Earth. Its eruption frequency ranges from about a half-hour to about two hours. Full-force eruption columns range from about 100 feet to less than 200 feet tall, and last from over a minute to about 5 minutes long.
The mottled whitish-grayish-brownish colored rocks at & around the Old Faithful 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 hot and superheated groundwater. Rhyolite is an abundant rock at Yellowstone.
This was a "long" eruption and lasted about 4 minutes. Long eruptions by Old Faithful Geyser result in a relatively long interval to the next eruption - typically about 90 minutes. "Short" eruptions, lasting about 2 minutes, result in a shorter interval to the next eruption - typically about one hour.