Giant Geyser is the central geyser of the Giant Group, which includes Mastiff Geyser and Bijou Geyser. In a “normal function eruption,” only Giant erupts, which means it will usually be shorter in terms of time and plume size, attaining 150 feet. Meanwhile, during a “Mastiff function eruption,” where Mastiff erupts alongside Giant (and sometimes Bijou), making Giant rocket upwards of 250 feet.
Giant Geyser has had a long history of amazing visitors and explorers alike—not only for the size of its eruptions, but also for its marvelous geyserite cone. The quick sketch above, done by Walter Trumbell of the 1870 Washburn expedition, is pretty much off the mark proportion-wise, but what’s more important is what it suggests: a stupendous display of thermal geology. A better representation of the cone’s size is evidenced in the picture above the headline, dating from 1909.
Giant was one of the most notable geysers, according to Washburn expedition member Nathaniel Pitt. Langford featured Giant prominently in his article “The Wonders of the Yellowstone,” published in Scribner’s Monthly:
“The Giant” has a rugged crater, ten feet in diameter on the outside, with an irregular orifice five or six feet in diameter. It discharges a vast body of water, and the only time we saw it in eruption the flow of water in a column five feet in diameter, and one hundred and forty feet in vertical height, continued uninterruptedly for nearly three hours. The crater resembles a miniature model of the Coliseum (124).
Later, Langford published the diary he kept on the expedition, which includes a very similar description of Giant Geyser, albeit with some more conservative measurements:
The “Giant” is a rugged deposit presenting in form a miniature model of the Colosseum [sic]. It has an opening three feet in diameter. A remarkable characteristic of this geyser is the duration of its discharges, which can continue for more than an hour in a steady stream about three feet in diameter and one hundred and forty feet high (110-111).
The 1870 Washburn expedition was responsible for the name “Giant,” as the Hayden party learned, according to Marlene Deahl Merrill, when they had to rename their Giant Geyser “Great Fountain.” And through the years, the name has been justified time and time again, captivating Yellowstone visitors throughout time, like in the 1953 photo above.
The video below shows Giant erupting on September 27, 2015 around 5:03 a.m. MST. You can see the eruption after the 11:30 mark in the video below.