Avoca Spring

Sapphire Pool was delightful to watch in the mid-20th century. It was crystal clear and a beautiful color. Visitors stood transfixed, watching a string of bubbles from far below, followed by an overall sizzling, and then a surging boil that engulfed the whole spring. Then, due to the 1959 Heb-gen Lake earthquake, Sapphire began erupting 125 feet (38 m). These violent eruptions increased the size of Sapphire’s crater, destroyed its unique biscuits, and changed its shape from circular to oval. By 1964, the eruptions were much smaller, and a few years later it ceased any significant activity. Take the boardwalk loop to the right beyond Sapphire to see several other features, including the deep cavernous vent of Black Pearl Geyser. Black Pearl was thickly studded with quarter-inch black knobs when it was named in the first part of the 20th century.

At the boardwalk curve is Mustard Spring, really two small geysers connected underground. Avoca Spring is the grotto-like cone located near Mystic Falls Trailhead. Avo-ca was a boiling spring until 1959, when it developed into an erratic geyser. Silver Globe Geyser, next to Avoca and connected to it, can mesmerize you when it has silvery white bubbles rising from deep within its clear pool. Here’s proof of the erudition of early Yellowstone namers, especially of tour guide George Henderson: he called the geyser-ite arch in and over Silver Globe Geyser the Zygomatic Arch, because it reminded him of a human cheekbone. A number of very hot features (including Avoca Spring and others to its south) are grouped into the Silver Globe Complex. Geyser expert T. Scott Bryan has suggested that a marked increase in activity here beginning in 1983 was linked to an Idaho earthquake of that year centered 150 miles (240 km) away at Borah Peak (Idaho’s highest point).

Starting across from Avoca Spring, the Mystic Falls Trail is about 3.5 miles (5.6 km) round trip, with steep sections near the falls. A left turn at the first junction would lead you through backcountry to Summit Lake in 7 miles (11 km) on Yellowstone’s western section of the new Continental Divide National Scenic Trail. To reach Mystic Falls on the Little Firehole River, go left at the next trail junction instead. (The right-hand trail takes you in 0.7 mile (1 km) to an overlook of Biscuit and Upper Geyser basins.) Trail junctions are not well marked except at the falls itself. If you stay left, you’ll follow the Little Firehole River to the 70-foot (21 m) drop of Mystic Falls. This delightful hike may reward you with a profusion of unusual wildflowers.

Avoca Spring Photo Gallery



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