File:Porkchop Geyser 4.jpg

English: Porkchop Geyser, western Back Basin, southern Norris Geyser Basin, Yellowstone Hotspot, northwestern Wyoming, USA.

Pre-1989 appearance of Porkchop Geyser’s vent area (from National Park Service trailside signage). The photo shows the very small vent (shadowed) - it was about 1 to 2 inches in size.

Porkchop Geyser was formerly known as Porkchop Spring. It’s basin was shallow, had a porkchop-shaped outline, a pustulose geyserite border, and a very small vent. Occasional to regular geyser eruptions occurred here during the 1970s and 1980s; spouts reached up to about 20 feet high. From 1985 to mid-1989, Porkchop exhibited perpetual spouter behavior, with a roaring column of water reaching over 30 feet high.

On 5 September 1989, during the annual basin-wide disturbance, a hydrothermal explosion event occurred at Porkchop Geyser, the first one in Norris Back Basin since the large 1878 event that created Steamboat Geyser. The explosion was immediately preceded by partial collapse of Porkchop Geyser’s very small vent, resulting in a brief, high geyser eruption, reaching to about 100 feet. This abnormal discharge of water lowered pressure in Porkchop Geyser’s subsurface reservoir. The lowered pressure caused a large volume of superheated reservoir water to instantly boil and convert to steam. The steam pressure was sufficiently high to explosively shatter geyserite around the vent and send large to small rock fragments up to 220 feet away (one rock was roughly 6 by 4 by 2.5 feet in size). Angular geyserite boulders and smaller rubble still surround the vent to this day. Geologists who examined the site a few hours after the explosion found some geyserite (= hydrothermal opal - hydrous silica) that was soft and gelatinous; this material was derived from the pre-explosion subsurface conduit walls.

Since the explosion, the feature has been a hot spring with milky blue-colored water and having gentle surface disruptions by rising gases. Occasional geyser eruptions have occurred here since 1989.

Water at Porkchop Geyser ranges from slightly hot to very hot. Unlike many Norris Back Basin features, Porkchop Geyser water is not acidic - it’s usually close to neutral or slightly alkaline.