File:Monarch Geyser (9 August 2011) 2.jpg

English: Monarch Geyser, northern Back Basin, southwest-central Norris Geyser Basin, Yellowstone Hotspot, northwestern Wyoming, USA on 9 August 2011. (looking ~E)

Many decades ago, Monarch Geyser was the 2nd most significant geyser in Norris Back Basin (Steamboat Geyser is # 1). In the late 1800s, Monarch erupted periodically and powerfully up to 200 feet high. This activity stopped in the early 1910s. Before it ceased activity, eruption columns from Monarch were observed to be dark-colored (muddy; sediment-rich), indicating subsurface conduit erosion. Such conduit erosion usually indicates that the ability to have additional geyser eruptions is diminished. Since 1913, Monarch has had no activity, except for sporadic activity during a few years in the 1920s, plus geyser eruptions up to 20 feet high from 1994 to 1996.

Monarch Geyser is located in a subcircular amphitheater with partially vegetated sloping walls. The walls expose rhyolitic ash-flow tuff of the Lava Creek Tuff (lower Middle Pleistocene, 640 ka). The amphitheater, or alcove, was presumably eroded through the volcanic tuff by the energetic hydrothermal activity of Monarch Geyser. It’s possible that this feature originated via a hydrothermal explosion event, similar to the explosions at Steamboat Geyser in 1878 and Porkchop Geyser in 1989.

Monarch Geyser’s irregularly-shaped pool has clear, green to pale-green water similar to Emerald Spring and Cistern Spring (see above). The water is hot, somewhat acidic, chloride-rich, and sulfate-rich. Its modern activity consists of overflow, plus minor surface disturbances and boils.