Which geyser in Yellowstone National Park erupts roughly every 90 minutes?

The Yellowstone geyser that erupts roughly every 90 minutes is Old Faithful.

Old Faithful is a cone geyser in Yellowstone National Park’s Upper Geyser Basin in Wyoming. Its eruptions typically occur about 65 to 91 minutes apart, depending partly on how long the previous eruption lasted, making it one of the world’s most predictable geysers. The average interval is commonly given as about 90–92 minutes.

The Washburn-Langford-Doane Expedition named it in 1870, and it became the first geyser in Yellowstone to receive a name. Each eruption can last from about 1½ to 5 minutes and send thousands of gallons of boiling water more than 100 feet into the air.

“Roughly” matters: Old Faithful does not erupt on an exact fixed timetable. Steamboat Geyser can produce taller eruptions, but its timing is far less predictable. Mammoth Hot Springs and Grand Prismatic Spring are thermal features, not regularly erupting geysers.