Yellowstone National Park's Steamboat Geyser, the world's largest, had its first major eruption Tuesday since Oct. 2, 1991, according to park officials.
Two people sleeping in a camper said they were awakened around 5 a.m. by what they thought was an earthquake, park spokeswoman Marsha Karle said.
Frightened, they drove south toward Madison, but looked back and saw Steamboat Geyser emitting a vapor plume about 500 feet high. A heavy mist meanwhile settled over the Norris Geyser Basin about 30 miles north of Old Faithful.
The unpredictable geyser can spout water more than 300 feet, although there was no estimate of the water plume height Tuesday, park officials said.
Steamboat Geyser's intervals between major eruptions can vary from three days to 50 years, including a dormant period from 1911-1961. Bursts of 10-14 feet are more common.
In recent years, the geyser had three major eruptions in 1989, one in 1990 and one in 1991. Major eruptions are typically followed by steam bursts that can last up to 12 hours after the water stops spouting.
A geyser is a hot spring encased in volcanic rock. Water trickles down through the rock until it touches lower rock layers heated by volcanic magma. The contact with the hot rocks heats the water, turning it into water vapor, and sending the steam upward through cracks and fissures in the rock.
Major eruptions occur when the steam cannot escape through the surface, and pressure builds up underground.
Geysers are very raresome scientists say there are only 700 on earthbut Yellowstone claims at least 400 in seven major geyser basins.
Yellowstone Park, the first national park established anywhere in the world, welcomed 3 million visitors in 1998.