Steamboat Geyser in Yellowstone National Park had a rare eruption last weekend, hurling mud and rocks up to 1,000 feet, a geologist said.

Its last major eruption was on Sept. 26, 1984, park research geologist Rick Hutchinson said Monday."All the evidence is there that it was a major eruption of what is the world's tallest and most powerful geyser," Hutchinson said.

Two snowmobilers in the Norris Geyser Basin witnessed what was probably the beginning of the eruption's steam phase, which immediately follows the water phase, about 2 p.m. Sunday, Hutchinson said.

Hutchinson said the water phase usually lasts three to 20 minutes, with the geyser typically shooting water 300-400 feet in the air in a northeasterly direction. The subsequent steam phase is "fantastic," he said.

"Imagine a rumbling sound so powerful it is painful to your ears and feels as if your chest is being pounded with padded gloves," he said.