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Mother nature started central Utah's Lighthouse Fire in mid-July, but fire officials say it wasn't lightning — how about two huge boulders?

"This is a first for anybody I am personally acquainted with," said Jason Curry, public information officer for the Utah Division of Forestry, Fire and State Lands.

The Lighthouse Fire burned 862 acres of land after starting on July 18, when a boulder the size of a refrigerator fell 800 feet from a cliff before smashing like a "hammer into an anvil" into another boulder the size of a large SUV, Curry said. It started in the Range Creek area in Emery County.

Curry and three investigators tried to make sense of what they found at the origin of the fire. There was green grass at the bottom of a sheer drop-off and freshly disturbed dirt.

"We found some pretty violent rockfall in the area," Curry said.

Witness accounts said there were no people in the area of the fire when it started, there was no lightning that day.

"We were pretty puzzled," said Curry, who added that he was "very hesitant" to assign the cause of the fire as rockfall, mostly because he had never heard of it.

Curry said that in talking with geologists and physicists they determined, although it is very rare, that it was plausible for a fire to start from the two boulders — composed of sandstone and a little bit of quartzite — hitting each other.

The sandstone boulder fell 200 feet and then bounced 10-12 times downhill, making another 600-foot descent before coming to an abrupt halt.

"Just the sheer amount of energy trapped in that boulder … it all came to a stop instantly," Curry said.

Investigators don't believe a human could have caused the boulder to come loose.

"It would be pretty much impossible to access the cliff where this boulder fell from," he said.

Curry said this kind of natural fire start "is very, very rare" and ranks up there as being as rare as spontaneous combustion, which can occur in moist debris like straw or other materials that stay humid.

Several archeological sites were threatened by the Lighthouse Fire, including rock art, granaries, pit and cliff dwellings, burial sites and other points of archaeological interest along Range Creek, but none were destroyed.

In the April 2000 edition of Wildfire Magazine, a report of a scientific case study includes at least eight incidents where wildfires were caused by rockfalls. The article states the "abrasive action can ignite fine fuel trapped between these objects." Many cases explain how boulders can dislodge in an earthquake, excavation or erosion from a cliff. One case involved a group of Boy Scouts on a canoeing trip in southern Utah by the Green River. The explorer scouts were hiking above the river and playing a game of who could get a boulder to go the farthest down the steep slope. Smoke began to appear in the rockfall area, the study states. The scouts, unaware of what caused the fire, left to continue rafting before fire crews arrived. The actual cause of the fire is officially unknown, according to the study.

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