A mysterious "boom" that resounded across Vancouver, Washington early Friday may have been an extraterrestrial wake-up call, theorizes a geophysicist with the U.S. Geological Survey in Vancouver.

"I can't think of any other explanation, other than a fairly substantial gravel quarry explosion," said Jeff Wynn, research geophysicist with the Cascades Volcano Observatory.

Local gravel quarries reported no activity, especially at 6 a.m.

Several online readers last week offered theories about the noise, which some reported rattling windows and spooking animals. But, in a story on Saturday, experts ruled out some of the obvious theories. It wasn't a thunderclap. It wasn't a volcanic eruption. As far as emergency managers know, nothing exploded on the ground.

Wynn said Monday he's reasonably confident that it was a relatively large meteorite known as a bolide blowing apart in the atmosphere miles above Vancouver. He said these arrivals are surprisingly common, though normally not in such a densely populated urban area where it's experienced by so many people.

People generally reported the noise in an area of no more than about 10 miles, from west Vancouver to Hazel Dell and Orchards.

"A relatively small object could do that," Wynn said. The object was probably "on the order of maybe a foot when it hit the upper atmosphere. It was probably pretty close to vertical" to be heard in such a confined area.

If it was any bigger?

"Portland wouldn't be here," he said.

Wynn personally studied the landscape impacted by an iron-nickel object that crashed down in a remote area of Saudi Arabia in 1863. It had "all the effects of a Hiroshima-scale atom bomb except one: no radiation," he wrote in an e-mail.

The objects enter our atmosphere at mind-bending speed - 7 to 25 kilometers a second, Wynn said - which causes air to stack up in front and a vacuum behind. When the bolide breaks apart, its now-exponentially larger surface area creates a blindingly bright flash and a sonic boom.

Wynn recalled witnessing one by happenstance while in the midst of a fierce sandstorm on the Arabian Peninsula in 1994.

He had bundled up against the storm inside his Land Cruiser, pulled a thick sleeping bag over his head and had his eyes shut. The flash penetrated the total darkness.

Depending on the size, composition and angle of entry, space rocks can do worse than create a loud noise or an interesting flash.

A bolide that detonated over the Siberian Taiga on June 30, 1908, leveled a forest the size of Rhode Island, Wynn said. Sixty kilometers south of the detonation point, a man in a remote trading post was assembling barrels with his back to the action.

"The first thing he knew, the back of his homespun wool shirt caught on fire," Wynn said. "As he pulled the shirt off, the concussion blast hit him and knocked him end over tea kettle."

Wynn said the man's wife, spotting her husband laying half-naked and unconscious at the base of a tree, lugged him inside their cabin and nursed him back to health.

In the case of the Vancouver boom, he said, the object had to be much smaller and composed of stony material rather than dense iron.

"If people find pieces of this thing on the ground, it will have a burned and pitted look," he said.

Wynn downplayed the chance that it was a sonic boom from an early-morning military operation, both because a spokesman for the Oregon Air National Guard discounted it and because the area affected was more confined than what Wynn would expect from a sonic boom from an aircraft.

"The idea that it would be a sonic boom from a military aircraft is pretty darn small now," he said. "It's a huge waste of energy, and you'd only do that if you're trying to chase someone down and shoot them."