Iowa’s oldest, rarest and most fearsome stone — the meteorite — has in ancient times left marks on the landscape detectable only by scientists and in modern times frightened hundreds of witnesses.
How fearsome can a rock be? Almost unimaginably so when it weighs 10 billion tons and comes at you traveling 45,000 miles per hour.
That’s the size and speed of the largest meteorite ever to strike Iowa, according to geologist Ray Anderson, who (until its recent cancellation) was among several meteorite experts slated to speak at the annual show of the Cedar Valley Rocks and Minerals Society.
When it struck 74 million years ago near the present town of Manson in northwest Iowa, it unleashed energy equivalent to 10 trillion tons of TNT, said Anderson, who retired five years ago after a career with the Iowa Geological Survey.
Simultaneously, he said, an electromagnetic pulse, radiating at nearly the speed of light, incinerated every flammable object within 130 miles, while a shock wave flattened trees within 300 miles and killed most animals within 650 miles.
A similar strike today, though infinitesimally unlikely, would kill everyone between Denver and Cincinnati, Anderson said.
Today, after eons of erosion and glaciation, there is no visible sign at the surface of the deeply buried crater.
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The area had been known as a geologic anomaly since the early 1900s when the drilling of a new municipal well encountered an unusual sequence of rocks that yielded Iowa’s only naturally soft groundwater.
By 1966 geologists had confirmed the structure originated with a meteorite impact. They theorized the impact vaporized limestone layers that give the rest of the state hard water. They also theorized it may have caused the extinction of dinosaurs 65 million years ago.
Scientists thoroughly investigated that possibility in the early 1990s, conducting isotopic age tests that proved the Manson impact preceded the extinction by about 9 million years.
“It probably killed lots of dinosaurs but not all of them,” Anderson said.
In the case of impact structures, the meteorite itself is typically vaporized, leaving no trace of the celestial body for later discovery, he said.
For many years, as part of his official duties, Anderson evaluated the authenticity of hundreds of suspected meteorites submitted to the Iowa Geological Survey. Almost all of them, he said, were “meteor-wrongs.”
Anderson said Iowa has two known impact structures: Manson, the earth’s 25th largest with a diameter of 24 miles; and the 458 million-year-old, 3-mile-wide structure at Decorah.
Of the thousands of meteorites that have fallen on Iowa, only seven have been recovered. Of those seven, four were observed falling, and the others were found later by farmers tilling their fields.
The Amana meteorite, the most spectacular of the four witnessed falls, traced a fiery course visible in the night sky from Omaha to Chicago and from St. Paul to St. Louis on Feb. 12, 1875.
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It exploded over Keokuk County, its two main fragments continuing north. No samples of the larger fragment, which exploded near the Iowa-Benton county line, were ever recovered. The smaller fragment exploded over Amana, scattering pieces over 15 square miles. Fragments totaling 800 pounds were recovered, the largest weighing 74 pounds.
Much of what is known of the Amana meteorite was recorded by Iowa City engineer Charles Irish, who interviewed witnesses, summarized their testimony and plotted its path.
Irish wrote that its visible flight lasted “not more than 10 seconds.” Its light, however, witnesses told him, “could be hardly tolerated by the naked eye turned full upon it.”
Having traced the path of the meteor for 210 miles, Irish calculated it traveled 21 miles per second.
Irish’s friend and colleague, University of Iowa science professor Gustavus Hinrichs, elaborated on Irish’s findings and later collected many of the meteorite’s fragments.
Writing under the headline, “The Great Iowa Meteor,” in the September 1875 edition of Popular Science Monthly, Hinrichs said “one of the most brilliant meteors of modern times” illuminated the night sky over all of Iowa and lit the southeast portion “bright as day.”
Its passage through the atmosphere generated “detonations of fearful intensity, which in a large portion of Iowa County shook the houses as if moved by an earthquake,” Hinrichs wrote.
At the sights, sounds and tremors, both humans and animals “were overcome with fear,” he wrote.