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The time: June 30, 1908.
The place: central Siberia, Imperial Russia.
A giant meteorite, blindingly incandescent, streaked across the sky and smashed to Earth near the Tunguska River, devastating a roughly circular region nearly 150 kilometers in diameter. Forests were flattened and several herds of reindeer killed. The earth was pitted by cone-shaped craters up to 50 meters across. Ground vibrations from the impact shattered windows scores of kilometers away. Heat seared the bark from trees, and smoke billowed many kilometers into the atmosphere. Shock waves from the blast were "heard" around the world by delicate microbarographs, instruments that measure pulsations in atmospheric pressure set up by very long sound waves.
Had the Tunguska meteorite fallen on New York City, the destruction would have extended east to the tip of Long Island, north to New Haven, Connecticut, West to Wilkes-Barre, Pennsylvania, and south to Atlantic City, New Jersey.
In 1947, another giant fell from space, its brilliance rivaling that of the sun. Known as Sikhote-Alin, this fireball strewed several tons of iron over eastern Siberia near Vladivostok, peppering an area about five kilometers long by 1.5 kilometers wide with more than 100 impact craters. Some were nearly 30 meters across by nine meters deep.
Astronomers estimate that these monsters weighed 100 tons or more before partially vaporizing when they entered the earth's atmosphere.
Because several popular disaster films often provoke interest in an end-of-the-world scenario involving giant meteorites, it seems timely to point out that anyone can visit the sites of such catastrophes. Before we take you to the impact craters of North America themselves, however, a look at their origins will make it clearer just how spectacular these now-quiet landscapes really are.
Origins of Craters
Just after the solar system formed about four billion years ago, the new planets swept millions of rocks and metal fragments from their paths. At that time, craters were gouged on the moon, Earth, Mars, and Venus a thousand times more often than they are today. Missiles, 1,000 kilometers across, plunged into the earth, rupturing newly solidified crustal rock.
The moon, Mercury, and Mars still have thousands of craters. Earth must surely have had as many in its youth. Over billions of years, however, the forces of geology and weather eroded many of them beyond recognition. Even so, geoscientists discover more giant meteorite craters each year. In Canada, some of the oldest known craters on our planet have survived because they were long protected under sheets of glacial ice. The Canadian Shield region around Hudson Bay boasts many well-preserved craters. The most ancient of these are a billion years old.
Astronomers have long suspected that many meteorites originate in the rubble of the asteroid belt, between Mars and Jupiter. More than 2,000 "midget planets" have been observed in this region. Comets have also been implicated as a source of meteorites, both large and small. Some 30 asteroid-like bodies called Apollo objects, or "earth grazers," swoop close to our planet periodically. Many astronomers presume they may be remnants of old comets rather than asteroids.
One Apollo object, named Adonis, comes within 2.5 million kilometers of Earth. Another, Hermes, approaches to 780,000 kilometers, only twice the distance of the moon from the earth. Others have come even closer.
Astronomers don't become nervous when Earth grazers sweep in close to Earth. The probability that one will plunge into our planet is very small: A direct hit occurs every 250,000 years. But if one should strike a bull's-eye, watch out. It could produce the impact of 100,000 ten-megaton hydrogen bombs and blast a crater 20 kilometers in diameter.
Craters to Visit
If you happen to be touring the United States or Canada, there are numerous giant impact craters of North America you can visit. It will take some hiking or better yet, a light plane to get to some of them, but they are well worth the effort.
The best-known impact site is the Arizona Crater, located in the desert just south of Highway 40, about 48 kilometers West of Winslow, Arizona. It is also known as the Barringer Crater, after D.M. Barringer, who stoutly defended its meteoric origin against a host of geological doubting Thomases around the turn of the century
The Arizona Crater is a model meteorite crater, closely resembling those on the moon. It is nearly circular, about 1,200 meters across and 200 meters deep. The upturned lip of its rim rises some 30 meters above the surrounding desert. Rock and metal fragments have been found scattered several kilometers from the impact site. Deep drilling in the crater has produced materials rich in nickel-iron, but no massive meteorite has ever been discovered. Astronomer E.J. Opik estimated that the Arizona meteorite weighed 2.6 million tons before it entered the atmosphere—a lump of iron the size of 200 ranch-style houses. It may have measured 100 meters in diameter.
Another spectacular meteorite crater, in New Quebec, Canada, is a colossal water-filled bowl 3.2 kilometers across. It was discovered in 1950. The highest point of its crater wall rises 410 meters above the lake bottom. While the Arizona Crater is about 50,000 years old, the New Quebec Crater is roughly 5 million years old. Unlike the Arizona Crater, the New Quebec Crater is not accessible by road.
All known meteorite craters have several features in common. They look like bowl-shaped excavations surrounded by uplifted rims. The outer crater slopes are gentle, but the inner walls are steep. The rim may be blanketed by rubble, ranging from fine dust to large boulders, also found in the bowl itself. Older craters tend to fill up with sediment, and the rim and outer debris erode away. The outer slopes and the surrounding plain of young and small craters, such as the Arizona site, are sprinkled with meteorite fragments, sand, and jagged rock.
While the remains of an iron meteorite are easily identified lumps of metal, fragments of a stony meteorite may be impossible to distinguish from local rock. Near a relatively young crater, the rock may be fused into glass or slag similar to volcanic cinders.
Meteorites striking at the rate of more than four kilometers per second produce geological features that may elude the untrained eye. Something called shock metamorphism creates igneous rock, recrystallized after being melted by the heat of impact. Also found are rocks containing high-density forms of quartz called Coesite and Stishovite, which are produced only by the combination of intense heat and pressure. You may also see shatter cones—large structures of quartzite that flare outward and down—produced by the meteorite's high velocity.
The Best Impact Craters of North America
Name: Brent, Ontario, Canada
Diameter (km): 3.8
Surface Features: Sediment-filled depression
Name: Crooked Creek, Montana
Diameter (km): 5.6
Surface Features: Oval area of disturbed rocks, shallow depression
Name: Decaturville, Montana
Diameter (km): 6.0
Surface Features: Slight oval depression
Name: Flynn Creek, Tennessee
Diameter (km): 3.8
Surface Features: Sediment-filled shallow depression with slight central elevation
Name: Haviland, Kansas
Diameter (km): 0.011
Surface Features: Evacuated depression
Name: Holleford, Ontario, Canada
Diameter (km): 2.0
Surface Features: Sediment-filled shallow depression
Name: Kentland, Indiana
Diameter (km): 13.0
Surface Features: Central uplift exposed in quarried, rest buried
Name: Lake Wanapitei, Ontario, Canada
Diameter (km): 8.5
Surface Features: Lake-filled, partly circular
Name: Middlesboro, Kentucky
Diameter (km): 6.0
Surface Features: Circular depression
Name: Odessa, Texas
Diameter (km): 0.17
Surface Features: Sediment-filled shallow depression with very slight rim
Name: Serpent Mound, Ohio
Diameter (km): 6.4
Surface Features: Circular area of disturbed rock, slight central elevation and surrounding depression
Name: Sierra Madera, Texas
Diameter (km): 13.0
Surface Features: Central hills, depression, outer rim of hills
Name: Wells Creek, Tennessee
Diameter (km): 14.0
Surface Features: Basin with central hill and inner and outer rings of valleys and ridges
Name: West Hawk Lake, Manitoba, Canada
Diameter (km): 2.7
Surface Features: Circular lake
These craters only cover meteor impacts in North America. To learn about some of the lesser-explored North American craters, or to educate yourself on craters throughout the world, read Meteorite Craters and Impact Structures of the Earth.
Meteorite Craters and Impact Structures of the Earth by Paul Hodge
Earth has been hit with countless meteorites and asteroids during its five billion year history. Meteorite Craters and Impact Structures of the Earth by Paul Hodge describes all 139 sites across the world where evidence of these impacts can be seen. Each site includes a summary table with location, age, size, and present condition, and maps are included as necessary.