The Solar System

574 PART 4^ |^ THE SOLAR SYSTEM

Barringer Crater

Barringer Crater near Flagstaff , Arizona, is 1.2 km ( 3 __
4

of a mile) in
diameter and 200 m (650 ft) deep. It seems quite large when you
stand on the edge, and the hike around it, though beautiful, is
long and dry (■ Figure 25-19). Barringer Crater was the subject
of controversy among geologists for years as to whether it was
caused by a volcanic event or a large meteorite impact. Finally, in
1963, Eugene Shoemaker proved in his doctoral thesis that the
crater must be the result of an impact because quartz crystals in
and around it had been subjected to pressures much higher than
can be produced by a volcano.
Further studies showed Barringer Crater was created approx-
imately 50,000 years ago by a meteorite estimated to have been
about 50 m (160 ft) in diameter, as large as a good-sized build-
ing, that hit at a speed of 11 km/s, releasing as much energy as a
large thermonuclear bomb. An object of that size could be called
either a large meteorite or a small asteroid. Debris at the site
shows that the impactor was composed of iron.

The Tunguska Event

On a summer morning in 1908, reindeer herders and home-
steaders in central Siberia were startled to see a brilliant blue-
white fi reball brighter than the sun streak across the sky. Still
descending, it exploded with a blinding fl ash and an intense
pulse of heat. One eyewitness account states:

Th e whole northern part of the sky appeared to be covered

with fi re... .I felt great heat as if my shirt had caught

fi re... there was a... mighty crash.... I was thrown

on the ground about [7 meters] from the porch.... A hot

wind, as from a cannon, blew past the huts from the

north.

Th e blast was heard up to 1000 km away, and the resulting

pulse of air pressure circled Earth twice. For a number of nights

following the blast, European astronomers, who knew nothing of

the explosion, observed a glowing reddish haze high in the

atmosphere.

When members of a scientifi c expedition arrived at the site

in 1927, they found that the blast had occurred above the Stony

Tunguska River valley and had fl attened trees in an irregular pat-

tern extending out 30 km (20 mi) (■ Figure 25-20). Th e trees

were knocked down pointing away from the center of the blast,

and limbs and leaves had been stripped away. Th e trunks of trees

at the very center of the area were still standing, although they

had lost all their limbs. No crater has been found, so it seems that

the explosion, estimated to have equaled 12 megatons (12 mil-

lion tons) of TNT, occurred at least a few kilometers above the

ground.

In the early 1980s, a detailed analysis of all the Tunguska

evidence suggested that the impactor’s speed and direction

resembled the orbits of Apollo objects. In 1993, astronomers

produced computer models of objects entering Earth’s atmo-

sphere at various speeds and concluded that the fragile icy head

of a comet would have exploded much too high in the

a

b

■ Figure 25-19

(a) The Barringer Meteorite Crater (near Flagstaff, Arizona) is nearly a mile
in diameter and was formed about 50,000 years ago by the impact of an iron
meteorite estimated to have been roughly 50 meters in diameter. Notice
the raised and deformed rock layers all around the crater. The brick museum
building visible on the far rim at right provides some idea of scale. (M. A.
Seeds) (b) Like all larger-impact features, the Barringer Meteorite Crater has
a raised rim and scattered ejecta. (USGS)