Geoscience Reference
In-Depth Information
explosions? The answer comes from a fundamental law of physics:
Kinetic energy is equal to one-half the mass of an object times the
square of its velocity. An object of little mass, if traveling fast enough,
can contain a vast amount of energy. Comets and asteroids move at
speeds far beyond our common experience—at cosmic velocities in
the range of 15 km/sec to 70 km/sec, or 33,500 mph to 157,000 mph.
On the average, comets travel at two to three times the speed of
asteroids. Even though ice is less dense than rock or metal, because
energy varies with the square of velocity and because of their greater
speed, comets can do just as much damage as asteroids.
When a comet or an asteroid strikes the solid earth, the energy
inherent in its great speed is converted into an enormous shock
wave. An impact that creates a crater 10 km wide releases about
10
2
5
ergs of energy (10
2
5
= 10 followed by 25 zeros; the erg is a
standard unit of energy; to lift a pound weight one foot requires
1.35 x 10
7
ergs). The impact that produces a 50-km crater releases
about 10
2
8
ergs. For comparison, the 1980 eruption of Mount St.
Helens released 6 x 10
2
3
ergs, the 1906 San Francisco earthquake
about 10
2
4
ergs. The energy budget of the entire earth for one
year—from internal heat flow, volcanic activity, and earthquakes—
is about 10
2
8
ergs. The asteroid envisioned by the Alvarezes would
have released almost 10
3
1
ergs. Because of their great speeds and
the inexorable laws of physics, the impact of comets and asteroids
releases more energy than any earthly process, placing impact in a
destructive class by itself.
During the 1960s and 1970s, impact cratering came to be seen
as a ubiquitous process in our solar system, one in which every solid
object has been struck countless times by impactors of all sizes.
Some of the projectiles were themselves the size of planets. As
knowledge of the scale, frequency, and ubiquity of impact began to
spread through the community of geologists, the notion that one
might have occurred at the end of the Cretaceous, and even that it
might have caused a mass extinction, no longer seemed quite so
heretical. The saving grace of the Alvarez theory, and the reason it
has proven so useful is that, in contrast to many other explanations
of mass extinction, it can be tested. If the Alvarezes are wrong and
the theory is false, the evidence would show it.
