Booming Sands:

(SFX: mysterious music under: "Evil desert spirits at times fill the air with the sounds of all
kinds of mucial instruments and also of drums and the clash of arms.")
MARCO POLO WROTE THAT AS HE TRAVELED ACROSS THE ASIAN DESERT TOWARDS
CHINA--AND HE WAS RIGHT.  WELL, HE WASN'T RIGHT THAT THERE WERE DEMONS LIVING
IN THE SAND, BUT HE WAS RIGHT ABOUT HOW MUCH NOISE THE SAND MADE. 

(SFX: booming sand noises.) 

THOSE ARE THE KINDS OF NOISES THAT CAN BE HEARD IN SOME SAND DUNES AROUND THE
WORLD.  SCIENTISTS ARE JUST STARTING TO FIGURE OUT WHAT CAUSES THESE BOOMING
NOISES.  THE SOUND IS MADE BY MINI-AVALANCHES THAT ALTERNATELY STICK AND SLIP
SAYS FRANCO NORI.

Nori: "It's similar to what happens when you take a chalk on the black board and
produce a very high pitched noise. . .at the microscopic level what's happening is like a
stick slip motion, which. . .people do not follow with their eyes but they can hear the
sound." 

NORI IS A PHYSICIST AT THE UNIVERSITY OF MICHIGAN AND HE'S USED A HIGH-POWERED
MICROSCOPE TO LOOK AT THE SHAPE OF THE SAND GRAINS AND SEE HOW THEY
CONTRIBUTE TO THIS AVALANCHING NOISE.   THE BOOMING SAND DUNES ALWAYS
OCCUR IN PLACES WHERE THE SANDS HAVE BEEN CARRIED A LONG DISTANCE BY THE
WINDS. 

Nori: "While they're carried , they get eroded, they get polished, they get rounder and
rounder.  So the grains. . . tend to be more spherical than typical sand grains, they have
a high surface smoothness."

THE WIND ALSO MAKES THE GRAINS EXTRA DRY.  THE COMBINATION OF SMOOTH AND
DRY CREATES THE PERFECT AMOUNT OF FRICTION FOR THE CRUCIAL STICK-SLIP MOTION
OF A BOOMING AVALANCHE.