Charles O'Brien, age 14, of Greenville, Miss., for his question:

WHAT IS GEOCHEMISTRY?

Geochemistry is a science that applies chemistry to the study of the earth. Geochemists investigate the distribution and quantities of chemical elements and their isotopes throughout the earth, including its crust, waters and atmosphere.

A geochemist also seeks to determine the natural processes that produce chemical elements.

Geochemistry helps locate deposits of ore, natural gas and petroleum. It also involves identifying and analyzing extremely small amounts of minerals that occur in surface, rocks, soil, plants and ground water. The presence of tiny samples could indicate that large deposits of the minerals lie just below the surface.

Geochemists classify the elements according to their chemical and physical relationships.