[Above] In the fiery eruption of a volcano, a plume of magma (melted rock) from the planet's mantle is brought to the surface and ejected as a mixture of hot gas, dust and lava.
for Geology

by Lynn Yarris

For most of the disciplines that come under the aegis of earth sciences, "guesstimates" are the rule. Scientists make educated guesses as to the physical composition of Earth's mantle and core, the hiding places of oil, coal and geothermal reservoirs, or the movement of water through underground rock. A notable exception is the discipline of isotope geochemistry where measurements are made at the atomic scale and are so exact they must be made in dust-free "ultraclean" environments.

"Isotope geochemistry is the experimental branch of the earth sciences," says Donald DePaolo, a geologist who directs the Center for Isotope Geochemistry (CIG) at Berkeley Lab. "With isotope geochemistry, we can tell you how many years a mountain exists before it gets washed to the sea."