The clues offered by ice cores, Cravens said, are kind of like parts of a puzzle. Snow that falls on a region may disappear in a number of ways. It may melt, sublimate or blow away. Melting is the process in which water turns from a solid into a liquid. Sublimation is the process in which it turns from a solid directly into a gas.
Glaciers form in places where more snow falls than disappears.
The snow that falls each season contains gas bubbles and dust particles from the atmosphere. As a glacier builds, each season s snow is squeezed into thin, dense layers. However dense these layers become, they retain information about what the world was like when they formed. These patterns result from different concentrations of dust in the ice. All snow contains a certain amount of dust, and, in polar regions, snow may fall throughout the year.