Annual layers of snowfall in ice cores can be counted as easily as tree rings, allowing precise dating of events such as volcanic eruptions. Distinct annual layers stand out because, in snow that falls in summer, crystals are larger and acidity higher than in winter snow. In some cases, scientists can even tell seasons apart, by using a laser to measure the concentration of dust particles. (Winds are generally stronger in springtime, meaning more dust gets blown into the atmosphere.) In this photograph of an ice core drilled in the Kunlun Mountains of western China, the thick, lighter bands indicate heavy snowfall during the monsoon season in the year 1167 AD, while the thinner, darker strips show layers of dust blown into the snowfield during the dry season.
Core photo courtesy of Lonnie G. Thompson, The Ohio University