The chemistry of the ash can often be tied directly to known volcanoes or ash layers in sediment.
Many historic eruptions over the past 2,000 years are confidently tied to core data, and others can be approximately tied to other historically or geologically dated eruptions.
Isotopes are thus an indicator for climate change.) in the air bubbles show that there have been changes in the concentration of these greenhouse gases over time.
The levels of both are much lower during glacial periods, lower than even pre-industrial levels in the modern atmosphere.
This drill recovers a core several metres long and 10 cm (4 inches) in diameter.
The core hole is kept open with solutions that do not freeze in order to combat the pressure of the ice that would otherwise force the hole closed as progressively deeper cores are extracted.
Crystal structure helps determine age, as well as changes in temperature, snowfall, seasonality, and potential ice movement.
Dust content is measured directly by lasers and by melting fractions of the ice and collecting residuals.
The record for the longest span of time recorded is found at Vostok, high on the Antarctic ice sheet, possibly extending back 400,000 years.
Dating of such records, however, must be done indirectly by correlating them to other records, such as marine oxygen isotope stages, meltwater pulses seen in sea-level curves, or pollen.
These successful but limited attempts were followed by the American-Danish-Swiss cooperative Greenland Ice Sheet Project (GISP-1) in southern Greenland from 1979 to 1981.