Tree-ring dating is helping to determine the ages of deposits and eruptive frequencies of several volcanoes in the Cascade Range. Willis Eggler, a geologist at Dartmouth College, cut cross sections from several pines near Paricutin and found that one to three rings were missing along selected radii in several disks. This showed that although annual rings are formed every growing season, rings may not form completely along all radii in years of limited growth. If a core is collected from a tree where one or more rings are missing, the age of the tree or the dates of volcanically induced narrow rings sequences will be inaccurate. Rainfall and daily temperatures vary from year to year.
Climatic fluctuations limit tree growth similarly over large geographical areas.
Though ring-width variation patterns will be similar from tree to tree, the actual widths of rings in different trees will vary. The ring-width variation patterns of different trees will match year for year up to the point where rings are missing in some trees because of volcanic activity or drought. Because of the possible occurrence of missing rings, tree samples should be cross-dated rather than dated by ring counting. Cross-dating with narrow rings can often identify missing rings. The series of missing and narrow rings starting with the 1482 ring were caused by tree injury during fallout of layer We .