Relationship between size of pyroclast flow eruptions and duration of the preceding dormancy period

The techniques of simple correlation and simple linear regression were used to as certain whether there is a relationship between the length of the period of dormancy of a volcano and the maximum distance travelled by pyroclast flow in the subsequent eruption. A positive correlation coefficient of 0.48 was obtained for a data set of 35 eruptions in various parts of the world. This is significant at the 1% level. Correlation and regression were also performed on the cube root of the total volume of pyroclast flow material emitted and the length of the preceding dormancy period. The correlation coefficient of 0.42, for 20 eruptions, was significant at the 10% level. However, when only eruptions for which the dormancy period is less than 200 years are considered, neither the maximum distance travelled by the pyroclast flows, nor their total volume, has a statistically significant linear relationship with dormancy. This suggests that there may be a tendency for large eruptions from volcanoes, which have been apparently quiescent for centuries, to be reported, where as small eruptions from such volcanoes may go unremarked. Inclusion of such biassed data may give misleadingly high positive correlations between pyroclast flow size and dormancy.