Determination of timing of volcanic events by secular variation and thermal modeling

The length of time between emplacement of successive lava flows is difficult to determine with high resolution except in regions of very recent or current activity. In paleomagnetic studies, a tacit assumption of the relative timing is made by considering the effects of secular variation to be averaged in samples from a series of flows, a condition which may not in fact be met. Through the combined use of numerical modeling of the cooling history of sequences of lava flows (erupted at any set of arbitrary time intervals) and detailed measurements of the magnetization directions from field samples, constraints can be placed on the length of time between flows. As individual flow units in a sequence cool, the position within each flow of the last point to pass through the blocking temperature is a function of the relative timing of the flows in the sequence. Additionally, the depth into an underlying flow to which magnetic directions are reset is also a function of this relative timing. In addition to the timing information, the approach appears promising for determination of rates of secular variation in the past.