Thermal interaction between lava lobes

Pahoehoe lava may be emplaced as lobes which flow on top of each other. A cross section was investigated through two lava lobes from the active Pu'u O'o vent, Kilauea, Hawaii, consisting of a 6.6-cm-thick top lobe and the upper part of a 3.2-cm-thick, underlying bottom lobe. The top lobe cooled at the contact with air and has a quenched, glassy margin which is almost I cm thick. The underlying lobe, although originally also in contact with air, is entirely crystalline, suggesting that the bottom lobe experienced crystallization during reheating by the top lobe. Based on published data, experimental results on reheating show that crystallization of basalt glass requires temperatures above 920 degreesC and, knowing that the Pu'u O'o melt is initially at about 1,160 degreesC, the minimum temperature of the bottom lobe should have been 700 degreesC prior to emplacement of the top lobe. Crystal-size distribution (CSD) theory demonstrates an oscillation of grain size which is superposed on an overall increase in size with depth. Cooling rates estimated from simple modeling and growth rates derived from CSD suggest a thermal interaction of the bottom lobe on the top lobe to about 2 cm from the contact.