HOLES IN OUR KNOWLEDGE: THERMAL PROPERTIES OF VOLCANIC 'HANGI' STONES USED IN MAORI EARTH OVENS

The thermal properties of igneous rocks are not comprehensively understood, especially in the subject of anthropogenic (not in-situ) heating and cooling of rocks. One topic that especially lacks understanding is the effect of vesicularity on the thermal properties of volcanic rocks. Hāngi, a traditional Maori cooking method, utilizes heated volcanic rocks to cook food in an earth pit, and in the region of New Zealand’s Banks Peninsula, vesicular volcanic rocks are preferred. This study investigates the Mātauranga Māori (Māori knowledge) of the hāngi process, reviews literature on thermal properties, and undertakes a laboratory simulation of the heating and cooling of traditionally sourced hāngi stones. Two experiments were performed using high precision thermal imaging to monitor the temperature trends during the heating and cooling of rocks with varying vesicularities. In both experiments, rocks with higher vesicularity responded more quickly to changes in surrounding temperatures than did rocks with lower vesicularity. This implies that vesicularity lowers the heat capacity of rocks. Our experiment also suggests that the size of vesicles influences heat capacity: a network of many small vesicles appears to lower heat capacity more than a network of fewer large vesicles. In applying this to Mātauranga Māori, vesicular stones may have been utilized as they require less firing (heating) to reach optimum temperature, yet supply enough residual heat for the hāngi process. These findings suggest that the Māori of Banks Peninsula may have chosen hāngi stones based on an understanding of their thermal properties, specifically heat capacity.