A detailed study was carried out on a single piece of land that had been affected by the impact with a thunderbolt during a violent rain-storm, with thunders and lightning that beat the Island of São Miguel, in the Azores Archipelago, in late October 2006.
Temperature and gas measurements (CO2, CO, H2S and CH4) were performed in four “study trenches”, dug in an area of ~3 m2, where soil combustion was going on following one stroke of lightning, with the emission of a column of vapour and smoke.
The soil under study was a well-pedogenized about 80 cm-thick layer, made of volcanic sandy tephra fallouts (Mdf = 1.05 – sf = 2.23) and contained 5.5% of organic matter. The combustion was monitored during one week and revealed a peak release of 404 ppm CO and 3.4% CO2 originating from a layer located at a depth of about 50 cm. Measurements of temperature gave maximum values of 326 °C inside the soil and 516.5 °C on the surface of a lava block buried at a depth of about 20 cm obtained one week after the impact. It was also observed that the combustion area migrated slightly toward SW during the observation period. The stratigraphic study of the same area showed that immediately under the surface there was an about 40 cm-thick layer of oxidized combusted soil and also a great amount of roots and scorched wooden material were present. Under this more superficial layer there was an about 55 cm-thick soil portion that continued to burn almost in the absence of oxygen due to the presence of up to 19.6% of organic material and of peat deposit, producing a grey-to-white ash without char.
The combustion process went on for about ten days, in spite of several other intense rain storms, until it was artificially extinguished through the excavations made to obtain “study trenches”. This particular circumstance enlightened even more the potential natural hazard represented by this kind of atmospheric event and the matter was, therefore, studied in detail and discussed.