This is the direct method of measuring lava temperatures. A thermocouple probe is inserted into the flow, and the temperature is read off by a hand-held device. This particular flow was not very active so it was easy to approach it and stay long enough for the thermocouple to equilibrate in the lava. Many times this is not the case and the temperature readings are not very accurate. In fact, George Walker (a famous volcanologist) has said "the temperature of a lava flow is inversely proportional to the comfort of the volcanologist making the measurement". (photo by P. Mouginis-Mark) The volcanologist must have been pretty comfortable this day because the measured temperature was almost 1160 degrees C (almost 2100 degrees F).
This is a remote method of determining the temperature of a lava flow. This geologist is holding a device that was originally invented to measure temperatures in industrial blast furnaces. Even at this distance of about 3 meters he still needs to wear a long sleeve fire-fighter's shirt and a knit hood over his head for protection against the radiant heat.