Conduit Processes at the Haukadalur Geyser‐Hosting Hydrothermal Field (Iceland) Revealed by In Situ Temperature and High‐Speed Camera Measurements.

Geysers fascinate scientists and visitors for several centuries. However, many driving mechanisms such as heat transfer in the conduit and in the subsurface remain poorly understood. We document for the first time transient temperature variations inside the active Strokkur's and nearby quasi‐dormant Great Geysir's conduits, Iceland. While recording temperature inside the conduit, we visually monitored Strokkur's activity at the vent with a high‐speed camera, providing a high temporal resolution of the eruptions. Our results reveal heat transfer from a bubble trap to and through the conduit. We propose a model for the eruptive cycle of Strokkur that includes vapor slug rise, eruption, and conduit refill. Each water jet of an eruption is marked by an initial pulse of liquid water and vapor, emitted at a velocity between 5 and 28 m/s and generally followed by a second pulse less than a second later. The timing of eruptions coincides with temperature maxima in the conduit. After the eruption, the conduit is refilled by water falling back in the pool and drained from neighboring groundwater‐saturated geological units. This results in a temperature drop, the amplitude of which increases with depth while its period is reduced. This reflects faster heat transfer in the deeper than shallower part of the conduit. The amplitude of temperature drop following an eruption also increases with the eruption order, implying larger heat release by higher‐order eruptions. Temperature in the conduit subsequently increases until the next eruption, starting then a new cycle. Plain Language Summary: Geysers are hot springs that erupt intermittently. Although they have been studied for several centuries, many aspects such as heat transfer in the conduit and subsurface remains poorly understood. We recorded the temperature evolution inside the active Strokkur and quasi‐dormant Great Geysir geysers, located 100 m apart, in the Haukadalur hydrothermal field (Iceland). Comparing the timing of eruptions, visually monitored at Strokkur with its temperature records allowed us to characterize the thermal cycle associated with eruptions. Each eruption is marked by a temperature drop and subsequent rise inside the conduit to which we refer as the cooling and warming phases, respectively. We characterize the duration of these phases and the associated temperature variations, which revealed a faster heat transfer in the deeper than shallower part of the conduit and larger heat release by higher‐order eruption. A high‐speed camera revealed the eruption of a water jet in details. Each jet is marked by a first pulse of liquid water and vapor, emitted at a velocity between 5 and 28 m/s, which is generally followed by a second pulse, less than a second later. We propose a new and complementary model for vapor rise, eruption and, conduit refill of Strokkur. Key Points: In situ temperature evolution from an active geyser reveals heat transfer from a bubble trap to the conduitTransient measurement reveals the conduit dynamics during the geyser's eruptive cycleDetails of water jet eruptions captured by high‐speed camera [ABSTRACT FROM AUTHOR]