THE ONSET OF DEGRADATION OF THE HOLUHRAUN SPATTER RAMPART

The long-term erosion of volcanic edifices associated with fissure eruptions, such as cinder cones and spatter ramparts, can be quantified by morphometric parameters and slope diffusion equations. These techniques have been used to estimate the ages of cinder cones within volcanic fields where only a small subset have been dated using radiometric or carbon dating techniques. The very earliest stages of degradation of cinder cones, or spatter ramparts, has not been as well studied, and is likely influenced by different factors than those that affect the long-term evolution of their morphology. Unlike radially symmetric cinder cones, spatter ramparts form as elongate pyroclastic deposits along a volcanic fissure vent. In most cases on Earth as well as on other planetary surfaces where the eruption was not able to be observed, the initial form of the vent edifice is lost, leaving the degraded state to be used to infer eruption dynamics. Depending on how rapid the initial rate of change is, this could lead to misinterpretation of eruption style based on the current state of the vent edifice.

A primary deposit of the August, 2014–February, 2015 Holuhraun fissure eruption in northern Iceland was a 500 m long by 50 m high spatter rampart along the main vent, called Baugur. The Holuhraun eruption was thoroughly documented over the 6-month duration of activity, providing a unique opportunity to compare the initial degradation of the spatter rampart to constructional dynamics. We acquired cm-scale topographic data of the vent region post-eruption during the summers of 2015, 2016, and 2018 with terrestrial scanning lidar and aerial photogrammetric surveys. Our topographic and numerical analyses of this time series quantify the year-to-year average rates of erosion. We identify areas of episodic collapse on the interior of the vent and diffusion effects on the exterior slopes. We discuss the earliest influences on erosion, such as oversteepened slopes, fumarolic activity, and cooling and contraction of the lava body. These factors, plus the lack of vegetation in the area, allow us to consider the degradation of Baugur as an analog for spatter ramparts in ultra-arid environments such as the Moon and post-Noachian Mars.