Topographic controls on fissure eruptions at Lakagigar and Eldgja, Iceland

The coupling between surface topography and subsurface magma dynamics in volcanic rift zones is a well-established concept; however, quantitative constraints on this interaction remain rare and not systematically explored. In this study, we integrate high-resolution geodetic data from satellite and drone-derived digital elevation models to study eruption vents, cones and associated fractures from the two largest fissure eruptions in historical time, i.e., the Laki (1783–1784) and Eldgja (939–940) eruptions, each tens of km long and hosting dozens of eruptive vents. Comparing cone morphometrics with analytical stress models reveals a statistically significant inverse correlation between topography-induced compressive stress and cone volume. We show that increased confining stress at higher elevations narrows feeder dykes, reducing eruptive efficiency and producing smaller cones. Conversely, larger cones dominate in topographic lows where loading is minimized. Furthermore, we find that steep slopes generate high stress gradients that drive fissure segmentation, arresting lateral propagation and trapping magma beneath mountains. Our models also help to explain why variations in topography correlate with a transition from symmetric grabens in flat terrain to asymmetric fault offsets in complex terrain due to topography-driven vertical shear stress. These findings move beyond conceptual models and establish topography as a predictive parameter for along-rift vent location, discharge patterns, and surface deformation, offering a quantitative framework for volcanic hazard assessment in rift zones.