Topographic stress controls fissure segmentation and eruptive cone formation during the Laki and Eldgjá eruptions.

Elongated dykes in tectonic rifts feed extensive basaltic fissure eruptions, yet surface eruption sites are highly discontinuous and unpredictable. Here we show that along-strike topographic variations, and the associated stress gradients, correlate with this localisation. We investigate eruptive cones at the two most productive historical basaltic eruptions in Iceland, Laki (1783-1784) and Eldgjá (~939-940). Morphological analyses of > 300 cones reveal volume variations (100-107 m3) and segmentation along ~ 100 km of fissures. Comparing cone distribution and volumes along the rift zone with pre-eruptive stress models, we demonstrate that high topographic stress gradients inhibit cone growth. We identify a critical stress gradient threshold predicting where larger cones are inhibited, while horizontal stress gradient peaks predict the location of fissure gaps. Our findings establish an empirical and physical link between topographic stress and volcanic morphologies, complementing previous models with a quantitative framework that aims at improving the forecasting of along-rift vent locations and development.