<p>Directly measuring the mechanical behavior of asperities on rock seam surfaces under normal loading is crucial to deepening our understanding of subsurface geological processes. However, quantitatively capturing this phenomenon at both site and laboratory scales remains a significant challenge. To address this, we conducted experimental loading tests on artificially created tight sandstone asperities with spherical caps to explore their nonlinear deformation characteristics and complex damage mechanisms under real-world loading conditions. By performing monotonic normal, ultimate, and cyclic loading tests, we innovatively measured and quantified the peak strain, normal stiffness, and plastic work of the spherical cap asperities. The results revealed that this nonlinear deformation primarily stems from the closure of pores and microcracks within the asperities. Notably, under low-load conditions, deformation accounts for approximately 25%–47% of the pre-crushing deformation. Additionally, asperities of different sizes exhibit both elastic‒plastic and elastic‒brittle‒plastic behaviors. Under loading, the asperities show strain-hardening behaviors similar to those of metals; repeated rupturing events lead to the formation of new load-bearing structures. This increased capacity is due to the asperities themselves rather than frictional resistance from shear cracks. Following cyclic loading and unloading, the hysteresis loops shift forward, with plastic work accumulating and stiffness increasing, both strengthening and damaging the asperities. This study provides a novel approach for examining asperity deformation under geological conditions, offers a new perspective for validating crack closure models, and enhances our understanding of fine-scale crack closure processes.</p>