Shear thinning and brittle failure of silicate melt control the dynamics of volcanic eruptions, but their molecular‐scale origin is still unclear. Here, we conducted tension and compression experiments on silicate melts, using time‐resolved X‐ray diffraction. Our experiments revealed that the intermediate‐range ordering of silicate structures, that is, the ring size formed by the SiO4 tetrahedra, demonstrated elastic and anisotropic dilation under tension and shrinkage under compression in the non‐Newtonian regime. In contrast, there were no significant changes in short‐range ordering, such as Si–O and Si–Si distances. Based on these findings, we inferred that shear thinning observed under high stress originates from the formation of anisotropically deformed large and small rings in silicate structures that are energetically unfavorable and unstable. Brittle failure occurred under high‐stress conditions, in both tension and compression. We propose a stress criterion as a necessary and sufficient condition for magma failure, rather than a strain rate criterion. Plain Language Summary Explosive volcanic eruptions occur when magma fragments and volcanic gases are released to the surface. However, magma is originally a continuous fluid that behaves like a liquid in the crust. To understand how magma fails during volcanic eruptions, we need to uncover the underlying mechanisms. This challenge was identified over four decades ago, and researchers proposed conducting molecular‐scale experiments on magma deformation to shed light on its complex behavior, including brittle failure. However, this has been difficult to achieve experimentally. In this study, we used powerful X‐ray sources at SPring‐8 in Japan to address this challenge. Our findings reveal that the complex behavior of magma originates from previously unrecognized molecular‐scale elastic and anisotropic deformation. Finally, we propose a criterion for magma failure that can help determine whether an eruption will be explosive. Key Points Molecular‐scale elastic and anisotropic deformation of the silicate melt was observed under tension (dilation) and compression (shrinkage) Formation of large and small rings by the SiO4 tetrahedra could be the origin of shear thinning and brittle failure in the silicate melt Stress condition is “the necessary and sufficient condition” for the magma failure criterion