Distorted olivines of enigmatic origin are ubiquitous in erupted products from a wide range of volcanic systems (e.g., Hawai'i, Iceland, Andes). Investigation of these features at Kīlauea Volcano, Hawai'i, using an integrative crystallographic and chemical approach places quantitative constraints on mush pile thicknesses. Electron backscatter diffraction (EBSD) reveals that the microstructural features of distorted olivines, whose chemical composition is distinct from undistorted olivines, are remarkably similar to olivines within deformed mantle peridotites, but inconsistent with an origin from dendritic growth. This, alongside the spatial distribution of distorted grains and the absence of adcumulate textures, suggests that olivines were deformed within melt-rich mush piles accumulating within the summit reservoir. Quantitative analysis of subgrain geometry reveals that olivines experienced differential stresses of ∼3-12 MPa, consistent with their storage in mush piles with thicknesses of a few hundred metres. Overall, our microstructural analysis of erupted crystals provides novel insights into mush-rich magmatic systems.