Excitatory synapses on spines of pyramidal neurons are considered a central locus of cortical memory traces. Here we introduce chronic in vivo STED nanoscopy to superresolve dendritic spines in mouse neocortex for up to one month and assess on-going spine remodeling at nanoscale resolution. We find that distinct features of spine geometry, such as neck length and head size exhibit essentially uncorrelated dynamics, indicating multiple independent drivers of spine remodeling. For neck length, neck width and head size, the magnitude of this remodeling indicates substantial fluctuations in synaptic strength, which is exaggerated in a mouse model of neurodegeneration. Despite this high degree of volatility, all spine features influencing synaptic strength also exhibit persistent components that are maintained over long periods of time. Thus, at the nanoscale, stable dendritic spines exhibit a delicate balance of stability and volatility.