<p>Flow of Earth’s upper mantle has long been considered to occur by slow, near-continuous creep. This behaviour is observed in classical high-temperature deformation experiments and is a fundamental component of geodynamic models. The latest generation of high-resolution experiments, however, have revealed that materials ranging from metals to ice exhibit a spectrum of behaviours, termed mild-to-wild plasticity, that range from this mild continuous flow to intermittent wild fluctuations in plastic strain rate. Here we show, using nanoindentation experiments, that olivine exhibits measurable wildness, even under conditions at which its behaviour is expected to be relatively mild. Specifically, during experiments on olivine single crystals at room temperature, continuous plastic flow is punctuated by intermittent bursts of displacement with log-normally distributed magnitudes, indicating avalanches of correlated dislocation motion that account for ~8 ± 6% of the plastic strain. The framework of mild-to-wild plasticity predicts that wildness should increase with depth in Earth, with flow of the asthenosphere occurring almost entirely by wild fluctuations of deformation at the grain scale. The recognition of intermittent plasticity in geological materials provides additional constraints on models of dislocation-mediated flow and raises questions about the mechanisms of transient instabilities in otherwise ductile regimes, such as deep earthquakes and slow-slip events.</p>