<p>The 1959 Kīlauea Iki eruption emplaced olivine-rich lava into a pre-existing pit crater to form a closed-system lava lake. It was drilled multiple times over the next 29 years, presenting a rare opportunity to study cumulate formation. We quantify olivine content and connectivity through the cumulate and overlying olivine-depleted zone by training deep learning segmentation models on µCT scans of quenched drill cores. In the cumulate, olivine approaches full connectivity at crystal fractions of 25–35 vol%, as predicted by percolation simulations. The depth interval over which the cumulate becomes interconnected aligns with geochemical evidence for diapiric extraction of ~ 30 wt% buoyant interstitial melt and resulting cumulate densification. We suggest, therefore, that early compaction of a deformable crystal network was driven primarily by cooling and crystallization rather than by the weight of overlying crystal mush. Early connectivity and crystal repacking, combined with basal cooling accelerated by foundered crust, explain the peak of olivine content toward the top of the cumulate zone, in contrast to basal olivine accumulation in mafic sills of similar thickness. Early connectivity development also provides an explanation for observed propagation of S-waves through the lava lake’s melt-rich (45% liquid) core in 1976.</p>