Thick, slow-moving block-lava flows are associated with extrusive activity in dacitic systems, where lava-core depressurization during flow-front collapse generates devastating block-and-ash flows. Dimensional and rare thermal data collected during January 2000 for an active dacitic block flow at Santiaguito (Guatemala) provide insight into cooling and emplacement mechanisms. Flow velocity was low (12.5 m·d−1), in spite of steep (≥10°) slopes, a result of the high viscosity (>4 × 109 Pa·s) that we calculate for this flow. The flow surface consisted of a thick (1.9–3.4 m), cool (40–111 °C) crust of meter-sized, subangular blocks. Extremely effective insulation by the thick crust results in model-derived core cooling of ≤0.08 °C·h−1. These low cooling rates make block flows the most thermally efficient of all styles of lava-flow emplacement, allowing cooling-limited flow lengths of several kilometers, in spite of low eruption rates (<0.5 m3·s−1). Flow-front observations along with a plug-flow model showed that collapse from the faster-moving flow top contributed to a caterpillar-track–type advance similar to that observed at basaltic ‘a‘a flows. Forward motion also caused toothpaste-like extrusions of the flow core through the frontal crust at basal and marginal shear zones. The axial part of the flow front was thicker than the marginal zones and was oversteepened. This geometry can be explained by a higher vertical velocity gradient in the axial zone, causing more frequent and larger-volume flow-front collapses. Axial-zone collapses also penetrate farther up flow, but not sufficiently to depressurize the flow core and generate a block-and-ash flow. For such a block-and- ash flow to occur, we calculate that an increase in velocity and/or thickness (due to increased slope or topographic confinement) must occur. Whereas low surface temperatures make block flows invisible to short-wave infrared sensors, the low velocity also contributes to the stealthy behavior of these flows. Their stealthy nature, however, masks the fact that they can extend many kilometers, moving block-and-ash flow sources closer to vulnerable communities.